APPARATUS FOR PREVENTING AND CONTROLLING SECONDARY GENERATION OF HYDRATES IN WELLBORE DURING DEPRESSURIZATION EXPLOITATION OF OFFSHORE NATURAL GAS HYDRATES AND PREVENTION AND CONTROL METHOD

Abstract

An apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates includes a gas recovery pipe column, a water recovery pipe column, a gas-liquid mixed transportation pipe section, a data collecting and processing unit, and a reaction control apparatus; according to characteristics of different exploitation pipe columns, three injection pipelines and three monitoring points are arranged to predict dynamic changes in a secondary generation risk of the hydrates throughout the wellbore; measures for preventing and controlling the secondary generation of the hydrates are taken at different pipe column positions by the integrated utilization of inhibitor injection, heating for pipe columns, the additional arrangement of electric submersible pumps, and other methods.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application Ser. CN2022111198097 filed 14 Sep. 2022.

FIELD OF THE INVENTION

The present disclosure relates to an apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates and a prevention and control method.

BACKGROUND OF THE INVENTION

Natural gas hydrates are ice-like cage compounds that are formed when water molecules and hydrocarbon gas molecules are combined at certain low-temperature and high-pressure conditions, which serve as new clean and efficient energy with huge reserves. According to incomplete statistics, organic carbon reserves in the natural gas hydrates are twice as large as the total reserves of fossil energy, such as oil gases, around the world. The natural gas hydrates are typically present in submarine sediments of the deep sea which is more than 300 m in depth, terrestrial permafrost regions, and other low-temperature and high-pressure regions. Vast deep sea regions are ideal environments for the stable existence of the natural gas hydrates, which contain more than 95% of the total reserve of natural gas hydrates, and thus, it is an important direction for energy development in the future.

In the existing exploitation methods (depressurization method, heat injection method, chemical agent injection method, CO₂ displacement method, solid fluidization method, and the like) of the natural gas hydrates, the depressurization method has the advantages of high gas recovery rate, easiness in operation, low costs, and the like, which is deemed as a preferred method for most possibly achieving the commercial exploitation of the natural gas hydrates in the future. During the depressurization exploitation of offshore natural gas hydrates, as seawater temperature will drop with an increase in the water depth (temperature may be as low as 2 to 4° C. at 1500 m beneath the seawater), temperature and pressure conditions for the secondary generation of the hydrates are easily satisfied in exploitation wellbores, which will pose the serious secondary generation risk of hydrates. Once the secondary generation risk of the hydrates is found in the exploitation wellbores, part of generated hydrates will be deposited onto pipe walls to form hydrate deposition layers, resulting in shrinking fluid flow channels, and even obstructing flow in severe cases, which, in turn, causes the safety problems of the flow. In 2017, during the pilot depressurization exploitation of offshore natural gas hydrates, carried out for the second time in Japan, the pilot exploitation process was interrupted twice due to the secondary hydrate generation and obstruction problems in exploitation pipe columns, and the hydrates were removed from the obstructed pipe columns for 31.25 h and 13.5 h, respectively, which severely affected the pilot exploitation schedule. Not only will the secondary generation of the hydrates in the exploitation wellbores of the offshore natural gas hydrates affect the pilot exploitation schedule, but it may affect the subsequent continuous depressurization. The fact that the bottom hole pressure in the pilot exploitation process for the second time in Japan was not reduced to the expected value may also be associated with the secondary generation, and in severe cases, safety accidents of pilot exploitation may even be caused. Currently, the injection of excessive thermodynamic inhibitors, as a major means of preventing and controlling the flow obstacle of the hydrates in deep water wellbores, is used to completely prevent hydrate generation throughout the wellbores. However, the method has defects of large using amounts (10% to 60%) of inhibitors, large storage area, high costs, and high requirements on injection equipment, and especially, the defects become more prominent when the water yield is high, and even the problems that the inhibitors cannot be injected and the like may be encountered, resulting in the failure in the prevention and control scheme for the secondary generation of the hydrates.

In conclusion, a method for preventing and controlling secondary generation of hydrates in depressurization exploitation of offshore natural gas hydrates economically and efficiently has not been found yet. This is a key difficulty that restricts the safe and efficient exploitation of offshore natural gas hydrates. Therefore, the present disclosure is proposed.

SUMMARY OF THE INVENTION

Aiming at defects in the prior art, and especially for problems of large consumption of an inhibitor and poor prevention and control effects of the existing method for preventing and controlling secondary generation of natural gas hydrates, the present disclosure provides an apparatus for preventing and controlling secondary generation of hydrates during depressurization exploitation of offshore natural gas hydrates. Based on the characteristics of different exploitation pipe columns, a combination of inhibitor injection, pipe column heating, the additional arrangement of an electric submersible pump, and other means has been developed to prevent and control the secondary generation of the hydrates during depressurization exploitation of offshore natural gas hydrates. This approach effectively improves the efficacy and economic benefits of the prevention and control of the secondary generation of the hydrates during depressurization exploitation of the offshore natural gas hydrates, providing a guarantee for achieving the flowing safety of offshore natural gas hydrates in the depressurization exploitation process.

A technical solution of the present disclosure is as follows:

The apparatus for preventing and controlling the secondary generation of the hydrates in a depressurization exploitation wellbore of offshore natural gas hydrates includes a gas recovery pipe column, a water recovery pipe column, a gas-liquid mixed transportation pipe section, a data collecting and processing unit, and a reaction control apparatus, and tail ends of the gas recovery pipe column and the water recovery pipe column are connected with a top of the gas-liquid mixed transportation pipe section; the gas-liquid mixed transportation pipe section is positioned in hydrate reservoirs; and the gas recovery pipe column and the water recovery pipe column recover gases and water decomposed by the natural gas hydrates in the reservoirs respectively;

• • the data collecting and processing unit includes a first data monitoring point, a second data monitoring point, a third data monitoring point, and a computer terminal; the first data monitoring point is positioned on a top of the gas recovery pipe column, and collects a temperature, pressure and gas flow of the top of the gas recovery pipe column; the second data monitoring point is positioned on a top of the water recovery pipe column, and collects a temperature, pressure and gas flow of the top of the water recovery pipe column; the third data monitoring point is positioned on a tail end of the gas-liquid mixed transportation pipe section, and collects a temperature and pressure of a well bottom; and the computer terminal receives and processes temperature, pressure, and flow data collected from the first data monitoring point, the second data monitoring point, and the third data monitoring point; and
  • the reaction control apparatus includes a signal actuator, a hydrate inhibitor storage tank, a hydrate inhibitor injection pump, a first inhibitor injection point, a second inhibitor injection point, a third inhibitor injection point, a first electric submersible pump, a second electric submersible pump, and a heater; one end of the signal actuator is connected with the computer terminal, and the other end of the signal actuator is connected with the hydrate inhibitor injection pump; the hydrate inhibitor injection pump is respectively connected with the first inhibitor injection point, the second inhibitor injection point, and the third inhibitor injection point via injection pipelines, and a control valve is arranged on each of the injection pipelines; the first inhibitor injection point is positioned on the top of the gas recovery pipe column, the second inhibitor injection point is positioned at a bottom of the gas recovery pipe column, and the third inhibitor injection point is positioned on the tail end of the gas-liquid mixed transportation pipe section; the first electric submersible pump is positioned at a bottom of the water recovery pipe column, and the second electric submersible pump is positioned in the middle of the water recovery pipe column; and the heater is positioned at the bottom of the gas recovery pipe column.

