Abstract:
Fractures are initiated or extended into earth formations from a well by providing pressured fluid in said wellbore at pressures exceeding the fracture gradient pressure of said formation and by generating a cyclic shock waves. Periodic shock waves applied simultaneously with a high pressured fluid on a formation increases the length of fractures/cracks in the formation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for fracturing the earth from a wellbore by overpressuring a fluid(s) and/or gases inside the wellbore and applying periodic shock waves. 
       BRIEF DESCRIPTION OF PRIOR ART 
       [0002]    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. 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. 
         [0003]    The use of high pressured gas or other pressured fluid(s) in a wellbore to clean perforations and for create fractures has been described in U.S. Pat. Nos. 5,669,448 and 5,131,472. 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; 5,145,013; 5,101,900; 4,936,385 and 2,740,478. Of more general interest is the disclosure in U.S. Pat. No. 3,029,732 and the principles of how a crack in the earth&#39;s formation appears under cyclic loading as described in “Erdogan F.“Crack Propagation Theories”. Fracture, N.Y., London, Academic Press, 1968, p. 70-77”. 
         [0004]    While there have been a variety of methods proposed for creating hydraulic fractures around the wellbore, there remains a need for an economical, high-pressure method which creates a pattern of fractures extending from all perforations into the formation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows a schematic illustration of a wellbore in which the method of the present invention is employed. 
           [0006]      FIG. 2  is a schematic illustration of alternative means for practicing the present invention. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0007]    Accordingly, a primary object of the present invention is to provide a method for fracturing a formation around a wellbore by means of applying periodic/cyclic shock waves to the formation surrounding wellbore which is undergoing pressures exceeding the fracture gradient pressure of the formation as a result of pumping of fluid into a wellbore. In accordance with the invention, a well is completed with casing cemented in place. Perforations are formed through the casing by conventional means. A fluid is pumped into the casing and through the perforations into the formation under pressures exceeding the fracture gradient pressure of the formation. The shock waves generating tool is installed in the casing near the part of formation to be fractured and generates a periodic/cyclic shock waves having an amplitude P a  in accordance with following expression: 
         [0000]      1.4 P   p −0.8ρg H≦P   a   ≦P   c ,
 
         [0000]    where P p  is the formation pore pressure, ρ is the formation density, g is a gravity acceleration, H is the depth of perforations, P a  is amplitude of shock wave and P c  is a collapse resistance stress of the shock wave generating tool. 
         [0008]    It is another object of the invention to provide a method for fracturing a formation around a wellbore in which the amplitude P a  of shock waves is at least not less than the fracture gradient pressure of the formation. 
         [0009]    It is another object of the invention to provide a method for fracturing a formation around a wellbore in which said shock wave generating tool is sealed from the perforations by means of packer(s). 
         [0010]    It is another object of the invention to provide a method for fracturing a formation around a wellbore in which the tool generating the shock waves is installed opposite not a non-perforated formation. 
       DETAILED DESCRIPTION 
       [0011]    Referring to  FIG. 1  and  FIG. 2 , there is shown a fluid producing or injecting well  9  extending downwardly into the earth to penetrate a subterranean hydrocarbon bearing or other type of formation  10 . The well includes a wellbore  11 , a casing string  1  and a cement sheath  12  isolating the casing string  1  from the bore hole  11 . A series of perforations  4  communicate between the formation  10  and interior of the casing string  1 . A tubing or production/injection string  2  extends downwardly into the casing string  1  and a packer  3  seals between the production and/or injection string  2  and the casing string  1 . The shock wave generating tool  6  is installed against perforations  4  on a conventional suspension means  7  and the fracturing fluid  5  is pumped into perforations  4  under pressure(s) exceeding the fracture gradient pressure of the formation  10 . The shock wave generating tool  6  generates the shock waves  8  affecting the fractures/cracks  14  thereby increasing their length. The shock wave generating tool  6  may be attached to the end of production or injection string/tubing  2 . Although the well  9  is of a conventional type, those skilled in the art will realize that this invention is equally usable on so called tubingless completions where the casing string also acts as the production or injection string. As shown on  FIG. 2  the shock wave generating tool  6  is installed in the fluid  13  below the formation  10  is sealed by means of packer  3 . 
         [0012]    Operation 
         [0013]      FIG. 1  shows a general arrangement of the fracturing procedure using the periodic/cyclic loading generated by the shock wave generating tool  6  installed on the suspension means  7  against perforations  4  in the well  10 . The shock wave generating tool  6  may be attached to the end of production or injection string  2  (not shown on figures). The fluid  5  is pumped into interior of casing string  1  and perforations  4  via production or injection tubing  2  under pressures exceeding the fracture gradient pressure of the formation  10  thereby creating the initial fractures/cracks  14 . As far as the pressures of fluid  5  exceeds the fracture gradient pressure of the productive formation  10 , the shock wave generating tool  6  starts to generate the cyclic/periodic shock waves  8  having an amplitude P a  of shock waves determined by following expression: 
         [0000]      1.4 P   p −0.9ρg H≦P   a   ≦P   c ,
 
