Nikolaos Markou | University of Cyprus (original) (raw)
articles by Nikolaos Markou
Rock Mech Rock Eng, 2024
Hydrocarbon reservoir structures are subjected to tectonic forces along the geological time that ... more Hydrocarbon reservoir structures are subjected to tectonic forces along the geological time that cause rock deformation and break into faulted zones. Faulted reservoirs, enclose certain complexity in terms of the distributed effective stresses, rock plastic alteration, slipping and fault block displacement. In this study, we develop a three-dimensional (3D) geomechanical reservoir model with faulted and compartmentalized geometry, located in the offshore deepwater environment of the Levantine basin in the Eastern Mediterranean, based on non-linear finite element analysis (FEA). A regional structural and stress map was also constructed, integrating various data sources, to present the regional stress setting to enhance this work. The assessment of the geomechanical impacts on the reservoir provides important information in reservoir studies, that can analyze potential stability issues during the depletion to optimize the field production planning. Stress–strain evolution in the reservoir is primarily affected by the in situ stresses, the geometry of faults, and the degree of compartmentalization. The results demonstrate clearly the mechanism of stress transfer transmission and the impact between the fault block compartments in the reservoir. Fault contacts exhibit a higher tendency for rock displacements and deformations. Plastic yielding develops at a narrow extent along the faults. The risk of fault slipping depends on the depletion strategy, but it is low in all cases. No significant reduction in permeability was found at the end of reservoir depletion. Overall, geomechanics integration enriches and improves the dynamic reservoir models and applications.
SPE Annual Technical Conference and Exhibition, 26-29 October, Virtual, 2020
This paper presents an optimum way to produce down to depletion a compartmentalized reservoir in ... more 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.
A fault stress analysis of a typical gas field in the Eastern Mediterranean is presented. The obj... more A fault stress analysis of a typical gas field in the Eastern Mediterranean is presented. The objective of this study is to provide estimates of the in situ stresses and pore pressure for populating a regional Mechanical Earth Model and to characterize the stability of faults under current and changing reservoir conditions. The fault stability analysis is based on the Mohr-Coulomb frictional faulting theory. The vertical in situ stress is estimated using seismic and density data and the bounds of the horizontal stresses were determined for different fault regimes. The pore pressure for determining the effective in situ stresses is estimated using the Bowers pore pressure prediction method. Fault stress analysis is performed in a series of calculations and the results are plotted on Mohr diagrams for shear failure. The fault stress analysis is performed on a wide range of alternative azimuth orientations for SHmax in order to capture the uncertainty on the actual orientation. Sensitivity with respect to reservoir pore pressure change suggests that pressure reduction in the reservoir improves the fault stress stability, ignoring in the current analysis any stress arching effects. Pore pressure increase decreases the normal stress on the fault leading to increasing risk of shear failure of the critically stressed faults. The case study examines eight faults on the Aphrodite gas field with the objective to characterize if the faults are active or remain dormant under current stress conditions and how the stability may change in reservoir injection or depletion conditions.
Papers by Nikolaos Markou
ARMA/DGS/SEG, 2022
In this study, we present a three-dimensional (3D) geomechanical reservoir model for a faulted an... more In this study, we present a three-dimensional (3D) geomechanical reservoir model for a faulted and compartmentalized reservoir in the Eastern Mediterranean. A series of alternative production scenarios performed using a simulation model that accounts for consolidation and plasticity deformation of the rocks. Plastic yielding is mainly developed in fault slip zones of narrow extent whereas it appears that there is low risk of plastic behavior in the main reservoir. The slip conditions become complex in the fault contact surfaces where local areas close to fault connections are more pronounced to slip creating localized areas of smaller faulted zones. Displacement magnitudes, are controlled by the structural boundary conditions and the geometrical shape of each fault block. Overall, 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, changes of normalized permeability can be drawn in the 3D space providing additional insights of heterogeneous distribution.
Energies, 2020
This paper examines the impact of the effective stresses that develop during depletion of a fault... more This paper examines the impact of the effective stresses that develop during depletion of a faulted reservoir. The study is based on finite element modeling using 2D plane strain deformation analysis with pore pressure and elastoplastic deformation of the reservoir and sealing shale layers governed by the Drucker–Prager plasticity model. The mechanical properties and response of the rock formations were derived from triaxial test data for the sandstone reservoirs and correlation functions for the shale layers. A normal fault model and a reverse fault model were built using seismic data and interpretation of field data. The estimated tectonic in-situ stress field was transformed to the plane of the modeled geometry. Sensitivity studies were performed for uncertainties on the values of the initial horizontal stress and for the friction of the fault surfaces. It was found that the stress path during depletion is mainly controlled by the initial lateral stress ratio (LSR). The developed effective stresses with depletion are influenced by the fault geometry of the compartmentalized blocks. Plastic deformation develops for low LSR whereas for high values the system tends to remain in the elastic region. When plastic deformation takes place, it affects mainly the region near the fault. The reservoir deformation is dominated by vertical displacement which is higher near the fault region and nearly uniform in the remote area. The volumetric strain is dominated by compaction. More volatile conditions in relation to change of the friction coefficient and LSR were found for the normal fault geometry.
