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.

Influence of Normal and Shear Stress on the Hydraulic Transmissivity of Thin Cracks in a Tight Quartz Sandstone, a Granite, and a Shale

“…These effects are well known from previous experimental studies (e.g., Chen et al, 2015;Cuss et al, 2011;Engelder & Scholz, 1981;Guo et al, 2013;Kranz et al, 1990;McKernan et al, 2017;Tanikawa et al, 2010;Zhang et al, 2013Zhang et al, , 2015Zimmerman & Bodvarsson, 1996). In experiments designed to investigate the effect of low amplitude fluid pressure oscillations of seismogenic origin on permeability of Berea sandstone, Elkhoury et al (2011) and Candela et al (2015) reported permeability enhancements by up to 50% following fluid pressure oscillations of 20 s period for 3 min.…”

“…At 10 MPa gas pore pressure also there is no significant enhancement of apparent permeability through the Klinkenberg effect (McKernan et al, 2017). The downstream volume measured for the matrix permeability measurements using gas on intact rock was 319 mm 3 and for the tests on the faulted samples was 440 mm 3 .…”

Hydraulic and Poroelastic Rock Properties From Oscillating Pore Pressure Experiments

“…The effective pressure dependence of permeability and specific storage is often neglected in fluid flow simulations, which may introduce significant errors in simulations. Hence, the knowledge of effective pressure behavior of these rock properties is critical in reservoir characterization and simulation (McKernan et al, , ; Hasanov & Prasad, ). As it has been suggested by numerous authors (Batzle et al, ; Guéguen et al, ; Walsh, ), microcracks significantly influence transport and elastic properties of rocks.…”

Fluid Injection Experiments in Shale at Elevated Confining Pressures: Determination of Flaw Sizes From Mechanical Experiments

“…For the model of Ito and Hayashi (1991), the T values were taken from Chandler et al (2016). McKernan et al (2017) found a mean value of = 0.71 ± 0.22 in Whitby mudstone during permeability measurements, and values of ≃0.7 were assumed in the other materials tested here. In our experiments 2 = 3 and P pore = 0, and the breakdown pressure according to Hubbert and Willis or Haimson and Fairhurst (from equations (2) and (3), respectively) should therefore be purely a function of the confining pressure for a material of known T , , and .…”