Fischer, Karsten (2013)
Geomechanical reservoir modeling – workflow and case study from the North German Basin.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
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Item Type: | Ph.D. Thesis | ||||
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Type of entry: | Primary publication | ||||
Title: | Geomechanical reservoir modeling – workflow and case study from the North German Basin | ||||
Language: | English | ||||
Referees: | Henk, Prof. Dr. Andreas ; Stein, PD Dr. Eckardt | ||||
Date: | 11 September 2013 | ||||
Place of Publication: | Darmstadt | ||||
Date of oral examination: | 18 October 2013 | ||||
Abstract: | There is an increasing importance for the optimal exploitation of conventional hydrocarbon reservoirs to have detailed knowledge of the specific state of stress in a reservoir and to gain clarity on the corresponding geomechanical implications. This knowledge is even becoming mandatory for most unconventional plays. The local stress field directly affects, for instance, wellbore stability, the orientation of hydraulically induced fractures, and – especially in fractured reservoirs – permeability anisotropies. Robust information on the locally prevailing stresses is thus ideally required prior to drilling. Numerical models based on the finite element (FE) method are able to cope with the complexity of real reservoirs. Acting as predictive tools, these models not only provide quantitative information on the stress distribution, but also a process-based understanding of geomechanical reservoir behavior. This study evaluates the potential of geomechanical FE models for the prediction of local in situ stress distribution and fracture networks in faulted reservoirs. The work of this study was conducted in cooperation with three major operators in the E&P industry and comprises two main parts. In the first methodological part, a generally applicable workflow is developed for building geomechanical FE models and calibrating them to field data. These models focus on spatial variations of in situ stress resulting from faults and contrasts in mechanical rock properties. Special techniques are elaborated regarding the transfer of the reservoir geometry from geological subsurface models to the numerical model and for the most effective application of boundary conditions. Complex fault geometries and the detailed topology of lithostratigraphic horizons can be considered on reservoir scale. In combination with reservoir-specific material parameters the incorporated horizons establish a mechanical stratigraphy inside the model. Faults are implemented as discrete planes by 2D interface elements. This allows fault-specific stresses and corresponding fault behavior to be analyzed. The resulting geomechanical models comprise high spatial resolution and several million elements. They are calculated in time spans of less than 20 hours by using high-performance computing. In addition, submodels resolving a detailed mechanical stratigraphy can be integrated into the reservoir-wide modeling for local focus. In the second part of the study, the workflow was successfully applied to an intensively faulted gas reservoir in the North German Basin. Comprehensive datasets are provided by the field operators and project partners for building and calibrating a detailed and truly field-scale geomechanical model covering more than 400km². It incorporates a network of 86 faults and a mechanical stratigraphy of three layers comprising reservoir-specific material parameters. For the static modeling approach, the present-day regional stress field is applied as boundary condition. Static modeling results are compared to local stress measurements, e.g. orientations from borehole breakouts and magnitudes from frac data. After iterative calibration, the best-fit model reveals the recent in situ stress distribution and individual fault behavior throughout the reservoir. The results show significant local perturbations of stress magnitudes (max. ±10MPa over 1-2km distance) and only minor deviations in stress orientation from the regional trend (max. ±25°). The strong dependency on the specific fault trace, offset and interactions precludes the derivation of generally valid rules for estimating stress variations and underlines the necessity of numerical modeling. Analysis of fault-specific results indicates that critical stress states occur most likely on NW-SE trending faults in the present-day stress field. Fracture information is inferred from a (geo-)dynamic model focusing on the major stages in the tectonic history of the reservoir and the respective past in situ stresses. Consequently, paleo-stress fields are applied as boundary condition and material parameters are adjusted. Correlation of fracture orientations and modeled paleo-stresses in the reservoir allows the formation of fracture sets to be assigned to Triassic and Late Jurassic to Early Cretaceous times. Increased perturbation intensity in the Late Jurassic to Early Cretaceous is related to potential reactivation of NW-SE trending faults and explains the variability of the corresponding fracture set. These results elucidate how stress perturbations can explain fracture variability without the need for complex tectonic histories. Furthermore, the dynamic model sheds light on fault zone permeability. Modeling indicates that if cataclasis is responsible for a reduced fault permeability, then it will most likely occur along E-W and NNE-SSW trending faults due to the high slip tendency values they experienced in the tectonic past. Modeling results show no such increased geomechanical exposure for NW-SE oriented faults. However, high dilation tendencies support the possibility of activity of these faults in Late Jurassic times – as proposed by fracture correlation. Low permeability of NW-SE trending faults is thus most likely the result of fluid entry and illitization, which is also observed at a wellbore close to such a fault set. The combination of static and dynamic modeling results suggests no significant impact of critically stressed natural fractures on the recent hydraulic behavior of the entire reservoir. Additionally tests of fault block refinements and submodels demonstrate their capability to provide further increased spatial resolution in areas of particular interest. The submodel generated for the northwestern part of the case study underlines the impact of the specific connections of the fault network on the modeling results. The outcome of this study confirms the high potential of geomechanical FE models to reveal the specific in situ stress and fault behavior, and to infer fracture characteristics from paleo-stresses. Beside the case study specific insights, the successfully applied and approved workflow can be used for future modeling of stress-sensitive reservoirs. Furthermore, the geomechanical models are not limited in application to the hydrocarbon industry. As general tools for stress prediction in undrilled rock formations, they can also be applied to deep geothermal reservoirs and underground engineering, for instance. The possibility of characterizing fault behavior makes the models additionally valuable in the fields of carbon capture and storage (CCS) and nuclear waste disposal. |
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URN: | urn:nbn:de:tuda-tuprints-36476 | ||||
Classification DDC: | 500 Science and mathematics > 550 Earth sciences and geology | ||||
Divisions: | 11 Department of Materials and Earth Sciences 11 Department of Materials and Earth Sciences > Earth Science 11 Department of Materials and Earth Sciences > Earth Science > Engineering Geology |
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Date Deposited: | 21 Nov 2013 14:38 | ||||
Last Modified: | 24 Jul 2020 11:09 | ||||
URI: | https://tuprints.ulb.tu-darmstadt.de/id/eprint/3647 | ||||
PPN: | 386308047 | ||||
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