Disproportionate Permeability Reduction of Water-Soluble Silicate Gelants: Importance of Formation Wettability

Abstract

Summary Disproportionate permeability reduction (DPR) may provide field solutions to address high volumes of water production and efficiency of oil recovery in noncommunicating layered reservoirs. This work evaluates the laboratory-scale DPR effectiveness at different formation-wettability conditions by use of an environmentally friendly, water-soluble, silicate gelant. A robust, time/temperature-stable and easy-to-design water-soluble silicate-gelant system is used to conduct DPR treatments in oil- and water-wet cores by use of a newly established steady-state, two-phase chemical-system placement. The experimental procedure is applied to ensure the presence of moveable oil saturation at which the injected DPR fluid (gelant) gels in the treated zone and to quantitatively control the placement-saturation conditions in the formation. DPR treatments are conducted by use of a steady-state, two-phase (oil/gelant) placement to better control the water/oil saturation at which the silicate gel sets. The performance of water-soluble, silicate-based DPR treatments is evaluated by use of pretreatment and post-treatment two-phase (brine/oil) steady-state and unsteady-state permeability measurements. Strongly water-wet Berea cores are chemically treated to alter their wettability to oil-wet, and measured-phase effective permeability curves are used to characterize the newly established core wettability. Treatment design should include filterability/injectivity and rheological studies of the DPR fluid to evaluate gelant interaction with the formation as well as gelation time and kinetics. Single-phase DPR fluid injectivity through Berea cores is excellent. At relatively high water cuts in water-wet cores, two-phase DPR fluid/oil injectivity is good and even better in oil-wet cores regardless of the water cut. At relatively low water cuts in water-wet cores, the injectivity is not as good as in higher water cuts, and the mobility reduction keeps increasing with the coinjection of the DPR fluid/oil. DPR fluid/oil-placement experiments conducted at the same saturation conditions and water/oil ratio (WOR) showed that the ultimate oil-residual-resistance factor (RRF) in oil-wet cores is significantly lower than that in water-wet cores. This is mainly because of more-favorable oil-phase continuity and distribution in oil-wet media compared with the corresponding ones in water-wet formations. In water-wet cores, encapsulation of oil by gel may cause oil-phase discontinuities and porous-medium-conductivity reduction. Wettability tests have shown that silicate gel is strongly water-wet. Therefore, in oil-wet DPR treatments, formed gel in porous media yields a mixed-wet formation and a lower trapped-oil saturation compared with the water-wet formation. In either wetting state, relative permeability hysteresis was insignificant during the post-DPR treatment-imbibition/drainage cycles. This also reflects stable gels during post-DPR treatment floods. DPR treatments conducted at high WOR in oil-wet cores have shown a minor gel “erosion” during the post-treatment two- and single-phase (water) injection; gel “erosion” ceased during oil injection. DPR treatments conducted at high WOR caused an increase in RRF of both water and oil phases regardless of the core's wetting conditions; the DPR effectiveness was more pronounced in oil-wet cores than in water-wet ones.