Oil Recovery by Sequential Waterflooding of Mixed-Wet Sandstone and Limestone

Duplicate tests on mixed-wet sandstone cores have been shown to give closely reproducible results. For studies with reservoir cores, duplicate tests are often precluded because of the limited availability of cores and their heterogeneity. In repeat flooding tests for recovery of crude oil, it has been found that each test affects the outcome of a subsequent test. Some consistent trends of increase in recovery have been observed. In this paper, results are presented for up to four cycles of flooding between initial water saturation and residual oil for reservoir sandstone, outcrop sandstone, and outcrop oolitic limestone. A single crude oil was used for most of the tests. Sequential flooding was also tested for low- and high-viscosity mineral oils. The brine was synthetic seawater for all but one of the floods. Each consecutive flood for the sandstones resulted in a systematic increase in the recovery of crude oil. Crude oil was sometimes produced as an emulsion of about 2% water. Trends for carbonate cores were comparable except that, for one of four of the tested cores, reduced recovery was observed for the second cycle. Implications for establishing baseline results for assessment of processes, such as low-salinity water flooding, are discussed.