Optimizing Disproportionate Permeability Reduction

Abstract

AbstractAn idealistic goal of water shutoff technology is identifying materials that can be injected into any production well (without zone isolation) and will substantially reduce the water productivity without significantly impairing hydrocarbon productivity. Although many polymers and gels reduce permeability to water more than to oil or gas, several factors currently limit widespread field applications of this disproportionate permeability reduction property. First, adsorbed polymers and weak gels (suspensions of gel particles) show large variations in performance. Second, in unfractured wells (i.e., radial flow into porous sand or rock), the oil residual resistance factor, Frro, (permeability reduction factor) must be reliably less than 2. Third, adsorbed polymers and weak gels reduce permeability by greater factors in low- permeability rock than high-permeability rock.Strong pore-filling gels were investigated to overcome these limitations. For porous media with pre-gel kw (at Sor) ranging from 120 to 6,500 md, one Cr(III)-acetate-HPAM gel (with 0.5% HPAM) consistently reduced kw to about 0.24 md (ranging from 0.12 to 0.37 md). In contrast, in Berea sandstone with kw (at Sor) ranging from 222 to 363 md, a weak gel (with 0.18% polymer) exhibited a much wider range of post-polymer kw values—from 0.75 to 202 md. Thus, strong pore-filling gels can provide greater reliability and behavior that is insensitive to the initial rock permeability.With sufficient oil throughput, pore-filling gels dehydrate, thus increasing permeability to oil. Several gel formulations provided water residual resistance factors greater than 2,000 and ultimate Frro values of 2 or less. These results provide hope that our current approach will identify gels that can successfully and reliably treat either fractured or unfractured production wells without zone isolation. Significant oil throughput was required to achieve low Frro values, suggesting that gelant penetration into porous rock must be small (a few feet or less) for existing pore-filling gels to provide effective disproportionate permeability reduction.