Geochemical insights for CO2 huff-n-puff process in shale oil reservoirs

Abstract

CO2 huff-n-puff process appears to be important to unlock hydrocarbon resources from shale oil reservoirs after multi-stage hydraulic fracturing. While existing literature shows that CO2 diffusion plays a significant role in kinetics of oil recovery, few studies have been able to draw on any systematic research into fluid-shale interaction due to water uptake of CO2 in connate water (water carbonation), which governs fluids flow in fractures thus CO2 huff-n-puff performance. We therefore hypothesized that water carbonation would depress ion exchange process between oil and organic matter (OM) thus forming a more water-wet system. Moreover, water carbonation would also decrease the electrostatic forces between oil and edge charge on OM, thereby further promoting water-wet system. To test the hypothesis, we measured contact angles in non‑carbonated high salinity brine (HS), carbonated high salinity brine (CO2 HS) and carbonated low salinity brine (CO2 LS) at temperature of 25 °C and under pressure of 3000 psi. Moreover, a geochemical modelling was conducted to evaluate ion exchange and surface complexation reactions in three different brines. Our contact angle measurements showed that HS gave a contact angle of 130°, while CO2 HS and CO2 LS resulted a contact angle of 23.5° and 23.0°, suggesting a more water-wet system. Geochemical modelling shows that ion exchange reactions between oil and shale surfaces are dramatically depressed in carbonated brine. In particular, the bridges number between oil and shale surfaces decreases from 5.2 × 10−4 μmol/m2 to 5.3 × 10−6 μmol/m2 in carbonated brines, supporting the contact angle measurements. Moreover, the computed surface potential at oil and rock surfaces increases from around −40 mV to 150 mV, suggesting more repulsive forces in the presence of carbonated brine, further supporting contact angle results. This work reveals for the first time that water carbonation during CO2 huff-n-puff process likely triggers a hydrophilic shale surface, which may significantly affects multiphase flow in natural and hydraulic fractures in particular.