EVALUATION METHOD FOR ACID FRACTURING EFFECT BASED ON THE THEORY OF ACID-FRAC "STIMULATED ZONE"

Abstract

The present invention discloses an evaluation method for acid fracturing effect based on the theory of acid-frac “stimulated zone”, comprising the following steps: establish a structured reservoir grid, and add initial artificial fractures to the structured reservoir grid; establish a fracture propagation model considering the acid-frac stimulated zone, on the basis of the structured reservoir grid, conduct numerical simulation according to the fracture propagation model, and work out the seepage parameters during acid fracturing; establish a gas well production model, and calculate the pore distribution and liquid saturation distribution of the reservoir in gas well production; calculate the cumulative production of the gas well, and calculate the multiple proportion of cumulative production increase of the construction plan according to the cumulative production of the gas well; the greater the multiple proportion, the better the acid fracturing effect.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims priority to Chinese patent application No. 202210537578.5, filed on May 18, 2022, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the technical field of oil and gas field development, in particular to an evaluation method for acid fracturing effect based on the theory of acid-frac “stimulated zone”.

BACKGROUND

Acidification is to filter acid fluid into reservoir matrix and dissolve minerals so as to improve matrix permeability, thus forming a stimulated zone near the wellbore. Acid fracturing is to break rock by hydraulic pressure and heterogeneously etch the wall of artificial fractures so as to form acid etched fractures with flow conductivity after fluid flowback and fracture closure to increase production. For far too long, when evaluating the acid fracturing effect, petroleum engineers mainly focus on the stimulation effect of acid etched fractures on oil wells and gas wells. They usually improved the credibility of the evaluation on improving the acid fracturing effect in reservoirs by considering the effect of acid etched fractures on fracture conductivity, fracture morphology and effective fracture length, while ignoring the stimulated zone formed by the dissolution of acid fluid filtering along the acid etched fractures in the reservoir matrix and the seepage pattern improvement and stimulation inside this stimulated zone. Therefore, the simulated acid fracturing effect is different from the actual acid fracturing effect, which cannot accurately reflect the actual acid fracturing effect of the construction plan.

SUMMARY

To address the above problems, the present invention aims to provide an evaluation method for acid fracturing effect based on the theory of acid-frac “stimulated zone”, which additionally considers the improvement of the seepage pattern in the “stimulated zone” near the acid etched fractures, and evaluates the acid fracturing effect by predicting the changes in porosity and permeability of the reservoir during stimulation stage and production stage and the cumulative gas production during a certain production period.

The technical solution of the present invention is as follows:

An evaluation method for acid fracturing effect based on the theory of acid-frac “stimulated zone”, comprising the following steps:

• • Step 1: Establishing a structured reservoir grid, and adding initial artificial fractures to the structured reservoir grid, wherein the initial artificial fractures are divided into multiple fracture units by the structured reservoir grid, the fracture units are numbered L=1, 2, 3, . . . , the length of each fracture unit is denoted as ξ_L and the total number of fracture units as n_f;
  • Step 2: Establishing a fracture propagation model considering the acid-frac stimulated zone;
  • Step 3: On the basis of the structured reservoir grid, conducting numerical simulation according to the fracture propagation model, and working out the seepage parameters at a certain moment during acid fracturing;
  • Step 4: Determining whether the fracture propagates at a given time according to the acid etched fracture propagation criterion: if there is no propagation, the total number n_f of fracture units remains unchanged; if there is propagation, the total number of fracture units is n_f=n_f+1;
  • Step 5: Taking the seepage parameters obtained in Step 3 and the total number of fracture units obtained in Step 4 as the initial conditions for the next time, and repeating Steps 3 to 5 until the completion of acid fracturing to obtain the seepage parameters at the end of acid fracturing;
  • Step 6: Establishing a gas well production model, and calculating the pore distribution and liquid saturation distribution of the reservoir in gas well production according to the gas well production model;
  • Step 7: Calculating the cumulative production of the gas well according to the results obtained in Steps 5 and 6;
  • Step 8: Calculating the multiple proportion of cumulative production increase of the construction plan according to the cumulative production of the gas well; the greater the multiple proportion, the better the acid fracturing effect.

Preferably, in Step 1, the establishment of the structured reservoir grid comprises the following sub-steps: collecting the geological exploration data of target reservoir, dividing the reservoir length L_x and width L_y into n_i and n_j segments respectively in a x-y rectangular coordinate system, so the entire reservoir can be divided into a n_i×n_j structured grid, where x_i,j and y_i,j represent the length and width of each grid respectively, and the subscripts i and j represent the position of each grid in the reservoir.

Preferably, in Step 1, when adding initial artificial fractures to the structured reservoir grid, the propagation direction of initial artificial fracture is designed as the x-axis direction and the propagation length as the total length of N grids, and the N is an integer greater than or equal to 3.

Preferably, in Step 2, the fracture propagation model includes:

(1) Calculation model considering fracture width and intra-fracture pressure in the acid-frac stimulated zone:

$\begin{array}{cc}W\left(x,t\right)=w\left(x\right)+\stackrel{\_}{w_e\left(t\right)}& \left(1\right)\end{array}$ $\begin{array}{cc}\frac{\pi E}{512\mu \left(1-{\nu }^2\right)}\frac{{\partial }^2{w}^4\left(x\right)}{u^2}-2v_{l}H=\frac{\pi H}{4}\frac{\partial w\left(x\right)}{\partial t}& \left(2\right)\end{array}$ $\begin{array}{cc}{v}_l=\frac{k_{mf}}{\mu \overline{d}}\left[\frac{w\left(x\right)E}{2\left(1-{\nu }^2\right)H}+{\sigma }_n-P_m\left(x\right)\right]& \left(3\right)\end{array}$ $\begin{array}{cc}\frac{\beta }{{\rho }_r\left(1-{\varphi }_m\right)}\left(2\eta v_l{C}_f+2k_c{C}_f\right)=\frac{\partial \stackrel{\_}{w_e\left(t\right)}}{\partial t}& \left(4\right)\end{array}$ $\begin{array}{cc}{P}_f\left(x,t\right)=\frac{W\left(x,t\right)E}{2\left(1-{\nu }^2\right)H}+{\sigma }_n& \left(5\right)\end{array}$

Where, W(x,t)—Width of the acid etched fracture at any time and at any position during acid fracturing, in m; w(x)—Width of acid etched fracture, in m; w_e(t)—Average width of acid etched fracture at acid fracturing time, in m; E—Young's modulus of reservoir rock sample, in MPa; μ—Viscosity of acidizing fluid, in mPa·s; v—Poisson's ratio of reservoir rock sample; x—Position of the structured reservoir grid along the X axis; v_l—Filtration rate of acidizing fluid, in m/s; H—Height of acid etched fracture, in m; t—Acid fracturing time, in s; k_mf—Average permeability between acid etched fracture and surrounding matrix, in mD; d—Distance from the acid etched fracture to the center point of the matrix grid where it is located, in m; σ_n—Minimum horizontal principal stress, in MPa; P_m(x)—Fluid pressure of the matrix around acid etched fracture, in MPa; β—Rock dissolution capacity of acidizing fluid, defined as the mass of rock dissolved by acidizing fluid per mole, in kg/mol; ρ_r—Rock density, in kg/m³; ϕ_m—Porosity of reservoir matrix, in %; η—Mass fraction of acidizing fluid in filtration that is involved in etching fracture wall; C_f—Acidizing fluid concentration in the fracture, in mol/m³; k_c—Mass transfer coefficient, in m/s; P_f(x,t)—Fluid pressure in the acid etched fracture at any time and at any position in the fracturing process, in MPa.

