Pore-scale Analysis of In Situ Contact Angle Measurements in Mixed-wet Rocks: Applications to Carbon Utilization in Oil Fields

Abstract

Carbon utilization in depleting oil reservoirs is considered as an important component in achieving the widespread commercial deployment of carbon capture and storage (CCS) technology. With absent strong climate policy, CO2 enhanced oil recovery (EOR) process adds a significant revenue stream and makes the subsequent carbon storage process economically feasible. This is reflected in the great majority of CO2 storage projects in oil fields utilizing CO2 for EOR. Therefore, early deployment of subsurface carbon dioxide storage is likely to focus on injection into depleted or producing oil fields. Observations and modelling have shown that capillary trapping of CO2 through capillary forces within the pore space of the water-wet rocks, typical of subsurface saline aquifers, is one of the most significant mechanisms for storage capacity. This important storage process is also a factor determining the ultimate extent of CO2 plume migration within the reservoir, enhancing the security of the storage process. In contrast, carbonate oil reservoirs are characterized by a mixed-wet state in which the capillary trapping of nonpolar fluids have been observed to be significantly reduced relative to trapping in water-wet rocks typical of saline aquifers unaltered by the presence of hydrocarbons. This study discuss the first observations of supercritical CO2 in a mixed-wet carbonate rock. Here we show that residual CO2 trapping of supercritical CO2 in a limestone altered to a mixed-wet state with crude oil is significantly less than trapping in water-wet systems characteristic of saline aquifers. The initial-residual CO2 saturations characteristic curve are reported for each system from core scale observations. While pore scale observations provided the first in situ contact angle measurements of supercritical CO2 in a mixed-wet rock and pore to pore arrangements of CO2 droplets. The measurements were compared with trapping of N2 and was similar in the water-wet rock. After altering the sample's wettability to a mixed-wet, however, trapping of CO2 was much less than N2. Here we provide a theoretical explanation of the different response in trapping between CO2 and N2 in the mixed-wet rock.