The present invention relates to viscoelastic surfactant fluids useful in subterranean operations, and more particularly, to dual-function additives that enhance fluid loss control and the stability of viscoelastic surfactant fluids, and their associated methods of use.
Treatment fluids may be used in a variety of subterranean treatments, including, but not limited to, stimulation treatments and sand control treatments. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The terms “treatment,” and “treating,” as used herein, do not imply any particular action by the fluid or any particular component thereof. These subterranean operations include, but are not limited to, hydraulic fracturing treatments, acidizing treatments, gravel-packing treatments, sand control treatments, and the like.
Maintaining sufficient viscosity in the treatment fluids used in these operations is important for a number of reasons. Maintaining sufficient viscosity is important in fracturing and sand control treatments for particulate transport and/or to create or enhance fracture width. Also, maintaining sufficient viscosity may be important to control and/or reduce fluid loss into the formation. At the same time, while maintaining sufficient viscosity of the treatment fluid often is desirable, it may also be desirable to maintain the viscosity of the treatment fluid in such a way that the viscosity also may be reduced easily at a particular time, inter alia, for subsequent recovery of the fluid from the formation.
To provide the desired viscosity, polymeric gelling agents commonly are added to the treatment fluids. The term “gelling agent” is defined herein to include any substance that is capable of increasing the viscosity of a fluid, for example, by forming a gel. Examples of commonly used polymeric gelling agents include, but are not limited to, guar gums and derivatives thereof, cellulose derivatives, biopolymers, and the like. To further increase the viscosity of a treatment fluid, often the polymeric gelling agent is crosslinked with the use of a crosslinking agent. Treatment fluids comprising crosslinked gelling agents also may exhibit elastic and/or viscoelastic properties, wherein the crosslinks between gelling agent molecules may be broken and reformed, allowing the viscosity of the fluid to vary with certain conditions such as temperature, pH, and the like.
The use of polymeric gelling agents, however, may be problematic. For instance, polymeric gelling agents may leave an undesirable gel residue in the subterranean formation after use, which can impact permeability of the formation. As a result, costly remedial operations may be required to clean up the fracture face and proppant pack. Foamed treatment fluids and emulsion-based treatment fluids have been employed to minimize residual damage, but increased expense and complexity often have resulted.
To combat perceived problems associated with polymeric gelling agents, some surfactants have been used as gelling agents. It is well understood that, when mixed with a fluid in a concentration above the critical micelle concentration, the molecules (or ions) of surfactants may associate to form micelles. The term “micelle” is defined to include any structure that minimizes the contact between the lyophobic (“solvent-repelling”) portion of a surfactant molecule and the solvent, for example, by aggregating the surfactant molecules into structures such as spheres, cylinders, or sheets, wherein the lyophobic portions are on the interior of the aggregate structure and the lyophilic (“solvent-attracting”) portions are on the exterior of the structure. These micelles may function, among other purposes, to stabilize emulsions, break emulsions, stabilize a foam, change the wettability of a surface, solubilize certain materials, and/or reduce surface tension.
When used as a gelling agent, the molecules (or ions) of the surfactants associate to form micelles of a certain micellar structure (e.g., rodlike, wormlike, vesicles, etc., which are referred to herein as “viscosifying micelles”) that, under certain conditions (e.g., concentration, ionic strength of the fluid, etc.) are capable of, inter alia, imparting increased viscosity to a particular fluid and/or forming a gel. Certain viscosifying micelles may impart increased viscosity to a fluid such that the fluid exhibits viscoelastic behavior (e.g., shear thinning properties) due, at least in part, to the association of the surfactant molecules contained therein. As used herein, the term “viscoelastic surfactant” refers to surfactants that impart or are capable of imparting viscoelastic behavior to a fluid due, at least in part, to the association of surfactant molecules to form viscosifying micelles. Moreover, because the viscosifying micelles may be sensitive to hydrocarbons, the viscosity of these surfactant fluids may be reduced after introduction into the subterranean formation without the need for certain types of gel breakers (e.g., oxidizers). The term “breaker” is defined herein to include any substance that is capable of decreasing the viscosity of a fluid. This may allow a substantial portion of the surfactant fluids to be produced back from the formation without the need for expensive remedial treatments. Moreover, these viscoelastic surfactants may not leave the undesirable gel residue in the subterranean formation found in uses of polymeric gelling agents, reducing or alleviating the need for costly remedial operations.
However, the use of viscoelastic surfactant fluids may be problematic in certain subterranean formations exhibiting high temperatures (e.g., above about 200° F.). Many viscoelastic surfactant fluids become unstable at these temperatures, which reduces the viscosity of the fluid. Moreover, the stability of viscosifying micelles in viscoelastic surfactant fluids may be extremely sensitive to various conditions (e.g., temperature, pH, presence of other additives in the fluid, composition of the subterranean formation, etc.), and thus the inclusion of other additives in the viscoelastic surfactant fluid that are needed for a given treatment using that fluid may detrimentally affect the rheological properties (e.g., viscosity) of the fluid. This inability to maintain a desired level of viscosity at higher temperatures, among other problems, may increase fluid loss and decrease the ability of the fluid to suspend and/or transport particulate materials.
Numerous additives are used in the art to help control fluid loss in subterranean operations. Numerous additives are also used in the art to maintain stability and/or viscosity of a treatment fluid at higher temperatures. However, the use of these conventional additives may give rise to other problems. First, the necessity of both a fluid loss control additive and a separate stabilizing or viscosifying additive in a treatment fluid may increase the complexity and cost of a treatment fluid and/or a subterranean application utilizing that fluid. Moreover, many conventional fluid loss control additives permanently reduce the permeability of a subterranean formation, affect the rheology of the treatment fluid in which they are used, and/or reduce the rate at which the fluid is allowed to penetrate or leak off into the subterranean formation. However, in some instances, while it may be desirable to control or prevent fluid loss for a given period of time, it may become desirable to allow the treatment fluid to penetrate or leak off into the subterranean formation, or to increase the permeability of the subterranean formation, at some later point in time. Costly and time-consuming operations may be required to reverse the effects of conventional fluid loss control additives on the treatment fluid and/or to restore permeability to those portions of the subterranean formation affected by the fluid loss control additives.