The Effects of Wall Slip in a Couette Rheometer When Measuring and Comparing Hydraulic Fracturing Fluids

Abstract

The most common apparatus to gather rheological properties on hydraulic fracturing fluids is the Couette rheometer. It functions by rotating a pressurized cup filled with a sample of the fluid around a bob. The rheometer calculates the apparent viscosity and power law values by measuring the friction between the bob, the fluid, and the cup based on the deflection of the bob at different shear rates of the rotating cup. With this sort of apparatus, the assumption is made that any "slippage" that the fluid encounters at the wall' s surface is negligible. Although this assumption may be valid with other types of fluids such as those within the food or cosmetics industry, due to the unique characteristics of hydraulic fracturing fluids this is not always the case. Based on a rhelogical study of three different types of hydraulic fracturing fluids presented in this paper, conclusions were drawn on the effects of wall slip when measuring and comparing different types fracturing fluids and how this wall slipping phenomenon can be minimized. In this study three types of fracturing fluids were tested: a borate crosslinked water based gel, a surfactant water based gel and a hydrocarbon based gel. The water based gels demonstrated a significant amount of wall slip due to a phase separation of the water and gel. No phase separation was found in the hydrocarbon based fracturing gels, as such no wall slippage was observed. Several different metals were tested with varying surface roughness in order to attempt to minimize slip. Non-corrosive metals with a roughened surface allowed for higher viscosity readings in the water based gels. The surface roughness played a minimal effect in the viscosity readings for hydrocarbon based gels.