Patent Publication Number: US-2006011342-A1

Title: Fluid loss control additives for use in fracturing subterranean formations

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to subterranean fracturing operations, and more particularly to fracturing fluids comprising an improved fluid loss control additive, and methods of using such fracturing fluids in fracturing subterranean formations.  
      Hydrocarbon-producing wells often are stimulated by hydraulic fracturing operations, wherein a fluid (often referred to as a “pad” fluid) is introduced into a portion of a subterranean formation at a hydraulic pressure sufficient to create or enhance at least one fracture therein. After the placement of the pad fluid, one or more subsequent fluids, laden with proppant particles, may be placed in the zone so that the proppant particles may be placed in the resultant fractures to maintain the integrity of the fractures (after the hydraulic pressure is released), thereby forming conductive channels within the formation through which hydrocarbons can flow. Once at least one fracture has been created and at least a portion of the proppant is substantially in place within the fracture, the viscosity of the pad fluid may be reduced, in some cases, to facilitate removal of the fracturing fluid from the formation.  
      In certain hydrocarbon-producing formations, much of the production may be derived from natural fractures. These natural fractures may exist in the reservoir prior to a fracturing operation, and, when contacted by an induced fracture (e.g., a fracture formed or enhanced during a fracturing treatment), may provide flow channels having a relatively high conductivity that may improve hydrocarbon production from the reservoir.  
      In certain circumstances, a portion of a pad fluid and/or a proppant-laden fluid may be lost into the subterranean formation during a fracturing operation. Conventional attempts to solve this problem have included adding a conventional fluid loss control additive to the pad fluid. Conventional fluid loss control additives generally comprise rigid particles having a spheroid shape, and often are used mainly to prevent the loss of the pad fluid and the proppant-laden fluid(s) to the formation, e.g., loss of the pad fluid and the proppant-laden fluid(s) to the natural fractures in the formation. While conventional fluid loss control additives may be able to withstand the closure stress of an induced fracture, they generally are not designed to withstand the stress exhibited by natural fractures in the formation. This may be particularly problematic at an intersection between an induced fracture and a natural fracture, because a natural fracture that is intersected by an induced fracture often will have a higher stress than the induced fracture. Consequently, when conventional fluid loss control additives enter, and potentially obstruct, natural fractures in the formation, the conventional fluid loss control additives often may be crushed within the natural fracture, which may be problematic because the crushed particles may partially or completely restrict any production from the natural fracture.  
     SUMMARY OF THE INVENTION  
      The present invention relates to subterranean fracturing operations, and more particularly to fracturing fluids comprising an improved fluid loss control additive, and methods of using such fracturing fluids in fracturing subterranean formations.  
      An example of a method of the present invention is a method of fracturing a subterranean formation comprising the steps of: providing a pad fluid comprising a base fluid and a fluid loss control additive comprising a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation; and contacting the formation with the pad fluid so as to create or enhance at least one fracture therein.  
      Another example of a method of the present invention is a method of controlling loss of a fracturing fluid during fracturing of a subterranean formation, comprising adding a fluid loss control additive to a pad fluid, wherein the fluid loss control additive comprises a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation.  
      Another example of a method of the present invention is a method of minimizing fluid loss in a subterranean formation comprising using a fluid loss control additive to obstruct at least one pore throat in the formation, wherein the fluid loss control additive comprises a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation.  
      An example of a composition of the present invention is a pad fluid for use in fracturing a subterranean formation comprising a fluid loss control additive comprising a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation.  
      Another example of a composition of the present invention is a fluid loss control additive for use in a fracturing fluid to be placed in a subterranean formation comprising a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the subterranean formation.  
      The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments, which follows.  
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
      The present invention relates to subterranean fracturing operations, and more particularly to fracturing fluids comprising an improved fluid loss control additive, and methods of using such fracturing fluids in fracturing subterranean formations.  
      The fluid loss control additives of the present invention generally comprise a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of a subterranean formation into which the fluid loss control additives may be placed. In certain embodiments, the fluid loss control additives may comprise a material having a size in the range of from greater than or equal to about 200 U.S. mesh to less than or equal to about 80 U.S. mesh and a compressive strength greater than the maximum stress of a subterranean formation into which the fluid loss control additives may be placed. Generally, the fluid loss control additives of the present invention will have a compressive strength greater than that of a natural fracture into which they may be placed. Generally, the fluid loss control additives of the present invention will have a compressive strength greater than that demonstrated by sand. In certain embodiments, the fluid loss control additives of the present invention may have a compressive strength such that when exposed to 2,000 psi pressure, less than about 1.7% by weight of fines are generated. As referred to herein, the term “fines” will be understood to mean particles having a size smaller than about 400 U.S. mesh. In certain embodiments, the fluid loss control additives of the present invention may have a compressive strength such that when exposed to 3,000 psi pressure, less than about 2.8% by weight of fines are generated. In certain embodiments, the fluid loss control additives of the present invention may have a compressive strength such that when exposed to 4,000 psi pressure, less than about 4.4% by weight of fines are generated. In certain embodiments, the fluid loss control additives of the present invention may have a compressive strength such that when exposed to 5,000 psi pressure, less than about 7% by weight of fines are generated. Examples of suitable materials that may be used in the fluid loss control additives of the present invention may include, but are not limited to, bauxite or bauxite-based materials, and ceramics or ceramic-based materials. Suitable materials are commercially available from Carboceramics, Inc., of Irving, Tex.; Sintex Minerals &amp; Services, Inc., of Houston, Tex.; and Norton-Alcoa Proppants, of Fort Smith, Ark. Examples of intermediate strength ceramic or ceramic-based materials that may be suitable include, but are not limited to, Econoprop®, Carbo Lite®, Carbo Prop®, Interprop®, Naplite®, and Valuprop®. Examples of high strength ceramic or ceramic-based materials that may be suitable include, but are not limited to, Carbo HSP®, Sintered Bauxite and SinterBall®. When a pad fluid comprising a fluid loss control additive of the present invention is placed in a subterranean formation, any portion of the fluid loss control additive that may be lost to the subterranean formation generally will be sufficiently strong to resist being damaged or crushed by the stress present within the formation, which may facilitate improved conductivity through at least the portion of the formation in which the fluid loss control additive resides. Generally, the fluid loss control additive is present in the pad fluids of the present invention in an amount sufficient to provide a desired degree of fluid loss control. More particularly, in certain embodiments, the fluid loss control additive is present in the pad fluids of the present invention in an amount in the range of from about 0.0006% to about 24% by weight of the pad fluid. In certain embodiments, the fluid loss control additive is present in the pad fluids of the present invention in an amount in the range of from about 0.002% to about 6.0% by weight of the pad fluid.  
      The pad fluids of the present invention generally comprise a base fluid, and a fluid loss control additive. A variety of base fluids may be included in the pad fluids of the present invention. For example, the base fluid may comprise water, acids, oils, or mixtures thereof. Examples of suitable acids include, but are not limited to, hydrochloric acid, acetic acid, formic acid, citric acid, or mixtures thereof. In certain embodiments, the base fluid may further comprise a gas (e.g., nitrogen, or carbon dioxide). Generally, the base fluid is present in the pad fluids of the present invention in an amount in the range of from about 30% to about 99% by weight of the pad fluid.  
      Optionally, the pad fluids of the present invention may comprise a viscosifier. Examples of suitable viscosifiers include, inter alia, biopolymers such as xanthan and succinoglycan, cellulose derivatives (e.g., hydroxyethylcellulose), and guar and its derivatives (e.g., hydroxypropyl guar). In certain embodiments of the present invention, the viscosifier comprises guar. More particularly, the viscosifier may be present in the pad fluids of the present invention in an amount in the range of from about 0.01% to about 1.0% by weight of the pad fluid. In certain embodiments, the viscosifier may be present in the pad fluid in an amount in the range of from about 0.2% to about 0.6% by weight.  
