Abstract:
A method for increasing productivity of oil and gas wells includes carrying out a secondary opening in formations which were opened before, by first using a hydro-sandblasting operation and thereafter using a hydro-fracturing operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to oil and gas excavation, and in particular it relates to methods for increasing productivity of oil and gas wells. 
         [0002]    It is known that near a well shaft of rock is in deformed-stressed condition under the action of annular tangential stresses. In condition of elastic compression these stresses, which concentrate in immediate vicinity to the walls of the well, are reducing with increase of a distance from it. These stresses cause intercrystalline deformations of rock collectors in near-shaft zone, redistribution of cementing substance, wedging and compacting of calmatating passages and cracks, compacting of clay material in a penetration zone, and closing of walls of the cracks. This leads to significant reduction of permeability of rock in the hear-well zone, leading to full loss of hydrodynamic communication of the formation with the well. 
         [0003]    In order to eliminate this phenomenon, various methods are used, such as for example, a cumulative perforation. They are used for establishing a communication between the well and the formation, in order to increase of oil and gas production. Nowadays however not more than 25-30% of oil and a little more gas is recovered from all formations. 
         [0004]    The well known method of cumulative perforation does not eliminate the problem of removing of compressive stresses and is not efficient for formation of openings, because of blocking of the formation by liquid, blocking by small particle of drilling solution, blocking my small particles of rocks, melting of openings, reducing of productivity of formations several times, crack formations in cement up and down over 5 m along the well shaft. Also, the most productive part of formation having high collector properties opens insufficiently with the use of known cumulative perforation. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly it is an object of the present invention to provide a new method for increasing the productivity of oil and gas wells. 
         [0006]    In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a method of increasing productivity of oil and gas wells, comprising the steps of carrying out a secondary opening in formations which were opened before, and performing the secondary opening first by exercising a hydro-slotting process, and, subsequently to hydro-slotting, by exercising a hydro-fracturing process. 
         [0007]    When the method is performed according to the present invention, the productivity of oil and gas wells is significantly increased and significantly higher quantities of oil and gas are recovered from the wells, in particular since the hydro-fracturing follows the hydro-slotting and starts from a plurality of points of formation of hydraulic fracturing. 
         [0008]    In accordance with another feature of the preset invention, the hydro-slotting forming slots along an axis of a well over a whole thickness of a formation with retention of self-destructible partitions in the formation. 
         [0009]    It is still a further feature of the present invention, that the hydro-slotting includes forming at least four slot cavities in one plane around a well shaft with the use of at least four perforator nozzles. 
         [0010]    Another feature of the present invention resides in that the hydro-slotting is carried out so that slot openings are oriented in a direction which is substantially perpendicular to a maximum horizontal stress. 
         [0011]    Still a further feature resides in that ecological powders, for example baking soda, are used during the hydro-slotting and hydro-fracturing of formations for dissolving a clay cement and of rocks which contain it. It can be used both in a cutting solution and during hydrocracking of the formation. 
         [0012]    The novel features of the present invention are set forth in particular in the appended claims. 
         [0013]    The invention itself however will best understood from the following description of preferred embodiments which is accompanied by the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  of the drawings is a view showing a well which is used for oil or gas excavation with increased productivity according to the invention, during the process of a hydro-slotting with the use of a perforator with nozzles used in the method according to the present invention; 
           [0015]      FIG. 2  is a view showing a well which is used for oil or gas excavation with increased productivity according to the invention, during the process of subsequent hydro-fracturing according to the present invention; and 
           [0016]      FIG. 3  is a view showing a prior art process of oil or gas recovery with the use of a hydraulic fracturing after a cumulative perforation. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]    A gas or oil well is shown in  FIG. 1  and identified with reference numeral  1 . It has a central shaft  2  having a circumferential cement wall  3 , and a near shaft zone  4 . A productive formation in surrounding rock are identified with reference numeral  5 . 
         [0018]    In accordance with the present invention slots  6  are produced in rock, in particular in the formation  5 , by a hydro-slotting process. The slots  6  are cut over a whole thickness of the formation with retention of partitions  7  between the slots  6  in the formation. The partitions are selected such that they are self-destructing. 
         [0019]    The slots  6  are initially cut in the upper part of the formation  5  along the well shaft  2  on several sides symmetrically to it axis. A perforator  8  with at least four nozzles  9  for cutting slots is moved along the well by a distance h. The slots  6  are cut with retention of the partitions  7  having a width which does not exceed the diameter of the well, and the partitions  7  are subsequently destroyed. Then the operation is repeated at a different height along the well over the whole width of the interval of the formation. 
         [0020]    The distance between the partitions is determined as follows: 
         [0000]        h&gt; 0.5· d·σ/σ   cube ·{1+[1+4·σ cube /σ]½}  (1)
 
         [0000]    wherein d is a diameter, m,
       σ is horizontal stress at a depth of formation, Mpa,   σ cube  is a cube strength of rock of formation, Mpa.       
 
