Top Seal Undrained Pore Pressure Response-Sensitivity to Skempton’s A

Summary Change of pore pressure in a reservoir modifies stress state in the reservoir itself and in its surrounding, which may further cause an undrained pore pressure response in reservoir’s overburden governed by Skempton’s parameters A and B. This sensitivity study aims to determine the importance of Skempton’s parameter A, often assumed to be 1/3 as predicted by isotropic linear elasticity, and explores the consequences of including experimental, angle-dependent values of this parameter in geomodelling. A top seal shale, assumed to be a representative example of impermeable overburden rocks, has been laboratory-tested in undrained conditions with different stress paths, i.e. ratios between the maximum and the minimum principal stresses. The results of the experimental data analysis have been used to model Skempton’s parameter A distribution and resultant undrained pore pressure response in an existing oil reservoir’s top seal. The modelling indicates that Skempton’s A values computed for a real reservoir geometry and stress state differ significantly from the typically assumed 1/3. The resultant differences in the undrained pore pressure response are significant and usually largest in the direct proximity of fault zones, suggesting that including anisotropic Skempton’s A in geomodelling may help in predicting failure risk in top seal rocks.