Recent studies have offered evidence of unique shear viscosity loss of borate crosslinked fracturing fluids when exposed to hydrostatic pressures, such as those encountered during deep water hydraulic fracturing. This phenomenon can have important implications for proppant transport and for the resulting fracture geometry that needs to be accounted for in fracture design. Another important aspect for fracture design is the fluid loss. Since crosslinked fluids have superior fluid loss characteristics compared to linear polymers, the question arises as to whether fluid loss of borate-crosslinked formulations is also affected by pressure. Fluid loss is a fundamental property in hydraulic fracturing treatments and may dictate the attainable fracture geometry and the retained conductivity. Prior knowledge of fluid loss is also important for designing special additives to overcome the effect of excessive fluid loss.
This experimental study was designed to determine the pressure effect on linear guar and both borate and zirconium-crosslinked polymers. The concept of melt point utilized for the viscosity dependence was incorporated to select fluids. A unique high pressure, high temperature fluid loss apparatus was developed for the experimental testing. Test parameters varied from 160 to 260°F, 1000 to 9000 psi and 0.1 to 10 mD sandstone cores. Spurt and fluid loss coefficients were determined and compared to determine the pressure effect.
The results indicate that fluid loss behavior of fluids comprising borate-crosslinked guar is susceptible to moderate pressure effects. In particular, spurt losses increased with pressure, whereas fluid loss coefficients were independent of pressure. The increase in spurt loss with pressure was more pronounced as the permeability of the core sample increased. Conversely, the fluid loss behavior for linear guar or derivatized guar crosslinked with zirconium was not influenced by pressure. The implications of these findings for hydraulic fracturing applications are also discussed.