(2) Matrix seepage model considering acid-frac stimulated zone during acid fracturing of gas reservoir:

$\begin{array}{cc}\frac{\partial }{\partial x}\left(\kappa \frac{k_{\mathrm{fw}}}{\mu B}\frac{\partial P_f\left(x,t\right)}{\partial x}\right)-{\delta }_m\frac{v_l{A}_{mf}}{V_f}=\frac{\partial }{\partial t}\left(\frac{{\varphi }_f}{B}\right)& \left(6\right)\end{array}$ $\begin{array}{cc}\frac{\partial }{\partial x}\left(\frac{k_m{k}_{\mathrm{mrw}}}{{\mu }_w{B}_w}\frac{\partial P_{\mathrm{mw}}}{\partial x}\right)+\frac{\partial }{\partial y}\left(\frac{k_m{k}_{\mathrm{mrw}}}{{\mu }_w{B}_w}\frac{\partial P_{\mathrm{mw}}}{\partial y}\right)+{\delta }_m\frac{v_l{A}_{mf}}{V_b}=\frac{\partial }{\partial t}\left(\frac{{\varphi }_m{S}_{\mathrm{mw}}}{B_w}\right)& \left(7\right)\end{array}$ $\begin{array}{cc}\frac{\partial }{\partial x}\left(\frac{k_m{k}_{mrg}}{{\mu }_g{B}_g}\frac{\partial P_{mg}}{\partial x}\right)+\frac{\partial }{\partial y}\left(\frac{k_m{k}_{mrg}}{{\mu }_g{B}_g}\frac{\partial P_{mg}}{\partial y}\right)=\frac{\partial }{\partial t}\left(\frac{{\varphi }_m\left(1-S_{mw}\right)}{B_g}\right)& \left(8\right)\end{array}$ $\begin{array}{cc}{P}_{mc}=P_{mg}-P_{m\nu }& \left(9\right)\end{array}$

Where, κ—Unit conversion coefficient, in 10⁻³; k_fw—Effective liquid permeability of acid etched fracture, in mD; B—Volume coefficient of acidizing fluid; δ_m—Judgment parameter, specifically δ_m=1 if there is fractures across the reservoir matrix grid and δ_m=0 if there is no fracture across the reservoir matrix grid; A_mf—Contact area between fracture and matrix, in m²; V_f—Volume of acid etched fracture unit, in m³; ϕ_f—Porosity of acid etched fracture; km Reservoir matrix permeability, in mD; k_mr—Relative liquid permeability of the reservoir matrix; k_mrg—Relative gas permeability of the reservoir matrix; w Liquid viscosity in the reservoir matrix, in mPa·s; μ_g—Gas viscosity in the reservoir matrix, in mPa·s; B_w—Bolume coefficient of liquid in the reservoir matrix; B_g—Volume coefficient of gas in the reservoir matrix; P_mw, P_mg—Liquid pressure and gas pressure in the reservoir matrix, in MPa; y—Position of the structured reservoir grid along the Y axis; V_b—Volume of reservoir matrix unit, in m³; S_mw—Liquid saturation in the reservoir matrix; P_mc—Capillary pressure in the reservoir matrix, in MPa.

The calculation model of acidizing fluid concentration distribution in reservoir matrix grid is as follows:

$\begin{array}{cc}\frac{\partial }{\partial t}\left({\varphi }_m{C}_m\right)=\left[\begin{array}{c}-\frac{\partial }{\partial x}\left(C_m\frac{k_m{k}_{mrw}}{{\mu }_w}\frac{\partial P_{mw}}{\partial x}\right)-\frac{\partial }{\partial y}\left(C_m\frac{k_m{k}_{mrw}}{{\mu }_w}\frac{\partial P_{mw}}{\partial y}\right)\\ +\frac{\partial }{\partial x}\left({\varphi }_m{D}_{\mathrm{ex}}\frac{\partial C_m}{\partial x}\right)+\frac{\partial }{\partial y}\left({\varphi }_m{D}_{\mathrm{ey}}\frac{\partial C_m}{\partial y}\right)\\ +{\delta }_m\frac{v_l{A}_{mf}{C}_f}{V_b}-k_s{C}_s{a}_v\end{array}\right]& \left(10\right)\end{array}$ $\begin{array}{cc}{C}_s=\frac{C_m}{1+\frac{k_s}{k_c}}& \left(11\right)\end{array}$ $\begin{array}{cc}{D}_{ei}={\alpha }_{os}{D}_m+{\lambda }_i❘v_l❘d_h& \left(12\right)\end{array}$

Where, C_m—Acidizing fluid concentration in matrix pores, in mol/m³; D_ex—Effective diffusion tensor in the x direction, in m²/s; D_ey—Effective diffusion tensor in the y direction, in m²/s; k_s—Reaction velocity constant, in m/s; C_s—Acidizing fluid concentration at the pore wall, in mol/m³; a_v—Rock specific surface area of reservoir matrix, in m²/m³; D_ei—Effective diffusion tensor in the i direction, in m²/s; α_os, λ_i—Pore structure constant, and α_os=1, λ_x≈0.5 and λy≤1 for spherical filling medium; D_m—Molecular diffusion coefficient, in m²/s; d_h—Hydraulic diameter of tubular pore, in m.

The calculation model of matrix porosity and permeability changes during acid-rock reaction is as follows:

$\begin{array}{cc}\frac{\partial {\varphi }_m}{\partial t}=\frac{k_s{C}_s\beta a_v}{{\rho }_r}& \left(13\right)\end{array}$ $\begin{array}{cc}\frac{k_m}{k_{m0}}=\frac{{\varphi }_m}{{\varphi }_{m0}}{\left(\frac{{\varphi }_m\left(1-{\varphi }_{m0}\right)}{{\varphi }_{m0}\left(1-{\varphi }_m\right)}\right)}^{2\gamma }& \left(14\right)\end{array}$ $\begin{array}{cc}\frac{a_v}{a_{v0}}=\frac{{\varphi }_m}{{{\varphi }_{m0}\left(\frac{k_m{\varphi }_{m0}}{k_{m0}{\varphi }_m}\right)}^{1/2}}& \left(15\right)\end{array}$

Where, k_m0—Initial permeability of reservoir matrix, in mD; ϕ_m0—Initial porosity of reservoir matrix; γ—Parameter related to pore structure; a_v0—Initial rock specific surface area of reservoir matrix, in m²/m³.

(3) Initial conditions for gas reservoir seepage:

Pmg(i,j,t)|_t=0=P₀  (16)

Where, P_mg(i,j,t)—Gas pressure in the reservoir matrix at the coordinates of positions i and j in the grid at time t, in MPa; P₀—Original formation pressure of gas reservoir, in MPa.

(4) Boundary conditions for fracture propagation:

$\begin{array}{cc}\{\begin{array}{cc}W\left(x,t\right)=0& x>x_{L=1}+\sum _{L=1}^{n_{f,t}}{\xi }_L\mathrm{Or}x<x_{L=1}\\ \frac{{\partial }^2{W\left(x,t\right)}^4}{\partial x}=\frac{256Q_{\mathrm{int}}\mu \left(1-v\right)}{\pi G}& x=0\end{array}& \left(17\right)\end{array}$ $\begin{array}{cc}{P}_{\mathrm{fL}=1,t}=P_{int}& \left(18\right)\end{array}$

Where, Q_int—Injection displacement of acid fracturing, in m³/min; G—Volume modulus of reservoir rock sample, in MPa; x_L=1—Rectangular coordinates of the first acid etched fracture unit; n_f,t—Total number of acid etched fracture units at time t; ξ_L—length of the L^th acid etched fracture unit, in m; P_fL=1,t—Fluid pressure in the acid etched fracture unit in Section 1 at time t, in MPa; P_int—Downhole pressure during acid fracturing, in MPa.

