Comparison of Secondary and Tertiary Recovery With Change in Injection Brine Composition for Crude-Oil/Sandstone Combinations

Abstract

Abstract Numerous examples of dependency of waterflood efficiency on brine composition have been observed for sandstones. Improved oil recovery depends on complex crude oil/brine/rock (COBR) interactions. Elucidation of the circumstances and mechanisms of increased oil recovery still requires extensive laboratory testing. Special attention is being given in laboratory and pilot tests to increased recovery by injection of low salinity brine in both secondary and tertiary modes. Investigation of connate brine composition as a key variable has been extended to variation in initial water saturation (Swi). Mixed-wettability (MXW) cores were prepared by aging with crude oil at reservoir temperature for a range of initial water saturations. Oil recovery as % original oil in place (OOIP) generally increased with initial water saturation for secondary recovery by injection of low salinity brine. CS crude oil/Berea sandstone combinations were essentially insensitive to displacement with low salinity brine in secondary mode even when the injection pressure was double that for oil recovery by injection of reservoir brine (RB). However, significant increase in tertiary mode (about 6% OOIP) was observed for the same COBR combination when low salinity brine was injected after establishing residual oil by injection of reservoir brine. Low salinity brine floods with Minnelusa oil and 400 md Berea sandstone showed more than 13% OOIP increase in recovery over that given by flooding with reservoir brine. For the Minnelusa oil, however, very little additional oil was produced in tertiary mode. From available data, response to injection of low salinity brine in tertiary mode was clearly highest for reservoir rock and crude oil. Overall, rock properties are the most important factor in improved recovery by injection of low salinity brine. Introduction Waterflooding accounts for a large fraction of worldwide oil production. In many mature reservoirs, two-thirds of the oil originally in place still remains as a target for additional recovery. Numerous examples of increased recovery by injection of low salinity brine have been reported for laboratory core-floods on outcrop and reservoir cores and for field tests.[1–12] The examples include secondary mode given by injection of low salinity brine starting at connate water saturation and tertiary mode whereby the injection brine is switched to low salinity brine at a mature stage of the flood. In the first reported pilot test of injection of low salinity brine,[10] reduction of residual saturation around a well-bore was demonstrated by log-inject-log measurements. Comparisons of recovery for high versus low salinity brine were reported for four sets of field trials on Alaska North Slope oilfields.[11] Single well tracer tests were used to evaluate the remaining oil saturation before and after the injection of low salinity brine. These tests showed that 6 to 12 % OOIP could be displaced in tertiary (post waterflood) mode; the field test results were generally consistent with tertiary mode recovery obtained for laboratory reservoir condition floods. Examples of significant response to low salinity brine for full reservoir condition waterfloods have also been reported for both secondary and tertiary mode.[12] However, increase in oil recovery by low salinity brine flooding is highly specific to COBR combinations and much remains to be learned about the recovery mechanisms under various circumstances. Connate water is a key variable in the development of mixed wettability by adsorption from crude oil. The question of the effect of the value of initial water saturation on recovery is addressed in the present work. Loss of availability of the type of Berea sandstone that had been essentially adopted as a model rock, necessitated testing of a wider range of types of Berea sandstone. The variation in performance for the different rock types provides further evidence of the dominant effect of rock properties on recovery of crude oil by injection of low salinity brine.