Abstract:
An accurate prediction of pore pressure using seismic velocities in the subsurface is
important in reducing the risk involved in a well planning and to avoid well control
problems. This is with the aim of predicting the abnormal pressure zones within
“safety” field, offshore, Niger Delta, Nigeria using seismically-derived velocities from
Amplitude variation with offset inversion and well log data. Ben Eaton’s method was
adopted for the estimation of pore pressure in this research work. Pore pressure
predicted from the seismic velocity has a better resolution than the pressure predicted
from the interval transit time. Pore pressure, overpressure and underpressure zones and
overpressure generation mechanism have been predicted and delineated within the
study area. The pore pressure within the field ranges from 14.7 psi to 3916 psi.
Overpressured and Underpressure zones were delineated on the field from the pressure
predicted from the seismic velocities. Within the field, the over-pressured zones were
delineated at depth 6855ft to 7802 ft. Over pressure top was delineated at a depth of
6855 ft with a pore pressure of 3053 psi and a corresponding hydrostatic pressure of
2722 psi. The under-pressured zones were also delineated at depth 7883 ft to 9288 ft.
The under pressure top was delineated at a depth of 7883 ft with a pressure of 1093 psi
and a corresponding hydrostatic pressure of 3122 psi. Porosity values within the over
pressure zone ranges between 23% – 53% which could be considered as relatively
high. This could be as a result of the fact that the pore fluid cannot be expelled rapidly
thereby causing the pore fluid to increase rapidly since they are no longer compacted;
thus leading to overpressure. As a result of overpressure top which is directly above
the reservoir top within the shale zone, drilling this reservoir vertically could not be
suggested so as to avoid possible blow out. It was also observed that the primary cause
or mechanism of overpressure within this field could be disequilibrium compaction.
