Geomechanics of Compartmentalized Reservoirs with Finite Element Analysis: Field Case Study from the Eastern Mediterranean

Abstract

This paper presents an optimum way to produce down to depletion a compartmentalized reservoir in offshore deep environment by considering geomechanical stress-deformation mechanisms and associated problems. The case study is for a faulted reservoir zone of the Aphrodite field, located in the Eastern Mediterranean. The study is based on finite element modelling using 2D plane strain analysis that incorporates pore pressure and elastoplastic deformation of reservoir and overburden rock formations using the Drucker-Prager plasticity model. The mechanical properties of the reservoir sandstones were derived from calibration of data obtained from triaxial tests and for the overburden shale layers from acoustic velocities and correlation functions. The compartmentalized geometry was constructed based on seismic data and logging data obtained at the exploration and appraisal phases. The estimated insitu stress field was transformed and applied on the boundaries of the model blocky geometry. Four different initial and equilibrium depletion scenarios were examined and the obtained results in terms of deformation and effective stresses are compared. The first scenario reflects the initial stress state, the next two intermediate scenarios present non-uniform depletion cases for each fault block, and the fourth scenario presents the case of a uniform depletion. It was found that the uniform depletion of the reservoir compartments creates the least stress contrast in the field and consequently, ensures better control of stress-related impacts during the production. The analysis highlights the local regions of a fault blocks system that potentially suffer by high shear strains that can cause fault reactivation or induced fractured zones but the over-all risk remains low. Furthermore, the analysis establishes relationships between the mean effective stress, volumetric strain, and permeability changes in order to predict the regions with improved transmissibility characteristics or the less permeable compacted rock regions of the reservoir. Overall, the analysis can provide an appreciation of the stress/strain-driven characteristics of the reservoir showing the area of rock compaction tendencies of the faulted blocks and the further deformation in depletion conditions. The presented work demonstrates clearly that a properly calibrated reservoir geomechanical model can be used as a screening tool for examining depletion scenarios of compartmentalized reservoirs, highlighting areas of potential problems such as fault activation, wellbore shearing, reservoir compaction, permeability changes and fault sealing.

3D Geomechanical Finite Element Analysis for a Deepwater Faulted Reservoir in the Eastern Mediterranean

“…Nevertheless, a reasonable simplicity for understanding the overall dynamic stress behavior of the formation is kept by considering uniform bodies with averaged rock properties, especially for the net-togross parameters, porosity, and permeability. Our model is calibrated based on the field pressure data (Markou and Papanastasiou 2018) and the triaxial tests conducted on reservoir sandstone core samples (Markou and Papanastasiou 2020a). The results were validated in the initial conditions from the in situ field data, and as the field is not a producer yet, the modeled depletion conditions were validated through the analytical method of Hooke's law and the outputs of triaxial tests.…”

“…In particular, in normal fault geometry, the footwall depletion causes the rock movement perpendicular to the fault surface, suggesting that the risk of fault activation is reduced. On the contrary, in the case of hanging-wall depletion, there is a differential displacement parallel to the fault surface indicating a risk of fault activation movement (Markou and Papanastasiou 2020a). The most predictable displacement conditions can exist in the uniform depletion scenario.…”

“…The orientation of applied in situ stresses is based on the borehole imaging presented in the reference work by Markou and Papanastasiou (2020a) using FMI resistivity and Stereonet plotting. As the three-dimensional model creates a blocky geometry, the original in situ stresses S h and S H were transformed to stresses that correspond to the current geometrical model axis by introducing the S h model and S H model , respectively.…”

“…The triaxial tests were performed on reservoir sandstone core samples at different levels of confinement and the data were used to calibrate the initial yield, the ultimate strength, and softening surfaces to the failure state of samples (Markou and Papanastasiou 2020a). The M-C material parameters model are obtained by plotting the Mohr circle for states of stress at initial yield and peak strength in the normal stress-shear stress diagram.…”

“…The fault blocks maintain uniform pressure equilibrium in scenario S1, while each block changes to isolated pressure conditions at the non-uniform depletion scenarios S2 and S3. The deformation model is based on the Terzaghi effective stresses derived from the equilibrium total stresses and pore pressure (Markou and Papanastasiou 2020a). The analysis examines three depletion scenarios as shown in Table 2 and compares the uniform and non-uniform depletion options.…”