Summary

This paper presents the results of a series of carefully conducted experiments to characterize the stress-dependent permeability of tight oil and gas shales. Specifically, the applicability of the Continuous Rate of Strain Consolidation (CRSC) test to obtain continuous stress-dependent permeability data on tight shales is rigorously investigated. The CRSC method has been widely used for geotechnical testing of very compressible sediments but has not been tested for very stiff and low permeability rocks. The CRSC method allows only for drainage from one end of the test sample and fluid flow is blocked at the other end while the sample is being compressed at a constant strain rate. An analytical solution is used to calculate the permeability from an idealized pore pressure distribution within the test sample. The tests were performed on a sample of Mancos shale trimmed in a thin disc-shape using a modified high pressure triaxial cell. The paper shows that the CRSC method can be used to reliably obtain continuous stress-dependent permeability data during consolidation of very low permeability shales provided poroelastic effects are accounted for in the pore pressure dissipation calculation. The permeability values from the CRSC tests were found to be comparable to those obtained from the Pressure-Pulse Decay Permeability (PDP) and the Constant Pressure Gradient Permeability (CPGP) tests. Klinkenberg non-Darcy inertial flow, and pore pressure variations effects were included in the gas permeability calculations. The paper shows that the use of the CRSC method for the measurement of stress-dependent permeability of tight shales can significantly reduce testing times while providing very reliable data.

Introduction

Reliability of productivity evaluation of shale gas and oil reservoirs depends on accurate characterization of low permeability shale formations. It is observed that the permeability of shale is significantly stress-dependent (Spencer 1989; Jones and Owens 1980; Gutierrez et al. 2000; Nygaard et al. 2004). The shale permeability decreases due to increase of effective confining stress, but it increases when the shear stress acting on the shale becomes high to start creating shear fractures (Nygaard et al. 2005). The permeability of shale needs to be characterized at the expected stress range. The range of shale permeability is typically very low from to m2 (Brace 1984; Neuzil 1991). The wide range of shale permeability can be attributed to the variation of shale microstructure as well as porosity. One of the main features of shale microstructure is strong anisotropy of microstructure attributed to the planar clay minerals aligned in a specific direction. Such structural anisotropy is also related to the fissility of shales along the bedding planes. At a single porosity, the shale matrix can be different in the degree of structural anisotropy. In addition, lithification in shale matrix affects the pore structure. The intact shale permeability should be function mainly of effective stress, porosity, the degree of anisotropy and the degree of lithification. Due to large variations in microstructure, shale permeability can vary significantly, e.g., by three orders of magnitude, even at a single porosity (Dewhurst et al. 1999).

URTeC 1619487

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