Abstract
The Lower Cretaceous Captain Sandstone Member of the Inner Moray Firth has significant potential for the injection and storage of anthropogenic CO 2 in saline aquifer parts of the formation. Pre-existing faults constitute a potential risk to storage security owing to the elevated pore pressures likely to result from large-scale fluid injection. Determination of the regional in situ stresses permits mapping of the stress tensor affecting these faults. Either normal or strike-slip faulting conditions are suggested to be prevalent, with the maximum horizontal stress orientated 33°–213°. Slip-tendency analysis indicates that some fault segments are close to being critically stressed under strike-slip stress conditions, with small pore-pressure perturbations of approximately 1.5 MPa potentially causing reactivation of those faults. Greater pore-pressure increases of approximately 5 MPa would be required to reactivate optimally orientated faults under normal faulting or transitional normal/strike-slip faulting conditions at average reservoir depths. The results provide a useful indication of the fault geometries most susceptible to reactivation under current stress conditions. To account for uncertainty in principal stress magnitudes, high differential stresses have been assumed, providing conservative fault-stability estimates. Detailed geological models and data pertaining to pore pressure, rock mechanics and stress will be required to more accurately investigate fault stability.
Highlights
The Lower Cretaceous Captain Sandstone Member of the Inner Moray Firth has significant potential for the injection and storage of anthropogenic CO2 in saline aquifer parts of the formation
Reactivation of previously stable faults caused by increasing pressure, and a reduction in the effective stress, could allow faults to become transmissive to buoyant fluids, such as supercritical CO2, due to the opening of flow pathways during failure (Streit & Hillis 2004). It is this aspect of fault stability that forms the focus of this study, with respect to the Captain Sandstone of the Inner Moray Firth, and utilizing an adaptation of the geological model presented by Jin et al (2012)
The stress tensor has been resolved onto 3D fault planes, derived from an existing geological model of the Captain Sandstone and its overburden, to evaluate the susceptibility of the larger faults to reactivation, the risk of which might be increased if pore-fluid pressure increases as a result of large-scale CO2 injection
Summary
A Shmin gradient of 18 MPa km–1 relative to the seabed appears to represent a suitable lower bound to the LOT data, and is taken here to represent Shmin in the fault-stability analysis (Fig. 8), the Tor Formation measurement indicates that the magnitude of Shmin is not a simple linear gradient through the overburden This gradient is similar to the assumed 18.09 MPa km– 1 fracture pressure gradient noted by Jin et al (2012). In the normal stress state, where SHmax = Shmin, faults dipping at 30° and striking in any orientation are those that are most susceptible to failure, with a required pore-pressure increase of approximately 5 MPa. Little change in the fault stability is seen once the poroelastic effect is considered, as the difference between the horizontal stress and Sv magnitude is reduced, resulting in a smaller Mohr circle (Fig. 12c). Such an in-depth study has been conducted for the proposed storage project at the Goldeneye Field (Shell 2011b), which found that geomechanical effects are not prohibitive to the proposed storage plans
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