Geomechanical characterization of the CO2CRC Otway Project site, Australia

• Main Author: Aruffo, Chiara Maria
• Format: Others
• Language: English; English; en
• Published: 2015
• Online Access: https://tuprints.ulb.tu-darmstadt.de/4679/1/Chiara_thesis_revised_July.pdf; https://tuprints.ulb.tu-darmstadt.de/4679/7/Appendix.zip; Aruffo, Chiara Maria <http://tuprints.ulb.tu-darmstadt.de/view/person/Aruffo=3AChiara_Maria=3A=3A.html> (2015): Geomechanical characterization of the CO2CRC Otway Project site, Australia.Darmstadt, Technische Universität, [Ph.D. Thesis]

Storage of CO2 in the subsurface is one of the options available to lower the amount of CO2 in the atmosphere, a general priority in mitigating effects of climate change. In this frame, a number of challenges need to be solved to ensure a safe storage containment by avoiding wellbore failure, fault reactivation, leakage of CO2 along faults, caprock failure and microseismicity. Risks related to those issues can be successfully addressed with an accurate geomechanical characterization prior to injection. The effectiveness of geomechanical methods has been recognized in production of hydrocarbon reservoirs as well as in fluid storage (i.e. waste water and gas). The case study chosen for this thesis is the CO2CRC Otway Project, launched in 2005 in the state of Victoria (Australia) as first pilot study for CO2 storage in the southern hemisphere. As international partner of CO2CRC, the PROTECT Research Group was established in 2011 to develop a seismo-mechanical workflow able to predict deformation at sub-seismic level. The work presented in this thesis contributes to the workflow by providing a geomechanical characterization of the storage site. More specifically, finite element forward modelling is used to obtain a description of the 3D state of stress. A joint effort with partners from PROTECT Research group led to the setup of a geological model based on a detailed 3D seismic interpretation. In particular, it provides the geometry information needed to the buildup of the geomechanical model at the core of this thesis. The inclusion of all lithostratigraphic layers up to the ground surface results in a strengthened reliability of the geomechanical model that can be potentially implemented as a reference in future well planning. Wells logs and a literature review provide rock and fault properties to populate the model, regional stress data are used as boundary conditions for the model and stress measurements from the wells allow to calibrate the model. In situ local stress is analyzed following two different approaches, both using finite element techniques, to provide a comprehensive knowledge of effective and total stresses in the injection area. The response of the in situ stress field to changes in pore pressure due to CO2 injection in the reservoir is studied through a one-way flow and geomechanics coupled simulation. The computed effective stresses acting on the reservoir allow to assess caprock integrity and potential fault reactivation in relation to CO2 injection operations. Vertical rock displacements are also derived from the modelling to understand compaction of the reservoir and subsidence/uplift at ground surface level during the initial gas production and the subsequent CO2 injection phase. In addition, a parametric study estimates the pore pressure needed to cause fault reactivation for both numerical and analytical models, along with the corresponding maximum allowable daily injection rate. The second approach consists of a structural analysis describing the tectonic present-day in situ stress distribution at reservoir scale. Resolution of the model allows to identify perturbation in stress magnitudes within the reservoir level, related mainly to the presence of faults. Stability of faults is analyzed from a structural point of view, estimating the slip and dilation tendency of each fault under the computed stress conditions. Identification and modelling of the major tectonic stages allows the reconstruction of the geomechanical evolution of the injection site. Evidences from the implemented models show some discrepancies in the outcome. Possible sources of divergence between numerical and analytical approach are explored, as well as factors affecting stress modelling using the two different geomechanical simulators. Combination of results aims to analyze and understand the occurrence of local stress rotations and its causes. Finally, the temporal evolution of the fracture network is studied by correlating observed fractures and modelled states of stress. Finally, a comparison with previous geomechanical models for the CO2CRC Otway Project is conducted in terms of critical pore pressure for fault reactivation. Increasing availability of data used to constrain the models is reflected in an enhanced level of accuracy. However, those models are purely analytical and do not consider variability in rock properties, topographic effects, presence of faults and interaction between adjacent cells. The degree of complexity handled by the numerical model presented in this thesis contributes to increase the confidence on risk analysis results. To summarize, this thesis presents the first 3D geomechanical model of the CO2CRC Otway project, with the aim to provide a comprehensive geomechanical characterization of the storage site. Description of the 3D state of state and fault stability analysis, taking into account both total and effective stresses, have particular relevance for storage performance and future well planning. Besides this specific case study, the proposed workflow can be potentially applied to other injection sites for pre-injection geomechanical assessment. More generally, the same methodology could be followed for understanding state of stress and faults behavior in hydrocarbon and geothermal reservoirs.