Abstract:
The method for enhancing of forming at least one fracture having a required width r and length L in the formation surrounding fracturing around a wellbore in the regime of resonance by means of applying vibrations to the formation which is undergoing pressurizing exceeding the fracture gradient pressure of the formation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for fracturing the earth from a wellbore by over pressuring a fluid(s) and/or gases inside a wellbore under conditions of resonance. 
     BRIEF DESCRIPTION OF PRIOR ART 
     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. One of method is described and claimed in U.S. Pat. No. 5,617,921 by Schmidt et al., herein incorporated by reference, wherein a method for initiating and/or extending a fracture in an earth&#39;s formation from a well penetrating the formation utilizing a source of high pressure fracturing fluid disposed on the earth&#39;s surface which is released to flow into and through the well at a predetermined time to initiate and/or extend the fracture. However, this method requires a significant amount of energy and to have a relatively large diameter tubing string in which to hold a sufficient charge of pressured gas to provide an adequate fracture fluid pressure and flow characteristics. 
     The use of high pressured gas or other pressured fluid(s) in a wellbore to clean perforations and/or create fractures has been described in U.S. Pat. Nos. 5,669,448 and 5,131,472, herein incorporated by reference. These references disclose a method of stimulating a well by suddenly applying pressure to the formation in excess of the fracture gradient pressure and thereafter pumping fluid into the well before the pressure declines below the fracture gradient pressure. In addition, there are other more expensive means of injecting treatment liquids that have been proposed. One such type of approach is to place the treatment liquid in the well and ignite a gas generating propellant in the production string, as shown in U.S. Pat. Nos. 6,138,753; 5,443,123; 5,101,900; 4,936,385 and 2,740,478, herein incorporated by reference. Of more general interest is the disclosure in U.S. Pat. No. 3,029,732, herein incorporated by reference, 
     While there have been a variety of methods proposed for creating hydraulic fractures around the wellbore, there remains a need for an effective, high-pressure method which creates a pattern of fractures extending from all perforations into the formation in particular with the required parameters of fractures. 
     SUMMARY OF THE INVENTION 
     Accordingly, a primary object of the present invention is to provide a method for enhancing of forming at least one fracture having a required width r and length L in the formation surrounding a wellbore in the regime of resonance by means of applying a vibrations to the formation which is undergoing a pressurizing process when a formation pressure exceeds the fracture gradient pressure of the formation as a result of pumping in of a fracturing fluid into a wellbore and farther into the formation through the perforations. In accordance with the invention, the method includes the steps of providing the pressurized fracturing fluid via tubing into a device for generating vibrations attached to the end of tubing inside the wellbore in the vicinity of the perforations and consisting of an elongated cylinder and plunger connected to a pumping means, and reciprocating upward and downward said plunger inside the elongated cylinder such that movement of plunger compresses liquid inside said tubing and generating the repetitive pulses of vibrations every time when said plunger exits out of a top of said elongated cylinder on upward movement of said plunger due to quick release of compressed liquid into said wellbore thereby generating vibrations having an amplitude varying between 15-35 MPa on a resonant frequency f r  in accordance with following expression: 
                 f   r     =       c     2   ⁢   π       ⁢       r     1.2   ⁢     HL   ⁡     (     r   +   W     )                 ,         
where f r  is the frequency of resonance, c is a speed of sound in the fracturing fluid, π equals 3.1415, r is the required width of fracture, H is a combined thickness of a casing and a cement bond surrounding the casing, W is a length of a casing arch between two neighboring perforations, L is the required length of fracture.
 
     It is another object of the present invention to provide the method for enhancing of forming at least one fracture having a required width r and length L in the formation surrounding a wellbore in the regime of resonance in which the repetitive pulses of vibrations provide with the rate from 10 times per hour to 20 times per minute. 
