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Investigating Controls on the Transport Properties of Mudstone: Implications for Shale-Gas Production

Abstract

Knowledge of mudstone permeability and sensitivity to stress is required to enhance interpretation of well logs and subsurface variability. This project provides an integrated study of both lab-measured permeability and mechanical properties, and sedimentological, diagenetic and burial histories of mudstones. Samples tested include a Jurassic mudstone (Whitby Mudstone Formation), a clay-bearing, silt-rich mudstone with 6–9% porosity, 1.5% TOC and an anisotropic texture. Samples from the carbonate-rich Eagle Ford Formation and clay-rich Marcellus Formation are also being tested to examine the relationship between permeability and porosity and the microstructural arrangement and elasticity of the component mineral phases in these different mudstones. Permeability was measured as a function of effective pressure (Peff) for flow of argon across 25 mm diameter cylindrical samples, using the oscillating pore pressure method. Alongside experiments, petrographical characterization of samples is used to explore the geological controls on fluid transport properties of mudstones. A large number of experiments have been performed that demonstrate the effect of pressure cycling on mudstone permeability and as a result the intrinsic sensitivity of permeability to variations in effective stress. Further, the validity of the effective pressure concept as defined by Terzaghi (1923) as simply the difference between total confining pressure and pore pressure has also been investigated by comparing variations of pore pressure under constant confining pressure with variations of confining pressure under constant pore pressure. Crucial to gas reservoir evaluation, results show that after reconditioning, variation of permeability with Peff is reproducible, and can be used to model the reduction in gas flow to be expected as a result of the increased Peff that results from in-situ pore pressure decay during gas extraction. Additionally, the influence of (hydrostatic) effective confining pressure on permeability, defined by the traditional effective stress law, must be modified by an additional term that depends only on pore pressure, of the form log k = A + Peff + b Pp where k is permeability, Peff is effective pressure, Pp is pore pressure and A and b are parameters. Such an approach is essential to the realistic application of laboratory-determined permeability data to gas reservoir evaluation.