Effect of WAG Injection and Rock Wettability on Oil Recovery During CO2 Flooding

Abstract

Summary Previous studies to determine the extent of oil trapping by water during CO2 water-alternating-gas (WAG) flooding have shown that rock wettability strongly affects this trapping. A significant trapping occurs in water-wet rock, and less trapping occurs in oil-wet rock. This paper presents laboratory results of Devonian crude oil displacement from watered-out Berea and reservoir cores by use of continuous CO2 injection, single-slug CO2 injection (followed by water), and CO2 WAG injection at miscible reservoir conditions of 120°F and 2,500 psig [49°C arid 17.2 MPa]. The reservoir cores used in this study were mixed- wet (Devonian and Muddy formations) and oil-wet (Tensleep formation). The Berea cores used had their wettability artificially altered to simulate these natural wettabilities. The X-ray photoelectron spectroscopic (XPS) method for measuring carbon content of rock surface was used to provide a qualitative measurement of wettability of the rock samples. The results of the study indicated that the experimental wettability-altering technique came close to duplicating reservoir rock wettability. The oil recovery data at the end of 1 PV fluid injection (continuous CO2 or WAG CO2) indicated that (1) in preferentially water-wet Berea cores, more than 45% of the waterfiood residual oil trapped by CO2 WAG; (2) in mixed-wettability Berea cores, 15 to 20% of the waterfiood residual oil was trapped; and (3) in oil-wet Berea cores, less than 5% residual oil was trapped.