(5) Boundary conditions for gas reservoir matrix seepage:

$\begin{array}{cc}\{\begin{array}{c}\frac{\partial P_{\mathrm{mg}}}{\partial x}{❘}_{x=0,L_x}=0\\ \frac{\partial P_{\mathrm{mg}}}{\partial y}{❘}_{y=0,L_y}=0\end{array}\&\{\begin{array}{c}\frac{\partial P_{\mathrm{mw}}}{\partial x}{❘}_{x=0,L_x}=0\\ \frac{\partial P_{\mathrm{mw}}}{\partial y}{❘}_{y=0,L_y}=0\end{array}& \left(19\right)\end{array}$

Where, L_x, L_y—Length and width of reservoir, in m;

(6) Boundary conditions and initial conditions for acidizing fluid migration reaction model:

$\begin{array}{cc}\{\begin{array}{c}{C}_f\left(0,t\right)=C_{0,t}\\ \begin{array}{cc}{C}_f\left(x_L,t\right)=C_{0,t}& 0<x_L<L_f\end{array}\\ C_f\left(L_f,t\right)=0\\ C_{m,t=0}=0\\ C_{s,t=0}=0\end{array}& \left(20\right)\end{array}$

Where, C_f(0,t)—Acidizing fluid concentration in initial artificial fracture unit at the acid fracturing time t, in mol/m³; C_f(x_L,t)—Acidizing fluid concentration in artificial fracture unit corresponding to the horizontal coordinate x_L at time t, in mol/m³; C_f(L_f,t)—Acidizing fluid concentration at the artificial fracture tip at time t, in mol/m³; C_m,t=0—Acidizing fluid concentration in the pore at the initial acid fracturing time, in mol/m³; C_s,t=0—Acidizing fluid concentration at the pore wall at the initial acid fracturing time, in mol/m³; L_f—Horizontal coordinate corresponding to the tip of artificial fracture unit at time t; C_0,t—Acidizing fluid concentration of construction fluid at time t, in mol/m³.

Preferably, in Step 4, the below is the criterion for determining acid etched fracture propagation:

• • When the stress intensity factor K_If,t at fracture tip is less than or equal to the fracture toughness K_IC of reservoir rock, the fracture will propagate;
  • When the stress intensity factor K_If,t at fracture tip is greater than the fracture toughness K_IC of reservoir rock, the fracture will not propagate.

Preferably, the stress intensity factor K_If,t at fracture tip is calculated by the following equation:

$\begin{array}{cc}{K}_{\mathrm{If},t}=\frac{0.806E\sqrt{\pi }{W}_{L=n_f,t}}{4\left(1-v^2\right)\sqrt{2\Delta x}}& \left(21\right)\end{array}$

Where, K_If,t—Stress intensity factor at fracture tip at time t, in MPa·m¹²; E—Young's modulus of reservoir rock sample, in MPa; W_L=n,t—Average width of structured reservoir grid, in m; v—Poisson's ratio of reservoir rock sample; Δx—Width of artificial fracture tip at time t, in m.

The fracture toughness K_IC of reservoir rock is calculated by the following equation:

$\begin{array}{cc}{K}_{IC}=0.3172{\rho }_r+\frac{0.0457}{V_c}+0.2131\ln \left(\mathrm{DT}\right)\times 0.5041& \left(22\right)\end{array}$

Where, K_IC—Type I fracture toughness of reservoir rock, in MPa·m^1/2; ρ_r—Rock density, in kg/m³; V_c—Average shaliness of reservoir rocks, in %; DT—Average interval transit time of the reservoir, in μs/m.

Preferably, in Step 6, the gas well production model includes:

(1) Differential equation of gas-water seepage in gas reservoir:

$\begin{array}{cc}\frac{\partial }{\partial x}\left(\kappa \frac{k_f{k}_{\mathrm{frw}}}{{\mu }_w{B}_w}\frac{\partial P_f}{\partial x}\right)+\frac{q_{\mathrm{fw}}}{V_f}+{\delta }_m\frac{Q_{\mathrm{mw}}}{V_f}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_f{S}_{\mathrm{fw}}}{B_w}\right)& \left(23\right)\end{array}$ $\begin{array}{cc}\frac{\partial }{\partial x}\left(\kappa \frac{k_f{k}_{\mathrm{frg}}}{{\mu }_g{B}_g}\frac{\partial P_f}{\partial x}\right)+\frac{q_{fg}}{V_f}+{\delta }_m\frac{Q_{mg}}{V_f}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_f\left(1-S_{\mathrm{fw}}\right)}{B_g}\right)& \left(24\right)\end{array}$ $\begin{array}{cc}\nabla \left(\beta \frac{k_m{k}_{mrw}}{{\mu }_w{B}_w}\nabla P_{mw}\right)-{\delta }_m\frac{Q_{mw}}{V_b}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_m{S}_{mw}}{B_w}\right)& \left(25\right)\end{array}$ $\begin{array}{cc}\nabla \left(\beta \frac{k_m{k}_{mrg}}{{\mu }_g{B}_g}\nabla P_{mg}\right)-{\delta }_m\frac{Q_{mg}}{V_b}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_m\left(1-S_{mw}\right)}{B_g}\right)& \left(26\right)\end{array}$ $\begin{array}{cc}{Q}_{mw}=\frac{2k_m{k}_{mw}·{\xi }_{L}H}{{\mu }_w\overline{d}}\left(P_{mw}-P_f\right)& \left(27\right)\end{array}$ $\begin{array}{cc}{Q}_{mg}=\frac{2k_m{k}_{mg}·{\xi }_{L}H}{{\mu }_g\overline{d}}\left(P_{mg}-P_f\right)& \left(28\right)\end{array}$

Where, k_f—Permeability of acid etched fracture, in mD; k_frw, k_frg—Relative permeability of liquid and gas in acid etched fracture; P_f—Pressure in artificial fracture, in MPa; q_fw, q_fg—Source and sink terms of liquid and gas in acid etched fracture, in m³/s; Q_mw, Q_mg—Liquid flow and gas flow between the main fracture and the matrix during gas well production, in m³/s; S_fw—Liquid saturation in acid etched fracture; t_p—Production time of gas well, in s; ∇—Gradient operator.

(2) Initial conditions:

Initial pressure distribution:

$\begin{array}{cc}\{\begin{array}{c}{P}_{\mathrm{fgL},t_p=0}=P_{\mathrm{fwL},t_p=0}=P_{\mathrm{fL},t_p=0}=P_{fL,t_{end}}\\ P_{mg}\left(i,j,t_p\right){❘}_{t_p=0}=P_{mg}\left(i,j,t\right){❘}_{t=t_{end}}\\ P_{mw}\left(i,j,t_p\right){❘}_{t_p=0}=P_{mw}\left(i,j,t\right){❘}_{t=t_{end}}\end{array}& \left(29\right)\end{array}$

Where, P_{fg L,p=0}—Initial gas pressure distribution of acid etched fracture in gas well production simulation, in MPa; P_{fw L,tp=0}—Initial liquid pressure distribution of acid etched fracture in gas well production simulation, in MPa; P_fL,tp=0—Initial pressure distribution of acid etched fracture in gas well production simulation, in MPa; P_fL,tend—Pressure distribution of artificial fracture at the end of acid fracturing, in MPa; P_mg(i,j,t_p)|_tp=0—Initial gas pressure distribution of reservoir matrix in gas well production simulation, in MPa; P_mg(i,j,t)|_t=tend—Pressure distribution in artificial fracture at the end of acid fracturing, in MPa; P_mw(i,j,tp)|_tp=0—Initial liquid pressure distribution of reservoir matrix in gas well production simulation, in MPa; P_mw(i,j,t)|_t-tend—Liquid pressure distribution in artificial fracture at the end of acid fracturing, in MPa.

Initial saturation distribution:

$\begin{array}{cc}\{\begin{array}{c}{S}_{\mathrm{fw}}\left(L,t_p\right){❘}_{t_p=0}=1\\ S_{mw}\left(i,j,t_p\right){❘}_{t_p=0}=S_{mw}\left(i,j,t\right){❘}_{t=t_{end}}\end{array}& \left(30\right)\end{array}$

Where, S_fw(L,t_p)|_tp=0—Initial liquid saturation of acid etched fracture in gas well production simulation; S_mw(i,j,tp)|_tp=0—Initial liquid saturation of reservoir matrix in gas well production simulation; S_mw(i,j,t)|_t=tend—Liquid saturation of reservoir matrix at the end of acid fracturing.

