Patent Application
20120211225
The present invention relates to hydrocarbon well stimulation and in particular to methods and apparatus to mobilize and remove fracturing fluids introduced into the fracture zone and surrounding porous media by means of applying periodic shock waves.
Fracturing the earth from a wellbore is a known technique for enhancing oil production and recovery from an oil bearing bed. A variety of methods have been proposed to create both short and long fractures near a wellbore. However hydraulic fracture treatments oftentimes underperform. In such cases the so-called Frac and Pack completions show a difference between the designed and-effective fracture length. This is due to creation of a positive skin effect caused in part by stagnant fluids (for instance polymers) retained in the fracture tip and fracture faces limiting hydrocarbon production (both in rate and capacity) from a given well. Numerous technologies have been developed to provide skin removal and fracture clean-up of such stagnant fluids. One of these methods is described and claimed in U.S. Pat. No. 6,069,118 wherein a well stimulation method coupled with methods and compositions to remove fluid introduced into a subsurface fracture are presented. In this patent methods are given to create then exploit chemical potential gradients at the fracture face to induce fluid flow from the fracture into the formation thereby increasing effective fracture length and improving fracture conductivity. In another U.S. Pat. No. 7,723,264 methods to increase recovery of treatment fluid following stimulation of a subterranean formation using cationic surfactant coated particles are disclosed.
These approaches provide the basis for numerous inventions as disclosed in U.S. Pat. Nos. 5,806,597; 5,875,843; 5,960,880; 5,964,289; and 6,439,309. Presently, the primary method for removal of stagnant fluids is a breaker fluid which is pumped into the fracture to lower viscosity of the stagnant fluids so they are more easily removed from the fracture during flowback. The main disadvantage of all above noted methods is the problem of delivering a breaker fluid deep enough into the fracture to provide the effective breaker action on stagnant fluids under the existing pressure gradient. One approach to resolve this problem is the use of vibrations or shock waves to increase the mobility of breaker fluid in the fracture thereby enhancing the process of fracture clean-up. One method for increasing fluid mobility is disclosed in U.S. Pat. No. 6,467,542 wherein high frequency vibrations are used for treatment of the near well zone to remove a skin effect. The disadvantage of this method is high attenuation of the high frequency vibrations in porous medium, limiting the distance over which they are effective. The use of shock waves for increasing oil mobility/recovery is disclosed in U.S. Pat. No. 6,899,175 and U.S. Pat. Nos. 125,783 and 140,004.
While there have been a variety of methods proposed for cleaning-up fractures around the wellbore, there remains a need for an economical method which provides effective clean-up of fractures.
Accordingly, a primary object of the present invention is to provide a method for enhancing fluid removal from a fracture in a geologic formation by applying periodic/cyclic shock waves to the fluids in the fracture and to the surrounding formation which has undergone fracturing. In accordance with the invention, the method includes the steps of arranging a device attached to the end of tubing inside the wellbore in the vicinity of said fracture for generating shock waves, providing a liquid via tubing into the device for generating shock waves with the amplitude Pa of shock waves determined by following expression:
0.3 MPa≦Pa≦1.4Pp−0.8ρgH,
where Pp is the formation pore pressure, ρ is the formation density, g is a gravity acceleration, H is the depth of said fracture, Pa is the amplitude of shock wave;
It is further object of the present invention to provide a method for enhancing of fluid removal from a fracture in a geologic formation in which a device for generating shock waves as described in U.S. Pat. No. 6,899,175 is installed in the wellbore of at least one offset well closest to the at least one well wherein a fracture is created.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation which includes a flow line at the surface supplying a liquid from breaker tank via a pump into the wellbore and the flow line having a check valve preventing flow of liquid from the wellbore back into the flow line, a tubing string connected to the flow line and extending downwardly into the wellbore, an elongated cylinder connected to the bottom of tubing string at the upper end and having an opening to wellbore, a plunger movably arranged within an elongated cylinder to move within the elongated cylinder, the pumping means connected with plunger for moving of plunger within the elongated cylinder and compressing the liquid contained between check valve inside the flow line and plunger inside the tubing and discharging said liquid into the wellbore via an opening when plunger exits out of the elongated cylinder on every upstroke of pumping means to generate a shock wave, a lubricator accommodating a pumping means to prevent the leakage of liquid from the tubing and flow line at the surface.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation in which the pumping means is a wire-line or slick-line.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation in which pumping means is a string of sucker rods connected to the pumping unit installed at the surface.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation in which the pumping means upward motion length Lp on every upstroke is determined by following formulae:
where Lp is a length of upstroke of pumping means, Pa is the required amplitude of shock wave, Vt is a volume of liquid contained between check valve inside the flow line and plunger inside the tubing, π equals 3.1415, β is a bulk modulus of pure water, φ is a coefficient accounting the difference in compressibility between pure water and liquid contained between check valve inside the flow line and plunger inside the tubing, Dp is diameter of plunger Pt is a pressure of liquid inside tubing, Pc is a pressure of liquid inside wellbore.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation in which the elongated cylinder consists of several cylinders having the same inside diameter and connected between each other.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation in which the plunger has a check valve configured to open for delivering a liquid into tubing above plunger on down stroke of pumping means.
