PhD thesis M. Zain-Ul-Abedin
SUBI – Safety of underground gas storage sites during cyclic loading: Functionality, integrity and monitoring of reservoir and wells
The project is motivated by the need for underground gas storage (UGS), which can compensate short-term high-amplitude fluctuations in energy demand on top of the summer-winter cycle. It is part of the larger research project SUBI funded by the BMBF, which also comprises borehole-scale studies and the analysis of salt caverns for UGS. Our focus is on coupled thermal-hydraulic-mechanical (THM) multi-phase models of porous reservoirs in order to assess the impact of frequently changing injection-production cycles onto fracture formation, fault reactivation and cap rock integrity. Among others, this will help to define the maximum safe injection pressure and, thus, the storage capacity of an UGS site.
Beside generic studies, work concentrates on a former gas field in the Bavarian Molasse Basin east of Munich for which a hypothetical transformation to an UGS is considered. Uniper SE kindly provided the various data sets relevant for this case study. The workflow combines thermo-hydraulic (TH) calculations based on Eclipse with mechanical (M) simulations using the Techlog and Visage software (Fig. 1). The geometry of the case study reservoir was build up through seismic interpretation, thickness maps as well as through well data. The subsurface model shows a fault-bounded structure with lateral dimensions of 4 km in N-S and 8 km in W-E direction, respectively. The pore pressure field at the beginning of the UGS stage is derived from an Eclipse simulation through history matching of pore pressure observed during the production and subsequent shut-in phase (Fig. 2). Mechanical properties (Young’s modulus, Poisson ratio, Biot’s coefficient, density), pore pressures as well as a first estimate for the vertical and horizontal stresses at well locations are derived using log data and 1D mechanical earth modeling (MEM) software Techlog, respectively. These data is also used to set up the 3D geomechanical model in Visage (Petrel Reservoir Geomechanics) which is coupled to the 3D Eclipse simulation through the mutual dependence of pore pressure, effective stresses, volumetric strain, porosity and permeability. It is expected that this combination of reservoir engineering and reservoir geomechanics provides valuable insights into the safe use of UGS sites also for injection-production cycles with high frequency (days to weeks).
The subproject “THM-multiphase modelling on reservoir scale for utilization of underground gas storage sites as short-term storage” within the joint project UG: SUBI is funded by the Federal Ministry of Education and Research (BMBF) in the program "Geo-Research for sustainability (GEO:N)“ under grant 03G0869B
Zain ul Abedin, M., Henk, A., Rudolph, T., 2019. Coupling of flow and geomechanical simulations for short-term underground gas storage – a case study from the Bavarian Molasse Basin, DGMK/ÖGEW-Früjahrstagung, 25-26 April 2019, Celle, Germany.
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