Effects of fines migration on residual oil during low-salinity waterflooding

Abstract

A novel mechanism of residual oil reduction during low-salinity water flooding by induced fines migration and consequent permeability damage is discovered. Dry corefloods (without oil) with piecewise-constant decreasing salinity are conducted to verify the presence of movable fines that yield permeability decrease. Sequential two-phase coreflood tests are performed using the displacement of nonpolar oil by high-salinity water, followed by full resaturation of the core by nonpolar oil and low-salinity waterflooding. The test then continued in tertiary mode, in which brines of decreasing salinity were injected in the presence of residual oil. Four Berea cores with high clay content, one Bentheimer core with low clay content, and two artificial cores with no clay content were used for two-phase waterflooding experiments. Reduction in permeability for water, fines production, and reduction in residual oil saturation accompanied the abrupt salinity decrease in all tests. This effect is attributed to fines mobilization that is due to salinity decrease, followed by fines migration and straining in thin pore throats that resulted in local hydraulic resistance and consequent pore-scale flux diversion, yielding Sor reduction. The Sor dependencies of the induced formation damage are derived from five series of laboratory tests. All the laboratory tests confirmed the proposed Sor reduction mechanism by fines-assisted low-salinity waterflooding.