Uncertainty of Stress Path in Fault Stability Assessment During Co2 Injection: Comparing Smeaheia 3d Geomechanics Model with Analytical Approaches

Abstract

Simplified stress path assumptions, particularly uniaxial strain conditions, are widely used in fault stability assessments when screening for suitable CO2 injection sites. We simulated a 3D distribution of CO2 injection-induced stress change in the Smeaheia fault block in Norwegian North Sea and compared it with a simplified stress path assumptions. Our results show that the uniaxial strain assumption underestimates the change of effective horizontal stress resulting from the injection-induced pressure change. The underestimation is generally larger for bounding faults subjected to a significant change in pore pressure gradient than for the intra reservoir faults with relative homogeneous pore pressure distribution; and the underestimation for the bounding fault is more prominent when the reservoir surroundings are softer than the reservoir. Rotations of maximum horizontal stresses parallel to soft surroundings are also observed along the bounding faults. The stability assessment shows that using the uniaxial strain approach tends to overestimate the fault stability up to 60% for an extreme case. Our study thus indicates that the uniaxial strain assumption, which limits to account for lateral deformation on the fault/reservoir boundary, will not capture critical changes of the effective horizontal stress, and associated critical scenarios for fault stability assessments are overlooked. When bounding faults are juxtaposed with low stiffness shale formation under a normal stress regime, calibrating the fault stability assessment by 30 % for a base case and 60 % for a conservative assessment can be a practical way to correct the stability uncertainties caused by using uniaxial strain assumption.

Keywords: Fault slip, Geological CO2 sequestration, Geomechanics, Stress path, Uniaxial strain condition