Patent Grant
8684076
The present invention relates to hydrocarbon well stimulation and in particular to methods and apparatus to mobilize and remove fracturing fluids introduced into a 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 a so-called Frac and Pack completions shows 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 a fluid flow from the fracture into the formation thereby increasing effective fracture length and improving fracture conductivity. In another patent (U.S. Pat. No. 7,723,264), herein incorporated by reference, 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, herein incorporated by reference. Presently, a primary method for removal of stagnant fluids is a breaker fluid which is pumped into the fracture to lower a viscosity of the stagnant fluids so they are more easily removed from the fracture during a flowback. The main disadvantage of all above noted methods is the problem of delivering the 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 a use of vibrations or shock waves to increase the mobility of the 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, herein incorporated by reference, wherein high frequency vibrations are used for treatment of a near well zone to remove the skin effect. The disadvantage of this method is a high attenuation of 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, herein incorporated by reference.
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 the fracture in a geologic formation by applying periodic/cyclic shock waves to fluids in the fracture and to a surrounding formation which has undergone fracturing. In accordance with the invention, the method includes the steps of arranging a device attached to an end of a tubing string inside a wellbore in a vicinity of said fracture for generating shock waves, providing a liquid via the tubing string into the device for generating shock waves with an amplitude Pa of shock waves determined by a following expression:
0.3 MPa≦Pa≦1.4Pp−0.8ρgH,
where Pp is a formation pore pressure, ρ is a formation density, g is a gravity acceleration, H is a depth of said fracture, Pa is the amplitude of shock wave;
It is further object of the present invention to provide the method for enhancing of fluid removal from the fracture in a geologic formation in which the device for generating shock waves includes a flow line at the surface supplying the liquid from a breaker tank via a pump into a wellbore and the flow line having a check valve, similar to the one described for instance in U.S. Pat. No. 6,899,375, preventing flow of the 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 the 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, a pumping means connected with the plunger for moving of the plunger within the elongated cylinder and compressing the liquid contained between said check valve inside the flow line and the plunger inside the elongated cylinder and discharging said liquid into the wellbore via the opening when the plunger exits out of the elongated cylinder on every upstroke of the pumping means to generate the shock wave, a lubricator accommodating the pumping means to prevent a leakage of liquid from the tubing and flow line at the surface, and said pumping means upward motion length Lp on every upstroke is determined by the following formulae:
where Lp is a length of upstroke of the pumping means, Pa is a required amplitude of the shock wave, Vt is a volume of liquid contained between the check valve inside the flow line and the plunger inside the tubing string, 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 the liquid contained between the check valve inside the flow line and the plunger inside the elongated cylinder, Dp is a diameter of the plunger, Pt is the pressure of the liquid inside the tubing string, Pc is the pressure of the liquid inside the wellbore.
It is further object of the present invention to provide an apparatus for enhancing of fluid removal from a fracture in the geologic formation comprising: the flow line at the surface supplying the liquid into wellbore, the tubing string connected with the flow line and extending downwardly into the wellbore, the elongated cylinder connected to the bottom of the tubing string at an upper end and having at least one opening into the wellbore on a side surface of the elongated cylinder, the plunger movably arranged within said elongated cylinder to move within said elongated cylinder, said plunger includes a lower portion having the diameter greater than upper portion of plunger, a spring installed between said lower portion of the plunger and a bottom of the elongated cylinder and said spring undergoes a compression displacement when the pressure inside the tubing string exceeds the pressure in the wellbore causing the lowering of the plunger inside the elongated cylinder and the discharging of the liquid contained inside the tubing string into the wellbore via at least one said opening as far as a top of the lower portion of moving downward 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 the tubing string equalizes with the wellbore liquid pressure and the process repeats itself as an auto-oscillation regime with a frequency of auto-oscillations in accordance with the formulae:
where ω is the frequency of auto-oscillations, Z is a spring constant, M is a weight of the plunger and λ is a coefficient of friction between the lower portion of the 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 the geologic formation in which said spring has the spring constant Z determined in accordance with the following formulae:
where Z is the spring constant, π equals 3.1415, Dp is the diameter of the lower portion of the plunger, Do is the diameter of the upper portion of the plunger, Pt is the pressure of the liquid inside the tubing string, Pc is the pressure of the liquid inside the wellbore.
a is a cross-sectional top view of the tubing string having at least one opening and the upper part of the 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 the 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 the generated shock waves 24 has to not exceed 33.6 MPa. The shock waves 24 propagating through the 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 the check valve 10 inside the flow line 11 and the plunger 4 inside the elongated cylinder 3 is discharged into the wellbore 1 via opening 8 during a fraction of a second.
As an option of pumping means 7, a wire line or a string of a 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 the check valve 10 inside the flow line 11 and the plunger 4 inside the elongated cylinder 3 is determined by the following formulae:
where Lp is the length of pumping means upstroke 7, Pa is the required amplitude of the shock wave 24, Vt is the volume of liquid contained between the check valve 10 inside the flow line 11 and the plunger 4 inside the tubing string 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 the liquid/breaker contained between the check valve 10 inside the flow line 11 and the plunger 4 inside the elongated cylinder 3, Dp is the diameter of the plunger 4, Pt is the pressure of the liquid inside the tubing string 2, Pc is the pressure of the liquid inside the 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 the tubing string 2 exceeds the pressure in the wellbore 1 causing the lowering of the plunger 21 inside the elongated cylinder 20 and discharging of the liquid contained inside tubing string 2 into the wellbore 1 via said at least one opening 14 as far as the top of the tower portion 19 of the 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 the tubing string 2 equalizes with the liquid pressure in the wellbore 1 and the process repeats itself as the auto-oscillation regime with the frequency of auto-oscillations in accordance with the formulae:
where ω is the frequency of auto-oscillations, Z is the spring constant, M is the weight of the plunger 21 and λ is the 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 the spring constant, π equals 3.1415, Dp is the diameter of the lower portion 19 of the plunger 21, Do is the diameter of the upper portion 15 of the plunger 21, Pt is the pressure of liquid inside tubing string 2, Pc is the pressure of the liquid inside the 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 the 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 a 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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2057859 | Thaheld | Oct 1936 | A |
| 5586602 | Vagin | Dec 1996 | A |
| 6899175 | Kostrov et al. | May 2005 | B2 |
| Number | Date | Country | |
|---|---|---|---|
| 20120211225 A1 | Aug 2012 | US |