Preferably, a joint of the water recovery pipe column and the gas-liquid mixed transportation pipe section and a joint of the gas recovery pipe column and the gas-liquid mixed transportation pipe section are provided with a casing pipe, the first electric submersible pump is positioned in the casing pipe, and a blowout preventer is arranged on a tail end of the gas recovery pipe column.

Preferably, a water storage pipe section is arranged in the middle of the water recovery pipe column, the middle of the water recovery pipe column is divided into a first half and a second half of the water recovery pipe column, a tail end of the first half of the water recovery pipe column and a top of the second half of the water recovery pipe column are positioned in the water storage pipe section, and the second electric submersible pump is positioned on the tail end of the first half of the water recovery pipe column.

A prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in wellbore during depressurization exploitation of offshore natural gas hydrates includes the following steps:

The three data collection points are installed on the top of the gas recovery pipe column, the top of the water recovery pipe column, and the tail end of the gas-liquid mixed transportation pipe section, which collect temperature, pressure and flow data at different positions; the different data collection points are connected with the computer terminal, and the collected data is transmitted to the computer terminal in real time; the computer terminal performs analysis and processing on the data collected from the different data collection points, sends instructions to the signal actuator to control inhibitor injection rates of different hydrate inhibitor injection points, and to control power of the heater in the gas recovery pipe column and power of the different electric submersible pumps in the water recovery pipe column to prevent and control the secondary generation of the hydrates in the gas recovery pipe column and the water recovery pipe column.

According to the present disclosure, preferably, the prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates includes the following steps:

• • (1) Real-Time Monitoring of Data at Different Positions
  • temperature, pressure and flow data is monitored at different positions via the first data monitoring point on the top of the gas recovery pipe column, the second data monitoring point on the top of the water recovery pipe column, and the third data monitoring point on the tail end of the gas-liquid mixed transportation pipe section, and the collected data is transmitted to the computer terminal in real time;
  • (2) Analysis of a Secondary Generation Risk of Hydrates Throughout Wellbore
  • the temperature and pressure distributions throughout the wellbore are obtained by the computer terminal via calculation according to the received temperature, pressure and flow data at different positions; the computer terminal judges whether the secondary generation of the hydrates happens at different positions in combination with the phase equilibrium calculation result of the natural gas hydrates, and the secondary generation risk of the hydrates throughout the wellbore is analyzed based on the judgment result, which provides a foundation for the prevention and control of the secondary generation of the hydrates in different pipe columns;
  • (3) Prevention and Control Reaction of Secondary Generation of Hydrates in Different Pipe Columns
  • according to the secondary generation risk of the hydrates in different pipe columns obtained by calculation, the computer terminal sends prevention and control instructions for the secondary generation of the hydrates, and corresponding measures of preventing and controlling the secondary generation of the hydrates are taken for different pipe columns; inhibitor injection is used as the measure for preventing and controlling the secondary generation of the hydrates at the gas-liquid mixed transportation pipe section, the collaborative prevention and control of the inhibitor injection+the heating of the pipe column bottom is used as the prevention and control measure for the secondary generation of hydrates in the gas recovery pipe column, and the collaborative prevention and control of depressurization by double pumps+inhibitor is used as at the prevention and control measure for secondary generation of the hydrates in the water recovery pipe column. These approaches may ensure multiphase flow safety in the exploitation wellbore for offshore natural gas hydrates.

According to the present disclosure, preferably, in step (2), as a significant temperature gradient exists in a stratum/seawater outside the exploitation wellbore for the offshore natural gas hydrates, a temperature difference exists between the fluid in the pipe columns and external environment. Furthermore, given differences in structures of pipe columns at different positions, distinct heat transfer processes are formed between the fluid flow in the exploitation wellbore and the external environment: {circle around (1)} well section below mud line—gas-liquid mixed transportation pipe section: heat transfer between the fluid in the gas-liquid mixed transportation pipe section and the external stratum; {circle around (2)} well section above mud line—gas recovery pipe column: heat transfer between the fluid in the gas recovery pipe column and external seawater; {circle around (3)} well section above mud line—water recovery pipe column: heat transfer between the fluid in the water recovery pipe column and the external seawater; the mud line is a seabed (i.e., a boundary of the seawater and a shallow layer of the seabed); and according to the characteristic of pipe columns for depressurization exploitation of offshore natural gas hydrates and in consideration of the influence of hydrate phase changes on temperature changes, the temperature distribution of the exploitation wellbore is calculated by the following formula based on the principle of conservation of energy:

$\begin{array}{cc}\frac{\partial }{\partial t}\left[A_{\mathrm{te}}{\rho }_m\left(C_{\mathrm{pm}}{T}_f+v_m^2/2\right)\right]+\frac{\partial }{\partial s}\left[A_{\mathrm{te}}{\rho }_m{v}_m\left(H+v_m^2/2\right)\right]=-A_{\mathrm{te}}{\rho }_m{v}_{m}g\sin \theta +\frac{\left(R_{\mathrm{hf}}-R_{\mathrm{hi}}\right)\Delta H}{M_h}+Q_{\mathrm{st}}& \left(1\right)\end{array}$

• • where, C_pm is a specific heat of a mixed fluid at constant pressure, J/(kg·° C.); T_f is a fluid temperature, ° C.; H is a specific enthalpy of the mixed fluid, J/kg; ΔH is a molar enthalpy of formation of the hydrates, J/mol; M_h is a molar molecular mass of the hydrates, kg/mol; ρ_m is a density of the mixed fluid, kg/m³; v_m is a flow velocity of the mixed fluid, m/s; Q_st indicates a heat exchange rate between the fluid in the pipe columns and ambient environment, J/(m·s); s is a position, m; A_te is a net sectional area of the pipe column, m²; R_hf is a generation rate of the hydrates, kg/(m·s); R_hi is a decomposition rate of the hydrates, kg/(m·s); and θ is an angle of inclination, °.

Due to the differences in structures of the exploitation pipe column at different well depths, the calculation of Q_st will vary with different well depth positions;

• • well section above mud line—gas recovery pipe column:

$\begin{array}{cc}{Q}_{\mathrm{st}}=\frac{2r_{\mathrm{tgo}}{U}_{\mathrm{tgo}}}{v_m{r}_{\mathrm{tgi}}^2}·\left(T_{\mathrm{sea}}-T_f\right)& \left(2\right)\end{array}$ $H_d\le H_{\mathrm{sea}}$

• • well section above mud line—water recovery pipe column:

$\begin{array}{cc}{Q}_{st}=\frac{2r_{\mathrm{two}}{U}_{\mathrm{two}}}{v_m{r}_{\mathrm{twi}}^2}·\left(T_{\mathrm{sea}}-T_f\right)& \left(3\right)\end{array}$ $H_d\le H_{\mathrm{sea}}$

• • well section below mud line—gas-liquid mixed transportation pipe section:

$\begin{array}{cc}{Q}_{st}=\frac{2}{v_m{r}_{\mathrm{ti}}^2}·\frac{r_{\mathrm{to}}{U}_{\mathrm{to}}{k}_e}{k_e+T_D{r}_{\mathrm{to}}{U}_{\mathrm{to}}}·\left(T_{\mathrm{ei}}-T_f\right)& \left(4\right)\end{array}$ $H_d>H_{\mathrm{sea}}$

• • where, r_tgo, r_two, and r_to are outer diameters of the gas recovery pipe column, the water recovery pipe column, and the gas-liquid mixed transportation pipe section, respectively, m; T_sea is a seawater temperature, ° C.; U_tgo, U_two, and U_to are overall coefficients of heat transfer based on outer surfaces of the gas recovery pipe column, the water recovery pipe column, and the gas-liquid mixed transportation pipe section as reference surfaces, respectively, W/(m²·K); H_d is a well depth, m; H_sea is a water depth, m; T_ei is an environment temperature, ° C.; r_tgi, r_twi, and r_ti are inner diameters of the gas recovery pipe column, the water recovery pipe column, and the gas-liquid mixed transportation pipe section, respectively, m; k_e is a stratum heat conductivity coefficient, W/(m·K); and T_D is a dimensionless temperature.

