Factors Affecting Gas Slippage in Tight Sandstones of Cretaceous Age in the Uinta Basin
Authors: Sampath K. , Keighin C. William 
Published: December 2007

Summary
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 lime 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, micro porosity 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, were not found in samples in which the epoxy was cured under a confining pressure of 34.5 MPa.

Introduction
The concept of slip during gas flow through capillaries with diameters small enough to be comparable to the mean free path of the gas was used by Klinkenberg to explain the discrepancies between gas and liquid permeabilities of porous media. He presented the following equation, which relates the apparent gas permeability, kg, of a gas flowing at a mean pore pressure, to the true permeability of the porous medium, k g.
..........................(1)
In this equation, b is a constant and is a function of the capillary radius, rb, and of the mean free path of the gas, at pressure .
...................................(2)
In Eq. 2, c is a constant approximately equal to one. Loeb showed that Poiseuille's law for viscous flow can be modified to account for slip and defined a coefficient of slip that can be shown equal to the term c in Eq. 2. As is evident from Eq. 2, the contribution resulting from slip is inversely proportional to the capillary radius. At mean pore pressures usually employed in the laboratory measurements of permeability. the slip effect is significant in the case of tight sands because of the small pore sizes. The dependence of tight sandstone permeability on confining pressure and partial water saturation has been documented by numerous investigators. It appears reasonable, therefore, that the slip effect also would be affected by confining pressure and water saturation. A series of experiments was conducted in conjunction with petrographic observations in an attempt to understand the possible alterations in pore dimensions that result from confining stress and partial water saturation. Samples investigated were from Well MAPCO RBU 11-17 F (Section 17, T10S, R20E) in the Uinta Basin, UT. They are petrographically similar to other low-permeability sandstones of late Cretaceous age from the region.