Non-equilibrium compaction and abnormal pore-fluid pressures: effects on rock properties

Knowledge of pore pressure using seismic data will help in planning the dri lling process to control potentially dangerous abnormal pressures. Various physical processes cause anomalous pressures on an underground fluid. Non-e quilibrium compaction is a significant process of overpressure generation. This occurs when the sedimentation rate is so rapid that the pore fluids do not have a chance to 'escape' from the pore space. The model assumes a closed system and that the pore space is filled with wa ter and hydrocarbon in a liquid state. Balancing mass and volume fractions yields the fluid pressure versus time of deposition and depth of burial. Th ermal effects are taken into account. The pore pressure, together with the confining pressure, determines the effective pressure which, in turn, deter mines the bulk moduli of the rock matrix. We assume a sandstone saturated with hydrocarbons and water, for which cali bration of the model with experimental data is possible. The seismic veloci ties and attenuation factors are computed by using Biot's theory of dynamic poroelasticity and the generalized linear solid. The example shows that th e formation can be overpressured or underpressured depending on the propert ies of the saturating fluid. Wave velocities and quality factors decrease w ith decreasing differential pressure. The effect is important below approxi mately 20 MPa. The model is in good agreement with experimental data for Be rea sandstone and provides a tool for predicting pore pressure from seismic attributes.