Stress controls on transport properties of the Mercia Mudstone Group: importance for hydrocarbon depletion and CO 2 injection

Harrington, J.F.; Graham, C.C.; Tamayo-Mas, E.; Parkes, D.. 2018 Stress controls on transport properties of the Mercia Mudstone Group: importance for hydrocarbon depletion and CO 2 injection. Marine and Petroleum Geology, 93. 391-408.

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The physical properties of the Mercia Mudstone Group (MMG) are of interest to Carbon Capture and Storage (CCS) in the UK, both in terms of the sealing capacity of certain horizons and in order to assess scenarios involving CO2 migration in the overburden above potential CCS sites. In this study, the hydromechanical properties of MMG samples from the Larne Basin, Northern Ireland, were directly measured under steady-state conditions. Test samples were found to be good seals, with hydraulic permeabilities ranging from ≈2.1 x 10−18 to 8.4 x 10−21 m2 (2.1 x 10−3 to 8.5 x 10−6 mD). A detailed examination of the consolidation behaviour of the material yielded values for compressibility, hydraulic permeability and specific storage, as a function of effective pressure. Consolidation testing also provided preconsolidation pressure values of between 30 and 37 MPa. Test data were fitted to a linear elastic model using a two-dimension finite element approach, yielding hydraulic permeability and Young's Modulus, as a function of effective pressure. Findings suggest localisation of flow, due to small-scale heterogeneity, may play a role even in relatively large test samples. Results also highlight the impact of methodology on resulting permeabilities and the importance of using values measured at boundary conditions appropriate for the specific application. Critical state envelopes were derived from test data and used to conduct a scenario analysis, considering a range of stress paths, to examine the impacts of depletion and reinflation during CO2 injection. Initial stress conditions, stress path gradient and caprock heterogeneity were all found to be influencing factors on the potential for yield during depletion and the resulting deformation mode. The response to CO2 injection is less clear, but will be impacted by the initial caprock permeability and resulting drainage response. An awareness of these controls on caprock performance during stress path changes may aid in the selection of depleted CCS sites with geomechanically favourable characteristics for reinjection.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 02648172
Date made live: 01 Oct 2018 10:47 +0 (UTC)

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