Abstract

The effective pressure sensitivity of gas flow through two shales (Bowland and Haynesville shales) and a tight gas sandstone (Pennant sandstone) was measured over the typical range of reservoir pressure conditions. These are low permeability rocks such as can be exploited as caprocks above reservoirs that might be developed to store compressed air, methane, hydrogen or to bury waste carbon dioxide, all of which may become important components of the forthcoming major changes in methods of energy generation and storage. Knowledge of the petrophysical properties of such tight rocks will be of great importance in such developments. All three rocks display only a small range in log10 permeability at low pressures, but these decrease at dramatically different rates with increasing effective pressure, and the rate of decrease itself decreases with pressure, as the rocks stiffen. The pressure sensitivity of the bulk moduli of each of these rocks was also measured, and used to formulate a description of the permeability decrease in terms of the progressive closure of narrow, crack-like pores with increasing pressure. In the case of the shales in particular, only a very small proportion of the total porosity takes part in the flow of gases, particularly along the bedding layering.

Highlights

  • Shales are of particular importance because their fine grain size and tight pore structure gives them a low matrix permeability and makes them excellent cap rocks for the containment of oil, water and gases

  • The first stage in a suite of permeability measurements covering a wide range of confining and pore pressures must be to take the sample to the maximum effective pressure to which it is to be exposed, to ensure closure of these inelastic cracks and pores up 279 to that pressure

  • The decrease of permeability with effective pressure is due to elastic closure of conductive cracks and pores, and this is expected to become more difficult as the porous material stiffens at higher pressure

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Summary

Introduction

Shales (laminated mudstones) are of particular importance because their fine grain size and tight pore structure gives them a low matrix permeability and makes them excellent cap rocks for the containment of oil, water and gases. This includes their future use as a sealant for the storage containment of fuel gases hydrogen and methane, compressed air storage and for the disposal deep underground of waste liquids and gases, including waste carbon dioxide. Compared to conventional reservoir rock materials (sandstones, limestones), shales are difficult to work with Their commonly laminated nature makes them often highly fissile, with a tendency to split along the layering. Coring and cutting operations for sample preparation are often difficult, and their physical properties (elasticity, mechanical strength, permeability, elastic wave velocities) are generally anisotropic

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