Porosity Compaction and Mean Effective Stress: Revisions to Pore Pressure and Fracture Gradient Estimation from Well Logs

Abstract Abnormal pore pressure in sedimentary basins is commonly estimated by identifying the deviations of porosity- indicator well logging data from their corresponding normal compaction trends. Conventional approaches attribute these deviations solely to changes in the vertical (overburden) component of the in-situ effective stress while neglecting the role of horizontal components. This study introduces a new framework based on Biot's theory of poroelasticity that accounts for the effects of both the vertical and horizontal components of in-situ stress on porosity change. On this basis, a modified form of Eaton's method for pore pressure estimation is developed that entails simultaneous estimation of pore pressure and fracture gradient from well logging data. The method is applied to a field case involving an offshore well located in a tectonically active region with high horizontal stress anisotropy. Results show that the revised workflow produces pore pressure and fracture gradient estimates that align more closely with downhole measurements of the same parameters, as well as with the mud weight profile of the well, compared to the conventional sequential approach in which porosity compaction is assumed to depend solely on overburden stress.