Multifunctional Boronic Acid Crosslinker for Fracturing Fluids

Abstract

Abstract Boron-crosslinked guar-based fluids are widely used in hydraulic fracturing applications and, unlike metal-crosslinked fluids, they are tolerant of high-shear conditions and often result in superior fracture conductivity because of better post-fracture cleanup. Guar is a natural polymer that contains insoluble residues, which can cause fracture conductivity damage. To help minimize this damage, lower-concentration polymer crosslinked fluids have been developed (18 to 20 lbm/1,000 gal). This approach is limited to polymer loadings above the polymer critical overlap concentration (C*). Below this concentration, unstable crosslinked gels are likely. The development of a new technology that permits the use of crosslinked guar at concentrations near or below C* is presented in this paper, as well as initial results obtained from field testing. Well-behaved crosslinked fracturing fluids have been formulated at guar concentrations at 8 to 10 lbm/1,000 gal with this technology. This new technology is based on the use of a water-soluble long chain polymer that includes boronic acid moieties, which can crosslink guar polymer below C* concentration. The chemistry of this polymeric crosslinker and the resulting gels are similar to that of conventional borate crosslinkers. Synthesis and optimization of the polymeric boronic acid crosslinker is presented, along with rheology and conductivity studies. This new technology provides the fracture design engineer with a new crosslinked fluid system that can reduce the potential for conductivity loss resulting from fracturing fluid damage by reducing the amount of guar gum (and insoluble residue) required to fracture a well by half or more. Background Hydraulic fracturing, sometimes simply referred to as fracturing, is widely used to enhance the production of oil and gas from subterranean formations. Fracturing treatments are performed at pressures above the fracture pressure of the reservoir formation and result in the creation of highly conductive flow paths connecting the reservoir to the wellbore. The fracturing operation not only creates new fractures but can also connect natural fractures that might already exist in the reservoir. The process greatly enhances the conductive area through which oil and gas can be produced. During hydraulic fracturing, fracturing fluid is pumped at a high rate and pressure into the wellbore and out into the formation to create fractures in the rock. The pumping of the fluid is continued to grow and expand the fracture. The newly created fracture will tend to close when pumping of the fracturing fluid is stopped. To prevent the fracture from closing, proppant is placed to keep the fracture propped open. The proppant is usually in the form of insoluble granular materials, which are suspended in the fracturing fluid, carried downhole, and deposited in the fracture. The proppant holds the fracture open while still allowing fluid flow through the permeability of the proppant. When deposited in the fracture, the proppant forms a "proppant pack," and, while holding the fracture apart, provides conductive channels through which fluids flow to the wellbore. The proppant is selected to withstand the closing pressure of the formation and the wellbore conditions, including fluids present. Generally, sand and manmade proppants, such as bauxite, are used to prop open the fracture. Proppant materials typically have a much higher density than water. Fracturing fluids with high viscosity are required to suspend and transport proppant deep into the fracture. When low-viscosity fluids are used, the proppant tends to separate quickly and settle, thereby giving poor proppant distribution or even causing premature "screenout." To help suspend the proppant in fracturing fluid, high viscosity is necessary, and viscosity-increasing additives are used to increase the viscosity of aqueous fluid.