Mark W. Fellgett

Carbon dioxide storage in the Captain Sandstone aquifer: determination of in situ stresses and fault-stability analysis

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. 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.

Borehole Imaging and its Impact on Unconventional Reservoir Characterisation in the United Kingdom

New exploration of unconventional reservoirs in the United Kingdom is seeking to establish the scale of potential hydrocarbon reserves. This presents both new opportunities for development of economic resources but also new challenges over whether such such resources can be developed in densely populated regions with societal license. The British Geological Survey is undertaking a review of the national in-situ stress regime to identify its potential implications on both the safety and efficacy of any future hydraulic fracturing operations. This is a unique application of imaging technology to undertake national scale investigation with potentially highly controversial impacts.