Evidence of Water-Wet Behavior for Supercritical CO2: Drainage and Imbibition Capillary Pressure Measurements Using the Semi-Dynamic Method

Abstract This paper investigates measuring supercritical carbon dioxide (scCO2) and brine drainage and imbibition capillary pressure at high-pressure and high-temperature conditions (1450 psi, 90 °C and 4350 psi, 73 °C) using the semi-dynamic method with commercially available equipment. Drainage represents carbon dioxide injection process, whereas imbibition is involved in post-injection plume migration. Neither drainage nor imbibition capillary pressure measurements are readily available for supercritical CO2 using traditional porous plate and centrifuge methods due to mutual miscibility, corrosive nature of CO2 in aqueous environment, and limitations of standard equipment. We measure both drainage and imbibition capillary pressure of scCO2, and compare the results with known wettability fluid behavior, such as mercury injection and nitrogen-brine porous plate methods. We use a temperature-controlled recirculating steady-state coreflooding system with in-situ saturation monitoring capability to measure capillary pressure via the semi-dynamic method. The method is based on directly measuring face saturation in steady-state gas (or brine) flow at different rates and obtaining capillary pressure from pressure drop. The semi-dynamic method results for Berea sandstone are compared to mercury injection capillary pressure (MICP) and nitrogen-brine porous plate measurements on sister samples. We address mutual miscibility of scCO2 and brine via continuous recirculation capabilities of the coreflooding system. The semi-dynamic method allows the measurement of both positive (spontaneous) imbibition as well as negative (forced) imbibition of capillary pressure. We assess the wettability state of supercritical CO2 by evaluating capillary pressure curve shape, cross-over saturation, and final trapped gas saturation. The results show that the drainage capillary pressure curve of scCO2 and brine measured with semi-dynamic method agrees with air-mercury capillary pressure from MICP test. Spontaneous imbibition shows strong re-saturation of the core with water, showing strongly water-wet characteristics. Cross-over saturation at near-zero capillary pressure shows high water saturation consistent with trapped gas. Forced imbibition resulted in the same trapped gas value as spontaneous imbibition, i.e. vertical capillary pressure at trapped gas, indicating a water-wet state. Complications during the test include complex phase behavior, such as evaporation and dissolution fronts, despite best efforts made to equilibrate fluids. Fractional flow theory is employed to demonstrate that even small deviations in phase behavior, such as difference of inlet and outlet pressures due to flow, will result in developing evaporation and dissolution fronts. To the best of our knowledge, we are presenting the first complete set of spontaneous and forced imbibition capillary pressure for scCO2 in consolidated sandstone. Moreover, we show measurements for both drainage and imbibition capillary pressure. The shape of capillary pressure curves suggests that scCO2 is strongly water-wet under tested conditions. This observation allows us to potentially consider using more traditional and readily available testing to characterize scCO2 capillary pressure, such as nitrogen porous plate and mercury injection.