      Optionally, the pad fluids of the present invention may comprise additional additives as deemed appropriate by one skilled in the art, with the benefit of this disclosure. Examples of such additives include, but are not limited to, de-emulsifiers, surfactants, salts, crosslinking agents, clay inhibitors, iron-control additives, breakers, bactericides, caustic, or the like. An example of a suitable de-emulsifier is commercially available from Halliburton Energy Services, Inc., under the trade name “LO-SURF 300.” An example of a suitable source of caustic is commercially available from Halliburton Energy Services, Inc., under the trade name “MO-67.” An example of a suitable crosslinking agent is commercially available from Halliburton Energy Services, Inc., under the trade name “CL-28M.” An example of a suitable breaker is commercially available from Halliburton Energy Services, Inc., under the trade name “VICON NF.” Examples of suitable bactericides are commercially available from Halliburton Energy Services, Inc., under the trade names “BE-3S” and “BE-6.” 
      Generally, the pad fluids of the present invention, comprising a fluid loss control additive of the present invention, may be introduced to a portion of a subterranean formation at a pressure sufficient to create or enhance at least one fracture therein. Optionally, one or more subsequent fluids that comprise proppant particles may be introduced to the chosen portion of the formation so as to deposit at least a portion of the proppant particles in at least one fracture therein. Proppant particles utilized in accordance with the present invention are generally of a size such that formation particulates that may migrate with produced fluids are prevented from being produced from the subterranean zone. Any suitable proppant may be utilized in the one or more subsequent fluids, including graded sand, bauxite, ceramic materials, glass materials, walnut hulls, polymer beads and the like. Generally, the proppant particles have a size in the range of from about 2 to about 400 U.S. mesh. In some embodiments of the present invention, the proppant is graded sand having a particle size in the range of from about 10 to about 70 U.S. mesh. Particle size distribution ranges are generally one or more of 10-20 U.S. mesh, 20-40 U.S. mesh, 40-60 U.S. mesh or 50-70 U.S. mesh, depending on factors including, inter alia, the particular size and distribution of formation particulates to be screened out by the consolidated proppant particles, the permeability of the formation, and the cost of the proppant particles. Generally, proppant particles may be included in the one or more subsequent fluids, in an amount in the range of from about 4% to about 70% by weight.  
      An example of a method of the present invention is a method of fracturing a subterranean formation comprising the steps of: providing a pad fluid comprising a base fluid and a fluid loss control additive comprising a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation; and contacting the formation with the pad fluid so as to create or enhance at least one fracture therein. Additional steps could include, inter alia, contacting the formation with one or more subsequent fluids that comprise proppant particles so as to deposit within the formation at least a portion of the proppant particles; “breaking” the pad fluid and/or the one or more subsequent fluids (e.g., reducing the viscosity of the pad fluid and/or the one or more subsequent fluids to a desired degree) with a suitable breaker; recovering at least a portion of the pad fluid from the subterranean formation; and recovering at least a portion of the one or more subsequent fluids from the subterranean formation.  
      Another example of a method of the present invention is a method of controlling loss of a fracturing fluid during fracturing of a subterranean formation, comprising adding a fluid loss control additive to a pad fluid, wherein the fluid loss control additive comprises a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation. Another example of a method of the present invention is a method of minimizing fluid loss in a subterranean formation comprising using a fluid loss control additive to obstruct at least one pore throat in the formation, wherein the fluid loss control additive comprises a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation.  
      An example of a composition of the present invention is a pad fluid for use in fracturing a subterranean formation comprising a fluid loss control additive comprising a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the formation. In one embodiment, a pad fluid of the present invention may comprise: water, 1% potassium chloride by weight, 0.05% LO-SURF 300 by weight, 0.15% of a fluid loss control additive of the present invention by weight, 0.2% guar by weight, 0.1% MO-67 by weight, 0.05% CL-28M by weight, 0.1% VICON NF by weight, 0.001% BE-3S by weight, and 0.001% BE-6 by weight.  
      Another example of a composition of the present invention is a fluid loss control additive for use in a fracturing fluid to be placed in a subterranean formation comprising a material that has a size in the range of from greater than or equal to about 400 U.S. mesh to less than or equal to about 70 U.S. mesh and a compressive strength greater than the maximum stress of the subterranean formation.  
      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 the invention has been depicted and described by reference to exemplary embodiments of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.