Effective stress, porosity, velocity and abnormal pore pressure prediction accounting for compaction disequilibrium and unloading

Abnormal pore pressures, mostly overpressures, exist in many sedimentary formations. The overpressures deteriorate drilling safety, causing borehole influx, kicks, and even blowout, if the pressures are not accurately predicted prior to drilling. Highly anomalous overpressures may also induce instability and reactivation of faults, causing fault weakness. Formation overpressures are primarily generated by compaction disequilibrium, which is often recognized by higher than expected porosities at a given depth and the porosities deviated from the normal porosity trend. Based on this mechanism, the paper proposes a new generalized theoretical model for porosity–depth relationship for both normally compacted and abnormally compacted formations, i.e., ϕ = ϕ 0 e − c Z ( σ e / σ n ) . This model leads to a new method for calculating effective stress and pore pressure in subsurface formations using porosity and compressional velocity. A new relationship of the transit time and depth has also been derived which extends the existing model (Chapmanʹs model). It demonstrates that the sonic/seismic travel time and effective stress have an exponential relationship (i.e., Δ t = Δ t m + ( Δ t m l − Δ t m ) e − c Z ( σ e / σ n ) ). unloading caused by formation uplift has a different path compared to compaction/loading curve of the stress and velocity, thus a different compaction constant. This causes a smaller effective stress and lower porosity than those in the loading case; i.e., unloading causes pore pressure increase. Effective stress and pore pressure calculations accounting for unloading are also proposed. Field data in several petroleum basins are analyzed and verify the theoretical relationship between effective stress and sonic transit time. Lab experimental data in sonic velocity and effective stress in both loading and unloading cases also verify the proposed effective stress and velocity relationship. Case study in an oil field is presented to examine the proposed model for pore pressure analysis in subsalt formations.