(3) Internal boundary conditions:

P _w(x_w,y_w,t_p)=P_wf(t_p)  (31)

Where, P_w(x_w, y_w, t_p)—Liquid pressure of well-corresponding grid at the simulated time t_p of gas well production, in MPa; P_wf(t_p)—Bottom hole flowing pressure at production time t_p, in MPa;

(4) External boundary conditions:

$\begin{array}{cc}\{\begin{array}{c}\frac{\partial P_{mw}\left(i,j,t_p\right)}{\partial x}{❘}_{x=0,L_x}=0\\ \frac{\partial P_{mg}\left(i,j,t_p\right)}{\partial y}{|}_{y=0,L_y}=0\end{array}\&\{\begin{array}{c}\frac{\partial P_{mw}\left(i,j,t_p\right)}{\partial x}{❘}_{x=0,L_x}=0\\ \frac{\partial P_{\mathrm{mw}}\left(i,j,t_p\right)}{\partial y}{|}_{y=0,L_y}=0\end{array}.& \left(32\right)\end{array}$

Preferably, in Step 7, the cumulative production of the gas well is calculated by the following equation:

$\begin{array}{cc}Q=\sum _{i=1}^{n_i}\sum _{j=1}^{n_j}\left(x_{i,j}·y_{i,j}·H\right)\left({\varphi }_m\left(i,j,t_p\right)·S_{\mathrm{mw}}\left(i,j,t_p\right)-{\varphi }_m\left(i,j,t_{end}\right)S_{\mathrm{mw}}\left(i,j,t_{end}\right)\right)+\sum _{L=1}^{n_{f,t_{\mathrm{end}}}}\left({\xi }_L·W_{L,t_{end}}·H\right)\left({\varphi }_f\left(L,t_{end}\right)S_{\mathrm{fw}}\left(L,t_{end}\right)-{\varphi }_f\left(L,t_p\right)S_{fw}\left(L,t_p\right)\right)& \left(33\right)\end{array}$

Where, Q—Cumulative production of gas well at time t_p, in m³; n_i, n_j—Total number of grids in x and y directions in the structured reservoir grid; x_i,j, y_i,j—Length and width of matrix grid at positions i and j, in m; ϕ_m(i,j,t_p)—Porosity of matrix grid at positions i and j at time t_p; S_mw(i,j,t_p) Liquid saturation of matrix grid at positions i and j at time t_p; ϕ_m(i,j,t_end)—Porosity of matrix grid at positions i and j at time t_p; S_mw(i,j,t_end)—Liquid saturation of matrix grid at positions i and j at time t_p; n_f,tend—Total number of acid etched fracture units at time t_end; W_L,tend—Width of acid etched fracture unit in Section L at time t_end, in m; ϕ^f(i,j,t_end)—Porosity of acid etched fracture unit in Section L at time t_end; S_fw(L,t_end)—Liquid saturation of acid etched fracture unit in Section L at time t_end; ϕ^f(L,t_p)—Porosity of acid etched fracture unit in Section L at time t_p; S_fw(L,t_p)—Liquid saturation of acid etched fracture unit in Section L at time t_p.

In Step 8, the multiple proportion of cumulative production increase is calculated by the following equation:

$\begin{array}{cc}S=\frac{Q_T}{Q_{0,T}}& \left(34\right)\end{array}$

Where, S—Multiple proportion of cumulative production increase; Q_T—Simulated cumulative production of the gas well at time T after acid fracturing, in m³; T—Time when the daily gas production after acid fracturing is equal to the daily gas production before acid fracturing, in d; Q_0,T—Estimated cumulative production of the gas well at T without acid-fracturing stimulation, in m³.

The present invention has the following beneficial effects:

In the present invention, structured grid and embedded discrete fracture model are used to simulate acid etched fracture propagation, stimulated zone formation and matrix seepage in stimulated zone during production, which not only significantly improves the computational efficiency of the model, but also effectively improves the evaluation accuracy of acid fracturing effect, so that the carbonate reservoir can be developed with reduced cost and enhanced efficiency.

BRIEF DESCRIPTION OF DRAWINGS

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following will make a brief introduction to the drawings needed in the description of the embodiments or the prior art. Obviously, the drawings in the following description are merely some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on the structures shown in these drawings without any creative effort.

FIG. 1 is a schematic diagram of deep acid-fracturing stimulation of carbonate reservoir in a specific embodiment;

FIG. 2 is the comparison results of simulated and actual cumulative productions of Well X in a specific embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is further described with reference to the drawings and embodiments.

It should be noted that the embodiments in this application and the technical features in the embodiments can be combined with each other without conflict. It is to be noted that, unless otherwise specified, all technical and scientific terms herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. “Include” or “comprise” and other similar words used in the present disclosure mean that the components or objects before the word cover the components or objects listed after the word and its equivalents, but do not exclude other components or objects.

The present invention provides an evaluation method for acid fracturing effect based on the theory of acid-frac “stimulated zone”, comprising the following steps.

Step 1: Establishing a structured reservoir grid, and adding initial artificial fractures to the structured reservoir grid, wherein the initial artificial fractures are divided into multiple fracture units by the structured reservoir grid, the fracture units are numbered L=1, 2, 3, . . . , the length of each fracture unit is denoted as ξ_L and the total number of fracture units as n_f;

In a specific embodiment, the establishment of a structured reservoir grid includes the following sub-steps: collecting the geological exploration data of target reservoir, dividing the reservoir length L_x and width L_y into n_i and n_j segments respectively in a x-y rectangular coordinate system, so the entire reservoir can be divided into a n_i×n_j structured grid. where x_i,j and y_i,j represent the length and width of each grid respectively, and the subscripts i and j represent the position of each grid in the reservoir. When adding initial artificial fractures to the structured reservoir grid, the propagation direction of initial artificial fracture is designed as the x-axis direction and the propagation length as the total length of N grids, and the N is an integer greater than or equal to 3.

Step 2: Establishing a fracture propagation model considering the acid-frac stimulated zone, wherein the fracture propagation model includes:

(1) Calculation model considering fracture width and intra-fracture pressure in the acid-frac stimulated zone:

$\begin{array}{cc}W\left(x,t\right)=w\left(x\right)+\stackrel{\_}{w_e\left(t\right)}& \left(1\right)\end{array}$ $\begin{array}{cc}\frac{\pi E}{512\mu \left(1-v^2\right)}\frac{{\partial }^2{w}^4\left(x\right)}{\partial x^2}-2v_{l}H=\frac{\pi H}{4}\frac{\partial w\left(x\right)}{\partial t}& \left(2\right)\end{array}$ $\begin{array}{cc}{v}_l=\frac{k_{mf}}{\mu \overline{d}}\left[\frac{w\left(x\right)E}{2\left(1-{𝓋}^2\right)H}+{\sigma }_n-P_m\left(x\right)\right]& \left(3\right)\end{array}$ $\begin{array}{cc}\frac{\beta }{{\rho }_r\left(1-{\varphi }_m\right)}\left(2\eta v_l{C}_f+2k_c{C}_f\right)=\frac{\partial \stackrel{\_}{w_e\left(t\right)}}{\partial t}& \left(4\right)\end{array}$ $\begin{array}{cc}{P}_f\left(x,t\right)=\frac{W\left(x,t\right)E}{2\left(1-v^2\right)H}+{\sigma }_n& \left(5\right)\end{array}$

Where, W(x,t)—Width of the acid etched fracture at any time and at any position during acid fracturing, in m; w(x)—Width of acid etched fracture, in m; w_e(t)—Average width of acid etched fracture at acid fracturing time, in m; E—Young's modulus of reservoir rock sample, in MPa; μ—Viscosity of acidizing fluid, in mPa·s; v—Poisson's ratio of reservoir rock sample; x—Position of the structured reservoir grid along the X axis; v_l—Filtration rate of acidizing fluid, in m/s; H—Height of acid etched fracture, in m; t—Acid fracturing time, in s; k_mf—Average permeability between acid etched fracture and surrounding matrix, in mD; d—Distance from the acid etched fracture to the center point of the matrix grid where it is located, in m; σ_n—Minimum horizontal principal stress, in MPa; P_m(x)—Fluid pressure of the matrix around acid etched fracture, in MPa; β—Rock dissolution capacity of acidizing fluid, defined as the mass of rock dissolved by acidizing fluid per mole, in kg/mol; ρ_r—Rock density, in kg/m³; ϕ_m—Porosity of reservoir matrix, in %; η—Mass fraction of acidizing fluid in filtration that is involved in etching fracture wall; C_f—Acidizing fluid concentration in the fracture, in mol/m³; k_e—Mass transfer coefficient, in m/s; P_f(x,t)—Fluid pressure in the acid etched fracture at any time and at any position in the fracturing process, in MPa.

