Rigorous Modeling for Data Analysis towards Accurate Determination of Shale Gas-Permeability by Multiple-Repeated Pressure-Pulse Transmission Tests on Crushed Samples

Abstract

Determination of the nanodarcy gas permeability and other parameters of shale by pressure-pulse transmission testing of core plugs, drill cuttings, and crushed samples is discussed. The methods available for interpretation of pressure-pulse decay tests are reviewed with emphasis on the difference between the intrinsic and apparent permeability. Improved formulation and analysis which honor the relevant physics of gas transport and interactions of flowing gas with the shale under the pore-proximity and elevated pressure conditions are presented. Modification of the shale and fluid properties under prevailing stress, and pore-size distribution, connectivity, and confinement conditions is shown to be important under any pressure conditions while the gas rarefaction and slippage effects diminish essentially at high pressures but become important at low pressures. The permeability and other parameters of shale samples are determined by numerical modeling and analysis of the pressure tests conducted under different conditions in order to accommodate for temporally and spatially variable conditions. Better design and analysis of experiments for simultaneous determination of several unknown parameters that impact transport calculations, including stress-deformation, adsorption, diffusion, and deviation from Darcy flow are described. The inherent limitations of the earlier methods which rely on the approximate analytical solutions of the simplified diffusivity equation based on the Darcy's law are delineated. It is pointed out that the permeability measured using a Darcy-type equation is the apparent permeability and not the intrinsic permeability. Thus, the primary reason for the contradictory values of permeability measured by application of the analytical models is explained by dependence of the permeability of shale to different testing conditions over which only different average permeability values can be obtained when applying the approximate analytical solutions obtained based on the assumption of a constant permeability value.

Experimental Investigation on Permeability and Porosity Hysteresis in Low-Permeability Formations

“…Transient permeability measurement approach can be performed on crashed cores, commonly known as Gas Research Institute (GRI) method (Luffel et al, 1993;API 1998;Cui et al, 2009;Passey et al, 2010;Profice et al, 2012;Tinni et al, 2012;Heller, et al, 2014;Civan and Devegowda, 2014), or on intact cores Lin, 1977;Haskett et al, 1988;Dicker and Smith 1988;Jones 1997;Akkutlu and Fathi, 2012;Mokhtari and Tutuncu, 2015;Jin et al, 2015;Zhou et al, 2016). Unsteady state (transient) methods are the alternatives to measure micro and nano-Darcy permeability formations at a reasonably short time.…”