Capillary pressure and wettability play crucial roles in controlling fluid flow and distribution in tight/shale reservoirs with its nano- to micrometer-scale pore sizes, such as the Bakken formation in the Williston Basin. Because wettability reflects the interactions between rock surfaces and fluids, the introduction of external fluids during drilling, completion, and enhanced oil recovery processes may possibly alter wettability through chemical adsorption on rock surfaces. Therefore, accurate determination of Bakken formation wettability, particularly its alteration caused by different injection fluids, is extremely important in the Bakken production process. In this study, a state-of-the-art ultra-high-speed centrifuge was employed to measure capillary pressures and examine the wettability of Bakken cores by calculating the USBM (United States Bureau of Mines) wettability indices. Cores saturated and aged with formation brine or surfactant solutions were first injected with Bakken crude oil (i.e., drainage process). Then the fluids in the cores were displaced by brine or surfactant solutions to obtain the imbibition capillary pressure curve. Multi-speed measurements were conducted using the centrifuge to determine the capillary pressure between the crude oil and brine/surfactant solutions. Wettability and its alteration after surfactant saturation were quantified by analyzing different sets of wettability indices calculated from capillary pressure curves. The experimental results revealed that wettability alteration caused by surfactant adsorption on rock surfaces tended to change the rocks from water-wet to a more neutral-wet status, while the aging process partially restored the cores to water-wet again.

For unconventional tight cores such as Bakken, the ultra-high-speed centrifuge method presents several distinct advantages over the commonly used mercury injection capillary pressure (MICP) method combined with other wettability measurement methods: (1) actual reservoir oil and brine are used, so that results are more representative of field conditions; (2) it is a non-destructive method so that core plugs can be reused to study wettability alteration caused by chemical additives; and (3) capillary pressure, wettability, and relative permeability can be obtained from the same set of measurements.

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