(2) Matrix seepage model considering acid-frac stimulated zone during acid fracturing of gas reservoir:

$\begin{array}{cc}\frac{\partial }{\partial x}\left(\kappa \frac{k_{\mathrm{fw}}}{\mu B}\frac{\partial P_f\left(x,t\right)}{\partial x}\right)-{\delta }_m\frac{v_l{A}_{mf}}{V_f}=\frac{\partial }{\partial t}\left(\frac{{\varphi }_f}{B}\right)& \left(6\right)\end{array}$ $\begin{array}{cc}\frac{\partial }{\partial x}\left(\frac{k_m{k}_{mrw}}{{\mu }_w{B}_w}\frac{\partial P_{mw}}{\partial x}\right)+\frac{\partial }{\partial y}\left(\frac{k_m{k}_{mrw}}{{\mu }_w{B}_w}\frac{\partial P_{mw}}{\partial y}\right)+{\delta }_m\frac{v_l{A}_{mf}}{V_b}=\frac{\partial }{\partial t}\left(\frac{{\varphi }_m{S}_{mw}}{B_w}\right)& \left(7\right)\end{array}$ $\begin{array}{cc}\frac{\partial }{\partial x}\left(\frac{k_m{k}_{mrg}}{{\mu }_g{B}_g}\frac{\partial P_{mg}}{\partial x}\right)+\frac{\partial }{\partial y}\left(\frac{k_m{k}_{mrg}}{{\mu }_g{B}_g}\frac{\partial P_{mg}}{\partial y}\right)=\frac{\partial }{\partial t}\left(\frac{{\varphi }_m\left(1-S_{mw}\right)}{B_g}\right)& \left(8\right)\end{array}$ $\begin{array}{cc}{P}_{mc}=P_{mg}-P_{\mathrm{mv}}& \left(9\right)\end{array}$

Where, κ—Unit conversion coefficient, in 10⁻³; k_fw—Effective liquid permeability of acid etched fracture, in mD; B—Volume coefficient of acidizing fluid; δ_m—Judgment parameter, specifically δ_m=1 if there is fractures across the reservoir matrix grid and δ_m=0 if there is no fracture across the reservoir matrix grid; A_mf—Contact area between fracture and matrix, in m²; V_f—Volume of acid etched fracture unit, in m³; ϕ_f—Porosity of acid etched fracture; k_m—Reservoir matrix permeability, in mD; k_mrw—Relative liquid permeability of the reservoir matrix; k_mrg—Relative gas permeability of the reservoir matrix; μ_w—Liquid viscosity in the reservoir matrix, in mPa·s; μ_g—Gas viscosity in the reservoir matrix, in mPa·s; B_w—Bolume coefficient of liquid in the reservoir matrix; B_g—Volume coefficient of gas in the reservoir matrix; P_mw, P_mg—Liquid pressure and gas pressure in the reservoir matrix, in MPa; y—Position of the structured reservoir grid along the Y axis; V_bVolume of reservoir matrix unit, in m³; S_mw—Liquid saturation in the reservoir matrix; P_mc—Capillary pressure in the reservoir matrix, in MPa.

The calculation model of acidizing fluid concentration distribution in reservoir matrix grid is as follows:

$\begin{array}{cc}\frac{\partial }{\partial t}\left({\varphi }_m{C}_m\right)=\left[\begin{array}{c}-\frac{\partial }{\partial x}\left(C_m\frac{k_m{k}_{\mathrm{mrw}}}{{\mu }_w}\frac{\partial P_{\mathrm{mw}}}{\partial x}\right)-\frac{\partial }{\partial y}\left(C_m\frac{k_m{k}_{\mathrm{mrw}}}{{\mu }_w}\frac{\partial P_{\mathrm{mw}}}{\partial y}\right)+\\ \frac{\partial }{\partial x}\left({\varphi }_m{D}_{\mathrm{ex}}\frac{\partial C_m}{\partial x}\right)+\frac{\partial }{\partial y}\left({\varphi }_m{D}_{\mathrm{ey}}\frac{\partial C_m}{\partial y}\right)+\\ {\delta }_m\frac{v_l{A}_{\mathrm{mf}}{C}_f}{V_b}-k_s{C}_s{a}_v\end{array}\right]& \left(10\right)\end{array}$ $\begin{array}{cc}{C}_s=\frac{C_m}{1+\frac{k_s}{k_c}}& \left(11\right)\end{array}$ $\begin{array}{cc}{D}_{\mathrm{ei}}={\alpha }_{\mathrm{os}}{D}_m+{\lambda }_i❘v_l❘d_h& \left(12\right)\end{array}$

Where, C_m—Acidizing fluid concentration in matrix pores, in mol/m³; D_ex—Effective diffusion tensor in the x direction, in m²/s; D_ey—Effective diffusion tensor in the y direction, in m²/s; k_s—Reaction velocity constant, in m/s; C_s—Acidizing fluid concentration at the pore wall, in mol/m³; a_v—Rock specific surface area of reservoir matrix, in m²/m³; D_ei—Effective diffusion tensor in the i direction, in m²/s; α_os, λ_i—Pore structure constant, and α_os=1, λ_x≈0.5 and λ_y≈1 for spherical filling medium; D_m—Molecular diffusion coefficient, in m²/s; d_h—Hydraulic diameter of tubular pore, in m. The calculation model of matrix porosity and permeability changes during acid-rock reaction is as follows:

$\begin{array}{cc}\frac{\partial {\varphi }_m}{\partial t}=\frac{k_s{C}_s\beta a_v}{{\rho }_r}& \left(13\right)\end{array}$ $\begin{array}{cc}\frac{k_m}{k_{m0}}=\frac{{\varphi }_m}{{\varphi }_{m0}}{\left(\frac{{\varphi }_m\left(1-{\varphi }_{m0}\right)}{{\varphi }_{m0}\left(1-{\varphi }_m\right)}\right)}^{2\gamma }& \left(14\right)\end{array}$ $\begin{array}{cc}\frac{a_v}{a_{v0}}=\frac{{\varphi }_m}{{{\varphi }_{m0}\left(\frac{k_m{\varphi }_{m0}}{k_{m0}{\varphi }_m}\right)}^{1/2}}& \text{(15)}\end{array}$

Where, k_m0—Initial permeability of reservoir matrix, in mD; ϕ_m0—Initial porosity of reservoir matrix; γ—Parameter related to pore structure; a_v0—Initial rock specific surface area of reservoir matrix, in m²/m³.

(3) Initial conditions for gas reservoir seepage:

P _mg(i,j,t)|_t=0  (16)

Where, P_mg(i,j,t)—Gas pressure in the reservoir matrix at the coordinates of positions i and j in the grid at time t, in MPa; P₀—Original formation pressure of gas reservoir, in MPa.

(4) Boundary conditions for fracture propagation:

$\begin{array}{cc}\{\begin{array}{cc}W\left(x,t\right)=0& x>x_{L=1}+\sum _{L=1}^{n_{f,t}}{\xi }_L\mathrm{Or}x<x_{L=1}\\ \frac{{\partial }^2{W\left(x,t\right)}^4}{\partial x}=\frac{256Q_{\mathrm{int}}\mu \left(1-v\right)}{\pi G}& x=0\end{array}& \left(17\right)\end{array}$ $\begin{array}{cc}{P}_{fL=1,t}=P_{\mathrm{int}}& \left(18\right)\end{array}$

Where, Q_int—Injection displacement of acid fracturing, in m³/min; G—Volume modulus of reservoir rock sample, in MPa; x_L=1—Rectangular coordinates of the first acid etched fracture unit; n_f,t—Total number of acid etched fracture units at time t; ξ_L—length of the L^th acid etched fracture unit, in m; P_fL=1,t—Fluid pressure in the acid etched fracture unit in Section 1 at time t, in MPa; P_int—Downhole pressure during acid fracturing, in MPa.

