Rock property measurements and measurement techniques for low permeability cores

Abstract

The advancement of technology in producing tight gas sands requires not only the rock property measurements, but also, the advancement of measurement techniques. In low permeability cores, conventional techniques for measuring permeability, porosity, and capillary pressure do not provide accurate results in a reasonable time frame. The purpose of this research was to develop better techniques for measuring absolute permeability, total porosity, capillary pressure, and relative permeability. A new method for measuring permeability and porosity simultaneously in low permeability cores has been developed. An analytical solution has been derived and has been coupled with a history matching routine in order to compute both permeability and porosity from the same pressure transient test across a core. Our transient method has been verified with numerical simulation and with actual transient data. The simultaneous determination provides the means of studying the slot-pore geometry common to most tight sands. A study of gas slippage was performed using the transient test method. Several parameters, including molecular weight, net stress, and flow rate, were varied to determine their relationship with gas slippage. Gas slippage appears to be affected by the slot-pore geometry. Two-phase rock property measurements are important if one wishes to model the effects of the water upon gas recovery. The measurement of capillary pressure and gas relative permeability have been investigated in this study. The ultracentrifuge presents a practical method for determining capillary pressures in low permeability cores. Several cores were run and the capillary pressure data are presented. A new technique for measuring gas relative permeabilities has been developed in this research. Under conditions where the water is considered immobile and incompressible, the analytical solution developed during this research project can be used to match effective gas permeability and gas porosity simultaneously. The system presents a rapid means of generating the entire gas relative permeability curve. The results of these measurements on absolute permeability, porosity, capillary pressure, and relative permeability present a description of the rock properties for studying fluid flow in a typical tight gas formation. The system of measurements can be routinely performed on any low permeability core.