DETERMINATION OF PRESSURE MECHANISMS AND PORE PRESSURE PREDICTION IN ‘MARIA’ FIELD, ONSHORE, NIGER DELTA

Abstract

Overpressure in a formation, caused by abnormally high fluid pressures, is a concern during all phases of oil field operations including exploration, drilling, well completion and reservoir management. Knowledge of formation pressure allows for more efficient selection of casing point, a reliable mud weight design, and provides insight into fluid behavior, seal integrity and reservoir presence. In 'Maria' Field, Offshore Niger Delta several wells have experienced pressure kicks. This development prompted this study that aimed at determining the pressure mechanism and predicting pore pressure in the study area. The data used for the investigation include wireline logs, pressure tests data, 3-D Seismic, and measurement while drilling (MWD) data. The methodology is hinged on the difference between a ‘normal trend’ in resistivity, sonic wave velocity, density and shale porosity and their observed values in overpressured zones. Overpressured zones were identified as pore pressures were predicted from seismic velocities and wireline logs across the field. In addition, seismic method and Repeat Formation Tester (RFT) were used for interpreting pressure and some other reserevoir characteristics in the four wells studied. The Empirical method of Eaton was used to estimate pore pressures from sonic, resistivity, and density logs which were calibrated to measure pore pressures from Repeat formation tester (RFT). Velocity reversals with density increase were observed in Maria-001, 014 and 015. Based on this, it was deduced that compaction disequilibrium is the governing porepressure mechanism in the field. The density and sonic velocity crossplots of the formation from Maria-004 well gave a good indication of the maximum compaction achieved by the rocks. At shallow depths, the subsurface stratigraphy followed normal compaction trend (NCT) from well and seismic velocity models. However, the shifts from the NCT, as observed from resistivity, sonic velocity and porosity, indicated under-compaction of sediments as the cause of the abnormal pressure in the field. The top of overpressure for Maria wells 001, 014 and 015 were identified at 12300ft (3749.04 m), 8950ft (2727.96 m) and 8900ft (2712.72 m) respectively. Maria-004 well revealed normal pressure trend from plotted pressure data. The 3-D pore pressure cube revealed the existence of isolated compartments with lower pressure below 2800 psi; the mild overpressure zone has values that vary from 2800 to 3200 psi, while the pressure for the hard overpressure varies from 3250 psi to 3600 psi. The predicted pore pressures were compared with offset well data and good match were observed. The study has provided information on depth to geopressure zones useful for casing and drilling design therefore reduces uncertainty and ensures safety and efficiency.