(5) Boundary conditions for gas reservoir matrix seepage:

$\begin{array}{cc}\{\begin{array}{c}{\frac{\partial P_{\mathrm{mg}}}{\partial x}❘}_{x=0,L_x}=0\\ {\frac{\partial P_{\mathrm{mg}}}{\partial y}❘}_{y=0,L_y}=0\end{array}\&\{\begin{array}{c}{\frac{\partial P_{\mathrm{mw}}}{\partial x}❘}_{x=0,L_x}=0\\ {\frac{\partial P_{\mathrm{mw}}}{\partial y}❘}_{y=0,L_y}=0\end{array}& \left(19\right)\end{array}$

Where, L_x, L_y—Length and width of reservoir, in m;

(6) Boundary conditions and initial conditions for acidizing fluid migration reaction model:

$\begin{array}{cc}\{\begin{array}{c}{C}_f\left(0,t\right)=C_{0,t}\\ C_f\left(x_L,t\right)=C_{0,t}\\ C_f\left(L_f,t\right)=0\\ C_{m,t=0}=0\\ C_{s,t=0}=0\end{array}\begin{array}{c}\\ 0<x_L<L_f\\ \\ \\ \end{array}& \left(20\right)\end{array}$

Where, C_f(0,t)—Acidizing fluid concentration in initial artificial fracture unit at the acid fracturing time t, in mol/m³; C_f(x_L,t)—Acidizing fluid concentration in artificial fracture unit corresponding to the horizontal coordinate x_L at time t, in mol/m³; C_f(L_f,t)—Acidizing fluid concentration at the artificial fracture tip at time t, in mol/m³; C_m,t=0—Acidizing fluid concentration in the pore at the initial acid fracturing time, in mol/m³; C_s,t=0—Acidizing fluid concentration at the pore wall at the initial acid fracturing time, in mol/m³; L_f—Horizontal coordinate corresponding to the tip of artificial fracture unit at time t; C_0,t—Acidizing fluid concentration of construction fluid at time t, in mol/m³.

Step 3: On the basis of the structured reservoir grid, conducting numerical simulation according to the fracture propagation model, and working out the seepage parameters at a certain moment during acid fracturing;

In a specific embodiment, Newton iteration method for solving nonlinear equations was used to solve the fracture propagation model, and the seepage parameters at a certain time was obtained, specifically including the width W_L,t, fluid pressure P_fL,t and porosity ϕ_f(i,j,t) of acid etched fracture unit and the porosity ϕ_m(i,j,t), gas pressure P_mg(i,j,t) and liquid saturation S_mw(i,j,t) of matrix grid, wherein the fracture width at the acid etched fracture is defined as W_L=nf,t.

Step 4: Determining whether the fracture propagates at a given time according to the acid etched fracture propagation criterion: if there is no propagation, the total number n_f of fracture units remains unchanged; if there is propagation, the total number of fracture units is n_f=n_f+1; the below is the criterion for determining acid etched fracture propagation:

• • When the stress intensity factor K_If,t at fracture tip is less than or equal to the fracture toughness K_IC of reservoir rock, the fracture will propagate;
  • When the stress intensity factor K_If,t at fracture tip is greater than the fracture toughness K_IC of reservoir rock, the fracture will not propagate.

In a specific embodiment, the stress intensity factor K₁,t at fracture tip is calculated by the following equation:

$\begin{array}{cc}{K}_{\mathrm{If},t}=\frac{0.806E\sqrt{\pi }{W}_{L=n_f,t}}{4\left(1-v^2\right)\sqrt{2\Delta x}}& \left(21\right)\end{array}$

Where, K_If,t—Stress intensity factor at fracture tip at time t, in MPa·m^1/2; E—Young's modulus of reservoir rock sample, in MPa; W_L=nf,t—Average width of structured reservoir grid, in m; v—Poisson's ratio of reservoir rock sample; Δx—Width of artificial fracture tip at time t, in m.

The fracture toughness K_IC of reservoir rock is calculated by the following equation:

$\begin{array}{cc}{K}_{\mathrm{IC}}=0.3172{\rho }_r+\frac{0.0457}{V_c}+0.2131\ln \left(\mathrm{DT}\right)\times 0.5041& \left(22\right)\end{array}$

Where, K_IC—Type I fracture toughness of reservoir rock, in MPa·m^1/2; ρ_r—Rock density, in kg/m³; Vc—Average shaliness of reservoir rocks, in %; DT—Average interval transit time of the reservoir, in μs/m.

It should be noted that the stress intensity factor at fracture tip and the fracture toughness of reservoir rock can also be calculated by other methods in the prior art in addition to the calculation method in the above embodiment.

Step 5: Taking the seepage parameters obtained in Step 3 and the total number of fracture units obtained in Step 4 as the initial conditions for the next time, and repeating Steps 3 to 5 until the completion of acid fracturing to obtain the seepage parameters at the end of acid fracturing; the seepage parameters at the end of acid fracturing include the total number n_f,tend of artificial fracture units, the width W_L,tend of each fracture unit, the fluid pressure P_fL,tend in each fracture unit, the porosity ϕ_m(i,j,t_end) of each fracture unit, the half length

$\sum _{L=1}^{n_{f,t_{\mathrm{end}}}}{\xi }_L$

of artificial fracture, the gas pressure P_mg(i,j,t_end) in each matrix grid, the porosity ϕ_f(i,j,t_end) of each matrix grid, and the liquid saturation S_mw(i,j,t_end) of each matrix grid.

Step 6: Establishing a gas well production model, and calculating the pore distribution and liquid saturation distribution of the reservoir in gas well production according to the gas well production model; the gas well production model includes:

(1) Differential equation of gas-water seepage in gas reservoir:

$\begin{array}{cc}\frac{\partial }{\partial x}\left(\kappa \frac{k_f{k}_{\mathrm{frw}}}{{\mu }_w{B}_w}\frac{\partial P_f}{\partial x}\right)+\frac{q_{\mathrm{fw}}}{V_f}+{\delta }_m\frac{Q_{\mathrm{mw}}}{V_f}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_f{S}_{\mathrm{fw}}}{B_w}\right)& \left(23\right)\end{array}$ $\begin{array}{cc}\frac{\partial }{\partial x}\left(\kappa \frac{k_f{k}_{\mathrm{frg}}}{{\mu }_g{B}_g}\frac{\partial P_f}{\partial x}\right)+\frac{q_{\mathrm{fg}}}{V_f}+{\delta }_m\frac{Q_{\mathrm{mg}}}{V_f}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_f\left(1-S_{\mathrm{fw}}\right)}{B_g}\right)& \left(24\right)\end{array}$ $\begin{array}{cc}\nabla \left(\beta \frac{k_m{k}_{\mathrm{mrw}}}{{\mu }_w{B}_w}\nabla P_{\mathrm{mw}}\right)-{\delta }_m\frac{Q_{\mathrm{mw}}}{V_b}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_m{S}_{\mathrm{mw}}}{B_w}\right)& \left(25\right)\end{array}$ $\begin{array}{cc}\nabla \left(\beta \frac{k_m{k}_{\mathrm{mrg}}}{{\mu }_g{B}_g}\nabla P_{\mathrm{mg}}\right)-{\delta }_m\frac{Q_{\mathrm{mg}}}{V_b}=\frac{\partial }{\partial t_p}\left(\frac{{\varphi }_m\left(1-S_{\mathrm{mw}}\right)}{B_g}\right)& \left(26\right)\end{array}$ $\begin{array}{cc}{Q}_{\mathrm{mw}}=\frac{2k_m{k}_{\mathrm{mw}}·{\xi }_{L}H}{{\mu }_w\stackrel{\_}{d}}\left(P_{\mathrm{mw}}-P_f\right)& \left(27\right)\end{array}$ $\begin{array}{cc}{Q}_{\mathrm{mg}}=\frac{2k_m{k}_{\mathrm{mg}}·{\xi }_{L}H}{{\mu }_g\stackrel{\_}{d}}\left(P_{\mathrm{mg}}-P_f\right)& \text{(28)}\end{array}$

Where, k_f—Permeability of acid etched fracture, in mD; k_fw, k_frg—Relative permeability of liquid and gas in acid etched fracture; P_f—Pressure in artificial fracture, in MPa; q_fw, q_fg—Source and sink terms of liquid and gas in acid etched fracture, in m³/s; Q_mw, Q_mg—Liquid flow and gas flow between the main fracture and the matrix during gas well production, in m³/s; S_fw—Liquid saturation in acid etched fracture; t_p—Production time of gas well, in s; ∇—Gradient operator.

