Pore radius and pore surface wettability affect capillary pressure and relative permeability functions which, in turn, affect fluid distribution in the flow channels and the outcome of improved and enhanced oil recovery in the reservoir. The contact angle between each pair of fluids and the pore surface is the main wettability determinant which is the focus of this paper; thus, we present measured contact angles for 16 samples from five unconventional reservoirs in the US and we correlate mineralogy of the facies and the crude oil composition to the reservoir rock wettability. Beyond enhanced oil recovery (EOR), the results may be significant in managing CO2 flooding and in determining the fraction of the sequestered CO2 in carbon capture utilization and storage (CCUS). Primary production, IOR, and EOR are affected by wettability and wettability alterations during the reservoir's life as a result of reservoir production methods used.
Contact angles were measured using a drop shape analyzer (DSA) at the reservoir temperature and pressure for reservoir samples from Wolfcamp, Eagle Ford, Niobrara, Codell, and Bakken: 1) unaged core slices surrounded with formation brine, 2) aged core slices surrounded with formation brine, and 3) aged core samples surrounded with formation brine and CO2. Core slices of 1-in width were prepared and polished using different grades of sandpaper and cleaned using toluene, chloroform, and methanol in a Soxhlet extractor, and dried in the oven. Sample saturation was accomplished in an ultrafast centrifuge.
Results show that the carbonate-dominated facies from Wolfcamp-A formation and Niobrara A-Chalk show a relatively higher contact angle even when the core sample is cleaned and is not aged. However, Eagle Ford, Niobrara B-chalk and C-chalk, Codell, Bakken, Three Forks indicate strong water-wet behavior when unaged as expected in ambient conditions. The same characteristics were observed for aged samples from the Eagle Ford. The results indicate the need for studying the wide variations for the formation facies and the need for evaluating the wettability of the reservoir facies prior to any EOR application in unconventional reservoirs, especially in Wolfcamp formation, where the facies mineralogy and wettability are significantly different in almost every foot.