         [0023]    When the slots are cut in one plane with partitions between them, the horizontal stresses are redistributed onto partitions to produce additional pressure which can reach an ultimate pressure that causes self-destruction of the partitions. As a result, one new slot is formed with a length which is equal to the length of two original slots and the partition. The sizes of partitions increase with the depth, as the pressure increases. 
         [0024]    Experimentally and analytically the values of sizes of partitions and distances between them to be self-destructive are determined as follows. 
         [0000]        h= 0.5· d·σ/σ   cube ·{1+[1+4·σ cube /σ]½}  (2)
 
         [0000]    wherein h and d are in mm. 
         [0025]    In accordance with the method of the invention, the width h of the self-destructing partition is determined from formula (1). In the upper part of the productive formation the slots are cut from several sides symmetrically to the axis of the well, in the hydro-slotting process, for example by a hydro-sandblasting perforation. The perforator is moved by distance h along the well. Then the next slot is cut. The operation of cutting is repeated with the retention of partitions which self-destruct to form several symmetrical vertical slots with the length equal to the thickness of the formation. The hydro-sandblasting perforator is lowered into the well for example with the speed 5 mm per min. Working fluid is supplied into the perforator with a quartz sand of for example 0.6-1 mm fraction with a pressure applied for 40 min, so that a slot of for example 200 mm is cut. Then the perforating tool is lifted over the size of the cut and a second cut is made. The remaining cuts are made sequentially in the same way. After the last cut the well is washed. 
         [0026]    In the inventive method at least four slots  6  which make cavities are made in one plane along the shaft of the well with the use of corresponding nozzles  9  of the perforator  8 . This provides a complete removal of tangential compressing stresses in the near-shaft zone. The perforator  8  which can have several, at least four nozzles  9  is shown in  FIG. 1 . It can have more than four nozzles as well. The construction of the perforator is known per se and not shown in detail. 
         [0027]    The slots  6  which make cavities are arranged in a direction A that is close to a perpendicular to a direction B of the maximum horizontal stress in the rock which surrounds the well  1 . In order to determine the direction of maximum horizontal stresses it is proposed to use determination of directions of cracks in accordance with maps of cracks for each particular area or to use determination of directions of compression of rock in the near-shaft zone by geophysical methods. As a result, it makes possible to create slot formation regions, in which hydro-fracturing of the formations is carried out in accordance with the method of the present invention. 
         [0028]    In the inventive method ecological powders can be used during the hydro-slotting and subsequent hydro-fracturing of the formations for dissolution of cement during the process of carrying out the method. For example a 8% solution of baking soda can be used for hydro-fracturing of the formation and in a cutting solution during hydro-slotting. In this case, with the use of the hydro-sandblasting in the hydro-slotting process according to the invention it is possible to pump in the fluid at lower pressures. 
         [0029]    After the hydro-slotting process, in accordance with the present invention the hydro-fracturing or hydro-blasting is used, as shown in  FIG. 2 . In the well  1  packers  21  are installed at upper and lower ends of the slotted formation. A tool  22 , for example in form of a pipe connected with a pumping-compressing equipment is introduced in the zone between the packers and the working medium is supplied under pressure into this zone to carry out the hydro-fracturing. A plurality of cracks are initiated from a plurality of points  23  of formation of hydraulic cracks during the initial hydro-slotting process and then propagate farther under the highly aggressive action of the hydro-fracturing as identified with reference  24 . 
         [0030]    It has been determined during experiments with the method according to the present invention that it becomes possible to recover of up to 80% of oil or gas, which cannot be achieved by other methods. 
         [0031]    In particular, as shown in  FIG. 3  which illustrates the method with the initial use of cumulative perforation, it can be seen that after the cumulative perforation the crack formation during the subsequent hydro-fracking process actually starts only from one point  31 , and as a result only a very limited hydra-cracking takes place in the formation. 
         [0032]    The present invention is not limited to the details shown since further modifications and structural changes are possible without departing from the spirit of the invention.