Abstract
Abstract. The optimal use of conventional and unconventional hydrocarbon reservoirs depends, amongst other things, on the local tectonic stress field. For example, wellbore stability, orientation of hydraulically induced fractures and – especially in fractured reservoirs – permeability anisotropies are controlled by the present-day in situ stresses. Faults and lithological changes can lead to stress perturbations and produce local stresses that can significantly deviate from the regional stress field. Geomechanical reservoir models aim for a robust, ideally "pre-drilling" prediction of the local variations in stress magnitude and orientation. This requires a numerical modelling approach that is capable to incorporate the specific geometry and mechanical properties of the subsurface reservoir. The workflow presented in this paper can be used to build 3-D geomechanical models based on the finite element (FE) method and ranging from field-scale models to smaller, detailed submodels of individual fault blocks. The approach is successfully applied to an intensively faulted gas reservoir in the North German Basin. The in situ stresses predicted by the geomechanical FE model were calibrated against stress data actually observed, e.g. borehole breakouts and extended leak-off tests. Such a validated model can provide insights into the stress perturbations in the inter-well space and undrilled parts of the reservoir. In addition, the tendency of the existing fault network to slip or dilate in the present-day stress regime can be addressed.
Highlights
The tectonic stress field strongly affects the optimal exploitation of conventional and unconventional hydrocarbon reservoirs
The workflow presented in this paper can be used to build 3-D geomechanical models based on the finite element (FE) method and ranging from field-scale models to smaller, detailed submodels of individual fault blocks
We present a detailed workflow utilising FE techniques to build and calibrate geomechanical reservoir models (Fig. 1)
Summary
The tectonic stress field strongly affects the optimal exploitation of conventional and unconventional hydrocarbon reservoirs. In some fault-controlled reservoirs, local stress reorientations of up to 90◦ relative to the regional trend have been reported (Maerten et al, 2002; Yale, 2003) In such cases, inference of local in situ stress orientations from regionalscale maps would inevitably lead to an incorrect pre-drilling prediction. Any robust prognosis has to incorporate the specific 3-D geological reservoir structure including faults as well as the specific rock mechanical behaviour of the reservoir under concern Such complexities can only be treated adequately by a numerical modelling approach. The reservoir rock is formed by a faulted aeolian sandstone from the Upper Rotliegend (Doornenbal and Stevenson, 2010) It makes an ideal case study for geomechanical modelling, as several data sets are available to set up the numerical simulation and compare model predictions to stress data observed
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