Impact of in situ stress and fault reactivation on seal integrity in the East Irish Sea Basin, UK

Abstract

Despite having been affected by several stages of exhumation during the Cretaceous and Cenozoic, the contemporary stress state of the East Irish Sea (EISB) is poorly characterised. As the basin is mature in terms of exploitation of hydrocarbons, future exploration beyond the conventional Sherwood Sandstone Group reservoir (Triassic) necessitates a greater understanding of the in situ stress field, while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information. Using petroleum well data, the in situ stress field of the EISB has been characterised to assess the mechanical seal integrity. A strike-slip stress regime most-likely prevails in the basin, meaning the Maximum Horizontal Stress (SHmax) is the greatest of the principal stresses. Interpretation of stress orientation data suggests that SHmax is oriented 152˚ ± 12˚, consistent with mean stress orientations across the wider region associated with plate boundary forces. Some degree of structural control appears to influence the orientation of SHmax, with orientations locally aligned sub-parallel to major Permo-Triassic basin-bounding faults. Fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre-existing faults. Vertical faults striking 30˚ from SHmax are optimally-oriented to become reactivated under elevated pore pressure conditions. For any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m, pore pressure increase should be less than 3.3 MPa to avoid reactivating pre-existing optimally-oriented faults. Higher pressure increases would be required to initiate reactivation of faults with other orientations. Vertical faults striking perpendicular to SHmax are least likely to become reactivated, and in the absence of halite, seal integrity would instead be limited by caprock strength and capillary-entry pressure. Major faults affecting the basin have been analysed for their slip tendency (ratio of shear to normal stress), which provides an indication of their susceptibility to become reactivated. Although the analysis is limited due to lack of an accurate 3D representation of the fault network, the results suggest that many of the fault orientations observed in the EISB exhibit high slip tendencies, including N-S striking faults to the north and west of the East Deemster Fault, where the SHmax orientation is NW-SE. Faults striking perpendicular to SHmax, such as the Lagman Fault, are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding, while faults striking parallel or very close to SHmax also exhibit low slip tendency as they are not subjected to significant shear stresses.

Effect of sedimentary heterogeneities in the sealing formation on predictive analysis of geological CO2 storage

“…Since the capillary pressure-controlled properties are associated with the pore size distribution and wettability, the lithology and mineral composition of the mudrock at the reservoir/seal interface becomes important when estimating the capillary sealing efficiency of the caprock overlying potential CO2 storage sites. The MMG, which overlies potential CO2 storage formations such as the Sherwood Sandstone Group and its North Sea equivalent, the Bunter Sandstone Formation (Noy et al, 2012;Williams et al, 2018), equally serves as a good example here. At the reservoir/seal interface, transitional lithologies commonly exist between the Sherwood Sandstone and the Mercia Mudstone (Newell and Shariatipour, 2016;Seedhouse and Racey, 1997;Shariatipour et al, 2016b).…”

“…The allowable bottom-hole-pressure (BHP) is set to 75% of a lithostatic pressure gradient assumed to be 22.5 MPa/km (after Noy et al, 2012). This is ~90% of the minimum horizontal stress magnitude in the East Irish Sea Basin as estimated by Williams et al (2018). In order to accurately approximate the magnitude of expected fluid pressure increase resulting from CO2 injection, cells towards the top of the reservoir and at the base of the caprock are thinner.…”

Stress magnitudes across UK regions: New analysis and legacy data across potentially prospective unconventional resource areas

“…The orientation of the UK and UKCS stress field has been studied extensively (Williams et al, 2015(Williams et al, , 2018Holford et al, 2016;Kingdon et al, 2016;Fellgett et al, 2017b)). Kingdon et al (2016) reported that the orientation of S HMax across the UK landmass is 150.9°± 13.1°, which is the result of ridge-push forces associated with the Mid Atlantic Ridge (Klein and Barr, 1986;Gölke and Coblentz, 1996).…”

“…The latest edition of the World Stress Map (WSM) includes all of the current openly available stress data for the UK (Heidbach et al, 2016) which has been greatly expanded by recent studies (Williams et al, 2015(Williams et al, , 2018Holford et al, 2016;Kingdon et al, 2016). Information from the WSM has previously been used to estimate stress magnitudes .…”

Application of three-dimensional fault stress models for assessment of fault stability for CO2 storage sites

“…The characterisation of fault properties in caprock strata is of particular relevance, as these structures may have a strong impact on the likely integrity of fluid traps, potentially allowing the development of fluid flow bypass systems through the seal units (Cartwright et al, 2007;IEAGHG, 2017b;Maia et al, 2016;Petrie et al, 2014). At a regional scale, the location of faults can be interpreted on 3D seismic data, allowing broad scale mapping of networks and other fluid-flow features (Backe et al, 2015;Streit et al, 2005;Williams et al, 2018). Reliably constraining other properties such as in situ stress relies upon a high density of subsurface data obtained from boreholes (Sibson, 1995;Williams et al, 2018).…”