Abstract:
The invention relates to methods for isolating near-wellbore zones and fractures and can be used for plugging fractures in the near-wellbore zone during the removal of the fracturing fluid, as well as for plugging different kinds of fractures and branches in the casing. The method for forming an isolating-plug includes the injection of a slurry containing dispersed fibers into a well and subsequent formation of a plug to isolate the relevant section of the well and to prevent the fluid penetration. The slurry is admixed with amphiphilic substances, and the fibers used are capable of adsorbing the amphiphilic substances on their surface.

Description:
FIELD OF INVENTION 
       [0001]    The invention relates to the oil and gas production industry, in particular, to methods for isolating near-wellbore zones and fractures, and can be used for plugging fractures in the near-wellbore zone during the removal of the fracturing fluid, as well as for plugging different kinds of fractures and branches in the casing. 
       BACKGROUND OF THE INVENTION 
       [0002]    The hydraulic fracturing is the main tool used for increasing the productive capacity of a well through creation or expansion of channels from the wellbore to the producing formation. This operation is generally accomplished by injecting a fracturing fluid into the wellbore which intersects an underground deposit, and by exposing the strata to the fluid pressure action. In order to increase the oil and gas production rates, it is necessary to solve the problem of how to remove the fracturing fluid and to plug the near-wellbore zones and fractures. A few methods are used for solving this problem, and these methods are usually based on addition of solid inclusions to fracturing-fluid solutions. The formation of an isolating plug starts from the formation of a bridge (so-called “bridging”) which is nothing but a cluster of solid inclusions stably captured from the solution on the fracture surface. At the same time, the fluid keeps on flowing through the fixed agglomerate of solid inclusions. As a result, the solids-containing solution (the slurry) is filtered, which gradually increases the density of the solids arrested and reduces the penetrability of the resulting structure and completely stops the flow. For example, U.S. Pat. No. 7,036,588 describes the use of ceramic particles and starch buildups for fluid loss control purposes; U.S. Pat. No. 7,318,481 describes shape-memory foams which are used as a withdrawal agent; International Application No. WO2007066254 describes the reversible plugging of a fracture or a well with a decomposable material. U.S. Pat. No. 7,331,391 describes the use of water-soluble fibers for drilling-mud loss control purposes. 
         [0003]    RU Patent No. 2330931 describes a method for forming an isolating plug, which includes the injection of a slurry containing dispersed fibers into the well and subsequent formation of a plug to isolate the relevant section of the well. When the slurry is injected, the device used in this patent accumulates fibers, thus forming an impermeable plug in the wellbore. Depending on the fiber material selected, it is possible to obtain a temporary or permanent plug. This method has a number of limitations on use, namely: relative design complexity and the fact that the plug is formed in the wellbore, which makes the access to the well areas behind the packer (access to the well end) difficult or impossible. 
         [0004]    A high fiber concentration is required for successful formation of a plug from fibers. Such an approach encounters a number of problems, namely: financial expenses related to the production/purchase/transportation of large amounts of fibers, and expenses related to the expansion of injection equipment capacities. At the same time, the equipment (pumps, mixers, etc.) may fail if used for processing a high fiber concentration. 
       SUMMARY OF THE INVENTION 
       [0005]    The technical result achieved with the implementation of the invention consists in providing efficient isolation of fractures in the near-wellbore zone, while reducing the fiber concentration and preventing the pumps and other equipment from fouling. 
         [0006]    To achieve the said technical result, we suggest a method for forming an isolating plug, which includes the injection of a slurry containing dispersed fibers into a well and subsequent formation of a plug to isolate the relevant section of the well and to prevent the fluid penetration. The slurry is admixed with amphiphilic substances, and the fibers used are capable of adsorbing the amphiphilic substances on their surface. 
         [0007]    When adsorbed on the fiber surface directly upstream of the isolation zone, the amphiphilic substances change the affinity of fibers to solvent and lead, as a consequence, to the formation of agglomerates from fibers. The agglomerates are bigger in size than the fibers, which results in the formation of a plug. The initial fiber concentration is low enough to ensure that the equipment will operate without failures. 
         [0008]    Vice versa, the amphiphilic substances promote a uniform distribution of hydrophobic fibers in the original fluid. After the decomposition of amphiphilic substances upstream of the isolation zone, this distribution will be disturbed, resulting in the formation of fiber agglomerates and then a plug. 
         [0009]    The amphiphilic substances can be added to the slurry during the preparation of the slurry. 
         [0010]    The amphiphilic substances can be injected into the well in parallel with the slurry injection, before the slurry injection or after the slurry injection. 
         [0011]    It is possible to use surfactants as amphiphilic substances. 
         [0012]    It is possible to use quaternized ammonium salts as amphiphilic substances. 
         [0013]    It is possible to use benzoic acid as amphiphilic substances. 
         [0014]    It is possible to use diblock copolymers with a polyelectrolyte block as amphiphilic substances. 
         [0015]    The amphiphilic substances can be added to the slurry in the capsular form and can be subsequently released at the plug formation point under the influence of the environmental conditions, such as the temperature, the pressure, the rate of shear in the flow, or chemical dissolution. Capsular amphiphilic substances are described in such patents as EP Patent No. 0107086, U.S. Pat. No. 5,480,577, etc. 
         [0016]    The amphiphilic substances can decompose under the influence of the environmental conditions, such as the temperature, the rate of shear in the flow, or due to chemical reactions. 
         [0017]    The amphiphilic substances can be added by injecting their precursors into the well, which precursors will be transformed into amphiphilic substances under the influence of the environmental conditions, such as the temperature or the flow. It is possible to use esters, antihydrides, heterocyclic acetates, amines, amides, and other chemically unstable substances forming amphiphilic substances as a result of a chemical reaction, as precursors. 
         [0018]    The amphiphilic substances, their precursors, and the chemicals which destruct the precursors or the capsules containing amphiphilic substances can be injected upstream of the isolation zone through coil tubing. 
         [0019]    It is possible to use PET fibers, polylactic acid, polyglycol acid, polyether, cotton, silica glass fiber, polyamide, protein, phenol-formaldehyde, polycarbonate, polyanhydride, epoxy resin as the fiber material. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In this invention, the coagulating properties of the fibers are controlled by adding amphiphilic molecules with charged or polarized groups. In case that fibers have a partially uncompensated charge on their surface, the charged or polarized groups will allow the molecules to adhere to (to be adsorbed on) the fiber surface, while the amphiphilicity of the molecules will ensure that the surface properties (hydrophobic/hydrophilic properties) of the fibers will be controlled. It is a well-known fact that hydrophilic particles coagulate in oil (a non-polar medium) and that hydrophobic particles coagulate in water (a polar medium). This results from the fact that the particles tend to reduce the area of unfavourable contacts with the environment. The main idea of the invention is to coat each fiber in the slurry with amphiphilic molecules, which will change the energy of the fiber-to-fiber contact and the fiber-to-fluid contact, making the former more favourable. As a result, the fibers will start aggregating in order to increase the number of more favourable contacts, thus making the slurry less homogeneous. 
         [0021]    The bridging ability of the slurry injected with fiber aggregates will be better than that of a homogeneous slurry at the same fiber concentration. 
         [0022]    Let us consider the embodiment where surfactants are used as amphiphilic substances. The method for forming an isolating plug is implemented as follows. The slurry is injected into a well and contains dispersed fibers which adsorb amphiphilic substances on their surface. It is possible to use PET fibers, polylactic acid, polyglycol acid, cotton, silica glass fiber, polyamide, protein, phenol-formaldehyde, polycarbonate, polyanhydride, epoxy resin as the fiber material. 
         [0023]    In parallel, the surfactants are injected. The fiber aggregation process presented herein and promoted by the surfactants has a simple physical explanation. Let us consider an undiluted slurry of fibers. The fibers come in mechanical contact with each other in such a slurry. Let us assume that each fiber has an uncompensated positive or negative charge on its surface (the nature of this charge can be of any kind, e.g. polarization of fiber-forming molecules). Let us assume that the suspending fluid is polar (e.g. water) and that the fibers in this system are weakly hydrophilic. Due to the presence of an uncompensated like charge on the fiber surface, they will be pushed away from each other. In order to initiate the coagulation and the formation of aggregates in such a system, it is necessary to add cationic or anionic surfactant molecules (the sign of the charged groups should be selected in such a way as to be opposite to the sign of the fiber surface charge). Due to electrostatic interactions, such surfactants are adsorbed with charged heads on the surface of the oppositely charged fiber, thus creating a crown of hydrophobic (non-polar) tails around it. As a result, the fibers coated with surfactants become hydrophobic (an evaluation of adhesion forces between the fibers and of the impact of certain surfactants on the adhesion forces is given in the following paper: E. A. Amelina, I. V. Videnskii, N. I. Ivanova, V. V. Pelekh, N. V. Altukhova, and E. D. Shchukin, Colloid Journal, vol. 63, No. 1, 2001, pp. 124-126). By using a great number of different surfactants (and even a greater number of possible amphiphilic molecules in the general case) at different concentrations, it is possible to obtain an immensely wide range of values for the hydrophobic properties of the fibers. 
         [0024]    In order to determine the optimum concentration of the surfactant molecules, it is necessary to take the following conditions into account:
       1) The number of the molecules should be great enough to coat the entire surface of the fibers.   2) The concentration should not exceed the critical micelle concentration (CMC) for these surfactants so that no self-transformation of the surfactants into micelles could take place.       
 
         [0027]    In case of cylindrical fibers, the concentration of the surfactant molecules can be evaluated by the following formula: 
         [0000]    
       
         
           
             
               m 
               S 
             
             = 
             
               
                 
                   4 
                    
                   M 
                 
                 
                   
                     N 
                     A 
                   
                    
                   
                     S 
                     0 
                   
                    
                   ρ 
                    
                   
                       
                   
                    
                   d 
                 
               
                
               
                 m 
                 f 
               
             
           
         
       
       
         
           
             
               C 
               S 
             
             = 
             
               
                 
                   4 
                    
                   M 
                 
                 
                   
                     N 
                     A 
                   
                    
                   
                     S 
                     0 
                   
                    
                   ρ 
                    
                   
                       
                   
                    
                   d 
                 
               
                
               
                 C 
                 f 
               
             
           
         
       
     
         [0028]    where M is the molar weight of the surfactant molecules, N A  is Avogadro&#39;s number, S 0  is the surface area of the surfactant molecule&#39;s head, d is the fiber diameter, ρ is the fiber material density. C f , m f  and C s , m s  are the concentrations and weights of the fibers and of the surfactant molecules, respectively. 
       Example 1 
       [0029]    Let us consider polylactic acid fibers having the following parameters: ρ=1.25 g/cm 3 , So=25 A 2 , d=12 μm, l=6 mm. For similar fibers: m s ˜10 −3  m f  and C s ˜10 −3  C f . 
         [0030]    Similar fibers contain a low uncompensated negative charge on their surface due to the polarization of the ester group. The fibers were suspended in a linear guar solution (at a concentration of 3.6 g/L), the fiber concentration was equal to 4.8 g/L. Trimethyl tetradecyl ammonium bromide at a concentration of 54.6 mg/L was added as a cationic surfactant to a similar slurry. After it had been mixed for 10 minutes in a mixer the propeller of which was rotating at 1,500 rpm, the presence of aggregates was detected in this slurry, as opposed to an equivalent slurry without surfactants added. The average aggregate size was equal to 25 mm. 
         [0031]    Amphiphilic substances can be added by injecting precursors into the well in parallel with the slurry injection, which precursors will be transformed into amphiphilic substances at the plug formation point under the influence of the environmental conditions. It allows you to inject a slurry containing homogeneously distributed fibers and to prevent the pumps from fouling and the wellbore from plugging near the surface, but to plug the fracture or the specified section of the wellbore below the ground. There are a few possibilities to achieve the described approach. 
         [0032]    For example, precursor molecules of cationic/anionic surfactants (which contain no charge) are injected together with the slurry at the initial stage. 
         [0033]    The precursor groups undergo a number of chemical transformations near the perforation or in the fracture, which results in the generation of a charge on the surfactant molecules&#39; polar (hydrophilic) groups. 
