TITLE:
Integrated Approach to Pore Pressure and Fracture Pressure Prediction Using Well Logs: Case Study of Onshore Niger-Delta Sedimentary Basin
AUTHORS:
Cyril Ngozi Nwankwo, Stephen Onoh Kalu
KEYWORDS:
Pore Pressure, Fracture Gradient, Overpressure, Models, Niger Delta
JOURNAL NAME:
Open Journal of Geology,
Vol.6 No.10,
October
28,
2016
ABSTRACT: This study investigated the cause of identified
zones of overpressure in some selected wells in a field in the Niger Delta
sedimentary basin. Two models were used each for predicting pore pressure and
the corresponding fracture pressure using well log and drilling data. Shale
lithology in Niger Delta is massive and characterized by high pore pressure;
hence shale compaction theory is utilized in this study. The petrophysical data
were evaluated using Ikon’s Science Rokdoc software. The two major pore pressure
prediction techniques employed are the Eaton’s and Bowers’ models while the
Eaton’s fracture pressure model and the Hubbert and Willis fracture pressure
prediction models were utilized for fracture prediction. The density and sonic
logs were used respectively to generate the shale trend and the shale normal
compaction trend used for the prediction. The wells studied showed
disequilibrium compaction of sediment to be the major mechanism that gave rise
to overpressure in the Niger Delta. Clay diagenesis and fluid expansion were
also observed as the secondary overpressure generation mechanism in well X-1.
This secondary overpressure mechanism was observed to start approximately at
depths of 10,000 ft (TVD). The top of overpressure and the pressure range in the
wells studied varied from 6000 to 11,017 ft (TVD) and 1796.70 to 5297.00 psi respectively. The Eaton’s model under-predicts pore pressure at the depth interval where unloading mechanism is witnessed.
Since the study revealed presence of secondary overpressure generation
mechanism, Bowers model was observed to be the most reliable pore pressure
prediction model in the area.