ABSTRACT:

Underground storage of carbon dioxide is considered one of the major mitigation methods to limit global warming. The mechanical performance of shales above injection formations is a key factor in ensuring safe disposal and preventing the upward migration of CO2. Pore pressure increase in the reservoir during injection of CO2 into the subsurface will also influence surrounding rocks and the undrained pore pressure response of low-permeable caprocks overlying such reservoirs can be estimated from Skempton's pore pressure parameters. We present results from a laboratory study investigating the anisotropic stiffness and poroelasticity of the North Sea Draupne shale. From undrained triaxial compression tests on samples with different orientation relative to rock layering, a trend of increasing stiffness and less pore pressure generation is seen on samples oriented parallel with layering. Whereas no orientational variation is seen for Skempton's B parameter, A clearly depends on sample orientation.

1. Introduction

Carbon capture and subsequent storage into geological formations (CCS) is considered a necessary measure to reduce CO2 emissions into the atmosphere [1]. After CO2 has been injected into sedimentary basins at depth, the containment relies, among other things, on the integrity of the low-permeable caprock overlying such reservoirs (e.g.[2–4]). Stress changes following fluid injection can occur both within and outside the reservoir itself. Shales are common caprocks and are well-known for being mechanically anisotropic because of the distribution of platy clay minerals. In this study, we examine the undrained stiffness anisotropy from undrained triaxial compression tests on the North Sea Draupne caprock. Furthermore, the undrained pore pressure response of low permeable caprocks overlying injection reservoirs may be estimated from the empirical equations by Skempton [5]. Therefore, the orientational variation in Skempton's A and B parameters with sample orientations is evaluated from the test campaign on the Draupne shale.

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