1. Field of the Invention
This invention relates to a fracturing fluid and to a method of fracturing a subterranean formation to increase the permeability of the formation.
More specifically, the invention provides a foamed, viscoelastic surfactant based fracturing fluid for fracturing a subterranean formation and transporting proppant into thus created fractures.
2. Discussion of the Prior Art
Hydraulic fracturing has been used for many years to stimulate the production of petroleum from subterranean formations. In hydraulic fracturing, a fracturing fluid is injected through a wellbore into the formation at a pressure and flow rate sufficient to overcome the overburden stress and to initiate a fracture in the formation. Frequently, a proppant, whose function is to prevent the created fractures from closing back down upon itself when the pressure is released, is suspended in the fracturing fluid for transport into a fracture. Proppants in use include, for example 20-40 mesh size sand and ceramics, but the most common proppant is sand. The proppant filled fractures provide permeable channels allowing petroleum to seep through the fractures into the wellbore from whence it is pumped to the surface. Accordingly, a desired fracturing fluid should have the following properties: (a) compatibility with the reservoir rock and reservoir fluids, (b) be sufficiently viscous and have a fluid structure capable of suspending proppants and transport them deep into the formation, (c) be stable enough to retain sufficient viscosity and fluid structure throughout proppant placement, (d) possess low fluid loss properties and low fluid flow friction pressures, (e) be easily removed from the formation with little residue, (f) be easily made under field conditions and (g) be relatively inexpensive. Production of petroleum can be enhanced significantly by the use of specialized fracturing fluids, which exhibit high levels of rheological performance.
Fracturing fluids in common use include various aqueous gels and hydrocarbon gels. The gels are formed by introducing cross-linkable polymers or surfactants into an aqueous or hydrocarbon fluid, followed by cross-linking of the polymer or surfactant molecules. The cross-linking give the fluid high viscoelastic properties that are necessary to transport and place proppants into the fractures.
Another widely used fracturing fluid is formed, water-based fracturing fluid. Such a fluid is described, for example, in U.S. Pat. No. 3,980,136, issued to R. A. Plummer et al on Sept. 14, 1976. Briefly, the foamed fracturing process involves generation of foams with a desired quality which arc pumped through a wellbore into a formation. Typically, for aqueous systems, a polymer has to be hydrated in water at the surface before being pumped into the formation. The process of polymer hydration is time consuming and often requires bulky equipment at the wellsite. Another problem common to polymer-based fracturing fluids is that a significant amount of polymer residue is left in the formation resulting in negative impact on formation permeability.
Viscoelastic surfactants have long been used for well stimulation. A surfactant is a type of substance, which contains both hydrophobic and hydrophilic groups in the same molecule. The hydrophobic group is usually one of a variety of alkyl groups and the hydrophilic group can be ionic, which may be positive (cationic), negative (anionic) or contain both positive and negative moieties (amphoteric), or nonioni—often consisting of a neutral polyoxyalkylene group. When dissolved in an aqueous medium, surfactants generally form various aggregates called micelles above a critical micelle concentration (cmc). At low concentration of surfactant, the micelles usually are small and spherical. Under certain conditions and surfactant concentrations, however, the spherical micelles grow in size and/or change their shape resulting in the formation of long flexible micelles. Above a certain concentration the long flexible micelles can become entangled and exhibit strong visoelastic behavior. Even though this feature has been observed in a number of systems containing nonionic and anionic surfactants, the effect is more pronounced in cationic surfactants, especially those containing an amine or quaternary ammonium group, in the presence of certain organic counterions such as, for example salicylate, benzonate and alkyl sulfonate. Viscoelastic surfactant fluids have been studied extensively in recent years and have found a wide variety of uses in many applications.
U.S. Pat. No.4,061,580, issued to R. W. Jahnke on Dec. 6, 1977 discloses surfactant gelled fracturing and acidizing fluids suitable for well stimulation. The gelled fluids are prepared by adding certain amine salts to aqueous acid or salt solutions. The amine salts used as thickeners are prepared by merely mixing one equivalent of amine per equivalent of acid or, in the case of polybasic acids such as sulfuric and phosphoric acids, as little as one-half equivalent of amine per equivalent of acid may be used resulting in the formation of an acidic salt. The aqueous acid or salt solution can be gelled by the addition of the above-described salts. For example, 15% by weight of HCl can be gelled by the addition of a small amount, usual 3-10% by weight and typically about 5% by weight of an amine or amine salt as described above. For fracturing fluids, aqueous solutions containing some inorganic salts can be gelled by the addition of 3-10% by weight, preferably about 5% by weight, of an amine salt described above. U.S. Pat. No.4,163,727, issued to C. G. Inks on Aug. 7, 1979 discloses an acidizing-gel composition which consists essentially of, for example, about 15% by weight of HCl, about 20% by weight of a suitable nonionic gel-forming surfactant containing oxyethylene and oxypropylene units, a corrosion inhibitor to the extent needed, and the balance water.
U.S. Pat. No. 5,551,516, issued to W. D. Norman et al on Sept. 3, 1996 and U.S. Pat. No. 5,964,295, issued to J. E. Brown et al on Oct. 12, 1999 disclose a fracturing fluid composition comprising a quaternary ammonium salt, erucyl bis (2-hydroxyethyl) methyl ammonium chloride, an organic salt such as sodium salicylate, inorganic salts such as ammonium chloride and potassium chloride and water. The patents state that the fluid has good visoelastic properties and is easily formulated and handled. Furthermore, no or very little residue is left in a formation after the completion of the fracturing process. It is worth noting, however, that cationic surfactants such as amine and quatenary ammonium salts usually degrade very slowly, both aerobically and anaerobically, and moreover are highly toxic to marine organisms. The combination of low biodegradability and high toxicity is a fundamental criterion for a product injurious to the environment. In addition, cationic surfactants tend to rend the formation, especially sandstone formations, oil-wet absorbing on the surface of clays and sands. The alteration of the formation wettability often reduces the relative permeability of petroleum leading to high water/petroleum ratio and low production rates. The strong adsorption of cationic surfactant on the clay and sands may also adversely affect fluid viscosity.