Experimental study of the effect of aging on fluid distribution in Middle Bakken cores

Abstract

Formation wettability is a critical parameter when designing enhanced oil recovery (EOR) for field applications both in conventional and unconventional reservoirs. In this study, we only focused on one unconventional resource—the Bakken. Specifically, we conducted laboratory experiments in five uncleaned Middle Bakken cores with different initial fluid saturations to determine (1) where the fluids reside within the pores, and (2) the effect of aging on wettability change.The experimental protocol included hydrocarbon-saturation of three uncleaned cores with Bakken oil. The cores were aged at 180 °F and 2500 psi pressure for four weeks, and stored in crude oil for 1 year at ambient laboratory conditions (about 12 psi and 66 °F). Synthetic brine was then used to produce oil from the aged cores by spontaneous imbibition in Amott cell. Additional oil was produced from the cores using a high-speed centrifuge.For the remaining two uncleaned cores, synthetic brine was used to saturate them, and brine was replaced with oil using a centrifuge. The cores were stored in crude oil at ambient laboratory conditions for four months; then, oil recovery from the cores was measured in Amott cell. NMR measurements were conducted on the cores after each fluid saturation/de-saturation experiment to determine the effect of aging time and temperature on the core wettability and fluid distribution in the pores. Also, porosity and permeability of cleaned twin cores were measured at several net confining stresses.The experimental results indicated that hydrocarbon-saturated cores become slightly more oil-wet than brine-saturated cores. NMR measurements indicated that brine resides in smaller pores and as a brine film on grains regardless of aging time, and the core fluids re-distributed with time indicating a strong rock-fluid interaction. Finally, the analysis of oil production indicated the preponderance of chemical osmosis over imbibition as the mechanism of oil displacement in formations with high brine salinity.