Predicting ground subsidence due to long term oil/gas production in a Niger Delta basin, Nigeria: implications for CO2 EOR and geosequestration

Abstract

Reservoir pressure depletion with production leading to porosity loss, compaction and surface subsidence are the effect of effective stress changes and the imposition of the overburden stress which was partly supported by the fluids on the rock grain skeleton. Ground subsidence is associated with environmental hazards, failure of operational facilities and damages to infrastructures amounting to huge economic losses. In this studies, subsidence has been assessed using the Geertsma nucleus of strain based on predicted reservoir pressure. Depletion was estimated as percentage pore pressure dissipation and dynamically derived geomechanical and petrophysical rock properties determined. Reservoir porosity vary from 15% – 32%, shale volume from 11.2% − 88%, bulk compressibility range from 2.52–2.53 x 10− 6 /mPa, and uniaxial compaction coefficient 1.15–1.8 x 10− 6/mPa. The vertical compaction in a typical reservoir interval with a thickness of 31.0m varies from 0.002mm to 0.05mm at 10% formation pressure depletion, − 0.005mm to 0.27mm at 50% formation pressure drawdown and 0.007 to 0.53mm at 99% production and reservoir pressure dissipation. surface subsidence would range from 0.045mm to 0.35mm at 10% pressure depletion, 0.058mm to 1.8mm at 50% pressure depletion and 0.045mm to -3.47mm at full reservoir pressure drawdown. At a distance of 92.45km from the Niger Delta coastline, subsidence in the oilfield can still spread to the coastline, the deformation causing damages to the environment, operational facilities and infrastructures that amount to huge economic losses to the operators and government. We recommend the use of CO2 in EOR to maximize production, mitigate subsidence through ground rebound and keep carbon securely sequestered.