Anisotropic Geomechanical Characterization of Sojuko Field, Shallow Offshore, Niger Delta

摘要

Borehole stability and hydraulic fracture issues are a major concern in the economic development of hydrocarbon reserves especially for deep targets which require drilling below well control. Characterizing geomechanical properties along a wellbore provides understanding of the vertical heterogeneity in the mechanical properties of the rocks, both in reservoirs and the bounding non-reservoir formations, and is critical to the operational planning and design of stable wellbores to successfully drill, complete and exploit proven hydrocarbon reserves even at shallow depths. In this work, velocity anisotropy, assuming vertical transverse isotropy with vertical axis of symmetry, was utilized to evaluate important geomechanical properties which include Young’s modulus and the Poisson’s ratio, in order to accurately determine rock strength and in situ horizontal stresses using geophysical well logs obtained from some wells in the Sojuko field, shallow Niger Delta offshore. The aim was to determine accurate parameters, by consideration of anisotropy, to aid well design and prevent formation failure during future developmental drilling in the field, and the subsequent landing of wells. The starting point was the estimation of the Thomsen’s delta anisotropic parameter from analysis of well and seismic interval velocities at a well location, which then aided derivation of the epsilon and gamma anisotropic parameters. The three anisotropy parameters were used in combination with bulk density and sonic log data to determine stiffness constants for the estimation of the geomechanical properties, which subsequently enabled the determination of rock strength and in situ stresses around the wellbore for analysis of rock failure and mudweight requirements for safe and cost effective drilling of the well. Computed in situ minimum horizontal stress in the area varies with depth from 727 psi to 7,500 psi, with an average gradient of 0.69 psi/ft, while the maximum horizontal stress is about 12.27% higher on the average. Minimum average safe drilling mudweight for the well is 0.529 psi/ft, giving an average overbalance of 655 psi mud pressure which is relatively higher in shale than sands. Predicted safe drilling mudweight window ranges from 0.529 psi/ft to 0.713 psi/ft. Comparison of the results with geomechanical data computed with isotropic assumption shows that the non-consideration of anisotropy results in under prediction of geomechanical data in subsurface formations where velocity anisotropy is present. This has serious safety and cost implication during drilling as most of the Niger Delta deep targets are located in geopressured formations where velocity anisotropy is a perennial problem.