         [0000]    where P p  is the formation pore pressure, ρ is the formation density, g is a gravity acceleration, H is the depth of perforations, P a  is amplitude of shock wave and P c  is a collapse resistance stress of shock wave generating tool thereby increasing the length of fractures/cracks  14  in the formation  10 .
 
In particular, for formation pore pressure P a , the formation density ρ, gravity acceleration g and depth of formation H accounting for 45 MPa, 2300 kg/m 3 , 9.81 m/s 2  and 3000 m, correspondingly, the amplitude of generated shock waves has to be not less than 2.1 MPa. The collapse resistance stress P a  of shock wave generating tool depends on material of wave generating tool is made and varies in 100 MPa to 150 Mpa range.
 
The shock wave generating tool  6  may start to generate the cyclic/periodic shock waves also simultaneously with pumping of fluid  5  into interior of casing string  1  and perforations  4  via production tubing  2 .
 
         [0014]    Referring now to  FIG. 2 , the shock wave generating tool  6  may be installed in the fluid  13  below (or above, not shown on figures) the packer  3  sealing the perforations  4  from the lower part of casing string  1  in accordance with invention in order to separate the shock wave generating tool  6  from the fracturing fluid  5 . 
         [0000]    It&#39;s a well-known fact that cyclic loading leads to the growth of cracks. The kinetic equation describing dependence of crack growth and cyclic loading is as follows (Erdogan F. “Crack Propagation Theories”. Fracture, N.Y., London, Academic Press, 1968, p. 70-77): 
         [0000]    
       
         
           
             
               
                 
                    
                   l 
                 
                 
                    
                   N 
                 
               
               = 
               
                 
                   c 
                    
                   
                     ( 
                     
                       Δ 
                        
                       
                           
                       
                        
                       k 
                     
                     ) 
                   
                 
                 n 
               
             
             , 
           
         
       
     
         [0000]    where l is the length of a crack, N is the number of cycles, Δk is the change of the stress intensity coefficient in the cycle, c is a coefficient depended on properties of the productive formation  10 , frequency of applied shock waves  8  and the average stress in the productive formation  10 , n is the material constant, i.e. productive formation  10 . It&#39;s obvious that the higher the frequency of the applied shock waves  8  the bigger the coefficient c. For simplicity purposes assume c=1.0, n=1.8, the static fracturing pressure of fluid  5  equal to 57 MPa and the amplitude of shock waves P a  accounts for 15 MPa, In this case the coefficient Δk equals (57+15)/(57−15)=1.71 and the length of fractures/cracks  14  will be by (1.71) 1.8 =2.63 times longer as compared with the length of the fractures created by applying only static fracturing pressure of fluid  5 . 
         [0015]    As a source of the shock wave generating tool  6 , for instance, can be devices described in U.S. Pat. No. 6,015,010 or U.S. Pat. No. 6,899,175. In this case the suspension means  7  is a production or injection string/tubing  2 . 
         [0016]    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.