(2) Initial conditions:

Initial pressure distribution:

$\begin{array}{cc}\{\begin{array}{c}{P}_{\mathrm{fg}L,t_p=0}=P_{\mathrm{fw}L,t_p=0}=P_{fL,t_p=0}=P_{fL,t_{\mathrm{end}}}\\ {P_{\mathrm{mg}}\left(i,j,t_p\right){❘}_{t_p=0}=P_{\mathrm{mg}}\left(i,j,t\right)❘}_{t=t_{\mathrm{end}}}\\ {P_{\mathrm{mw}}\left(i,j,t_p\right){❘}_{t_p=0}=P_{\mathrm{mw}}\left(i,j,t\right)❘}_{t=t_{\mathrm{end}}}\end{array}& \left(29\right)\end{array}$

Where, P_{fg L,tp=0}—Initial gas pressure distribution of acid etched fracture in gas well production simulation, in MPa; P_fwL,tp=0—Initial liquid pressure distribution of acid etched fracture in gas well production simulation, in MPa; P_fL,tp=0—Initial pressure distribution of acid etched fracture in gas well production simulation, in MPa; P_fL,tend—Pressure distribution of artificial fracture at the end of acid fracturing, in MPa; P_mg(i,j,t_p)|t_p=0—Initial gas pressure distribution of reservoir matrix in gas well production simulation, in MPa; P_mg(i,j,t)|_t=tend—Pressure distribution in artificial fracture at the end of acid fracturing, in MPa; P_mw(i,j,t_p)|_tp=0—Initial liquid pressure distribution of reservoir matrix in gas well production simulation, in MPa; P_mw(i,j,t)_|t=tend—Liquid pressure distribution in artificial fracture at the end of acid fracturing, in MPa.

Initial saturation distribution:

$\begin{array}{cc}\{\begin{array}{c}{S_{\mathrm{fw}}\left(L,t_p\right)❘}_{t_p=0}=1\\ {S_{\mathrm{mw}}\left(i,j,t_p\right){❘}_{t_p=0}=S_{\mathrm{mw}}\left(i,j,t\right)❘}_{t=t_{\mathrm{end}}}\end{array}& \left(30\right)\end{array}$

Where, S_fw(L,t_p)|_tp=0—Initial liquid saturation of acid etched fracture in gas well production simulation; S_mw(i,j,t_p)|_tp=0—Initial liquid saturation of reservoir matrix in gas well production simulation; S_mw(i,j,t)|_t=tend—Liquid saturation of reservoir matrix at the end of acid fracturing.

(3) Internal boundary conditions:

P _w(x_w,y_w,t_p)=P_wf(t_p)  (31)

Where, P_w(x_w, y_w, t_p)—Liquid pressure of well-corresponding grid at the simulated time t_p of gas well production, in MPa; P_wf(t_p)—Bottom hole flowing pressure at production time t_p, in MPa;

(4) External boundary conditions:

$\begin{array}{cc}\{\begin{array}{c}{\frac{\partial P_{\mathrm{mg}}\left(i,j,t_p\right)}{\partial x}❘}_{x=0,L_x}=0\\ {\frac{\partial P_{\mathrm{mg}}\left(i,j,t_p\right)}{\partial y}❘}_{y=0,L_y}=0\end{array}\&\{\begin{array}{c}{\frac{\partial P_{\mathrm{mw}}\left(i,j,t_p\right)}{\partial x}❘}_{x=0,L_x}=0\\ {\frac{\partial P_{\mathrm{mw}}\left(i,j,t_p\right)}{\partial y}❘}_{y=0,L_y}=0\end{array}.& \left(32\right)\end{array}$

Acidized wormholes will change the porosity and permeability of the matrix grid, and then affect the seepage pattern. The gas well production model described in the present invention is built by an embedded discrete fracture model only considering the acid etched fracture, the matrix, and the fluid seepage between acid etched fracture and matrix.

Specifically applying the gas well production model to calculate the pore distribution and fluid saturation distribution of the reservoir during gas well production, the following parameters are taken as the initial parameters of gas well production model: the pressure distribution of each phase of the acid etched fracture unit, the saturation distribution of each phase of the acid etched fracture unit, the pressure distribution of each phase of the matrix grid and the saturation distribution of each phase of the matrix grid at the end of acid fracturing operation. With finite difference discretization and programming, the following parameters can be worked out: the porosity ϕ(i,j,t_p) and liquid saturation S_fw(L,t_p) of each etched fracture unit in the reservoir, and the porosity ϕ_m(i,j,t_p) and liquid saturation S_mw(L,t_p) of each matrix grid at any instant in production. It should be noted that the solution method of the gas well production model is in the prior art, and the specific steps will not be described herein.

Step 7: Calculating the cumulative production of the gas well according to the results obtained in Steps 5 and 6; the cumulative production of the gas well is calculated by the following equation:

$\begin{array}{cc}Q=\sum _{i=1}^{n_i}\sum _{j=1}^{n_j}\left(x_{i,j}·y_{i,j}·H\right)\left({\varphi }_m\left(i,j,t_p\right)·S_{\mathrm{mw}}\left(i,j,t_p\right)-{\varphi }_m\left(i,j,t_{\mathrm{end}}\right)S_{\mathrm{mw}}\left(i,j,t_{\mathrm{end}}\right)\right)+\sum _{L=1}^{n_{f,t_{\mathrm{end}}}}\left({\xi }_L·W_{L,t_{\mathrm{end}}}·H\right)\left({\varphi }_f\left(L,t_{\mathrm{end}}\right)S_{\mathrm{fw}}\left(L,t_{\mathrm{end}}\right)-{\varphi }_f\left(L,t_p\right)S_{\mathrm{fw}}\left(L,t_p\right)\right)& \left(33\right)\end{array}$

Where, Q—Cumulative production of gas well at time t_p, in m³; n_i, n_j—Total number of grids in x and y directions in the structured reservoir grid; x_i,j, y_i,j—Length and width of matrix grid at positions i and j, in m; #m(i,j,t_p)—Porosity of matrix grid at positions i and j at time t_p; S_mw(i,j,t_p) Liquid saturation of matrix grid at positions i and j at time t_p; +m(i,j,t_end)—Porosity of matrix grid at positions i and j at time t_p; S_mw(i,j,t_end)—Liquid saturation of matrix grid at positions i and j at time t_p; n_f,tend—Total number of acid etched fracture units at time t_end; W_L,tend—Width of acid etched fracture unit in Section L at time t_end, in m; ϕ^f(i,j,t_end)—Porosity of acid etched fracture unit in Section L at time t_end; S_fw(L,t_end)—Liquid saturation of acid etched fracture unit in Section L at time t_end; ϕ^f(L,t_p)—Porosity of acid etched fracture unit in Section L at time t_p; S_fw(L,t_p)—Liquid saturation of acid etched fracture unit in Section L at time t_p.