     It is another object of the present invention to provide the method for enhancing of forming at least one fracture having a required width r and length L in the formation surrounding a wellbore in the regime of resonance in which for known formation pore pressure P p , formation density ρ, depth of perforations H and formation Poisson&#39;s ratio ν the amplitude of the repetitive pulses of vibrations P a  is defined by the following expression: 
               P   a     =         1   -   v         (     1   +   v     )     ⁢     (     1   -     2   ⁢           ⁢   v       )         ⁢     (       1.8   ⁢     P   p       -     0.9   ⁢   ρ   ⁢           ⁢   gH       )             
where g is a gravity acceleration.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic illustration of the wellbore in which the method and the apparatus of the present invention is employed. 
         FIG. 2  is a cross-sectional top view of the wellbore and the formation with the fractures. 
         FIG. 3  is a measured dynamometer diagram of repetitive pulses of load provided by device for generating vibrations. 
         FIG. 4  is a waveform and spectrum of a single burst/pulse provided by device for generating vibrations. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1  and  FIG. 2 , there is shown the wellbore  1  having perforations  5  and fractures  6 .  FIG. 1  shows a general arrangement of a device for generating vibrations and procedure using the vibrations, the flow line  11  at the surface supplying the pressurized fracturing fluid from tank  13  via pump  12  into wellbore  1 , the check valve  10  which is closed when the pressure of fracturing fluid inside the tubing  2  is greater than the one in flow line  11  thereby preventing flow of fracturing fluid from the tubing  2  back into the flow line  11 , the tubing string  2  connected to flow line  11  and extending downwardly into the wellbore  1 , a device for generating vibrations consisting of the elongated cylinder  3  connected with the bottom of tubing string  2  at the upper end and having the opening  8  to wellbore  1 , the plunger  4  having the taper  16  movably arranged within the elongated cylinder  3  to move within said elongated cylinder  3 , the pumping means  7  (for instance, sucker rods which, in turn, are connected to pumping unit) connected with the plunger  4  for moving the plunger  4  within the elongated cylinder  3  and compressing the fracturing fluid contained between the check valve  10  inside the flow line  11  and plunger  4  inside the elongated cylinder  3  and discharging the compressed fracturing fluid into the wellbore  1  via the opening  8  when plunger  4  exits out of the elongated cylinder  3  on every upstroke of the pumping means  7  to generate the vibrations due to the creating a periodic vortices in accordance with well known phenomenon of an auto-oscillations discovered by V. Strouhal in 19 th  century. A lubricator  9  accommodates a pumping means  7  to prevent the leakage of the compressed fracturing fluid from the tubing string  2  and flow line  11  at the surface. More details about phenomenon of auto-oscillations could be found for example in the articles: Sobey, Ian J. (1982). “Oscillatory flows at intermediate Strouhal number in asymmetry channels”. Journal of Fluid Mechanics, N. 125: 359-373, herein incorporated by reference, and Sakamoto, H.; Haniu, H. (1990). “A study on vortex shedding from spheres in uniform flow”. Journal of Fluids Engineering, N 112 (December 1992): 386-392, herein incorporated by reference. 
     The generation of vibrations is repetitive and occurs in the form of bursts or so called hydro-impact waves/pulses at the moment when plunger  4  exits out of the elongated cylinder  3  due to the upward motion of the pumping means  7 . The pumping means  7  provide a reciprocating upward and downward movements of the plunger  4  inside the elongated cylinder  3 . The number or rate of reciprocating movements may vary from a few per hour to a dozens per minute depending on the particular fracturing operation.  FIG. 3  shows the typical measured dynamometer diagram of repetitive load pulses created by device for generating vibrations. The amplitude of bursts/pulses may vary between 15-35 MPa depending on the type of formation undergoing fracturing. For relatively soft formation like, for instance, the unconsolidated sandstones this amplitude should be lower compared to hard formation like the deep shales. In case the characteristics of formation are known, i.e. the formation pore pressure P p , the formation density ρ, the depth of perforations H and the formation Poisson&#39;s ratio ν the amplitude of the repetitive pulses of vibrations P a  is defined by the following expression: 
                 P   a     =         1   -   v         (     1   +   v     )     ⁢     (     1   -     2   ⁢           ⁢   v       )         ⁢     (       1.8   ⁢     P   p       -     0.9   ⁢   ρ   ⁢           ⁢   gH       )         ,         
where g is a gravity acceleration. In particular, for formation pore pressure P a , the formation density ρ, the gravity acceleration g, the depth of formation H and Poisson&#39;s ratio ν accounting for 45 MPa, 2300 kg/m 3 , 9.81 m/s 2 , 3000 m and 0.25, correspondingly, the amplitude of the repetitive pulses of vibrations P a  accounts for 24 MPa.
