Laboratory Measurements Relevant to Two-Phase Flow in a Tight Gas Sand Matrix

Abstract

Abstract True drainage measurements of specific water permeability, gas entry pressure, critical gas saturation and effective gas permeability at in-situ stress conditions have been conducted on several tight sandstones, mostly from the Mesaverde formation. The results have been used to develop some general rules useful in making meaningful estimates of both drainage and imbibition relative permeability curves for use in reservoir simulation. Introduction Conventional core analysis measurements are unable to provide adequate data on gas-water capillary pressure and relative permeabilities at reservoir confining stress conditions for use as inputs in numerical simulation of flow in a tight sand matrix. Difficulties in measurement are caused primarily by the high capillary pressures and extremely low flow rates encountered in work with tight sands. In recent years, both the pulse technique and the steady state technique have been used to make gas permeability measurements on rocks having permeabilities as low as 1 microdarcy. The same techniques have been used for measuring effective permeability at different water saturation levels, evaporation being used to vary the water content of the sample. By far the most popular techniques for obtaining capillary pressure data are mercury porosimetry and the centrifuge technique, neither which are conducted with a confining stress on the sample. It is obvious that the reservoir engineer seeking capillary pressure and relative permeability inputs for tight sands simulation is faced with laboratory data that are inadequate either in terms of stress conditions, or in having pore water distribution that can be classified neither as true drainage nor as true imbibition. More recently a technique has been described for making coordinated gas-water capillary pressure and effective gas phase permeability measurements on tight sands at in-situ net stress and under conditions of true drainage. While this technique provides data under very well controlled conditions, movement of water in tight sands is so slow that capillary equilibration times of as much as a week may be needed for each data point. Such long scale experiments may be too slow for providing data within a reasonable time frame for some applications. One objective of this study was to develop techniques for making relevant measurements more quickly in the laboratory. In the context of two-phase flow, attention was focused on measuring specific permeability to water, gas entry capillary pressure, critical gas saturation, and effective gas phase permeability at gas saturations near critical. It was felt that these measurements, when combined with dry sample Klinkenberg data and mercury porosimetry data would provide a good basis for making reasonable estimates of capillary pressure and relative permeability curves under reservoir conditions. A second objective of the study was to work towards building up a data base and understanding of the phenomena with the ultimate aim of arriving at guidelines or correlations which could be used to estimate reservoir rock properties based on only a limited amount of experimental data. Such an approach would be useful in situations, ouch as at the well site, where it may not be possible to wait for results from the laboratory. EQUIPMENT AND PROCEDURE Drainage Experiments The measurements of water permeability, gas entry pressure, critical gas saturation and effective gas phase permeability under true drainage conditions were made in an apparatus called the EXTRA (Experimental Tight Rock Apparatus). P. 125^