Determining CO2-brine relative permeability and capillary pressure simultaneously: an insight to capillary entrance and end effects

Abstract:

CO₂-brine relative permeability relations are important parameters in modeling scenarios such as CO₂ sequestration in saline aquifers and CO₂ enhanced recovery in oil reservoir. Many steady-state experimental studies on CO₂-brine relative permeability showed that the CO₂-brine relative permeability differs greatly from typical oil-brine relative permeability. Particularly, they reported a very small endpoint CO₂ relative permeability of 0.1~0.2 at a relative high residual water saturation of 0.4~0.6.

In this study, we hypothesize the measured low endpoint CO₂ relative permeability in previous studies was an experimental artifact that is primary due to low CO₂ viscosity. We conducted steady-state CO₂ drainage experiments by co-injecting equlibrated CO₂ and brine into a long (60.8 cm) and low permeability (116-mD) Berea sandstone core at 20 °C and 1500 psi. During every experiment, both the overall pressure drop across the core and the pressure drops of the five independent and continuous sections of the core were monitored. The in-situ saturation was measured with a medical X-ray Computed Tomography (CT) scanner. In the center three sections where saturation was uniform, we determined the relative permeability to both brine and CO₂ phases. In the entrance and exit sections, both measured pressure gradients and saturation were non-uniform. To cope with this, we make several self-consistent assumptions that reveal the nature of capillary entrance and effect in steady-state two-phase core flooding experiments. Based on these assumptions we determined the relative permeability to CO₂ and CO₂-brine capillary pressure simultaneously using measured pressure drops. We found: (1) a much higher endpoint CO₂ relative permeability of 0.58 at a water saturation of 48%, (2) the entrance region with non-uniform saturation expanded CO₂ relative permeability data to much lower water saturation, (3) the determined CO₂-brine capillary pressure curve is self-consistent and matches with an independent Mercury Injection Capillary Pressure (MICP) curve measured with a plug from the same Berea sandstone after appropriate scaling. Herein we propose a dimensionless number that estimates how much the measured overall pressure drop during steady-state two-phase flow is affected by capillary effects.