Guar-borate crosslinked fluids have been successfully used in hydraulic fracturing operations for decades. These fluids are often preferred because of cost, ease of use, and robustness. Additionally, borate fluids are renowned for their ability to recover viscosity after exposure to high shear, a process commonly referred to as “rehealing.” With the prevalence and reputation of these fluids, it is easy to become complacent and rely on the assumption of rehealability when pushing borate fluids to their operational limits. This is particularly true in the current climate of cost minimization. However, there are situations in which borate fluids might not reheal, or the rehealing process is significantly retarded. Because common assumptions about borate fluids cannot be assured in all situations, understanding the variables that affect performance is imperative.
This paper explores the properties of instant and delayed borate crosslinked fluids under different shear rates, shear histories, and a range of pH values. Various experiments were conducted to investigate the viscosity and the ability of viscosity to recover under different shear histories, temperatures, and pH values. This paper focuses on optimizing the fluid chemistry to provide the desired viscosities from the surface to deep within the fracture.
The work completed demonstrates how several borate fluid formulations can generate nearly identical viscosity profiles under a single shear rate, but vastly different profiles after exposure to high shear. If shear history is not considered in fluid design, some formulations could appear feasible during initial testing but fail to provide the desired viscosity in the near-wellbore (NWB) region during field operations. Assuming borates will simply reheal without consideration of pH and shear history on rehealing time could give rise to premature screenouts.
Borate crosslinked fluids are viewed as simple and forgiving. This is true to a point, but there are limitations that can be overlooked if the appropriate testing is not performed and formulations are stretched to their limits. The information presented in this paper demonstrates where assumptions about borate fluids' ability to reheal fail, while providing recommendations that can help ensure the desired viscosity is maintained throughout the treatment.