Step 8: Calculating the multiple proportion of cumulative production increase of the construction plan according to the cumulative production of the gas well; the greater the multiple proportion, the better the acid fracturing effect; the multiple proportion of cumulative production increase is calculated by the following equation:

$\begin{array}{cc}S=\frac{Q_T}{Q_{0,T}}& \left(34\right)\end{array}$

Where, S—Multiple proportion of cumulative production increase; Q_T—Simulated cumulative production of the gas well at time T after acid fracturing, in m³; T—Time when the daily gas production after acid fracturing is equal to the daily gas production before acid fracturing, in d; Q_0,T—Estimated cumulative production of the gas well at T without acid-fracturing stimulation, in m³.

It should be noted that gas-water flow is considered in the establishment of each model in the above embodiment and the models are applicable to evaluating the acid fracturing effect of the gas reservoir. The present invention can also adopt the same idea for establishing relevant models of the reservoir based on oil-water flow, so as to evaluate the acid fracturing effect of the reservoir.

In a specific embodiment, in a study case of Well X in a marine carbonate gas reservoir in eastern Sichuan, the present invention was applied to evaluate the acid fracturing effect of the well.

The reservoir depth of Well X is 4,479.5 to 4,502 m, and it is mainly composed of crystal powder dolomite, developed with intergranular pores and dissolved pores; the porosity ranges from 1.6 to 7.4%, 5.1% on average; the permeability ranges from 0.53 mD to 0.74 mD, 0.65 mD on average; the gas saturation ranges 370.36% to 480.59%, 450.52% on average; the temperature is 100.3° C. at the well depth of 4,488.75 m (vertical depth of 4,416.4 m) in the middle of reservoir, and the formation pressure is 40.6 MPa. After the conventional acid-frac stimulation in the early stage of development, the length of acid etched fractures was short, the conductivity was low, the maximum daily production of a single well was 12.27×10⁴ m³/d during well testing, and the average daily production was 10.26×10⁴ m³/d in the first week. It is planned to conduct deep acid-frac stimulation on the reservoir where Well X is located to increase the production of a single well. The working medium is composed of 100 m³ slick water, 180 m³ gelled acid and 140 m³ diverting acid, with the displacement of 4 m³/min, 3 m³/min and 3 m³/min, respectively.

The present invention and the conventional numerical method (numerical method in Numerical Simulation of Productivity of Fractured Gas Well with Start-up Pressure Gradient Considered) are used to simulate acid etched fracture propagation, acidizing fluid flow reaction, acidizing fluid filtration and gas-water seepage in the stimulated zone of Well X with and without consideration of “stimulated zone”. The deep acid fracturing of carbonate reservoir considering the “stimulated zone” is shown in FIG. 1. When simulating the acid fracturing and production of Well X, it was necessary to first establish a structured reservoir grid and initial artificial fractures; then, according to the initial and boundary conditions in (16) to (20), equations (1) to (5) were used to calculate the width and intra-fracture pressure of artificial fracture at a certain time during acid fracturing, and the permeability, porosity, gas pressure and water content in the stimulated zone formed by acidizing fluid filtration were calculated by equations (6) to (15); next, the stress intensity factor K_If,t at fracture tip and the fracture toughness K_IC of reservoir rock were calculated by equations (21) and (22), and the fracture propagation was determined according to the acid etched fracture propagation criterion; finally, the calculation results at that time were taken as the initial conditions for the next time, and the calculation and determination were repeated until the completion of acid fracturing to obtain the seepage parameters at the end of acid fracturing; the seepage parameters at the end of acid fracturing include the total number n_f,tend of artificial fracture units, the width W_L,tend of each fracture unit, the fluid pressure P_fL,tend in each fracture unit, the porosity ϕ_m(i,j,t_end) of each fracture unit, the half length

$\sum _{L=1}^{n_{f,t_{\mathrm{end}}}}{\xi }_L$

of artificial fracture, the gas pressure P_mg(i,j,t_end) in each matrix grid, the porosity ϕ_f(i,j,t_end) of each matrix grid, and the liquid saturation S_mw(i,j,t_end) of each matrix grid; These parameters were taken as the initial conditions of gas well production, the porosity distribution and liquid saturation distribution of gas well at different production times were calculated by equations (23) to (28) under the initial conditions and boundary conditions shown in (29) to (32), and then equations (33) to (34) were used to work out the simulated cumulative production at time T of gas well production and the multiple proportion of cumulative production increase after deep acid fracturing.

The simulated cumulative production result of this embodiment is shown in FIG. 2. It can be seen from FIG. 2 that the maximum daily production of Well X after stimulation is 23.16×10⁴ m³/d, and the cumulative production after 720 days is 7.319×10⁷ m³. Before stimulation, Well X is simulated to product for 720 days, and the simulated cumulative production is 7.952×10⁷ m³ and 5.806×10⁷ m³ with or without consideration of the “stimulated zone”, respectively. Therefore, with consideration of the seepage pattern improvement and stimulation effect by the acid-frac “stimulated zone”, the present invention can predict the cumulative production of single well in a certain acid fracturing plan more accurately, and effectively improve the accuracy of the evaluation results of acid fracturing effect, which severs the purpose of optimizing the acid fracturing plan, and significantly guiding the development of carbonate reservoir with reduced cost and enhanced efficiency. It is a significant improvement over the prior art.

The above are only the preferred embodiments, which are not intended to limit the present invention in any form. Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Those skilled in the art, within the scope of the technical solution of the present invention, can use the disclosed technical content to make a few changes or modify the equivalent embodiment with equivalent changes. Within the scope of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention are still regarded as a part of the technical solution of the present invention.

Claims

1. A deep acid-frac stimulation method based on an evaluation for acid fracturing effect, comprising the following steps: Step 1: collecting a geological exploration data of a target reservoir, dividing the target reservoir into a ni×nj structured grid to establishing a structured reservoir grid, and adding an initial artificial fractures to the structured reservoir grid, wherein the initial artificial fractures are divided into multiple fracture units by the structured reservoir grid, the multiple fracture units are numbered by consecutive positive integers, a length of each fracture unit of the multiple fracture units is denoted as ξL and a total number of fracture units as nf;Step 2: Establishing a fracture propagation model considering an acid-frac stimulated zone formed by a dissolution of acid fluid filtering along a acid etched fracture in the target reservoir, wherein the fracture propagation model comprises:(1) a first calculation model considering fracture width and intra-fracture pressure in the acid-frac stimulated zone:

2. The deep acid-frac stimulation method according to claim 1, wherein in Step 1, the establishment of a structured reservoir grid comprises the following sub-steps: collecting the geological exploration data of target reservoir, dividing the reservoir length Lx and reservoir width Ly into ni and nj segments respectively in an x-y rectangular coordinate system, so the entire reservoir can be divided into a ni×nj structured grid, where xi,j and yi,j represent the length and width of each grid respectively, and the subscripts i and j represent the position of each grid in the reservoir.

3. The deep acid-frac stimulation method according to claim 1, wherein in Step 1, when adding initial artificial fractures to the structured reservoir grid, a propagation direction of initial artificial fracture is designed as the x-axis direction and a propagation length as the total length of N grids, wherein N is an integer greater than or equal to 3.

4. The deep acid-frac stimulation method according to claim 1, wherein in Step 4, the acid etched fracture propagation criterion comprising: When a stress intensity factor KIf,t at fracture tip is less than or equal to a fracture toughness KIC of reservoir rock, the fracture will propagate;When the stress intensity factor KIf,t at fracture tip is greater than the fracture toughness KIC of reservoir rock, the fracture will not propagate.

5. The deep acid-frac stimulation method according to claim 4, wherein the stress intensity factor KIf,t at fracture tip is calculated by the following equation:

6. The deep acid-frac stimulation method according to claim 1, wherein in Step 6, the gas well production model comprises: (1) Differential equation of gas-water seepage in gas reservoir:

7. The evaluation method for acid fracturing effect based on the theory of acid-frac “stimulated zone” according to claim 6, wherein in Step 7, the cumulative production of the gas well is calculated by the following equation: