Patent Publication Number: US-7712531-B2

Title: Methods for controlling particulate migration

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/862,986 filed Jun. 8, 2004 now U.S. Pat. No. 7,299,875, entitled “Methods For Controlling Particulate Migration,” by Philip Nguyen, et al., which is incorporated by reference herein for all purposes, from which priority is claimed pursuant to 35 U.S.C. § 120. 

   BACKGROUND 
   The present invention relates to methods for controlling the migration of unconsolidated particulates in a subterranean formation. More particularly, the present invention relates to the use of treatment fluids comprising relatively dilute resin compositions for controlling the migration of unconsolidated or weakly consolidated particulates in a portion of a subterranean formation wherein the relatively dilute resin compositions comprise an aqueous dissolvable solvent. 
   Hydrocarbon wells are often located in subterranean formations that comprise unconsolidated portions, that is, portions of a subterranean formation that contain particulate matter capable of migrating out of the formation with produced fluids. Unconsolidated portions of subterranean formations include those that contain loose particulates that are readily entrained by produced fluids and those wherein the particulates are bonded together with insufficient bond strength to withstand the forces produced by the production of fluids through the zones. The presence of particulate matter, such as sand, in produced fluids may be disadvantageous and undesirable in that such particulates may abrade pumping equipment and other producing equipment and may reduce the fluid production capabilities of the producing portions of the subterranean formation. 
   One method of controlling unconsolidated particulates involves placing a filtration bed of gravel near the well bore to prevent the transport of unconsolidated formation particulates with produced fluids. Typically, such operations are referred to as “gravel packing operations,” and they usually involve pumping and placing a quantity of particulates adjacent to a portion of an unconsolidated formation so as to form a gravel pack between the open well bore and the formation walls. Although used frequently, such methods can be time-consuming and expensive to perform. 
   Another conventional method used to control loose formation particulates in unconsolidated formations involves consolidating a portion of a subterranean formation from which the unconsolidated particulates tend to flow by applying a curable resin composition to that portion. In one example of such a technique, an operator pre-flushes the formation, applies a resin composition, and then applies an after-flush fluid to remove excess resin from the pore spaces within the formation. Such resin consolidation methods, however, have not been practicable on formations containing reactive mineral clays such as smectite and kaolinite. Resin consolidating treatments performed on such reactive clay-laden formations tend to yield little or no improvement in consolidation strength and/or may cause significant loss of permeability. 
   SUMMARY OF THE INVENTION 
   The present invention relates to methods for controlling the migration of unconsolidated particulates in a subterranean formation. More particularly, the present invention relates to the use of treatment fluids comprising relatively dilute resin compositions for controlling the migration of unconsolidated or weakly consolidated particulates in a portion of a subterranean formation wherein the relatively dilute resin compositions comprise an aqueous dissolvable solvent. 
   One embodiment of the present invention provides a method of stabilizing a portion of a subterranean formation comprising contacting the portion of the subterranean formation with a pre-flush fluid; contacting the portion of the subterranean formation with a consolidation fluid comprising a resin and an aqueous dissolvable solvent; and, contacting the portion of the subterranean formation with an after-flush fluid. 
   Another embodiment of the present invention provides a method of reducing the production of particulates from a portion of a subterranean formation comprising contacting the portion of the subterranean formation with a pre-flush fluid; contacting the portion of the subterranean formation with a consolidation fluid comprising a resin and an aqueous dissolvable solvent; and, contacting the portion of the subterranean formation with an after-flush fluid. 
   Another embodiment of the present invention provides a method of fracturing a portion of a subterranean formation while controlling particulates comprising contacting the portion of the subterranean formation with a pre-flush fluid; contacting the portion of the subterranean formation with a consolidation fluid comprising a resin and an aqueous dissolvable solvent; and, contacting the portion of the subterranean formation with a fracturing fluid at a pressure sufficient to create or enhance a fracture in the subterranean formation. 
   Other and further features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows. 

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention relates to methods for controlling the migration of unconsolidated particulates in a subterranean formation. More particularly, the present invention relates to the use of treatment fluids comprising relatively dilute resin compositions for controlling the migration of unconsolidated or weakly consolidated particulates in a portion of a subterranean formation wherein the relatively dilute resin compositions comprise an aqueous dissolvable solvent. 
   Some embodiments of the present invention provide improved methods for preventing the migration of unconsolidated particulates within chosen portions of subterranean formations, particularly in portions surrounding a fracture or other void space such as a well bore. Some methods of the present invention comprise contacting a portion of a subterranean formation with a pre-flush fluid, followed by a consolidation fluid diluted with an aqueous dissolvable solvent, followed by an after-flush fluid. 
   Some methods of the present invention are particularly well suited, inter alia, for use in consolidating unconsolidated particulates in a portion of a formation that includes reactive mineral clays. In particular, embodiments of the present invention may be well suited for portions of subterranean formations comprising at least about 0.1 weight % reactive mineral clay, and particularly well suited for portions of subterranean formations comprising at least about 1 weight % reactive mineral clay. 
   Pre-flush fluids suitable for use in the methods of the present invention may comprise any combination of an aqueous liquid, a surfactant, and a glycol ether solvent. That is, in some embodiments, the pre-flush fluid may comprise an aqueous liquid and a surfactant, in other embodiments, the pre-flush fluid may comprise a glycol ether, in still other embodiments the pre-flush fluid may comprise an aqueous solvent, a surfactant, and a glycol ether. Other potential combinations of an aqueous liquid, a surfactant, and a glycol ether solvent are also suitable. The pre-flush fluid, inter alia, readies the formation to receive the consolidation fluid and aids in removing oils that may impede the consolidation fluid from making contact with formation particulates. 
   In pre-flush fluids comprising an aqueous liquid, the aqueous liquid may comprise fresh water, salt water (e.g., water containing one or more salts dissolved therein), brine (e.g., saturated salt water), seawater, or any other aqueous liquid that does not adversely react with the other components used in accordance with the methods of the present invention. A preferred aqueous component of a pre-flush fluid is a brine. 
   Where the pre-flush fluid comprises a surfactant, any surfactant compatible with the aqueous liquid and capable of aiding the consolidation fluid in coating the surfaces of the unconsolidated particles may be suitable for use in the present invention. Suitable such surfactants include, but are not limited to, ethoxylated nonyl phenol phosphate esters, one or more cationic surfactants, and one or more non-ionic surfactants and an alkyl phosphonate surfactant. Mixtures of one or more cationic and nonionic surfactants also may be suitable. Examples of such surfactant mixtures are described in U.S. Pat. No. 6,311,773 issued to Todd et al. on Nov. 6, 2001, the relevant disclosure of which is incorporated herein by reference. A C 12 -C 22  alkyl phosphonate surfactant is preferred. While surfactants chosen to aid in coating the formation particulates will generally be cationic, mixtures of surfactants comprising non-ionic surfactants, anionic surfactants, or combinations thereof may be desirable in some embodiments to aid in fluid compatibility. For example, mixtures of surfactants may be beneficial in preventing the formation of viscous, damaging emulsions that may otherwise may form when the relatively dilute resin compositions of the present invention contact the formation fluids. 
   Where the pre-flush fluid comprises a glycol ether, any glycol ether capable of aiding the consolidation fluid in coating the surfaces of the unconsolidated particles may be suitable for use in the present invention. Suitable such glycol ethers include, but are not limited to, diethylene glycol methyl ether, dipropylene glycol methyl ether, 2-butoxy ethanol, ethers of a C 2  to C 6  dihydric alkanol that comprise at least one C 1  to C 6  alkyl group, mono ethers of dihydric alkanols, methoxypropanol, butoxyethanol, hexoxyethanol, isomers thereof, and combinations thereof. 
   Consolidation fluids suitable for use in the present invention comprise an aqueous dissolvable solvent and a suitable resin. Suitable consolidation fluids are capable of imparting consolidation strength to the formation without substantially affecting the formation&#39;s permeability or the producability of desired fluids. The viscosity of the consolidation fluid should preferably be controlled to ensure that it is able to sufficiently penetrate the unconsolidated portions of the subterranean formation. For example, where the portion of the subterranean formation being consolidated is a portion neighboring a well bore, from about 3 inches to about 1.5 feet of penetration into the portion neighboring the well bore may be desired. Where the portion of the subterranean formation being consolidated is a portion neighboring a propped fracture, for example, at least about 0.25 inches of penetration into a neighboring fracture wall may be sufficient. To achieve these penetration levels, the viscosity of the consolidation fluid is important. Generally speaking, the consolidation fluid viscosity may be kept below 100 cP, more preferably below 40 cP, and most preferably below 10 cP; wherein the viscosity is measured at room temperature, using a Brookfield DV-II viscometer, with a No. 2 spindle at 100 RPM. 
   Achieving the desired viscosity will generally dictate a resin to aqueous dissolvable solvent ratio ranging from about 1:0.2 to about 1:20, preferably from about 1:1 to about 1:3. It is within the ability of one skilled in the art, with the benefit of this disclosure, to use a sufficient amount of a suitable aqueous dissolvable solvent to achieve the desired viscosity for the consolidation fluid and, thus, to achieve a desired degree of penetration into the subterranean formation. 
   Selection of an appropriate aqueous dissolvable solvent is an important component of the present invention. While prior resin-based consolidation treatments have not been able to impart significant consolidation strength to many formations, particularly formations containing reactive mineral clays, careful selection of a solvent renders consolidation of such formations achievable. While traditional consolidation treatments use high flash point solvents that are not readily dissolvable in aqueous fluids, the methods of the present invention dilute a consolidating resin with an aqueous dissolvable solvent. Suitable aqueous dissolvable solvents, inter alia, tend to enhance the removal of an aqueous phase of fluid surrounding the formation particulates and to allow the diluted resin to coat the particulates while absorbing onto the particulate surfaces. 
   Any aqueous dissolvable solvent that is compatible with the other consolidating fluid components and that achieves the desired viscosity effect is suitable for use in the present invention. Such aqueous dissolvable solvents include, but are not limited to, methanol, isopropanol, butanol, glycol ether solvents, and combinations thereof. Suitable glycol ether solvents include, but are not limited to, diethylene glycol methyl ether, dipropylene glycol methyl ether, 2-butoxy ethanol, ethers of a C 2  to C 6  dihydric alkanol containing at least one C 1  to C 6  alkyl group, mono ethers of dihydric alkanols, methoxypropanol, butoxyethanol, hexoxyethanol, and isomers thereof. Selection of an appropriate solvent is dependent on the resin composition chosen and is within the ability of one skilled in the art with the benefit of this disclosure. 
   Suitable consolidation fluids include all resins known in the art that are capable of forming a hardened, consolidated mass. Many such resins are commonly used in subterranean consolidation operations, and some suitable resins include two component epoxy based resins, novolak resins, polyepoxide resins, phenol-aldehyde resins, urea-aldehyde resins, urethane resins, phenolic resins, furan resins, furan/furfuryl alcohol resins, phenolic/latex resins, phenol formaldehyde resins, polyester resins and hybrids and copolymers thereof, polyurethane resins and hybrids and copolymers thereof, acrylate resins, and mixtures thereof. Some suitable resins, such as epoxy resins, may be cured with an internal catalyst or activator so that when pumped down hole, they may be cured using only time and temperature. Other suitable resins, such as furan resins generally require a time-delayed catalyst or an external catalyst to help activate the polymerization of the resins if the cure temperature is low (i.e., less than 250° F.), but will cure under the effect of time and temperature if the formation temperature is above about 250° F., preferably above about 300° F. It is within the ability of one skilled in the art, with the benefit of this disclosure, to select a suitable resin for use in embodiments of the present invention and to determine whether a catalyst is preferred to trigger curing. 
   After-flush fluids suitable for use in the methods of the present invention may be either aqueous liquids or inert gases. Where the after-flush fluid is an aqueous liquid, it may be salt water, brine, or any other aqueous liquid that does not adversely react with the other components used in accordance with this invention. A preferred aqueous after-flush fluid solution is brine. A volume of about 1 to about 5 times the volume of the consolidation fluid used is generally a suitable volume of after-flush fluid. In some subterranean formations, particularly gas-producing subterranean formations, it may be advantageous to after-flush using an inert gas, such as nitrogen, rather than an aqueous solution to prevent interaction between the after-flush fluid and the formation. The after-flush fluid acts, inter alia, to displace the curable resin from the well bore and to remove curable resin from the pore spaces inside the subterranean formation, thereby restoring permeability while leaving behind resin at the contact points between formation particulates. 
   One embodiment of the methods of the present invention of stabilizing a portion of a subterranean formation comprises the steps of applying a consolidation fluid diluted with an aqueous dissolvable solvent to the portion of the subterranean formation, and applying an after-flush fluid to the portion of the subterranean formation. 
   Another embodiment of the methods of the present invention for reducing the production of particulates from a portion of a subterranean formation comprises the steps of applying a consolidation fluid diluted with an aqueous dissolvable solvent to the portion of the subterranean formation, and applying an after-flush fluid to the portion of the subterranean formation. 
   The methods of the present invention also may be used to consolidate fracture faces before proppant is placed in those fractures. Such methods comprise applying a consolidation fluid diluted with an aqueous dissolvable solvent to a portion of a subterranean formation as a pre-pad or pad, placing an after-flush fluid to the portion of the subterranean formation, and then placing a fracturing fluid into the subterranean formation at a pressure sufficient to create or enhance at least one fracture therein. In such methods, a portion of the fracturing fluid that leaks off into the formation during the fracturing treatment may provide an effective means of over-displacing the resin from the pore space, and thus, a separate after-flush may not be necessary. 
   Once a method of the present invention is complete, the resin should be allowed time to cure. The required time will depend on the consolidation fluid used, the temperature of the portion of the formation, and the unconfined compressive strength (UCS) needed in the particular application. Generally, the cure time will be between about 0.5 hours and about 72 hours, preferably between about 6 hours and about 48 hours. Determining the proper cure time is within the ability of one skilled in the art with the benefit of this disclosure. 
   To facilitate a better understanding of the present invention, the following examples of some of the preferred embodiments are given. In no way should such examples be read to limit, or define, the scope of the invention. 
   EXAMPLES 
   Unconsolidated cores were prepared in 1-inch inner diameter, 5-inch long Teflon sleeves. Some of the cores comprised Brazos River sand, others comprised mixtures of 70/170-mesh sand, silica flour, and either kaolin or smectite clay. The chosen core material was packed between 0.5-inch 40/60-mesh sand packs and 80-mesh stainless steel metal screens. 
   A low-viscosity curable resin was prepared by combining one part hardenable resin component and one part hardening agent component to create a two-component epoxy consolidation fluid, and then combining one part of the two-component epoxy consolidation fluid with one part methanol (an aqueous dissolvable solvent). 
   The unconsolidated cores were first treated with 5% NH 4 Cl brine containing 0.5% non-ionic surfactant. During that treatment, the initial permeability of each of the cores was calculated. Next, the low-viscosity curable resin was applied to the unconsolidated core from the top of the core. Finally, an after-flush of 5% NH 4 Cl brine was applied to the core in the same direction. The cores were allowed to cure for 24 hours at 200° F. 
   After the cure time had lapsed, a mixture of 5% NH 4 Cl brine was again applied to the cores in the direction from the bottom of the core to determine the retained permeability of the treated cores. Consolidated cores of top and bottom portions of the cores were obtained and tested to determine their unconfined compressive strengths (UCS). 
   Table 1 below displays the consolidation and permeability results of these tests. These results appear to indicate that unconsolidated core materials are transformed into permeable consolidated masses, regardless of the amount of clay existing in the core materials. However, as the clay content increases, the permeability of packed core decreases. The top portion of the core tends to have lower UCS value than that of the bottom portion. This may indicate that the resin is displaced from the pore spaces in the direction from the top portion downward, and as a result of displacement, more resin is moving toward the bottom portion of the core. 
   