According to the present disclosure, preferably, in step (2), the fluid in the hydrate exploitation pipe column is primarily affected by forces of gravity, pressure difference, and frictional resistance during the flowing process. According to the principle of conservation of momentum and in consideration of factors, such as changes in gas volume fraction and changes in gas-water volume fraction distribution arising from gas expansion, a calculation formula of pressure field distribution in the pilot exploitation pipe column of the hydrates is as follows:

$\begin{array}{cc}\frac{\partial }{\partial t}\left(A_{\mathrm{te}}{\rho }_m{v}_m\right)+\frac{\partial }{\partial s}\left(A_{\mathrm{te}}{\rho }_m{v}_m^2\right)+\frac{d\left(A_{\mathrm{te}}{P}_f\right)}{\mathrm{ds}}+A_{\mathrm{te}}g{\rho }_m\cos \alpha +\frac{d\left(A_{\mathrm{te}}\mathrm{Fr}\right)}{ds}=0& \left(5\right)\end{array}$

• • where, P_f is a fluid pressure in the pilot exploitation pipe column, Pa; α is an angle of inclination, rad; and Fr is a frictional pressure drop, Pa.

According to the present disclosure, preferably, in step (2), phase equilibrium temperature and pressure conditions of the natural gas hydrates are calculated by the following formula:

$\begin{array}{cc}{P}_e=10^6\exp \left({\sum }_{n=0}^5{a_n\left(T_f+\Delta T_d\right)}^n\right)& \left(6\right)\end{array}$ $\mathrm{where}.$ $\begin{array}{cc}\{\begin{array}{c}{a}_0=-1.94138504464560\times 10^5\\ a_1=3.31018213397926\times 10^3\\ a_2=-2.25540264493806\times 10^1\\ a_3=7.67559117787059\times 10^{-2}\\ a_4=-1.30465829788791\times 10^{-4}\\ a_5=8.86065316687571\times 10^{-8}\end{array}& \left(7\right)\end{array}$

• • where, ΔT_d is a temperature at which a decline in a hydrate equilibrium is caused by a hydrate inhibitor, K, which may be calculated by the following formula:

$\begin{array}{cc}\Delta T_d=\Delta T_{d,r}\frac{\ln \left(1-x\right)}{\ln \left(1-x_r\right)}& \left(8\right)\end{array}$

• • where, P_e is a phase equilibrium pressure of hydrates, Pa; x is a molar fraction of the hydrate inhibitor in a water phase, which is dimensionless; x_r is a reference molar fraction of the hydrate inhibitor in the water phase, which is dimensionless; and ΔT_d,r is a temperature at which the decline in the hydrate equilibrium is caused under the molar fraction of the inhibitor as x_r, K.

Preferably, in step (2), the secondary generation risk of the hydrates in different pipe columns is determined by comparing the temperature of the pipe columns with the phase equilibrium temperature of the natural gas hydrates; a natural gas hydrate phase equilibrium temperature-pressure curve under the condition of the produced fluid component is converted into a temperature-depth curve by taking into account a temperature and pressure distribution curve of the wellbore and a hydrate phase equilibrium curve, for which coordinate conversion is performed; and when the temperature on the wellbore temperature curve at a certain depth is lower than that on the hydrate phase equilibrium curve, the fluid temperature in the wellbore at the depth satisfies the secondary generation condition of the hydrates, that is, there is the secondary generation risk of the hydrates. A discriminant formula of the secondary generation of the hydrates is as follows:

P_e>P_f or T_e<T_f   (9)

• • where, Te is a phase equilibrium temperature of the hydrates, ° C.

Preferably, in step (3), different prevention and control measures of the secondary generation of the hydrates are taken for different pipe columns in the wellbore; at the gas-liquid mixed transportation pipe section, when the processing result from the computer terminal indicates that the secondary generation risk of the hydrates is found in a horizontal pipe section of the gas-liquid mixed transportation pipe section at the well bottom, the concentration of the hydrate inhibitor as required for preventing and controlling the secondary generation of the hydrates is obtained via calculation according to the prevention and control requirement for the secondary generation of the hydrates, which may be determined according to formulas (6), (7) and (8); the higher the concentration of the hydrate inhibitor, the higher the temperature and the lower the pressure at which the hydrate phase equilibrium is achieved are perceived to be; the concentration of the inhibitor is designed to make the phase equilibrium temperature of the hydrates higher than a fluid temperature or make the phase equilibrium pressure thereof lower than a fluid pressure, thereby avoiding the secondary generation of the hydrates in the wellbore; as an injection rate is associated with the concentration, the inhibitor injection rate is obtained by multiplying the amount of recovered water by the concentration; and then, the inhibitor injection instructions are sent to the third inhibitor injection point on the tail end of the horizontal pipe section, and the control valve on the injection pipeline is opened, thereby effectively preventing and controlling the secondary generation of the hydrates in the gas-liquid mixed transportation pipe section;

• • for the water recovery pipe column, when the processing result from the computer terminal indicates that there is the secondary generation risk of the hydrates in the water recovery pipe column, it is required to take into account the concentration of the hydrate inhibitor which has possibly been present in an aqueous solution, and the concentration of the hydrate inhibitor in the water recovery pipe column is the same as that of the hydrate inhibitor at the gas-liquid mixed transportation pipe section; water in the water recovery pipe column is pumped from the gas-liquid mixed transportation pipe section; if the hydrate inhibitor is not injected into the third inhibitor injection point, the concentration of the existing hydrate inhibitor in the water recovery pipe column is 0; if the hydrate inhibitor is injected into the third inhibitor injection point, the concentration of the existing hydrate inhibitor in the water recovery pipe column is the concentration of the hydrate inhibitor at the gas-liquid mixed transportation pipe section; if the hydrate inhibitor is not injected into the third inhibitor injection point, the computer terminal controls, based on the processing result, the operating power of the first electric submersible pump and the operating power of the second electric submersible pump on the water recovery pipe column to reduce the pressure throughout the water recovery pipe column until the pressure in the pipe column drops to below the phase equilibrium pressure of the hydrates, thereby preventing and controlling the secondary generation of the hydrates therein. Meanwhile, the output power of the first electric submersible pump and the output power of the second electric submersible pump are maintained at a consistent level, which ensures that the liquid level in the second electric submersible pump module stays above the second electric submersible pump to ensure the safety of the fluid flow in the water recovery pipe column; if it is unable to make the pressure of the water recovery pipe column drop to below the phase equilibrium pressure of the hydrates, the hydrate inhibitor needs to be injected into the third inhibitor injection point, and if the hydrate inhibitor has been injected into the third inhibitor injection point, the concentration of the inhibitor in the water recovery pipe column is the same as that of the inhibitor at the gas-liquid mixed transportation pipe section, based on which the operating power of the first electric submersible pump and the operating power of the second electric submersible pump on the water recovery pipe column are controlled to reduce the pressure throughout the water recovery pipe column, making the pressure in the pipe column drop to below the phase equilibrium pressure of the hydrates; meanwhile, the output power of the first electric submersible pump and the output power of the second electric submersible pump are maintained at a consistent level, so that the liquid level in the second electric submersible pump module is stably maintained above the second electric submersible pump; and if it is unable to meet the prevention and control requirement of the hydrates by the depressurization of the electric submersible pumps and the existing inhibitor concentration, a certain concentration of hydrate inhibitor continues to be injected into the third inhibitor injection point to avoid the generation of the hydrate; and
  • for the gas recovery pipe column, when the processing result from the computer terminal indicates that there is the secondary generation risk of the hydrates in the gas recovery pipe column, the computer terminal sends the heating instructions to the heater at the bottom of the gas recovery pipe column to elevate gas temperature in the gas recovery pipe column. After heating, the concentration of the hydrate inhibitor required for preventing and controlling the secondary generation of the hydrates is calculated according to the prevention and control requirement for the secondary generation of the hydrates, and the secondary generation of the hydrates is determined according to formulas (6), (7) and (8); the inhibitor injection instructions are then sent to the first inhibitor injection point and the second inhibitor injection point, and the control valve on the injection pipeline is opened; the injection flow rate of the first inhibitor injection point is independent of that of the second inhibitor injection point, the latter is used specifically to prevent the secondary generation of the hydrates in the gas recovery pipe column. The former, however, is used to stabilize the concentration of the inhibitor and avoid the generation risk of the hydrates arising from throttling and temperature drops of the produced fluid that flows into the platform pipeline; and a heating temperature is encouraged to be at the highest level possible, but an ideal state of being above the phase equilibrium temperature of the hydrates after heating is impossible to achieve for the heating apparatus on site. As such, the secondary generation risk of the hydrates is prevented in the gas recovery pipe column by combining heating and inhibitor injection, that is, heating is performed, and then, the concentration of the injected inhibitor and the injection rate are determined based on the temperature after heating, thereby achieving the prevention and control of the secondary generation risk of the hydrates in the gas recovery pipe column.

