Chemistry and Rheology of Borate-Crosslinked Fluids at Temperatures to 300F

Summary This paper explains the variables involved in borate chemistry as they affect crosslink formation, stability, and breaking properties. The borate-ion concentration in solution is a function of pH and temperature. A method for calculating borate-ion equilibria is used to help establish the required final pH value for application of a borate-crosslinked gel at a specific temperature. Introduction Many of the properties of borate-crosslinked fluids can be explained in terms of the shift in borate-ion equilibria with pH and temperature. Syneresis, the process of forming a dense polymer mass and excluding liquid from the structure, is easily understood as a product of increased borate-ion concentration when a fluid is cooled. Proppant-settling behavior that is pH-dependent also can be explained. As the equilibrium concentration of borate ion increases with increasing pH, the number of network crosslinks increases and hence the proppant-settling rate decreases. Rheological measurements from flow-loop and Fann Model 50 viscometers are used to help identify the chemical variables that control such fluid properties as crosslinking time, viscosity development, and the rehearing character of borate fluids. A new theological procedure using steady-state shear rate is presented for obtaining viscosity data for fluids used in fracture design programs. Background Borate-crosslinked fluids have been gaining popularity in the fracturing industry. Even though such fluids have been used for more than 20 years, new developments in borate-fluid technology are occurring rapidly, particularly with respect to high-temperature capability. For years, borate-crosslinked fluids were considered inexpensive, high-viscosity fluids limited to low temperature. More recently, wells up to 300F have been treated with borates. To understand the extension to high temperature, we should reconsider the chemistry of borate-crosslinked fluids. Borate Crosslinking Borate-crosslinked fluids used for fracturing consist of polysaccharide polymers crosslinked with borate salts. These gels are difficult to investigate with analytical instruments, so model diol chemical systems and model sugars have been used to study the formation of borate esters and crosslinking reactions. Borate crosslinks are viewed as rapid exchange equilibria of monoborate ion on the cis-diols of galactomannan polymers (guar).