Microstructural controls on the pressure-dependent permeability of Whitby Mudstone

Abstract

A combination of permeability and ultrasonic velocity measurements allied with image analysis is used to distinguish the primary microstructural controls on effective-pressure dependent permeability. Permeabilities of cylindrical samples of Whitby Mudstone were measured using the oscillating pore pressure method at confining pressures ranging between 30-95 MPa and pore pressures ranging between 1-80 MPa. The permeability-effective pressure relationship is empirically described using a modified effective pressure law in terms of confining pressure, pore pressure and a Klinkenberg effect. Measured permeability ranges between 3×10 -21 m 2 and 2 ×10 -19 m 2 (3 and 200 nd), and decreases by ~1 order of magnitude across the applied effective pressure range. Permeability is shown to be less sensitive to changes in pore pressure than changes in confining pressure, yielding permeability effective pressure coefficients ( ) between 0.42 and 0.97. Based on a pore-conductivity model which considers the measured changes in acoustic wave velocity and pore volume with pressure, the observed loss of permeability with increasing effective pressure is attributed dominantly to the progressive closure of bedding-parallel, crack-like pores associated with grain boundaries.Despite only constituting a fraction of the total porosity, these pores form an interconnected network that significantly enhances permeability at low effective pressures.The pre-publication reference is: Mckernan, R., Mecklenburgh, J., Rutter, E. H. and Taylor Progress in understanding fluid transport properties of mudstones is currently hindered by a scarcity of published experimental data. However, the ongoing expansion of the hydrocarbon industry into low-permeability unconventional resources is driving the demand for research and development in this field. Even when hydraulic fracture treatment is used to enhance production, flow of hydrocarbons into the fractures will be controlled ultimately by the microporous, low permeability matrix. Furthermore, during reservoir production pore-fluid pressure is progressively reduced, which acts to increase the in-situ Terzaghi effective pressure (defined as overburden pressure minus pore pressure), thereby decreasing permeability. The evolution of permeability of the matrix therefore determines the prediction of long-term production, which must take into account the effects of flow regime, multiphase flow, sorption effects, permeability anisotropy and, most importantly, the pressuredependence of permeability.Laboratory measurements of permeability of intact mudstone samples performed under reservoir conditions has yielded values between 10 -22 m 2 and 10 -12 m 2 (0.1 nD and 1 D) for flow both parallel and normal to layering (Morrow et al.