All aspects not fully described in the present disclosure shall be referenced to the prior art.

The present disclosure has beneficial effects that

• • 1. According to the present disclosure, dynamic changes in the secondary generation risk of the hydrates throughout the wellbore can be predicted in real time based on the temperature, pressure, and flow data monitored at the different positions on site in real time in combination of a wellbore temperature field calculation model and a natural gas hydrate phase equilibrium prediction model. By this method, the possible specific positions of the secondary generation of the hydrates in different pipe columns can be determined, enabling the accurate positioning of the secondary generation of the hydrates to facilitate more accurate monitoring. This method lays a foundation for the efficient prevention and control of the secondary generation risk of the hydrates in different pipe columns.
  • 2. According to the present disclosure, the exploitation of the offshore natural gas hydrates is divided into two categories: the gas recovery pipe column and the water recovery pipe column. Furthermore, distinct measures will be applied to prevent and control the secondary generation of hydrates in the two respective pipe columns during the exploitation of the offshore natural gas hydrate: the injection of the hydrate inhibitor will be implemented at the gas-liquid mixed transportation pipe section, the collaborative prevention and control of hydrate inhibitor injection+the heating of the pipe column bottom will be implemented at the gas recovery pipe column, and the collaborative prevention and control of depressurization by double pumps+inhibitor will be implemented at the water recovery pipe column. With the combination of the three prevention and control measures, the safe and efficient prevention and control of the secondary generation of the hydrates in the exploitation process of the offshore natural gas hydrates can be achieved to guarantee the multiphase flow safety throughout the wellbore. On one hand, the present disclosure can obviously reduce the using amount of the hydrate inhibitor. On the other hand, the present disclosure can efficiently prevent the secondary generation of the hydrates in the pilot exploitation wellbore, and the multiphase flow safety throughout the hydrate exploitation wellbore can be ensured under the combined action of multiple methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus for preventing and controlling secondary generation of hydrates during depressurization exploitation of offshore natural gas hydrates;

FIG. 2 is an enlarged schematic diagram of a second electric submersible pump module; and

FIG. 3 is a schematic diagram of a secondary generation zone of hydrates in a wellbore.

In the drawings, 1: computer terminal; 2: signal actuator; 3: hydrate inhibitor storage tank; 4: hydrate inhibitor injection pump; 5: control valve 1; 6: control valve 2; 7: control valve 3; 8: first data monitoring point; 9: first inhibitor injection point; 10: gas recovery pipe column; 11: heater; 12: blowout preventer; 13: second inhibitor injection point; 14: second data monitoring point; 15: water recovery pipe column; 16: second electric submersible pump module; 17: first electric submersible pump; 18: casing pipe; 19: gas-liquid mixed transportation pipe section; 20: third inhibitor injection point; 21: third data monitoring point; 22: second electric submersible pump; 23: water storage pipe section; 24: second half of water recovery pipe column; 25: first half of water recovery pipe column; 26: sea level; 27: seawater; 28: shallow seabed; and 29: hydrate reservoir.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure will be further explained with reference to embodiments and drawings, but the embodiments of the present disclosure are not limited thereto.

Embodiment 1

An apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates includes a gas recovery pipe column, a water recovery pipe column, a gas-liquid mixed transportation pipe section, a data collecting and processing unit, and a reaction control apparatus, and tail ends of the gas recovery pipe column and the water recovery pipe column are connected with a top of the gas-liquid mixed transportation pipe section; the gas-liquid mixed transportation pipe section is positioned in hydrate reservoirs; and the gas recovery pipe column and the water recovery pipe column recover gases and water decomposed by the natural gas hydrates in the reservoirs respectively;

• • the data collecting and processing unit includes a first data monitoring point, a second data monitoring point, a third data monitoring point, and a computer terminal; the first data monitoring point is positioned on a top of the gas recovery pipe column, and collects a temperature, pressure and gas flow of the top of the gas recovery pipe column; the second data monitoring point is positioned on a top of the water recovery pipe column, and collects a temperature, pressure and gas flow of the top of the water recovery pipe column; the third data monitoring point is positioned on a tail end of the gas-liquid mixed transportation pipe section, and collects a temperature and pressure of a well bottom; and the computer terminal receives and processes temperature, pressure, and flow data collected from the first data monitoring point, the second data monitoring point, and the third data monitoring point;
  • the reaction control apparatus includes a signal actuator, a hydrate inhibitor storage tank, a hydrate inhibitor injection pump, a first inhibitor injection point, a second inhibitor injection point, a third inhibitor injection point, a first electric submersible pump, a second electric submersible pump, and a heater; one end of the signal actuator is connected with the computer terminal, and the other end of the signal actuator is connected with the hydrate inhibitor injection pump; the hydrate inhibitor injection pump is respectively connected with the first inhibitor injection point, the second inhibitor injection point, and the third inhibitor injection point via injection pipelines, and a control valve is arranged on each of the injection pipelines; the first inhibitor injection point is positioned on the top of the gas recovery pipe column, the second inhibitor injection point is positioned at a bottom of the gas recovery pipe column, and the third inhibitor injection point is positioned on the tail end of the gas-liquid mixed transportation pipe section; the first electric submersible pump is positioned at a bottom of the water recovery pipe column, and the second electric submersible pump is positioned in the middle of the water recovery pipe column; and the heater is positioned at the bottom of the gas recovery pipe column.

