Pressure constraints on the CO2 storage capacity of the saline water-bearing parts of the Bunter Sandstone Formation in the UK Southern North Sea

The Bunter Sandstone Formation (BSF) in the UK sector of the Southern North Sea is thought to have a significant potential for the injection and storage of anthropogenic CO2 within periclines that lie above salt domes and pillows formed by halokinesis in underlying Zechstein strata. During the formation of the periclines, the BSF and its overlying top seals were subjected to extensional stresses and, in consequence, are commonly cut by seismically resolvable faults that present a risk to the containment of gas and buoyant fluids such as supercritical CO2. Although most of the closed structures in the BSF are saline water-bearing, eight gas fields (total gas initially-in-place >72 bcm (billion cubic metres)) have been discovered to date. The seismically resolved structure of these gas fields demonstrates that two different top seals, the Haisborough Group and the Speeton Clay, can seal gas columns of up to 128 and 104 m respectively, despite the presence of faults with small displacements above the field gas–water contacts. The observed gas columns are equivalent to CO2 columns of up to around 100 m in height. Simple geomechanical modelling suggests that existing optimally orientated faults may dilate or be reactivated if the pore-fluid pressure increase as a result of CO2 injection exceeds a gradient of about 13.4 MPa km–1, potentially resulting in loss of storage integrity.