Hysteretic trapping and relative permeability of CO2 in sandstone at reservoir conditions

Abstract

Hysteretic nonwetting phase trapping behavior is predicted to be a significant factor determining long term immobilization of injected CO2. This paper presents results from an experimental investigation of hysteresis in residual trapping and relative permeability of CO2 in a CO2/water system at 50°C and 9MPa in a Berea sandstone core. Carbon dioxide saturation data were collected using the steady-state method in a horizontal core-flooding apparatus with X-ray computed tomography. Three flooding cycles were completed at a constant total volumetric flow rate by incrementally increasing and decreasing the fractional flow rates of supercritical CO2 and water. The fractional flow rates were chosen such that the maximum CO2 saturation of each cycle was greater than that of the previous cycle. During the injection stage of a cycle, CO2 displaced water in a wetting-phase drainage process (water saturation decrease). Following injection, water displaced CO2 in a wetting-phase imbibition process (water saturation increase). The fractional flow of CO2 was then increased during the next cycle. Results showed the CO2 saturations trapped during wetting-phase imbibition increased with the maximum CO2 saturations reached during each cycle. The residually trapped CO2 saturation is a hysteretic function, depending on the maximum saturation reached by the core before imbibition. A linear model with coefficient 0.5 describes the nonwetting trapping relationship. The experimental relative permeability data for CO2 fit a single bounding primary drainage curve while CO2 saturations increased, but followed three different scanning curves during wetting-phase imbibition. The CO2 relative permeability data can be fit by making a minor modification in the Van Genuchten–Burdine relative permeability data to account for hysteresis.