According to a prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates during depressurization exploitation of offshore natural gas hydrates, three data collection points are installed on the top of the gas recovery pipe column, the top of the water recovery pipe column, and the tail end of the gas-liquid mixed transportation pipe section, which collect temperature, pressure and flow data at different positions; the different data collection points are connected with the computer terminal, and the collected data is transmitted to the computer terminal in real time; the computer terminal performs analysis and processing on the data collected from the different data collection points, and sends instructions to the signal actuator to control inhibitor injection rates of different hydrate inhibitor injection points, and to control power of the heater in the gas recovery pipe column and power of the different electric submersible pumps in the water recovery pipe column to prevent and control the secondary generation of the hydrates in the gas recovery pipe column and the water recovery pipe column.

Embodiment 2

An apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates is different from Embodiment 1 in that a joint of the water recovery pipe column and the gas-liquid mixed transportation pipe section and a joint of the gas recovery pipe column and the gas-liquid mixed transportation pipe section are provided with a casing pipe, the first electric submersible pump is positioned in the casing pipe, and a blowout preventer is arranged on a tail end of the gas recovery pipe column.

Embodiment 3

An apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates is different from Embodiment 1 in that a water storage pipe section is arranged in the middle of the water recovery pipe column, as shown in FIG. 2, the middle of the water recovery pipe column is divided into a first half and a second half of the water recovery pipe column, a tail end of the first half of the water recovery pipe column and a top of the second half of the water recovery pipe column are positioned in the water storage pipe section, and the second electric submersible pump is positioned on the tail end of the first half of the water recovery pipe column.

Embodiment 4

A prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates as described in Embodiment 1 includes the following steps:

• • (1) Real-Time Monitoring of Data at Different Positions
  • temperature, pressure and flow data is monitored at different positions via the first data monitoring point on the top of the gas recovery pipe column, the second data monitoring point on the top of the water recovery pipe column, and the third data monitoring point on the tail end of the gas-liquid mixed transportation pipe section, and the collected data is transmitted to the computer terminal in real time;
  • (2) Analysis of Secondary Generation Risk of Hydrates Throughout Wellbore
  • the temperature and pressure distributions throughout the wellbore are obtained by the computer terminal via calculation according to the received temperature, pressure and flow data at different positions; the computer terminal judges whether the secondary generation of the hydrates happens at different positions in combination with the phase equilibrium calculation result of the natural gas hydrates, and the secondary generation risk of the hydrates throughout the wellbore is analyzed based on the judgment result, which provides a foundation for the prevention and control of the secondary generation of the hydrates in different pipe columns;
  • as a significant temperature gradient exists in a stratum/seawater outside the exploitation wellbore for the offshore natural gas hydrates, a temperature difference exists between the fluid in the pipe columns and external environment. Furthermore, given differences in structures of pipe columns at different positions, distinct heat transfer processes are formed between the fluid flow in the exploitation wellbore and the external environment: {circle around (1)} well section below mud line—gas-liquid mixed transportation pipe section: heat transfer between the fluid in the gas-liquid mixed transportation pipe section and the external stratum; {circle around (2)} well section above mud line—gas recovery pipe column: heat transfer between the fluid in the gas recovery pipe column and external seawater; {circle around (3)} well section above mud line—water recovery pipe column: heat transfer between the fluid in the water recovery pipe column and the external seawater; the mud line is a seabed (i.e., a boundary of the seawater and a shallow layer of the seabed); and according to characteristic of pipe columns for depressurization exploitation of offshore natural gas hydrates and in consideration of the influence of hydrate phase changes on temperature changes, the temperature distribution of the exploitation wellbore is calculated by the following formula based on the principle of conservation of energy:

$\begin{array}{cc}\frac{\partial }{\partial t}\left[A_{\mathrm{te}}{\rho }_m\left(C_{\mathrm{pm}}{T}_f+v_m^2/2\right)\right]+\frac{\partial }{\partial s}\left[A_{\mathrm{te}}{\rho }_m{v}_m\left(H+v_m^2/2\right)\right]=-A_{\mathrm{te}}{\rho }_m{v}_{m}g\sin \theta +\frac{\left(R_{\mathrm{hf}}-R_{\mathrm{hi}}\right)\Delta H}{M_h}+Q_{\mathrm{st}}& \left(1\right)\end{array}$

• • where, C_pm is a specific heat of a mixed fluid at constant pressure, J/(kg·° C.); T_f is a fluid temperature, ° C.; H is a specific enthalpy of the mixed fluid, J/kg; ΔH is a molar enthalpy of formation of the hydrates, J/mol; M_h is a molar molecular mass of the hydrates, kg/mol; ρ_m is a density of the mixed fluid, kg/m³; v_m is a flow velocity of the mixed fluid, m/s; Q_st indicates a heat exchange rate between the fluid in the pipe columns and ambient environment, J/(m·s); s is a position, m; A_te is a net sectional area of the pipe column, m²; R_hf is a generation rate of the hydrates, kg/(m·s); R_hi is a decomposition rate of the hydrates, kg/(m·s); and θ is an angle of inclination, °.

Due to the differences in the structures of exploitation pipe columns at different well depths, the calculation of Q_st will vary with different well depth positions;

• • well section above mud line—gas recovery pipe column:

$\begin{array}{cc}{Q}_{\mathrm{st}}=\frac{2r_{\mathrm{tgo}}{U}_{\mathrm{tgo}}}{v_m{r}_{\mathrm{tgi}}^2}·\left(T_{\mathrm{sea}}-T_f\right)& \left(2\right)\end{array}$ $H_d\le H_{\mathrm{sea}}$

• • well section above mud line—water recovery pipe column:

$\begin{array}{cc}{Q}_{\mathrm{st}}=\frac{2r_{\mathrm{two}}{U}_{\mathrm{two}}}{v_m{r}_{\mathrm{twi}}^2}·\left(T_{\mathrm{sea}}-T_f\right)& \left(3\right)\end{array}$ $H_d\le H_{\mathrm{sea}}$

• • well section below mud line—gas-liquid mixed transportation pipe section:

$\begin{array}{cc}{Q}_{\mathrm{st}}=\frac{2}{v_m{r}_{\mathrm{ti}}^2}·\frac{r_{\mathrm{to}}{U}_{\mathrm{to}}{k}_e}{k_e+T_D{r}_{\mathrm{to}}{U}_{\mathrm{to}}}·\left(T_{\mathrm{ei}}-T_f\right)& \left(4\right)\end{array}$ $H_d>H_{\mathrm{sea}}$

• • where, r_tgo, r_two, and r_to are outer diameters of the gas recovery pipe column, the water recovery pipe column, and the gas-liquid mixed transportation pipe section, respectively, m; T_sea is a seawater temperature, ° C.; U_tgo, U_two, and U_to are overall coefficients of heat transfer based on outer surfaces of the gas recovery pipe column, the water recovery pipe column, and the gas-liquid mixed transportation pipe section as reference surfaces, respectively, W/(m²·K); H_d is a well depth, m; H_sea is a water depth, m; T_ei is an environment temperature, ° C.; r_tgi, r_twi, and r_ti are inner diameters of the gas recovery pipe column, the water recovery pipe column, and the gas-liquid mixed transportation pipe section, respectively, m; k_e is a stratum heat conductivity coefficient, W/(m·K); and T_D is a dimensionless temperature.

