Abnormal Pressure Detection Using Integrated Approach in “Oluku” Field, East Niger-Delta, Nigeria

Abstract

This research reports overpressure occurrence in the eastern part of Niger Delta by using an integrated approach that combines the use of equivalent depth method, the Eaton equation with seismic reflection data. Sonic logs from four wells in the Oluku field were used. Pore pressure was calculated using Eaton equation and equivalent depth method and the result compared with pressure measurement data. The result showed that the eastern part of the delta is highly pressured with pressure regime that followed a typical ledged-tiered triple configuration with three episodes of overpressure. Prominent overpressure zones occur at depths of 4965 ft to 5099.5 ft, 7826 ft to 8337 ft, and 9424 ft to 10129 ft in Oluku-01 well, 4251.5 ft to 4408 ft, 7132.5 ft to 7495 ft and 9212.5 ft to 10000 ft in Oluku-02 well, 6322 ft to 6750 ft, and 9706ft to 10020 ft in Oluku-03 well and 4136.5 ft to 4275.5 ft, 7560 ft to 8000 ft, and 9038.5 ft to 9350 ft in Oluku-04 well. When compared with available pressure measurement data, computation using the Eaton equation was found to be closer to the pressure measurement data with the equivalent method underestimating the pore pressure. Seismic signature in the vicinity of the overpressure shows evidence of low velocity close to the shallow over-pressure zone probably due to under-compaction of sediments at shallow depth. Four regional faults in the study area contribute to abnormally high pressure at depth due to their sealing of permeable bed in the vicinity of kerogen-rich formation. This work reveals that the eastern flank of the Niger Delta exhibit higher degree of overpressure that the western flank. The knowledge of this is necessary for adequate planning before drilling into the formation in order to guard against dangerous drilling problems such as excessive cost overrun, well kicks and blowouts, lost circulation, stuck pipe and wellbore instability.