Factors Affecting Gas Slippage in Tight Sandstones of Cretaceous Age in the Uinta Basin

The gas permeability of some sandstones from Uinta County, UT was measured in the laboratory. The effect of confining pressure and partial water saturation on gas permeability was investigated. Measurements were conducted at pressures of up to 34.5 MPa and partial liquid saturations of up to 60%. We noted that the slope of the straight line relating apparent gas permeability to reciprocal mean pore pressure decreased with increasing net confining pressure and with increasing partial water saturation. Gas permeabilities extrapolated to infinite pore pressure were often higher than liquid permeabilities under the same confining pressure conditions; scanning electron microscopic (SEM) observations indicated rock/fluid interactions. Geologic studies of end pieces cut off from core plugs revealed that porosity is reduced by quartz overgrowths, authigenic clays (kaolinite, illite, and minor quantities of chlorite), carbonate cements (calcite and dolomite), and physical compaction of labile rock fragments. In some samples, organic-rich laminae were found to lie parallel to bedding surfaces. Apparently, microporosity had been formed by partial leaching of feldspars and rock fragments. Most of the pores were lined with authigenic clays that form highly irregular pore walls. Thin film intergranular pores, which were observed in samples impregnated with blue epoxy at ambient pressure,more » were not found in samples in which the epoxy was cured under a confining pressure of 34.5 MPa.« less