Reservoir Geomechanics: Applications in the Natural Gas Fields of Eastern Mediterranean

Abstract

Hydrocarbon reservoir structures are subjected to tectonic forces along the geological time that cause rock deformation and break to faulted zones. Faulted reservoirs, enclose certain complexity in terms of the distributed effective stresses, rock plastic alteration, slipping and displacement. In this thesis, we present the application of two-dimensional (2D) and three-dimensional (3D) geomechanical reservoir models based on Finite Element Analysis (FEA) for a faulted and compartmentalized geometry, located in the offshore deep-water environment of the Levantine basin in the Eastern Mediterranean. A refined model for the wellbore geometry was used to examine the in-situ stress anisotropy and the impact of reservoir draw-down on wellbore stability and sand production. The geometry is derived from existing knowledge of the Aphrodite gas reservoir in the Cyprus Exclusive Economic Zone. The geometry of the model is constructed to provide a simple and comprehensive understanding of mechanical impact on the reservoir geobody, where complex stress-strain evolution of the reservoir is primarily affected by the generated in-situ stresses, the geometry of faults and the degree of compartmentalization of the geological structure. The Drucker-Prager plasticity model was used for the rock deformation and the Mohr-Coulomb theory validated the plastic model, which is also used to assess the shear failure of faults slipping. Mechanical properties of reservoir sandstones were derived from calibration of data obtained from triaxial tests and for the overburden shale layers the acoustic velocity and correlation functions were used. The compartmentalized geometry was constructed based on seismic data and logging data obtained at the exploration and appraisal phases. The estimated in-situ stress field was transformed and applied to the boundaries of the model blocky geometry. A series of alternative production scenarios of the field is analyzed using a simulation model that accounts for consolidation and plastic deformation of the rocks. The model estimates the displacement, strain magnitudes, and the associated plastic deformation in the reservoir formation and particularly examines the fault surface conditions and block interactions. The analysis assesses the impact of effective stress change distribution on the fault slip tendency and normalized permeability distribution. Plastic yielding is mainly developed in fault slip zones of narrow extent, whereas it appears that there is a low risk of plastic behavior in the main reservoir body. The slip conditions become complex in the fault contact surfaces where local areas close to fault connections are more pronounced to slip creating more unstable conditions with the creation of localized areas of smaller faulted zones. Displacement magnitudes are controlled by the structural boundary conditions and the geometrical shape of each fault block compartment. The higher displacements develop in the near-fault region while in the remote from the fault area, the vertical displacement is nearly constant as it is clearly governed by the reservoir depletion. Furthermore, using the volumetric changes we can estimate the changes of permeability in rock formations. In our study, the risk of fault slip is low, and no significant reduction impact in permeability is expected. The presented work demonstrates the advantages that a properly calibrated reservoir geomechanical model provides in assessing field depletion scenarios in compartmentalized reservoirs and in identifying local reservoir regions of potential problems, such as fault re-activation and slipping, wellbore shearing, reservoir compaction, and permeability changes. Overall, geomechanics integration enriches and improves the dynamic reservoir models and applications.