 
     The typical measured waveform of a single burst and corresponding spectrum are shown on  FIG. 4 . The diagram/waveform on the left part of  FIG. 4  shows the signal from device for generating vibrations recorded by a geophone and two geophones, namely, horizontal and vertical ones in the offset well located on the distance of 1385 feet from the well wherein the device for generating vibrations was installed. The duration of the bursts accounts for 40-100 milliseconds depending on pressure of the compressed fracturing fluid between the plunger  4  and the check valve  10 . A main or resonance frequency of generated vibrations can be “moved” along the frequency axis to the left or right on the spectrum diagram by providing the device for generating vibrations having an ability to create vibrations on a particular resonant frequency, i.e. the frequency matching so called eigen frequency of fractures with predetermined or required width r and length L. As it is seen from  FIG. 4  the amplitude of the resonant frequency is by about 30-50 times higher compared with the rest of frequencies in spectrum (the units on vertical axis are in decibels). The check valve  10  installed on a flow line  11  could have a simple design having a seat with round hole in the center of said seat and a ball having bigger diameter and matching said hole in such manner that when the pressure of fracturing fluid in front of ball is greater than behind the one the ball closes said round hole of said seat thereby preventing any backward flow of fracturing from flow line  11  into the tank  13 . It should be noted that valve  18  during fracturing has to be either closed or at least one standard packer (not shown) is installed between tubing  2  and casing  15  above the perforations  5 .  FIG. 2  shows the cross-sectional top view of the wellbore  1 , a casing  15 , cement bond  17 , and the formation with the perforations  5  and the fractures  6 . The eigen, natural or resonant frequency of such fractures (or slots in acoustics) is determined by the following formulae: 
                 f   r     =       c     2   ⁢   π       ⁢       r     1.2   ⁢     HL   ⁡     (     r   +   W     )                 ,         
where f r  is the frequency of resonance, c is a speed of sound in the fracturing fluid, π equals 3.1415, r is the required width of fracture  6 , H is the combined thickness of the casing  15  and the cement bond  17  surrounding the casing, L is the required length of fracture  6 , W is a length of the casing arch between two neighboring perforations  5 . In particularly, for fracturing event shown on  FIG. 3  (four fractures  6 ) W=πD/4, where D is the diameter of the casing  15 . Thus, in order to get the fracture(s)  6  with particular parameters, i.e. the required width r and length L, the affecting vibrations have to be supplied on corresponding resonant frequency. For instance for the following parameters: r=0.02 m, H=0.05 m, L=100 m, W=0.13 m (corresponds to 7.0 inch casing and four fractures), c=1600 msec (corresponds to 70 MPa hydrostatic pressure under 20° C. temperature in wellbore) the resonant frequency equals 38 Hz. It should be noted that under conditions of resonance the fractures  6  will have predetermined, required width r and length L. It&#39;s important from the point of view of fracturing process when in case of too wide width r the excessive usage of proppant could be prohibitively expensive, and the fracture having too long length L can reach the highly water saturated sublayer of formation leading to the production of excessive portion of water instead of oil or gas from fractured well.
 
     As an alternative source of the said vortices generating vibrations on resonant frequency, for instance, can be used the devices described in U.S. Pat. No. 8,459,351, herein incorporated by reference. 
     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.