It is further object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation comprising: a flow line at the surface supplying a liquid into wellbore, a tubing string connected with flow line and extending downwardly into the wellbore, an elongated cylinder connected to the bottom of tubing string at the upper end and having at least one opening into wellbore on the side surface of the elongated cylinder, a plunger movably arranged within said elongated cylinder to move within said elongated cylinder, said plunger includes a lower portion having a diameter greater than upper portion of plunger, a spring installed between said lower portion of plunger and the bottom of elongated cylinder and said spring undergoes a compression displacement when pressure inside tubing exceeds the pressure in wellbore causing the lowering of plunger inside the elongated cylinder and the discharging of liquid contained inside tubing into the wellbore via at least one said opening as far as a top of the lower portion of moving down plunger reaches at least one said opening thereby generating a shock wave, then said spring returns to its initial position as far as the liquid pressure inside tubing equalizes with the wellbore liquid pressure and the process repeats itself as an auto-oscillation regime with the frequency of auto-oscillations in accordance with formulae:
where ω is a frequency of auto-oscillations, Z is spring constant, M is a weight of plunger and λ is a coefficient of friction between the lower portion of plunger and the elongated cylinder.
It is another object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in geologic formation in which said spring has a spring constant Z determined in accordance with the following formulae:
where Z is spring constant, π equals 3.1415, Dp is a diameter of the lower portion of plunger, Do is a diameter of the upper portion of plunger, Pt is a pressure of liquid inside tubing, Pc is a pressure of liquid inside wellbore.
a is a cross-sectional top view of tubing having at least one opening and upper part of plunger.
Referring to
0.3 MPa≦Pa≦1.4Pp−0.8ρgH,
where Pp is the formation pore pressure, ρ is the formation density, g is a gravity acceleration, H is the depth of said fracture 6, Pa is the amplitude of shock wave 24. In particular, for formation pore pressure Pp, the formation density ρ, gravity acceleration g and depth of formation H accounting for 45 MPa, 2300 kg/m3, 9.81 m/s2 and 3000 m, correspondingly, the amplitude of generated shock waves 24 has to not exceed 33.6 MPa. The shock waves 24 propagating through fracture (s) 6 enhance the process of clean-up by breaking the high molecular chains and enhancing the movement of breaker inside the fracture (s) 6 and in the formation thereby increasing the effective fracture length. The generation of shock waves 24 described above is based on classic hydro-impact phenomenon when compressed liquid contained between check valve 10 inside the flow line 11 and plunger 4 inside the tubing 2 is discharged into wellbore 1 via opening 8 during a fraction of a second.
As pumping means 7 a wire line or a string of sucker rods connected to the pumping unit installed at the surface could be used. The length of pumping means upstroke Lp to compress the liquid contained between check valve 10 inside the flow line 11 and plunger 4 inside the tubing 2 is determined by the following formulae:
where Lp is a length of upstroke of pumping means 7, Pa is the required amplitude of shock wave 24, Vt is a volume of liquid contained between check valve 10 inside the flow line 11 and plunger 4 inside the tubing 2, π equals 3.1415, β is a bulk modulus of pure water, φ is a coefficient accounting the difference in compressibility between pure water and liquid/breaker contained between check valve 10 inside the flow line 11 and plunger 4 inside the tubing 2, Dp is diameter of plunger 4, Pt is a pressure of liquid inside tubing 2, Pc is a pressure of liquid inside wellbore 1. In particular, for Dp=0.06985 m, Pa=10 MPa, Vt=8.5 m3, β=2.2*109 Pa, φ=0.8, Pt=12.5 MPa and Pc=12 MPa the length of upstroke Lp accounts for 12 m.
The generation of shock waves could be provided without using a pumping means. Referring to
The spring 16 undergoes a compression displacement when pressure inside tubing 2 exceeds the pressure in the wellbore 1 causing the lowering of plunger 21 inside the elongated cylinder 20 and discharging of the liquid contained inside tubing 2 into the wellbore 1 via said at least one opening 14 as far as the top of the lower portion 19 of downward moving plunger 21 reaches at least one opening 14 thereby generating a shock wave, then spring 16 returns to its initial position as far as the liquid pressure inside tubing 2 equalizes with the liquid pressure in wellbore 1 and the process repeats itself as an auto-oscillation regime with the frequency of auto-oscillations in accordance with formulae:
where ω is a frequency of auto-oscillations, Z is spring constant, M is a weight of plunger 21 and λ is a coefficient of friction between the lower portion of plunger 19 and the elongated cylinder 20. In particular, for Z=163000 N/m, M=120 kg, and λ=350 kg/sec the frequency of auto-oscillations ω accounts for 36.8 Hz. The spring constant Z, in turn, is determined in accordance with the following formulae:
where Z is spring constant, π equals 3.1415, Dp is a diameter of the lower portion 19 of plunger 21, Do is a diameter of the upper portion 15 of plunger 21, Pt is a pressure of liquid inside tubing 2, Pc is a pressure of the liquid inside wellbore 1. In particular, for Dp=0.06985 m, Do=0.03985 m, Pt=12.5 MPa and Pc=12 MPa the spring constant accounts for 163000 N/m.
The elongated cylinder 20 has also the opening 18 at its bottom to avoid the compressing of liquid below plunger 21.
Plunger 21 can be installed and retrieved after clean-up procedure by means for instance of a wire-line or a slick-line technique using the corresponding fishing neck 17 at the top of plunger 21.
Referring to
While in accordance with the provisions of the Patent Statutes the preferred forms and the embodiments of the invention have been illustrated and described, it will be apparent to those of ordinary skill in the art various changes and modifications may be made without deviating from the inventive concepts set forth above.
