<i>In situ</i> stresses in the North Sea and their applications: petroleum geomechanics from exploration to development

Abstract

Present-day maximum horizontal stress (σ H ) is oriented northwest–southeast onshore North West Europe, reflecting the first-order control on intraplate stresses exerted by plate boundary forces. Stresses associated with deglaciation appear to influence the stress regime in the Northern North Sea, with σ H oriented E–W and a contemporary stress regime close to the transition between strike-slip and reverse faulting, i.e. σ H &gt; σ v ~ σ h (where σ v and σ h are vertical and minimum horizontal stress respectively). Maximum horizontal stress orientations are highly variable in the Central North Sea and the stress regime within the sedimentary sequence appears to be detached from that in the basement. The stress regime in the sedimentary sequence of the Central North Sea is predominantly one of normal faulting with almost isotropic horizontal stresses (σ v &gt; σ H ~ σ h ). Geomechanical risking of the likelihood of seal breach due to fracturing should incorporate the risk of reactivating sealing faults or fracturing intact cap seal rocks in shear or in tension. The risk of fracture-related seal breach is considered for stress regimes representative of the Northern and Central North Seas. Generally, reactivation of optimally oriented faults and/or shear fracturing of intact cap rock are the most likely mechanisms of fracture-related seal breach. However, in the Central North Sea overpressured scenario, tensile failure is the most likely mechanism of fracture-related seal breach. In situ stress data have value throughout the life cycle of a field. The implications of the stress regimes representative of the Northern and Central North Seas for naturally fractured reservoirs, wellbore stability, water flooding and fracture stimulation are analysed. These issues are stress-sensitive and the conclusions with respect to each issue differ significantly between the Northern and the Central North Sea because of their differing stress regimes.