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 cap rock above depleted reservoirs has not been well characterized. Several casing shoe tests performed in the cap rocks showed that fracture gradients in the cap rock above depleted reservoirs also significantly declined with reservoir depletion. This gives rise to the problem of determining a safe casing shoe set position without inducing lost circulation problems.
To answer the above questions, parametric analysis of stress reduction in the cap rock above compacting reservoirs were performed using a finite element structural model with fluid flow from cap rocks. It was found that two factors dominate the in-situ stress change in cap rocks. One factor is pore pressure change due to dehydration from shale to reservoir sections as reservoir pressure is depleted and the other factor is the horizontal stress reduction due to the roof effect of cap rocks. Although cap rock permeability is generally very low, shale pore fluid can drain over time into reservoir sections due to 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 to the shear modulus of reservoir formation, the horizontal stress in the cap rock reduces due to 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 cap rock above depleted reservoirs is proposed in this paper.