Produced Water Reinjection with Polysulphate Additive for Enhanced Oil Recovery from Carbonate Reservoirs

Abstract

When naturally occurring polysulphate salt dissolves in water, sulphate and calcium ions are released into the water, making it a potential enhanced oil recovery (EOR) fluid. In mature fields the handling of produced water (PW) presents increasing economic and environmental challenges. PW reinjection (PWRI) can reduce these challenges, but because of the PW composition its efficiency as an EOR method is limited. Polysulphate was explored as an additive for PWRI and EOR from carbonate reservoirs. Seawater is known to be a successful injection fluid in carbonates. However, in areas where seawater is not available, nearby water-sources or reservoir PW are used for waterflooding, potentially limiting the recovery factor. In this work polysulphate salt was added to PW and used as imbibition fluid to test its ability of altering wettability of carbonate rock material and improving oil recovery beyond that obtained by PW alone. Complimentary tests including spontaneous imbibition oil recovery tests and chromatographic wettability tests were used to compare wettability alteration and increase in water-wet rock surfaces in mixed-wet chalk. Oil recovery by spontaneous imbibition tests in mixed-wet outcrop chalk showed that PW with added PS (PW-PS) improved oil recovery beyond that obtained by PW only in both secondary and tertiary mode at 90 °C and 110 °C. In tertiary mode at 90 °C a 22 %OOIP increase in oil recovery was obtained by PW-PS imbibition succeeding the secondary mode PW imbibition, while an additional 15 %OOIP was obtained at 110 °C. By adding PS to PW, wettability alteration was induced by the presence of sulphate, generating positive capillary forces, increased spontaneous imbibition of water and improved displacement of crude oil. In these experiments PW-PS was more efficient than seawater and other PS-brines in the oil recovery process. Compared to seawater, which is considered a Smart Water in chalk, PW-PS increased the ultimate oil recovery in secondary mode with 10 %OOIP at 90 °C and 3 %OOIP at 110 °C. Chromatographic wettability tests, performed after the oil recovery tests, confirmed that the PW-PS brine had altered the wettability of the mixed-wet chalk to more water-wet, supporting the high oil recovery results observed by spontaneous imbibition. These findings suggest that the naturally occurring polysulphate salt is a promising additive for improving PW as an injection fluid in carbonate reservoirs. This is especially important in areas where seawater is not available or PWRI is mandatory or necessary for reducing the challenges associated with its processing and disposal.