Apparent permeability of gas shales - Superposition of fluid-dynamic and poro-elastic effects

•Experimental measurements of apparent gas permeability on gas shales.•Pitfalls in the evaluation of high pressure apparent gas permeability data.•Terzaghi's principle is not valid for permeability of gas shales.•A permeability minimum occurs in the Pp range from 2 to 10 MPa.•Poro-elastic and fluid-dynamic effects are superposed (0 to >20 MPa Pp).

The permeability of low-permeable gas shales is affected by both, fluid-dynamic (slip flow) and poro-elastic effects over a large pore pressure range. To analyse and separate the influence of these superposed effects, an apparent permeability model has been set up. The model's poro-elastic and fluid-dynamic parameters were adjusted simultaneously to match own experimental data for an intact Bossier Shale (“matrix”) sample, a fractured Haynesville Shale sample and previously published literature data.The effective stress-permeability relationship can only be described by a modified effective stress law:σ′=Pc-χPpHere the fitted permeability effective stress coefficients χ, were consistently ≤1, indicating that pore pressure has a lesser influence on effective stress than confining pressure. Fluid-dynamic gas slippage effects were found to be significant up to pore pressures of 20 MPa in low permeable (<10 μD) matrix samples.Pitfalls in the separation of fluid-dynamic and poro-elastic effects are wrong a priori assumptions. These are neglecting gas slippage above a certain pore pressure and assuming effective stress conditions to be constant in the Klinkenberg evaluation. Ignoring gas slippage in the evaluation of stress effects results in underestimation of χ values whereas undetected stress effects (by wrong a priori χ values) lead to incorrect predictions of the fluid-dynamic effects with increasing pore pressures.The predictions of the apparent permeability model were validated and checked for consistency and plausibility by (1) visualization in a k(Pp, Pc) diagram, (2) preparation of Klinkenberg plots over large pore pressure ranges (>10 MPa) and (3) analysis of the different slippage behaviour of He and Ar.The apparent permeability model predicts that during depletion of a shale gas reservoir apparent permeability passes through a minimum in the pressure range from 2 to 10 MPa due to the transition from a poro-elastic to a fluid-dynamic dominated realm.