According to the present disclosure, preferably, in step (2), the fluid in the hydrate exploitation pipe column is primarily affected by forces of gravity, pressure difference, and frictional resistance during the flowing process. According to the principle of conservation of momentum and in consideration of factors, such as changes in gas volume fraction and changes in gas-water volume fraction distribution arising from gas expansion, a calculation formula of pressure field distribution in the pilot exploitation pipe column of the hydrates is as follows:

$\begin{array}{cc}\frac{\partial }{\partial t}\left(A_{\mathrm{te}}{\rho }_m{v}_m\right)+\frac{\partial }{\partial s}\left(A_{\mathrm{te}}{\rho }_m{v}_m^2\right)+\frac{d\left(A_{te}{P}_f\right)}{ds}+A_{\mathrm{te}}g{\rho }_m\cos \alpha +\frac{d\left(A_{te}\mathrm{Fr}\right)}{ds}=0& \left(5\right)\end{array}$

• • where, P_f is a fluid pressure in the pilot exploitation pipe column, Pa; α is an angle of inclination, rad; and Fr is a frictional pressure drop, Pa.

According to the present disclosure, preferably, in step (2), phase equilibrium temperature and pressure conditions of the natural gas hydrates are calculated by the following formula:

$\begin{array}{cc}{P}_e=10^6\exp \left({\sum }_{n=0}^5{a_n\left(T_f+\Delta T_d\right)}^n\right)& \left(6\right)\end{array}$ $\mathrm{where},$ $\begin{array}{cc}\{\begin{array}{c}{a}_0=-1.94138504464560\times 10^5\\ a_1=3.31018213397926\times 10^3\\ a_2=-2.25540264493806\times 10^1\\ a_3=7.67559117787059\times 10^{-2}\\ a_4=-1.30465829788791\times 10^{-4}\\ a_5=8.86065316687571\times 10^{-8}\end{array}& \left(7\right)\end{array}$

• • where, ΔT_d is a temperature at which a decline in a hydrate equilibrium is caused by a hydrate inhibitor, K, which may be calculated by the following formula:

$\begin{array}{cc}\Delta T_d=\Delta T_{d,r}\frac{\ln \left(1-x\right)}{\ln \left(1-x_r\right)}& \left(8\right)\end{array}$

• • where, P_e is a phase equilibrium pressure of hydrates, Pa; x is a molar fraction of the hydrate inhibitor in a water phase, which is dimensionless; x_r is a reference molar fraction of the hydrate inhibitor in the water phase, which is dimensionless; and ΔT_d,r is a temperature at which the decline in the hydrate equilibrium is caused under the molar fraction of the inhibitor as x_r, K.

Preferably, in step (2), the secondary generation risk of the hydrates in different pipe columns is determined by comparing the temperature of the pipe columns with the phase equilibrium temperature of the natural gas hydrates; a natural gas hydrate phase equilibrium temperature-pressure curve under the condition of the produced fluid component is converted into a temperature-depth curve by taking into account a temperature and pressure distribution curve of the wellbore and a hydrate phase equilibrium curve, for which coordinate conversion is performed; and when the temperature on the wellbore temperature curve at a certain depth is lower than that on the hydrate phase equilibrium curve, the fluid temperature in the wellbore at the depth satisfies the secondary generation condition of the hydrates, that is, there is the secondary generation risk of the hydrates. A discriminant formula of the secondary generation of the hydrates is as follows:

P_e>P_f or T_e<T_f   (9)

• • where, Te is a phase equilibrium temperature of the hydrates, ° C.

Therefore, when the hydrate phase equilibrium curve is on the right side of the wellbore temperature curve, an area where the hydrate phase equilibrium curve intersects with the wellbore temperature curve is a secondary generation zone of the hydrates, as shown in FIG. 3. Meanwhile, the longer the length the area where the hydrate phase equilibrium curve intersects with the wellbore temperature curve is in the longitudinal direction, the more extensive the secondary generation zone of the hydrates in the exploitation wellbore will be. Additionally, the wider area in the transverse direction will result in the higher degree of supercooling of the secondary generation of the hydrates, making it easier for the secondary generation of the hydrates. Accordingly, the secondary generation risk of the hydrates in different pipe columns may be determined.

• • (3) Prevention and Control Reaction of Secondary Generation of Hydrates in Different Pipe Columns

According to the secondary generation risk of the hydrates in different pipe columns obtained by calculation, the computer terminal sends prevention and control instructions for the secondary generation of the hydrates, and corresponding measures of preventing and controlling the secondary generation of the hydrates are taken for different pipe columns; inhibitor injection is used as the measure for preventing and controlling the secondary generation of the hydrates at the gas-liquid mixed transportation pipe section, the collaborative prevention and control of the inhibitor injection+the heating of the pipe column bottom is used as the measure for preventing and controlling the secondary generation of the hydrates in the gas recovery pipe column, and the collaborative prevention and control of depressurization by double pumps+inhibitor is used as at the measure for preventing and controlling the secondary generation of the hydrates in the water recovery pipe column. These approaches may ensure the multiphase flow safety in the exploitation wellbore for the offshore natural gas hydrates.

Different prevention and control measures for the secondary generation of the hydrates are taken for different pipe columns in the wellbore; at the gas-liquid mixed transportation pipe section, when the processing result from the computer terminal indicates that the secondary generation risk of the hydrates is found in a horizontal pipe section of the gas-liquid mixed transportation pipe section at the well bottom, the concentration of the hydrate inhibitor as required for preventing and controlling the secondary generation of the hydrates is obtained via calculation according to the prevention and control requirement for the secondary generation of the hydrates, which may be determined according to formulas (6), (7) and (8); the higher the concentration of the hydrate inhibitor, the higher the temperature and the lower the pressure at which a hydrate phase equilibrium is achieved are perceived to be; the concentration of the inhibitor is designed to make the temperature of the hydrate phase equilibrium higher than a fluid temperature or make the pressure thereof lower than a fluid pressure, thereby avoiding the secondary generation of the hydrates in the wellbore; as an injection rate is associated with the concentration, the inhibitor injection rate is obtained by multiplying the amount of recovered water by the concentration; and then, the inhibitor injection instructions are sent to the third inhibitor injection point on the tail end of the horizontal pipe section, and the control valve on the injection pipeline is opened, thereby effectively preventing and controlling the secondary generation of the hydrates in the gas-liquid mixed transportation pipe section;

