Static leakoff experiments with filterpaper, using borate- and zirconate-crosslinked hydroxypropylguar (HPG) fluids, have resulted in practically the same Ieakoff coefficients. This is in contrast to previous, dynamic, Ieakoff tests suggesting that borate-crosslinked fluids perform better. Within the temperature range where the polymers are thermally stable (200°F), there were no temperature effects after correcting for the filtrate viscosity. Under constant pressure differentials, both fluids exhibited compressible filtercake behavior with the Ieakoff coefficient approximately proportional to powers of 0.2 for borates and 0.17 for zirconates. When pressure was increased, imitating pumping and, then decreased, imitating closure, characterization of the pressure-dependent fluid loss using the Ieakoff coefficient as the descriptive parameter indicated a fluid-loss hysteresis for both fluids. During the pressure increase from 0 to 1400 psi both fluids showed a two-range response: incompressible behavior at low pressures and compressive behavior at higher pressures. In this range, the Ieakoff coefficient was practically constant. A comparison of the stress-sensitive properties has shown that while zirconate filtercakes exhibit decidedly viscoelastic properties, borate filtercakes are merely elastic. This can be attributed to the differences in the nature of the bonding of these polymers. Finally, filterpaper and core experiments, done with noncrosslinked fluids, have shown no filtercake-type behavior for a large range of core permeabilities, but rather a viscous flow dependent on porous medium characteristics.

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