High Resolution Overpressure Prediction From Seismic Velocity And Field Derived Formation Parameter in the Onshore Niger Delta.

Abstract

Occurrence of overpressured formation is common in Tertiary basins (e.g. Niger Delta), which can result in geomechanical problems, such as “kicks”, loss of expensive drilling mud, blowout, stuck pipe, reservoir quality damage, wellbore instability etc. if it is not accurately predicted prior to drilling. This study did a critically re-evaluation of the two commonly utilized methods and practices involve in pore pressure work in the industry. The main aim was to accurately predict pore pressure ahead of drilling using seismic velocity. This involve analyzing ten offset wells to probing the overpressure origin, deriving a site specific Bower’s parameter, establishing a relationship between velocity and pressure using well data, and accurate velocity analysis for pore pressure prediction. The velocity – density crossplot for all the wells analyzed showed a compaction disequilibrium as the origin of the overpressure. The lithology log derived from GR showed a typical sand dominated Benin Formation to a depth of about 9,200ft (TVD) and beyond that depth a shale dominated Agbada Formation. The overburden gradient ranges from 0.78 psi/ft. to 0.96 psi/ft. contrary to 1.0 psi/ft. fixed gradient sometimes used. Bower’s parameter (A = 7.5, B = 0.68) were derived specifically for the field, instead of the regional parameters (A = 8, B = 0.57) in use. The velocity pressure relationship established using the offset well data gave a good match with the measured pressure data where it’s available, which affirms the reliability of using acoustic velocity for pore pressure prediction. The onset of overpressure (at 10,200ft) corresponds with the 15.0 maximum flooding surface (which hinders fluid expulsion), further confirms compaction disequilibrium as the origin of the overpressure. Bower’s method at the shallow section (for Kappa 39 ST 1) gave a pressure ranging from 0.54 psi/ft. to 0.66 psi/ft. which is far greater than the mud pressure of 0.52 psi/ft. to 0.53 psi/ft. that was used in drilling the well (i.e. overestimate pore pressure). Whereas Eaton’s method gave a range of 0.44 psi/ft. to 0.46 psi/ft. for the same depth interval, which evidence that the offset well was drilled overbalanced, and not underbalanced as suggested by Bower’s prediction at that shallow depth. Seismic velocity analysis for pore pressure prediction and calibration with well data results in a good match of predicted pressure from seismic data with measured pressure for the offset well, which strengthens the assurance that seismic velocity can be relied upon to give an accurate pore pressure estimate where log data are not available. A safe drilling mud window interval between predicted pressure and fracture pressure was established also. It is therefore important to suggest based on the result of this work that Eaton’s and Bowers’ methods should be integrated for any deep exploration well, such that, the result of Eaton’s method can be relied upon at shallow section, where Bowers method tend to overestimate pore pressure. The origin of overpressures at the deeper section of the Onshore Niger Delta is an unresolved question, which should be given a priority as more deep exploration wells are been drilled.