Understanding Subsurface Reservoir Pressure through Accurate Geo-Mechanical Characterization

Abstract

So far, Bulk modulus, Shear modulus, combined modulus of strength and shear modulus to compressibility ratio have been actively employed as geo-mechanical parameters that have been used in characterizing subsurface reservoir in Niger delta wells in Nigeria. These can be further utilized to enhance understanding of the pressure in the formation with depth. To successfully maximize hydrocarbon recovery, to minimize sanding rate, and to ensure safety of personnel during and after drilling, pore pressure gradient and its associated fracture pressure has been predicted for each of the abnormally pressured intervals discovered in one of the Niger Delta oil fields by applying Eaton’s method. This is aimed at identifying fragile sections. However, empirical relations have been established using geo-mechanical principles.

From the two wells studied, geo-mechanical strength increases with depth and tends toward the acceptable range for a competent formation. In the second well, the fracture pressure increases with increasing pore pressure and decreasing Bulk modulus at various depths, while abnormal formation pressure occurred throughout the formation especially at greater depths.

The relationship established between geo-mechanical factors and fracture pressure is such that as one decreases, the other increases vice versa.

Keywords: Formation strength; fracture; mud weight; pore pressure; sand production and sand control