Core Tests for Relative Permeability of Unconventional Gas Reservoirs

Abstract

Abstract A combination of up to seven or more core tests provides a practical approach to krg and krw function definition even when absolute permeability is at the sub-microdarcy level. The method is designed to provide data needed to define function parameters required by traditional engineering equations for relative permeability. This is accomplished with cores by determining end point specific permeabilities, end point non-wetting phase trapping capacity, critical gas saturation, and a small but sufficient number of effective gas permeabilities to define both drainage and imbibition krg as functions of water saturation. Assessment of the krw function is handled variously and is described. This combination-method is used because traditional core tests involving somewhat continuous displacement are invalid or not practical when cores have absolute permeability approaching a microdarcy and lower. Conventional analysis of gas shale and tight gas sand cores provides a snap-shot of reservoir permeability. The more complete view of flow capacity needed by engineers and petrophysicists comes with the availability of gas-water relative permeability. Measured relative permeability data are used for reservoir simulation, production forecasting, interpretation of formation damage, and to develop detailed reservoir permeability versus depth profiles. Sample and test conditions are described. Data for several tight gas sand and gas shale cores are presented along with the required data integration steps to combine the separate test results and to generate relative permeability functions of water saturation. Results show these unconventional rocks can be characterized with well-known relative permeability equations and that traditional rock-property logic can be followed during data integration. The significance of this work lies in presentation of a novel yet practical solution for acquiring rock curves from unconventional rocks. It is believed to represent the first treatment of core tests for krg hysteresis in tight gas sands and of core-based development of relative permeability functions in gas shales.