HR: 1340h
AN: S23A-1113    [Abstracts]
TI: Overpressure Prediction From Seismic Data: Implications on Drilling Safety
AU: * Osinowo, O O
EM: wale.osinowo@mail.ui.edu.ng
AF: Department of Geology, University of Ibadan, Ibadan, Nigeria, Department of Geology, University of Ibadan, Ibadan, Oyo 234, Nigeria
AU: Oladunjoye, M A
EM: ma.oladunjoye@mail.ui.edu.ng
AF: Department of Geology, University of Ibadan, Ibadan, Nigeria, Department of Geology, University of Ibadan, Ibadan, Oyo 234, Nigeria
AU: Olayinka, A I
EM: ai.olayinka@mail.ui.edu.ng
AF: Department of Geology, University of Ibadan, Ibadan, Nigeria, Department of Geology, University of Ibadan, Ibadan, Oyo 234, Nigeria
AB: High rate of sediment influx into the Niger Delta via river Niger coupled with high rate of basin subsidence, very thick clayey members of Agbada and Akata Formations as well as prevailing presence of growth faults had been identified as the main factors responsible for overpressure generation and preservation in the Niger Delta basin. Analysis of porosity dependent parameters such as interval transit times and interval velocities derived from the seismic records of a field in the Western Niger Delta revealed the presence of overpressured formation at depth of 8670 feet, which is the top of the overpressured zone. The plot of interval transit times against depth gave a positive deflection from normal at the region of overpressure while interval velocity plot gave negative deflection; the ratio of this deviation in both cases is as high as 1.52. Pressure gradient in the upper, normally pressured part of the field was determined to be 0.465 psi/ft., which is within the established normal pressure gradient range in Niger Delta, while the abnormal formation pressure gradient in the overpressured region was determined to be 0.96 psi/ft., and this is also within the published abnormal pressure gradient range of 0.71 to 1.1 psi/ft. in Niger Delta. Formation fracture pressure gradients were determined from the formation pressure information to be 0.66psi/ft. in the upper part of the field and 1.2psi/ft. in the overpressured horizon. Mud weight window (MWW); mud density range necessary to prevent formation kick without initiating hydraulic fracturing was determined to be 10.2 to 12.5lbm/gal in the upper part of the field and 22.1 to 22.63lbm/gal in the overpressured horizon. MWW is indispensable for the selection of the mud pump type, capacity, pumping rate and mud densities at different formation pressure regimes. Overpressure prediction is also requisite for drilling program design, casing design as well as rig capacity choice before spudding. It is necessary to reduce well construction risk, save drilling hour as well as cut down drilling cost. If adequate predictions are not taken however, drilling hazards known as blowout may occur. Blowout, an uncontrollable flow of formation fluid into the well has made oil exploration and exploitation activities in Niger Delta, Southern Nigeria, a curse for the people rather than a blessing because considerable numbers of wells blew out during well construction activities, hence the characteristic oil spill which had degraded the environment, making fishing operation, a source of livelihood of the people difficult. Therefore the need for overpressure prediction as a guide for safe drilling, especially in unfamiliar exploration environments.
DE: 3025 Marine seismics (0935, 7294)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting