Measurement of Hysteretic Shale Capillary Pressure – Saturation Relationships using a Water Activity Meter

Capillary pressure is the pressure difference across the interface of two immiscible fluids within a porous medium due to the interfacial tension between fluids and is related to both the properties of the fluids and the porous medium. Capillary pressure within a porous medium will change depending upon its degree of saturation. Understanding the relationship between capillary pressure and saturation for a rock allows for the modeling of multi-phase flow. Many traditional methods of measuring capillary pressure are unsuitable for the characterization of shale due to their inability to measure the high capillary pressures found within the small pores. Furthermore, the mercury injection method used to determine shale pore-size distribution may be problematic due to both compression and contamination of the sample, as well as difficulty in converting the mercury capillary pressure to reservoir fluid capillary pressure. A possible alternative to the mercury injection method is the water activity meter which has been utilized extensively in the soil sciences for measuring capillary pressure. However, its application to lithified material has been limited. This study used a water activity meter to collect capillary pressure measurements (ranging from 2-200 MPa) at several saturation levels (ranging from 10-100%) for seven types of oil and gas producing shale. Nonlinear regression was used to fit the capillary pressure-saturation data for each shale type to the Brooks and Corey model which describes the relationship between capillary pressure and saturation using four parameters. Six of the seven shale types investigated were successfully parameterized indicating that the water activity meter may be a viable method for characterizing the capillary pressure-saturation relationship of shale for inclusion in numerical reservoir models. There were no significant differences between the wetting and drying Brooks and Corey parameters for the different shales, indicating that hysteresis was not a major factor. As expected, the different shale types had significantly different Brooks and Corey parameters for a given drying/ wetting regime. Bulk density, matrix density, and porosity measurements were also made on each shale type. These properties were correlated with total organic carbon content and were also statistically different between the examined shale types.