Chemical Evolution of a High- Temperature Fracturing Fluid

Reservoirs with bottomhole temperatures (BHT's) in excess of 250 deg. F [121 deg. C] and permeabilities of less than 1.0 md are commonly encountered in drilling and completing geothermal and deep gas wells. Successful stimulation of these wells often requires the use of massive hydraulic fracturing (MHF) treatments. Fracturing fluids chosen for these large treatments must possess shear and thermal stability at high BHT'S. The use of conventional fracturing fluids has been limited traditionally to wells with BHT's of 250 deg. F [121 deg. C] or less. Above 250 deg. F [121 deg. C], high polymer concentrations and/or large fluid volumes are required to maintain effective fluid viscosities in the fracture. However, high polymer concentrations lead to high friction pressures, high costs, and high gel residue levels. The large fluid volumes also increase significantly the cost of the treatment. Greater understanding of fracturing fluid properties has led to the development of a crosslinked fracturing fluid designed specifically for wells with BHT's above 250 deg F [121 deg C). The specialized chemistry of this fluid combines a high-ph hydroxypropyl guar gum (HPG) solution with a high-temperature gel stabilizer and a proprietary crosslinker. The fluid remains stable at 250 to proprietary crosslinker. The fluid remains stable at 250 to 350 deg. F [121 to 177 deg. C] for extended periods of time under shear. This paper describes the rheologial evaluations used in the systematic development of this fracturing fluid. In field applications, this fracturing fluid has been used to stimulate successfully wells with BHT's ranging from 250 to 540 deg. F [121 to 282 deg C). Case histories that include pretreatment and posttreatment production data are presented. Introduction Temperatures exceeding 250 deg F [121 deg C) and permeabilities less than 1.0 md are frequently encountered in permeabilities less than 1.0 md are frequently encountered in deep gas and geothermal wells. Successful economic completion of these wells requires that long, conductive fractures with optimal proppant distribution be created. Ultimately, the amount of production from these formations depends on the propped fracture length created. One successful stimulation technique used to create these long fractures is MHF. In these treatments, the fracturing fluids are often exposed to shear in the fracture for prolonged periods of time at high temperatures. Thus the fracturing fluids must exhibit extended shear and thermal stability at the high BHT'S. In addition, the fracturing fluid must not leak off rapidly into the formation, or the fracture may not be extended to the desired length. Many early treatments were limited by fracturing fluids that lost viscosity rapidly at high BHT's because of excessive thermal and/or shear degradation. Creation of a narrow fracture width, excessive fluid loss to the formation, and insufficient proppant transport resulted from the use of these low viscosity fluids. The solution to conventional fracturing fluid deficiencies was to develop a more efficient fracturing fluid (low polymer concentrations) with greater viscosity retention under shear at high temperatures, better fluid-loss control, and lower friction pressures. Generally, the components that make up crosslinked fracturing fluids include a polymer, buffer, gel stabilizer, and crosslinker. Each of these components is critical to the development of the desired fracturing fluid properties. The role of polymers in fracturing fluids is to properties. The role of polymers in fracturing fluids is to provide fracture width, to suspend proppants, to help provide fracture width, to suspend proppants, to help control fluid loss to the formation, and to reduce friction pressure in the tubular goods. Guar gum and cellulosic pressure in the tubular goods. Guar gum and cellulosic derivatives are the most common types of polymers used in fracturing fluids. The cellulosic derivatives are residue-free and thus help minimize fracturing fluid damage to the formation. However, the cellulosic derivatives are difficult to disperse because of their rapid rate of hydration. Guar gum and its derivatives are easily dispersed but produce some residue when broken. Buffers are used in conjunction with polymers so that the optimal pH for polymer hydration can be attained. When the optimal pH is reached, the maximal viscosity yield from the polymer is more likely to be obtained. The most common example of fracturing fluid buffers is a weak-acid/weak-base blend, whose ratios can be adjusted to that the desired ph is reached. However, some of these buffers dissolve slowly, particularly at cooler temperatures. Gel stabilizers are added to polymer solutions to inhibit chemical degradation. Examples of gel stabilizers used in fracturing fluids include methanol and various inorganic sulfur compounds. Other stabilizers are useful in inhibiting the chemical degradation process, but many interfere with the mechanism of crosslinking. The sulfur containing stabilizers possess an advantage over methanol, which is flammable, toxic, and expensive. SPEJ P. 623