Rock Mech Rock Eng, 2024
Hydrocarbon reservoir structures are subjected to tectonic forces along the geological time that ... more Hydrocarbon reservoir structures are subjected to tectonic forces along the geological time that cause rock deformation and break into faulted zones. Faulted reservoirs, enclose certain complexity in terms of the distributed effective stresses, rock plastic alteration, slipping and fault block displacement. In this study, we develop a three-dimensional (3D) geomechanical reservoir model with faulted and compartmentalized geometry, located in the offshore deepwater environment of the Levantine basin in the Eastern Mediterranean, based on non-linear finite element analysis (FEA). A regional structural and stress map was also constructed, integrating various data sources, to present the regional stress setting to enhance this work. The assessment of the geomechanical impacts on the reservoir provides important information in reservoir studies, that can analyze potential stability issues during the depletion to optimize the field production planning. Stress–strain evolution in the reservoir is primarily affected by the in situ stresses, the geometry of faults, and the degree of compartmentalization. The results demonstrate clearly the mechanism of stress transfer transmission and the impact between the fault block compartments in the reservoir. Fault contacts exhibit a higher tendency for rock displacements and deformations. Plastic yielding develops at a narrow extent along the faults. The risk of fault slipping depends on the depletion strategy, but it is low in all cases. No significant reduction in permeability was found at the end of reservoir depletion. Overall, geomechanics integration enriches and improves the dynamic reservoir models and applications.
SPE Annual Technical Conference and Exhibition, 26-29 October, Virtual, 2020
This paper presents an optimum way to produce down to depletion a compartmentalized reservoir in ... more 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.
A fault stress analysis of a typical gas field in the Eastern Mediterranean is presented. The obj... more A fault stress analysis of a typical gas field in the Eastern Mediterranean is presented. The objective of this study is to provide estimates of the in situ stresses and pore pressure for populating a regional Mechanical Earth Model and to characterize the stability of faults under current and changing reservoir conditions. The fault stability analysis is based on the Mohr-Coulomb frictional faulting theory. The vertical in situ stress is estimated using seismic and density data and the bounds of the horizontal stresses were determined for different fault regimes. The pore pressure for determining the effective in situ stresses is estimated using the Bowers pore pressure prediction method. Fault stress analysis is performed in a series of calculations and the results are plotted on Mohr diagrams for shear failure. The fault stress analysis is performed on a wide range of alternative azimuth orientations for SHmax in order to capture the uncertainty on the actual orientation. Sensitivity with respect to reservoir pore pressure change suggests that pressure reduction in the reservoir improves the fault stress stability, ignoring in the current analysis any stress arching effects. Pore pressure increase decreases the normal stress on the fault leading to increasing risk of shear failure of the critically stressed faults. The case study examines eight faults on the Aphrodite gas field with the objective to characterize if the faults are active or remain dormant under current stress conditions and how the stability may change in reservoir injection or depletion conditions.
ARMA/DGS/SEG, 2022
In this study, we present a three-dimensional (3D) geomechanical reservoir model for a faulted an... more In this study, we present a three-dimensional (3D) geomechanical reservoir model for a faulted and compartmentalized reservoir in the Eastern Mediterranean. A series of alternative production scenarios performed using a simulation model that accounts for consolidation and plasticity deformation of the rocks. Plastic yielding is mainly developed in fault slip zones of narrow extent whereas it appears that there is low risk of plastic behavior in the main reservoir. The slip conditions become complex in the fault contact surfaces where local areas close to fault connections are more pronounced to slip creating localized areas of smaller faulted zones. Displacement magnitudes, are controlled by the structural boundary conditions and the geometrical shape of each fault block. Overall, 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, changes of normalized permeability can be drawn in the 3D space providing additional insights of heterogeneous distribution.
Energies, 2020
This paper examines the impact of the effective stresses that develop during depletion of a fault... more This paper examines the impact of the effective stresses that develop during depletion of a faulted reservoir. The study is based on finite element modeling using 2D plane strain deformation analysis with pore pressure and elastoplastic deformation of the reservoir and sealing shale layers governed by the Drucker–Prager plasticity model. The mechanical properties and response of the rock formations were derived from triaxial test data for the sandstone reservoirs and correlation functions for the shale layers. A normal fault model and a reverse fault model were built using seismic data and interpretation of field data. The estimated tectonic in-situ stress field was transformed to the plane of the modeled geometry. Sensitivity studies were performed for uncertainties on the values of the initial horizontal stress and for the friction of the fault surfaces. It was found that the stress path during depletion is mainly controlled by the initial lateral stress ratio (LSR). The developed effective stresses with depletion are influenced by the fault geometry of the compartmentalized blocks. Plastic deformation develops for low LSR whereas for high values the system tends to remain in the elastic region. When plastic deformation takes place, it affects mainly the region near the fault. The reservoir deformation is dominated by vertical displacement which is higher near the fault region and nearly uniform in the remote area. The volumetric strain is dominated by compaction. More volatile conditions in relation to change of the friction coefficient and LSR were found for the normal fault geometry.