Parametric Analysis of Stress Reduction in the Caprock Above Compacting Reservoirs

Abstract

Abstract It is well known that the fracture gradient is significantly reduced when drilling through depleted reservoirs; hence, mud density must be adjusted or a casing must be set before drilling through depleted reservoirs. However, the magnitude and extent of fracture-gradient reduction in the caprock above depleted reservoirs have not been well characterized. Several casing-shoe tests performed in the cap-rocks showed that fracture gradients in the caprock above depleted reservoirs also declined significantly with reservoir depletion. This gives rise to the problem of determining a safe casing-shoe set position without inducing lost-circulation problems. To address these issues, parametric analysis of stress reduction in the caprock above compacting reservoirs was performed using a finite-element structural model with fluid flow from caprocks. It was found that two factors dominate the in-situ stress change in caprocks. One factor is pore-pressure change caused by dehydration from shale to reservoir sections as reservoir pressure is depleted, and the other factor is the horizontal-stress reduction because of the roof effect of caprocks. Although caprock permeability is generally very low, shale pore fluid can drain over time into reservoir sections because of large pressure differences after reservoir-pressure depletion. The loss of pore fluid causes a fracture-gradient reduction in shale sections. By contrast, if the shear modulus in the shale section is high compared with the shear modulus of the reservoir formation, the horizontal stress in the caprock reduces because of the roof effect. Using the correlations of shale elastic modulus and permeability with respect to shale porosity, a practical method to estimate the extent and magnitudes of stress reduction in the caprock above depleted reservoirs is proposed in this paper.