Integrated Approach to Pore Pressure and Fracture Pressure Prediction Using Well Logs: Case Study of Onshore Niger-Delta Sedimentary Basin

Cyril Ngozi Nwankwo,Stephen Onoh Kalu

doi:10.4236/ojg.2016.610094

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.

Highlights

One of the requirements to safe drilling is the knowledge of the formation pore pressure gradient and the corresponding fracture gradient
This study focuses on comparing wire line log data with that of Wire line Formation Tester (WFT) in predicting the pore pressure in some selected wells in the Niger Delta oil field
Estimation of Pore Pressure by Different Methods Eaton’s exponential and Bowers pore prediction models were the fundamental pore pressure prediction approaches employed in this work

Summary

Introduction

One of the requirements to safe drilling is the knowledge of the formation pore pressure gradient and the corresponding fracture gradient. When the pore pressure of a formation at any depth is above the hydrostatic pressure for such formation, the pressure is considered to be over-pressured. On the other hand when the formation pressure is below the hydrostatic pressure at any depth, the formation is said to be under-pressured. The normal pressure is the hydrostatic pressure, or the pressure exerted by a column of water from the formation depth to sea level. When impermeable rocks such as shale sediments are compacted, their pore fluids cannot always escape and must support the total overlying rock column leading to anomalous formation pressure [1]

Methods

Results

Discussion

Conclusion