Polymer-Free Fluid for Hydraulic Fracturing

Abstract

Abstract Polymer residues that stay in the fracture after a hydraulic fracturing treatment can limit treatment effectiveness. A new and easy-to-prepare, polymer-free fluid that consists of a quaternary ammonium salt derived from a long-chain fatty acid is described. In brine, it builds viscosity due to the formation of highly entangled worm-like micelles. The micelles have a gross structure similar to a polymer chain. Since the viscosity of the fluid depends on the nature of micelles, the fluid can be broken by changing this micellar structure. The breaking occurs when the fluid is exposed to hydrocarbons or diluted with formation waters. Therefore, conventional breakers are not required, and the produced oil or gas can act as breakers for this fluid system. The structural characteristics of the polymer-free surfactant fluid to its chemical and physical properties are correlated in this paper. Structure, rheology, fluid loss and conductivity of this surfactant fluid together with its production data are presented. Introduction Fracturing fluid is a very critical component of a hydraulic fracturing treatment. Selection of the fracturing fluid, job design, and well turnaround procedure all help to determine the production of a well after a stimulation by hydraulic fracturing. A fracturing fluid should provide sufficient viscosity to suspend and transport proppant into the fracture, and should break into a low-viscosity fluid after the job is completed. This will facilitate the fracture to clean up by allowing rapid flowback of fluid to the surface. Analysis of the fluid returned to the surface (flowback fluid) after hydraulic fracturing indicates that as little as 30 to 45% of the guar-based polymer pumped during the treatment returned from the well during the flowback period. Polymer residues that remain in the fracture significantly contribute to a lowered proppant-pack permeability leading to a loss in fracture treatment effectiveness. The field success of a viscoelastic surfactant-based (VES) polymer-free fluid in frac-pack applications led to the development of a similar fluid for hydraulic fracturing. The VES fluid can be used for the fracturing treatment of potentially all gas and oil wells below 240 F. The principal advantage of this fluid system is its operational simplicity. This fluid is easy to prepare and requires less equipment at the wellsite This fluid does not require polymer hydration, biocides, crosslinkers, or breakers. The hydrocarbons produced, or dilution of VES gel by other formation fluids, can break this fluid. This new fluid has been used for the successful execution of more than 250 fracturing jobs. Results from representative treatments are presented. Theory The presence of two structurally dissimilar groups (a lyophilic and a lyophobic) within a single molecule is the most fundamental characteristic of all surfactants These molecules are composed of groups of opposing solubility tendencies, typically an oil-soluble hydrocarbon chain (hydrophobic) and a water-soluble ionic group (hydrophilic). In aqueous solution these molecules self-associate in an attempt to sequester their apolar regions from contact with the aqueous phase. Micelles can be small spheres, disks, or long cylindrical structures. When dissolved in brine. a small group of surfactants is able to form structures other than the most commonly encountered spheres or disks. This includes the family of quaternary ammonium compounds, which is the subject of this paper. The geometry of the micelles is similar to polymer molecules. This network of micellar structure resists distortion, whereby the viscosity increases and imparts viscoelastic properties to the fluid. In this system. unlike guar-based fluids, no crosslinker is necessary. There is a repulsive interaction in micelle structure, primarily between the positively charged head groups. P. 553^