• • for the water recovery pipe column, when the processing result from the computer terminal indicates that there is the secondary generation risk of the hydrates in the water recovery pipe column, it is required to take into account the concentration of the hydrate inhibitor which has possibly been present in an aqueous solution, and the concentration of the hydrate inhibitor in the water recovery pipe column is the same as that of the hydrate inhibitor at the gas-liquid mixed transportation pipe section; water in the water recovery pipe column is pumped from the gas-liquid mixed transportation pipe section; if the hydrate inhibitor is not injected into the third inhibitor injection point, the concentration of the existing hydrate inhibitor in the water recovery pipe column is 0; if the hydrate inhibitor is injected into the third inhibitor injection point, the concentration of the existing hydrate inhibitor in the water recovery pipe column is the concentration of the hydrate inhibitor at the gas-liquid mixed transportation pipe section; if the hydrate inhibitor is not injected into the third inhibitor injection point, the computer terminal controls, based on the processing result, the operating power of the first electric submersible pump and the operating power of the second electric submersible pump on the water recovery pipe column to reduce the pressure throughout the water recovery pipe column until the pressure in the pipe column drops to below the pressure of the hydrate phase equilibrium, thereby preventing and controlling the secondary generation of the hydrates therein. Meanwhile, the output power of the first electric submersible pump and the output power of the second electric submersible pump are maintained at a consistent level, which ensures that the liquid level in the second electric submersible pump module stays above the second electric submersible pump (the whole water recovery pipe column is filled with water, and the liquid level refers to a liquid level of the water storage pipe section, as shown in FIG. 2, which is simply positioned above the second electric submersible pump to avoid the idling of the electric submersible pump), thus maintaining the flow safety of the fluid in the water recovery pipe column; if the pressure in the water recovery pipe column is unable to drop below the hydrate phase equilibrium pressure, the hydrate inhibitor needs to be injected into the third inhibitor injection point; once the hydrate inhibitor has been injected into the third inhibitor injection point, the inhibitor concentration in the water recovery pipe column remains consistent with that at the gas-liquid mixed transportation pipe section; in this case, to reduce the pressure throughout the water recovery pipe column, the operating power of the first electric submersible pump and the operating power of the second electric submersible pump on the column are regulated, allowing the pressure in the pipe columns to drop below the hydrate phase equilibrium pressure; and meanwhile, the output power of the first electric submersible pump and the output power of the second electric submersible pump are maintained at a consistent level, which ensures that the liquid level in the second electric submersible pump module stays above the second electric submersible pump. If depressurization by the electric submersible pumps and the existing inhibitor concentration may not meet the prevention and control requirement of the hydrates, it is critical to continue injecting a certain concentration of hydrate inhibitor into the third inhibitor injection point additionally to avoid the generation of the hydrates; and
  • if the inhibitor is not injected into the bottom (the third inhibitor injection point) of the gas-liquid mixed transportation pipe section, the concentration of the inhibitor in the water recovery pipe column is zero, as a result, if the prevention and control requirement of the hydrates may be met only by the depressurization by the electric submersible pumps, it is unnecessary to inject the hydrate inhibitor from the third inhibitor injection point, or else, it is critical to additionally inject a certain concentration of hydrate inhibitor into the third inhibitor injection point to avoid the generation of the hydrates; and if the inhibitor is injected into the bottom (the third inhibitor injection point) of the gas-liquid mixed transportation pipe section, the concentration of the inhibitor in the water recovery pipe column is consistent with that of the inhibitor at the gas-liquid mixed transportation pipe section. In this case, if the depressurization by the electric submersible pumps and the existing inhibitor concentration may meet the prevention and control requirement of the hydrates, it is unnecessary to inject the hydrate inhibitor into the third inhibitor injection point, or else, it is imperative to continue injecting a certain concentration of hydrate inhibitor into the third inhibitor injection point additionally to avoid the generation of the hydrates. The existing inhibitor concentration requires less depressurization as the inhibitor present in water maintains the higher pressure required for producing the hydrates. This makes the hydrates more difficult to generate.

For the gas recovery pipe column, when the processing result from the computer terminal indicates that there is the secondary generation risk of the hydrates in the gas recovery pipe column, the computer terminal sends the heating instructions to the heater at the bottom of the gas recovery pipe column to elevate gas temperature in the gas recovery pipe column, and after heating, the concentration of the hydrate inhibitor required for preventing and controlling the secondary generation of the hydrates is calculated according to the prevention and control requirement for the secondary generation of the hydrates, and the secondary generation of the hydrates is determined according to formulas (6), (7) and (8); the inhibitor injection instructions are sent to the first inhibitor injection point and the second inhibitor injection point, and the control valve on the injection pipeline is opened; the injection flow rate of the first inhibitor injection point is independent of that of the second inhibitor injection point; the latter is used specifically to prevent the secondary generation of the hydrates in the gas recovery pipe column. The former, however, is used to stabilize the concentration of the inhibitor and avoid the generation risk of the hydrates arising from throttling and temperature drops of the produced fluid that flows into the platform pipeline; and a heating temperature is encouraged to be at the highest level possible, but an ideal state of being above the phase equilibrium temperature of the hydrates after heating is impossible to achieve for the heating apparatus on site. As such, the secondary generation risk of the hydrates is prevented in the gas recovery pipe column by combining heating and inhibitor injection, that is, heating is performed, and then, the concentration of the injected inhibitor and the injection rate are determined based on the temperature after heating, thereby achieving the prevention and control of the secondary generation risk of the hydrates in the gas recovery pipe column.

Claims

1. An apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates, wherein the apparatus comprises a gas recovery pipe column, a water recovery pipe column, a gas-liquid mixed transportation pipe section, a data collecting and processing unit, and a reaction control apparatus; tail ends of the gas recovery pipe column and the water recovery pipe column are connected with a top of the gas-liquid mixed transportation pipe section; the gas-liquid mixed transportation pipe section is positioned in hydrate reservoirs; and the gas recovery pipe column and the water recovery pipe column recover gases and water decomposed by the natural gas hydrates in the reservoirs respectively; the data collecting and processing unit comprises a first data monitoring point, a second data monitoring point, a third data monitoring point, and a computer terminal; the first data monitoring point is positioned on a top of the gas recovery pipe column, and collects a temperature, pressure and gas flow of the top of the gas recovery pipe column; the second data monitoring point is positioned on a top of the water recovery pipe column, and collects a temperature, pressure and gas flow of the top of the water recovery pipe column; the third data monitoring point is positioned on a tail end of the gas-liquid mixed transportation pipe section, and collects a temperature and pressure of a well bottom; and the computer terminal receives and processes temperature, pressure, and flow data collected from the first data monitoring point, the second data monitoring point, and the third data monitoring point;the reaction control apparatus comprises a signal actuator, a hydrate inhibitor storage tank, a hydrate inhibitor injection pump, a first inhibitor injection point, a second inhibitor injection point, a third inhibitor injection point, a first electric submersible pump, a second electric submersible pump, and a heater; one end of the signal actuator is connected with the computer terminal, and the other end of the signal actuator is connected with the hydrate inhibitor injection pump; the hydrate inhibitor injection pump is respectively connected with the first inhibitor injection point, the second inhibitor injection point, and the third inhibitor injection point via injection pipelines, and a control valve is arranged on each of the injection pipelines; the first inhibitor injection point is positioned on the top of the gas recovery pipe column, the second inhibitor injection point is positioned at a bottom of the gas recovery pipe column, and the third inhibitor injection point is positioned on the tail end of the gas-liquid mixed transportation pipe section; the first electric submersible pump is positioned at a bottom of the water recovery pipe column, and the second electric submersible pump is positioned in the middle of the water recovery pipe column; and the heater is positioned at the bottom of the gas recovery pipe column.

2. The apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 1, wherein a joint of the water recovery pipe column and the gas-liquid mixed transportation pipe section and a joint of the gas recovery pipe column and the gas-liquid mixed transportation pipe section are provided with a casing pipe, the first electric submersible pump is positioned in the casing pipe, and a blowout preventer is arranged on a tail end of the gas recovery pipe column.

3. The apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 1, wherein a water storage pipe section is arranged in the middle of the water recovery pipe column, the middle of the water recovery pipe column is divided into a first half and a second half of the water recovery pipe column, a tail end of the first half of the water recovery pipe column and a top of the second half of the water recovery pipe column are positioned in the water storage pipe section, and the second electric submersible pump is positioned on the tail end of the first half of the water recovery pipe column.

4. A prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 1, wherein the method comprises the following steps: collecting, by the three data collection points installed on the top of the gas recovery pipe column, the top of the water recovery pipe column, and the tail end of the gas-liquid mixed transportation pipe section, temperature, pressure and flow data at different positions, the different data collection points being connected with the computer terminal for transmitting the collected data to the computer terminal in real time; performing, by the computer terminal, analysis and processing on the data collected from the different data collection points, sending instructions to the signal actuator to control inhibitor injection rates of different hydrate inhibitor injection points, and to control power of the heater in the gas recovery pipe column and power of the different electric submersible pumps in the water recovery pipe column to prevent and control the secondary generation of the hydrates in the gas recovery pipe column and the water recovery pipe column.

