Implications of Pressure-Induced Thinning in Crosslinked Fluids for Fracturing and Frac Pack Operations

Abstract

Several decades of fracturing treatments using borate-crosslinked gels have led to a strong understanding of the optimization and applicability of these fluids. Conventional HPHT rheological measurements of these fluids follow recommended practices defined by the API and ISO (involving low applied pressure of 400 psi). However, more recent measurements have shown that the viscosity of such a gel may be up to 80% lower when exposed to actual fracturing conditions (applied pressure > 5,000 psi) versus the standard low-pressure measurements of viscosity. The current study demonstrates further investigation into the variables that impact this phenomenon, using rheology measurements at ultrahigh-pressure on a series of controlled fluid-compositions. This work includes focus on the impact of pressure on gelled fluids used for frac pack operations, as productivity from deepwater frac pack operations is highly sensitive to predictable delivery of the intended fracture design. Initial tests were established to confirm the magnitude of viscosity-reduction as a function of increasing pressure. Advanced testing was also used to define the pressure-dependence of the reversible "melt" behavior of borate-crosslinked gels. A test matrix of crosslinked fluids was established to look at the individual and the combined impact of gel loading, fluid salinity, shear-rate, and applied pressure on fluid stability in a series of gel formulations. Finally, the corrected viscosity measurements were used in fracture simulations to determine the effects of pressure-corrected viscosity (compared to viscosity measured using conventional API/ISO methods) on fracture geometry and conductivity. The current study identifies several critical findings. First, the application of ultrahigh pressure on borate gels reversibly destabilizes the gel, leading to a shift in the melt-temperature to cooler temperatures by >30degF. Additionally, high salinity mix-waters (such as heavy brines) shift or partially suppress the pressure-induced destabilization of these gels. Finally, fracture modelling demonstrates a potentially significant negative impact to fracture geometry and an increased risk of screen-out using the pressure-corrected gel viscosity. The findings of the current work will have sizeable impact on the execution of gelled fracturing and frac pack treatments. The new findings regarding the effects of salinity are crucial to understanding pressure-corrections in frac pack design, as those gels are often weighted using heavy brine to add hydrostatic pressure. To that end, understanding the boundaries of pressure effect occurrence in these gels can improve the gel design to avoid the unintended consequences of high-pressure.