Abstract

Detailed discussions on the poroelastic behavior of stiff shales are provided with the aim of developing a fast reliable method to characterize stress-dependent shale permeability using the constant rate of strain (CRS) consolidation test. Shales usually have dense stiff matrices composed of soft grains. Characterizing the hydro-mechanical properties of shales in fully-saturated condition is challenging due to their very low permeability. For reliable development of unconventional reservoirs in terms of borehole stability and hydraulic fracturing, the effect of hydromechanical interactions between shale matrix and pore fluid is an important factor. Thus, fast, reliable and simple test methods to characterize the hydro-mechanical properties of saturated shales are needed.

Applying the CRS test standardized for soft soils to shales confronts some critical technical issues. Satisfying the required test condition, that is one-dimensional consolidation with satisfactory one-dimensional fluid flow in test sample, is highly difficult for shales. Thus, the consolidation is carried out in isotropic stress condition. Stress-dependent anisotropy of deformation of tested shale attributed to their layered structure is evaluated based on the stiffness matrix determined in a previous study. In the standard CRS procedure, the permeability during CRS test is related to the strain rate, sample thickness, and excess pore pressure. However, in shales, non-negligible compressibility of solid grains is considered to cause a certain loss of excess pore pressure, which is not considered in the standard procedure. In this study, the loss of excess pore pressure is compensated by assuming that the ratio of excess pore pressure for a porous medium composed of compressible solid grains to that for a porous medium having negligible solid grain compressibility is equivalent to the value of Skitalicpton's pore pressure coefficient, B, of the former medium. To extract the B-value of shale sample from that experimentally-determined in the presence of porous metal filters, the B-value of multi-layered porous media is formulated in relation to the fluid saturation, porosity, Biot coefficient, and solid grain bulk modulus of each layer. The extracted B-value based on the estimated value of fluid saturation of sample, 0.94, overcompensates the loss. It is found that the CRS permeability values obtained by assuming full saturation are highly consistent with the steady-state permeability values.

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