Measurements of capillary pressure and electric permittivity of gas-water systems in porous media at elevated pressures

Abstract

Sequestration of CO2 in aquifers and coal layers is a promising technique to reduce greenhouse gas emissions. Considering the reservoir properties, e.g. wettability, heterogeneity and the caprocks sealing capacity, the capillary pressure is an important measure to evaluate the efficiency, the success and the safety of storage applications. In this research, the capillary pressure behavior was investigated for the CO2-water system in quartz and coal. Measurements were conducted at pressures and temperatures, ranging from ambient to reservoir conditions, where CO2 is present as supercritical fluid. Furthermore, the relation between capillary pressure hysteresis and interfacial area was investigated, measuring the capillary pressure and the electric permittivity as a function of frequency, simultaneously. The results from the quartz samples, showed a dependence of the capillary pressure on the CO2 pressure. Moreover, only dissolution rate effects for gaseous CO2 in the water were observed. Significant capillary pressure fluctuations and negative values during imbibition were observed at near supercritical conditions. From the coal experiments it was observed that with increasing CO2 pressures the wettability of medium rank coal altered from water-wet to CO2-wet. High rank coal was CO2-wet during primary imbibition experiments in the entire pressure range. The relation between capillary pressure and interfacial area has been investigated by measuring the capillary pressure and the electric permittivity at 100 kHz as function of the water saturation. The permittivity data showed hysteresis between drainage and imbibition. Furthermore, non-monotonic behavior was observed which was attributed to polarization of the gas-water and water-solid interfaces. The permittivity hysteresis is provoked by the different phase distributions and geometries. From these results it was concluded that the capillary pressure is a unique function of the permittivity and the water saturation.