Influence of Effective Pressure on Shale Matrix Permeability: Implications for Shale Gas Production

Abstract

With the onset of the shale gas revolution, enhancing production from unconventional reservoirs has become the subject of intensive investigation worldwide. Even with large amounts of gas in place, achieving economic flow rates through such low permeability strata is a challenge. Despite permeability having a large effect on estimates of gas-in-place and productivity, its high stress-sensitivity and anisotropy (observed from measurements on shale samples) are often overlooked during analysis and interpretation of well test results. This study explores the link between lab-measured mechanical properties and sedimentological, diagenetic and burial histories of shales. Measured here, are the effects of both microstructure and changes in effective stress on matrix permeability. Samples of Jurassic Whitby Mudstone comprise silt-bearing clay-rich shales with 6-9% porosity and >2% TOC. They exhibit a planar microfabric identified as elongate clay rich lenses and oriented mica grains. Unconfined acoustic velocities and permeability measurements attest to this transverse isotropy. Permeability (k) was measured as a function of effective pressure (P_eff) for flow of argon using the oscillating pore pressure method. Pressure cycling between P_eff = 10-70 MPa was repeated on each sample until a reproducible pattern of permeability variation was observed. Pressure cycling initially reduced the permeability by 2-3 orders of magnitude, where it stabilised at ∼10^-18 m (at P_eff 10 MPa). After cycling, permeability of core plugs parallel to the foliation varies with P_eff according to k = 10^-18x P_eff ^(−1.2). Over this pressure range pore closure is purely elastic and there is no permanent pore collapse. Results show that a meaningful description of the permeability can only be obtained after several pressure cycles have been applied, to recompact the rock to its state at depth. Removal of shale from depth inevitably results in some degree of volumetric expansion and consequent increase in permeability. After recompaction, variation of permeability with P_eff is reproducible and can be used to model the reduction in gas flow to be expected as a result of the increased P_eff that results from in-situ pore pressure decay during gas extraction. Such insight will improve the calculation of in-situ effective stresses and their effect on permeability, and is thus integral to optimising models of reservoir depletion.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014