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 S or reduction. The S or dependencies of the induced formation damage are derived from five series of laboratory tests. All the laboratory tests confirmed the proposed S or reduction mechanism by fines-assisted low-salinity waterflooding.

This study presents a novel mechanism of enhancing oil recovery by fines migration during low salinity waterflooding. Formation damage is isolated from other low salinity mechanisms in the experimental tests performed in this work. Therefore, the reduction in residual oil saturation is attributed to fines migration mechanism only that is caused by improved microscale sweep efficiency via water flux diversion due to fine particles straining. Corefloods were performed on Berea cores with high clay content, Bentheimer cores with low clay content, and artificial clean sand cores with no clay to investigate the effect of clay presence on residual oil saturation. HSW and LSW took place after drainage displacements that resulted in the same initial conditions of connate water saturation and oil relative permeability. Non-polar oil is used to ensure water-wetness in the cores and to avoid possible wettability alteration by low salinity waterflooding. Single phase corefloods were also performed to study the effect of piecewise decreasing salinity on permeability. The results show a permeability decline with low salinity water injection in the single phase tests of clay-rich cores accompanied by fine particles production and pH increase. The same effect is observed in the two phase tests. In addition, incremental oil production is observed along with the permeability decrease and fines production. This is due to detachment of clay particles by weakened attraction as a result of LSW, which leads to fines migration and straining in water filled pores. Therefore, water flux diversion into trapped oil pores takes place, which displaces the residual oil in these pores. A relationship between formation damage, βΔσ, and residual oil saturation has been introduced and it can be applied in reservoir simulation for LSW.

Fines mobilization during the injection of low-salinity water yields a decrease in well injectivity and productivity but may cause an increase in the reservoir sweep efficiency during oilfield waterflooding. We investigate the stability of clay fines under the combined alteration of Na and Ca concentrations. Consequent injections of Ca and Na solutions in natural and engineered cores intercalated by deionized water (DIW) injections have been performed. Fines migration has not been observed during DIW injection after preflush by CaCl 2 solution; further consequent injection of NaCl and DIW yields a significant fines mobilization and permeability decline. The tests demonstrate a strong hysteretic behavior of mutual adsorption−desorption of Ca and Na cations on the reservoir clays and rock. The same phenomena have been observed in the presence of residual oil, where fines migration has been accompanied by incremental oil production.

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