Applications of a New, Efficient Hydraulic Fracturing Fluid System

Abstract

Abstract A new, borate-crosslinked hydraulic fracturing fluid system has been developed. This new, optimized fluid system provides higher viscosity with lower gelling-agent concentrations compared to conventional, borate-crosslinked fluids. Application of the optimized low-gel borate (OLGB) fluid system at low temperatures is discussed. Viscosity. proppant transport. and fluid-loss data of the OLGB fluid system are compared to conventional, borate-crosslinked fracturing fluids. Treatment designs are also presented. Introduction The use of borate-crosslinked, hydraulic fracturing fluids has become routine in the oilfield-service industry. Although these fluids are successfully used on a daily basis, returned fluids can recrosslink at the surface. At low temperatures. high concentrations of oxidizing breakers are required for a complete break. Generally. a catalyst must be used to obtain the necessary break profile. Enzyme breakers can be used in conventional, borate-crosslinked fluids at low temperatures, but these fluids require an additive (acid) to lower the fluids' pH before the enzyme breakers can function properly. If such acids are not delayed or modified to be controlled-release, fluid stability is compromised. The pH of a borate-crosslinked fracturing fluid is critical to its successful use. Adjusting the pH of the fracturing fluid to maintain the correct pH value is often difficult. One way to overcome these difficulties is to adjust the pH of the fracturing fluid using a buffering agent. instead of a base such as sodium hydroxide. Buffering agents resist changes in fluid pH. The optimized low-gel borate (OLGB) fluid system combines a buffer with a crosslinker into a single component to adjust fluid pH to an optimum value for crosslinking, consequently allowing for simplicity and reliability in field applications. To assist in resolving the difficulties of breaking borate-crosslinked fluids in low-temperature applications and maintaining proper fluid pH, the OLGB fluid system has been developed. In addition to reducing the aforementioned difficulties, the OLGB fluid system allows a lower gel concentration to be used compared to conventional borate-crosslinked fluid systems. Discussion The OLGB fluid system consists of two primary components: a guar gelling agent and a crosslinking agent. The difference between the OLGB fluid system and most crosslinked fluid systems is that a third component is normally necessary to adjust the pH of most borate-crosslinked fluid systems. System pH is important to all crosslinked fracturing fluids, particularly in borate-crosslinked fluids. The use of a single component buffer/crosslinker additive has been important to the successful application of OLGB in fracturing applications. The inability to adjust and maintain the proper fluid-system pH can result in job failures. The importance of maintaining the correct fluid pH for a borate fluid system has been discussed by Harris. The relationship of temperature. system pH. gelling agent concentration, and crosslinker concentration is critical to the stability and ultimate viscosity of the fluid. This complex relationship is true whether the fluid is used in high- or low-temperature applications. This paper discusses low-temperature applications involving a borate-crosslinked fluid system that does not rely on a separate component to maintain system pH. In this system. the crosslinker and buffer are combined into a single component. The concentrations of gelling agents for fracturing treatments have decreased over the past several years. Lower amounts of gelling agents provide better and more complete breaks in a fracture. Several papers have discussed the merits and applications of reduced gel-concentration fluid systems. In this paper. laboratory test results and field applications that use 15 to 25 lb/Mgal of guar concentrations will be discussed. These lower gel concentrations are for fracturing applications at 70 to 140 F.