This paper presents the results of an industry consortium established to understand and allow modeling of the fluid-leakoff process in hydraulic-fracturing applications. Fluid-loss data was measured with a state-of-the-art experimental setup that included fluid preconditioning loops for high-shear and low-shear regimes and a novel cell design that minimizes flow irregularities. Previous papers 1,2 have presented the effects of experimental designs, shear rate, permeability, and pressure on the fluid loss of water-based systems with no fluid-loss control aids. This paper presents the results of experiments that used linear hydroxypropylguar (HPG) gel and HPG/borate fluid systems with various types of hydrocarbon and/or particulate fluid-loss additives in similar testing procedures.
This study has found that each fluid-loss additive has an optimum performance range that depends on the fluid type and core permeability. Hydrocarbon fluid-loss additives, even at low concentrations (0.5% by volume), were found to be effective in reducing the filter-cake permeability of crosslinked gel systems in low-permeability cores (less than 10 md). Unless effective particulate fluid-loss additives were also used, hydrocarbon additives were ineffective in reducing the spurt loss or the filter-cake permeability in higher-permeability cores. Since hydrocarbon additives principally work on reducing the filtercake permeability, they exhibited more shear-rate sensitivity than "neat" fluids. However, in contrast to the pressure-independent behavior of neat fluids, the fluid-loss coefficients with hydrocarbon additives increases with pressure.
Particulate fluid-loss additives were very effective in reducing spurt loss, especially for low-viscosity fluids. Inert and oil-soluble materials out-performed most water-soluble particu lates, particularly in higher-permeability formations. An investigation of the particulate particle size indicated that the optimum particulate size distribution must incorporate particles one to two times larger than the peak core pore size. Shear rate did not alter the particulates' effectiveness in controlling spurt loss; however, it did affect the filter-cake properties (Cw and vn values) in most situations similar to neat fluids. The addition of particulates did not change the compressibility of filter cakes that was observed earlier with neat fluids. The spurt loss with inert particulate additives was relatively independent of pressure, in contrast to the pressure-dependent spurt loss of neat fluids.