         [0034]    The precursor groups may include esters, antihydrides, heterocyclic acetates, amines, amides and other chemically unstable groups. 
         [0035]    The following factors may be responsible for the transformation of the precursors: the temperature below the ground, the rock surface properties, the flow velocity, acids/bases (in case of hydrolysis or protonation), alkylating agents or other chemicals. The sign of the resulting charge of a portion of the surfactant molecules must be opposite to the sign of the fiber surface, which results in favourable contact between the surfactant molecules and the fibers and in subsequent coagulation of the fibers. 
         [0036]    It is also possible to use capsular surfactant molecules with charged groups. In this case, the surfactant molecules are released from capsules under the influence of the temperature, the flow, the pressure, or the chemical dissolution of the capsules. The released surfactant molecules behave just as described above. 
         [0037]    It is possible that the surfactant molecules will reduce the interactions (attractions) between the fibers in the slurry. In this case, the slurry is more homogeneous and can be injected more easily into the well. If a certain number of the surfactant molecules are removed from the system, the slurry will become less homogeneous and will have a greater tendency to plug the well. 
         [0038]    A similar approach can be implemented with decomposable surfactants. In this situation, a so-called double layer will be formed on the fiber surface due to a high concentration of the surfactant molecules. Having been adsorbed on the fiber surface due to electrostatic interactions, the surfactant molecules with charged groups (the sign of this charge is opposite to that of the fiber surface charge) form an internal layer. The surfactant molecules left in the solution in order to minimize the number of unfavourable contacts between the non-polar groups and the polar fluid (or vice versa) will form an external portion of the layer, with the polar heads being directed outwards. As a result, the fibers surrounded by the double layer will be hydrophilic, which promotes a better dispersion of the fibers in the slurry. The surfactant molecules in the external layer will decompose under the relevant conditions. After the decomposition, the fibers will become hydrophobic and will start aggregating. 
         [0039]    The surfactants which decompose below the ground can contain, as hydrophilic heads, the following decomposable chemical groups: esters, amides, anhydrides, quaternized ammonium salts, amines, etc. 
         [0040]    The surfactants which decompose below the ground can contain, as hydrophobic tails, the following decomposable chemical groups: groups containing double bonds, alcohols, disulfides, aza groups, esters, amides, amines, etc. The decomposition can be caused by the hydrolysis or oxidation, the presence of acids or bases, the presence of other chemicals, the action of temperature or flow. 
         [0041]    Decomposable surfactants are described, for instance, in the following papers: Rairkar Maithili E.; Diaz M. Elena; Torriggiani Mauro; Cerro Ramon L.; Harris J. Milton; Rogers Sarah E.; Eastoe Julian; Gomez Del Rio Javier A.; Hayes Douglas G.; Colloids and surfaces. A, Physicochemical and engineering aspects ISSN 0927-7757; 2007, vol. 301, no. 1-3, pp. 394-403, or International Application No. WO2006120422. 
         [0042]    The described methods can be used with any amphiphilic substances. For example, it is possible to use quaternized ammonium salts the amphiphilic chains of which are shorter than those of the surfactant molecules but are long enough to provide the hydrophobic properties. Another example is benzoic acid having a charged carboxyl group and an aromatic hydrophobic fragment. Such substances can also be delayed-action or decomposable ones. Also, it is possible to use diblock copolymers with a polyelectrolyte block.