5. The prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 4, wherein the method comprises the following steps: (i) real-time monitoring of data at different positionsmonitoring temperature, pressure and flow data at different positions via the first data monitoring point on the top of the gas recovery pipe column, the second data monitoring point on the top of the water recovery pipe column, and the third data monitoring point on the tail end of the gas-liquid mixed transportation pipe section, and transmitting the collected data to the computer terminal in real time;(ii) analysis of a secondary generation risk of hydrates throughout wellboreperforming real-time calculation in real time by the computer terminal according to the temperature, pressure and flow data received thereby at different positions to obtain temperature and pressure distributions throughout the wellbore; judging, by combining a phase equilibrium calculation result of the natural gas hydrates, whether the secondary generation of the hydrates happens at different positions, and determining the secondary generation risk of the hydrates throughout the wellbore; and(iii) prevention and control reaction of secondary generation of hydrates in different pipe columnssending, by the computer terminal, prevention and control instructions of the secondary generation of the hydrates according to the secondary generation risk of the hydrates in different pipe columns obtained by calculation, and taking corresponding measures for preventing and controlling the secondary generation of the hydrates for different pipe columns, wherein inhibitor injection is used as the measure for preventing and controlling the secondary generation of the hydrates at the gas-liquid mixed transportation pipe section, the collaborative prevention and control of the inhibitor injection+the heating of the pipe column bottom is used as the measure for preventing and controlling the secondary generation of the hydrates in the gas recovery pipe column, and the collaborative prevention and control of depressurization by double pumps+inhibitor is used as at the measure for preventing and controlling the secondary generation of the hydrates in the water recovery pipe column.

6. The prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 5, wherein in step (ii), distinct heat transfer processes are set between a fluid flow in the exploitation wellbore and an external environment: (1) well section below mud line—gas-liquid mixed transportation pipe section: heat transfer between a fluid in the gas-liquid mixed transportation pipe section and an external stratum; (2) well section above mud line—gas recovery pipe column: heat transfer between the fluid in the gas recovery pipe column and external seawater; (3) well section above mud line—water recovery pipe column: heat transfer between the fluid in the water recovery pipe column and the external seawater; the mud line is a boundary of the seawater and a shallow layer of a seabed; and the temperature distribution of the exploitation wellbore is calculated by the following formula:

7. The prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 6, wherein in step (2), a pressure field distribution in the hydrate pilot exploitation pipe column is calculated by the following formula:

8. The prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 7, wherein in step (ii), phase equilibrium temperature and pressure conditions of the natural gas hydrates are calculated by the following formula:

9. The prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 8, wherein in step (ii), the secondary generation risk of the hydrates in different pipe columns is determined by comparing a phase equilibrium temperature of the pipe columns with a temperature of the natural gas hydrates; a natural gas hydrate phase equilibrium-pressure curve under the condition of the produced fluid component is converted into a temperature-depth curve by taking into account a temperature and pressure distribution curve of the wellbore and a hydrate phase equilibrium curve, for which coordinate conversion is performed; and when the temperature on the wellbore temperature curve at a certain depth is lower than that on the hydrate phase equilibrium curve, a fluid temperature in the wellbore at the depth satisfies the secondary generation condition of the hydrates, that is, there is the secondary generation risk of the hydrates, a discriminant formula of the secondary generation of the hydrates is as follows: Pe>Pf or Te<Tf   (9)where, Te is a phase equilibrium temperature of the hydrates, ° C.

10. The prevention and control method of the apparatus for preventing and controlling secondary generation of hydrates in a wellbore during depressurization exploitation of offshore natural gas hydrates according to claim 9, wherein in step (iii), different prevention and control measures of the secondary generation of the hydrates are taken for different pipe columns in the wellbore; at the gas-liquid mixed transportation pipe section, when the processing result from the computer terminal indicates that the secondary generation risk of the hydrates is found in a horizontal pipe section of the gas-liquid mixed transportation pipe section at the well bottom, the concentration of a hydrate inhibitor as required for preventing and controlling the secondary generation of the hydrates is obtained via calculation according to the prevention and control requirement for the secondary generation of the hydrates, which may be determined according to formulas (6), (7) and (8); the higher the concentration of the hydrate inhibitor, the higher the temperature and the lower the pressure at which a hydrate phase equilibrium is achieved are perceived to be; the concentration of the inhibitor is designed to make the phase equilibrium temperature of the hydrates higher than a fluid temperature or make the phase equilibrium pressure thereof lower than a fluid pressure; an inhibitor injection rate is obtained by multiplying the amount of recovered water by the concentration; and then, the inhibitor injection instructions are sent to the third inhibitor injection point on the tail end of the horizontal pipe section, and the control valve on the injection pipeline is opened; for the water recovery pipe column, when the processing result from the computer terminal indicates that there is the secondary generation risk of the hydrates in the water recovery pipe column, the concentration of the hydrate inhibitor which has possibly been present in an aqueous solution needs to be considered, and the concentration of the hydrate inhibitor in the water recovery pipe column is the same as that of the hydrate inhibitor at the gas-liquid mixed transportation pipe section; if the hydrate inhibitor is not injected into the third inhibitor injection point, the concentration of the existing hydrate inhibitor in the water recovery pipe column is 0; if the hydrate inhibitor is injected into the third inhibitor injection point, the concentration of the existing hydrate inhibitor in the water recovery pipe column is the concentration of the hydrate inhibitor at the gas-liquid mixed transportation pipe section; if the hydrate inhibitor is not injected into the third inhibitor injection point, the computer terminal controls, based on the processing result, the operating power of the first electric submersible pump and the operating power of the second electric submersible pump on the water recovery pipe column to reduce the pressure throughout the water recovery pipe column until the pressure in the pipe column drops to below the pressure of the hydrate phase equilibrium, and the output power of the first electric submersible pump and the output power of the second electric submersible pump are maintained at a consistent level, which ensures that a liquid level in a second electric submersible pump module stays above the second electric submersible pump; if a pressure of the water recovery pipe column is unable to drop to below the hydrate phase equilibrium pressure, the hydrate inhibitor needs to be injected into the third inhibitor injection point; if the hydrate inhibitor is injected into the third inhibitor injection point, the operating power of the first electric submersible pump and the operating power of the second electric submersible pump on the water recovery pipe column are controlled to reduce the pressure throughout the water recovery pipe column until the pressure in the pipe column drops to below the hydrate phase equilibrium pressure, and the output power of the first electric submersible pump and the output power of the second electric submersible pump are maintained at a consistent level, which ensures that the liquid level in the second electric submersible pump module stays above the second electric submersible pump; and if depressurization by the electric submersible pumps and the existing inhibitor concentration may not meet the prevention and control requirement of the hydrates, the hydrate inhibitor continues to be injected into the third inhibitor injection point additionally; andfor the gas recovery pipe column, when the processing result from the computer terminal indicates that there is the secondary generation risk of the hydrates in the gas recovery pipe column, heating instructions are sent to a heater at the bottom of the gas recovery pipe column to elevate gas temperature in the gas recovery pipe column, and after heating, the concentration of the hydrate inhibitor required for preventing and controlling the secondary generation of the hydrates is calculated according to the prevention and control requirement for the secondary generation of the hydrates, and the secondary generation of the hydrates is determined according to formulas (6), (7) and (8); and inhibitor injection instructions are sent to the first inhibitor injection point and the second inhibitor injection point, and the control valve on the injection pipeline is opened.