Estimation of overpressures from porosity based method : a theoretical approach applied to the central/coastal swamp depo-belts of the Niger delta basin

Ubon Mkpese

doi:10.14419/ijag.v6i1.8547

Abstract

The Depth-dependent compaction theory that variations in certain geophysical properties with depth; bulk density, formation resistivity together with sonic velocity being a reflection of the pressure regime is the basis for pore pressure prognosis study. Pore pressure prediction (PPP), when done accurately can be used to avert disaster and helps in safe drilling. A porosity-based model has been applied to predict overpressured zones in an onshore environment of the Niger delta basin. Zones with hard overpressures greater than a magnitude of 0.7 psi/ft are generally within 10000ft and below. Top of overpressures for studied wells ranges between 7000ft and 10000ft. Porosities in shale are of typical values ranging between 0.05 to 0.46. A robust concordance between PPP and MPP profiles for each of the wells validates the results here and confirms suitability of model to the studied area.

Highlights

Abnormal formation pressures would usually pose a major challenge to drillers and as such is one area that requires careful and in-depth studies
The Depth-dependent compaction theory that variations in certain geophysical properties with depth; bulk density, formation resistivity together with sonic velocity being a reflection of the pressure regime is the basis for pore pressure prognosis study
3) Estimate overpressures within the sediments of study locations using porosity based pore pressure prediction (PPP) model 4) Compare the predicted overpressures obtained within the shale zones to measured pore pressures (MPPs)

Summary

Introduction

Abnormal formation pressures would usually pose a major challenge to drillers and as such is one area that requires careful and in-depth studies. Such case scenario where fluid pressure within the pore exceeds, in the significant amount, or is below that of the normal pressure (the hydrostatic pressure) at a particular depth of consideration is said to be abnormal (Jiao et al, 1998). They have been propositioned of several of such prediction models in recent years These are based either on graphical extrapolation, velocity calibration or empirical power laws. That the choice of any prediction model for any sedimentary basin is dependent on the causal mechanism of overpressure and on the researcher’s experience (Zijian, 2015)

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Conclusion