Experimental measurement of the exclusion-diffusion potential in sandstone and shaly sand samples at full and partial water saturation

Abstract

The exclusion-diffusion potential (EDP) arises in response to concentration gradients and is one of the key components of the self-potential (SP) observed in subsurface environments. However, few data are available to characterize the EDP in reservoir rocks saturated with complex natural brines and at partial brine saturation. We report experimental measurements of the EDP across fully and partially brine-saturated sandstone samples and fully brine-saturated shaly sand samples, using natural saline brine (2.2 Mol/L), seawater (0.5 Mol/L), and natural crude oil. When fully saturated, the EDP across the samples is diffusion-dominated, with the shaly samples exhibiting a small contribution from charge exclusion. The contribution of charge exclusion to the measured EDP increases as permeability decreases. At the residual oil saturation, the measured EDP is diffusion-dominated, consistent with our findings at full saturation. However, the EDP is more exclusive at the irreducible water saturation, and the polarity of the excluded ions depends on the polarity of the charged oil-brine interface. A charge exclusion is more significant at lower irreducible water saturation. Our results support recent studies suggesting that the oil-brine interface can be positively charged in contact with natural brines. The EDP trends reported have not been observed previously and have broad implications for geophysical monitoring of subsurface flow and transport, such as transport of nonaqueous phase contaminants in aquifers and water flow during hydrocarbon production.