This work is a novel attempt to analyze rock/brine and oil/brine interactions in heterogeneous unconventional liquid reservoir rocks and the effect of these interactions on oil recovery. There is very limited literature on the impact of brine salinity on shale wettability alteration and spontaneous imbibition experiments. This study includes a comprehensive approach that analyzes rock-fluid interactions through contact angle and zeta potential measurements followed by imbibition studies monitored by CT scan technology to understand the effect of salinity on CT penetration profiles and the resultant oil recovery.
The rock samples were obtained from sidewall coring of an ultra-tight liquid rich shale in South Texas composed predominantly of carbonate while sodium chloride and calcium chloride brines of varying concentrations were used as testing fluids. Contact angle and IFT experiments were conducted on unaltered samples aged in oil obtained from the same well the sidewall samples were retrieved. The captive bubble method was used to measure the contact angles of oil droplets on the rock while IFT was measured using the pendant drop technique. Zeta potentials were measured to assess the stability of thin brine films on the powdered rock samples and oil droplets. Finally, spontaneous imbibition was carried out at optimum salt concentration that resulted in maximum water wetness to measure oil recovery while Computed Tomography (CT) based imaging was used to analyze imbibition profiles and quantify penetration of fluids into the rock.
Experimental results suggest that both sodium and calcium chloride were able to alter wettability of samples from an initial intermediate-wet state to water-wet with the maximum water wetness observed for sodium chloride. Increasing salt concentration also lowered the crude oil/brine IFT marginally. Zeta potential measurements suggested sodium brine films were more stable compared to calcium brines while double layer expansion does not completely explain the wetting changes observed. Sodium and calcium brines at optimum salinities recovered more oil compared to water alone while maximum recovery was obtained with sodium chloride which also penetrated the most into the rock matrix as found by analyzing the CT scan images. This study therefore validates the potential of affordable low salinity injection brines which have the capability to alter shale rock wettability and improve oil recovery by penetrating deeper into the reservoir rock.