Geomechanics in Depleted Faulted Reservoirs

Abstract

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.

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

“…Joints develop in both tensile and compressive stress regimes. Structural geometry of faults is constructed using the interpretation of available seismic data while the 3D representation of fault blocks and their mechanical reactions can create technical modeling challenges in meshing and shear failure (Healy et al 2015;Markou and Papanastasiou 2020b) Fault displacement and slip tendency can support the estimation of stress state in a faulted system and Alpak et al (2019) presented a model for structural connectivity of faults in a reservoir. Markou and Papanastasiou (2018) supported that the existing structural geometry of the fault blocks in reservoirs can indicate that the deformations occurred in the reservoir through the geological time driven by paleostresses.…”

“…The faulting regimes are primarily classified as normal faults, strike-slipping, and reserve faults; however, in actual conditions, a combination of the primary classes exists. Also, the horizontal stress anisotropy is defined by the ratio of S H /S h and is examined by Hettema (2020) and Markou and Papanastasiou (2020b).…”

“…The reservoir geology consists of the Early Miocene-Oligocene sandstones covered by Mid-Early Miocene mudstones and shales. Figure 1a and b shows two seismic sections from the study area, where a normal fault and a reverse fault of the Aphrodite field in the Eastern Mediterranean are illustrated (Markou and Papanastasiou 2020b). The reservoir zone is defined between the horizons in yellow and the faults are delineated with the black lines.…”

“…In sedimentary rocks, the lateral constraints generate horizontal stresses different from the vertical stress (Fjaer et al 2008) and the horizontal stress determination can be expressed by the lateral stress ratio (LSR) as the ratio of horizontal stress to vertical stress. In this case, the ratio of the horizontal effective stresses to the vertical effective stress is expressed by K 0 , as a function of the Poisson ratio, K 0 = ν/ (1−ν) (Markou and Papanastasiou 2020b). Alternatively, if rock is assumed to be in limit, equilibrium K 0 can be determined from the Jacky equation, K 0 = 1−sinφ.…”

“…Fig. 1 Seismic profiles illustrate two different fault geometries from the Aphrodite field where a normal fault is shown in (a), and a reverse fault is shown in (b) fromMarkou and Papanastasiou (2020b). Reservoir section of the Aphrodite field (c) afterMarkou and Papanastasiou (2018) …”