Density and Vertical Stress Variation across the Niger Delta Depobelts: Implications for Geopressure Prognosis

Abstract

Accurate model of the vertical stress is important for precise prediction of pre-drill formation pore pressure, and hence fracture pressure, whether at the well location or on a regional scale. This has the overall effect of a safe, cost effective and successful drilling through the high pressure, high temperature targets which currently form part of the Niger Delta main exploration frontiers. In this study, we modeled density and vertical stress in the Niger Delta from twenty-five (25) wells, spanning the six (6) depobelts in the Niger Delta. The aim is to study variation in the vertical stress across the Niger Delta, with the underlying implication for proper well planning, exploratory and developmental drilling specific to the respective areas. The study shows variation in vertical stress gradient between 0.40 and 0.94 psi/ft in the northern Delta depobelt, and between 0.40 and 0.90 psi/ft in the deep offshore depobelt, indicating a slight overall decrease in the gradient from the onshore towards the Niger Delta deep offshore. The variation is attributable to changes in lithology which result in variations in bulk density with depth and spatially across the Niger Delta as a result of differences in the rate of sediment compaction. Average vertical stress gradient across the Niger Delta is 0.94 psi/ft ±0.02 psi/ft. Bulk density varies with depth from 1.87 to 2.48 g/cm3, and 1.84 to 2.36 g/cm3 in the northern Delta and deep offshore depobelts, respectively. Analytical model between bulk density and vertical stress has been derived for the respective depobelts to aid real-time pore and fracture pressure predictions, including formation strength estimates in the specific depobelts, especially where relevant data are not available or data quality is questionable.