     
       
         
             
             
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
                 
               Approx. 
                 
                 
                 
                 
             
             
                 
               Resin 
               Top 
             
             
                 
               Volume 
               UCS 
               Bottom 
               Initial 
               Treated 
             
             
               Core Composition 
               (cc) 
               (psi) 
               UCS (psi) 
               Permeability 
               Permeability 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
          
             
               Brazos River sand 
               80 
               100 
               280 
               1120 
               1190 
             
             
               Brazos River sand 
               80 
               370 
               720 
               850 
               470 
             
             
               Brazos River sand 
               40 
               40 
               230 
               1060 
               960 
             
             
               70/170-mesh sand 
               80 
               210 
               2470 
               6910 
               6140 
             
             
               90 weight % 70/170-mesh 
               80 
               500 
               1260 
               335 
               370 
             
             
               sand, and 
             
             
               10 weight % silica flour 
             
             
               88 weight % 70/170-mesh 
               80 
               1220 
               1240 
               55 
               220 
             
             
               sand, 
             
             
               10 weight % silica flour, and 
             
             
               2 weight % kaoline clay 
             
             
               85 weight % 70/170-mesh 
               80 
               3020 
               3520 
               15 
               110 
             
             
               sand, 
             
             
               10 weight % silica flour, and 
             
             
               5 weight % kaoline clay 
             
             
               80 weight % 70/170-mesh 
               80 
               4790 
               4300 
               10 
               &lt;5 
             
             
               sand, 
             
             
               10 weight % silica flour, and 
             
             
               10 weight % kaoline clay 
             
             
               88 weight % 70/170-mesh 
               80 
               305 
               1270 
               295 
               655 
             
             
               sand, 
             
             
               10 weight % silica flour, and 
             
             
               2 weight % smectite clay 
             
             
               85 weight % 70/170-mesh 
               40 
               225 
               270 
               240 
               85 
             
             
               sand, 
             
             
               10 weight % silica flour, and 
             
             
               5 weight % smectite clay 
             
             
               80 weight % 70/170-mesh 
               40 
               — 
               223 
               75 
               20 
             
             
               sand, 
             
             
               10 weight % silica flour, and 
             
             
               10 weight % smectite clay 
             
             
                 
             
          
         
       
     
   
   Therefore, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those that are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit and scope of this invention as defined by the appended claims.