Present-day stress field and stress path behaviour of the depleted Mishrif reservoir from the super-giant Zubair oilfield, Iraq – A geomechanical case study

Abstract

Accurate understanding of the reservoir pore pressure and in-situ stress behaviour in response to depletion has critical effects in production optimization and field redevelopment while ensuring reservoir integrity. This work presents the first reservoir geomechanical modeling from the supergiant Zubair oil field, southern Iraq to decipher the pore pressure, in-situ stresses and depletion stress path of the middle Cretaceous Mishrif limestone reservoir. Vertical stress (Sv) has 0.97–0.99 psi/ft gradient. Minimum horizontal stress (Sh) estimated from effective stress ratio and Poisson's ratio-based approaches yielded an about 0.58–0.76 psi/ft gradient and validated with C-category fracture closure pressure data available from mini-frac tests. In absence of image log-based breakout widths, maximum horizontal stress (SH) is estimated from three approaches, which provided a gradient range of 0.89–1.21 psi/feet. Relative stress magnitudes (SH ≥ Sv >Sh) indicate a normal to strike-slip faulting transitional tectonic state in the study area. C-quality wellbore breakouts, interpreted from the oriented four arm caliper data infer a mean N60°E SH orientation, which is parallel to the movement direction of the Arabian plate towards the Eurasian plate. Recent downhole measurements indicate that the primary producer Mishrif limestone is in depleted condition. Stress path, interpreted from the reduction of pore pressure (Δpp) and minimum horizontal stress (ΔSh), yields average values of 0.65 and 0.62 for upper and lower Mishrif, respectively. Deformation analysis in reservoir space indicates stress path in depleted Mishrif formation of Zubair oil field is less than the normal faulting limit (0.71) and depletion-induced faulting is therefore unlikely to occur.