In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open source initiative for numerical simulation of thermo-hydro-mechanical-chemical (THMC) processes in porous media. The basic concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multi-field problems in porous and fractured media for applications in geoscience and hydrology. To this purpose OGS is based on an object-oriented FEM concept including a broad spectrum of interfaces for pre-and post-processing. The OGS idea has been in development since the mid eighties. We provide a short historical note about the continuous process of concept and software development having evolved through Fortran, C, and C++ implementations. The idea behind OGS is to provide an open platform to the community, outfitted with professional software engineering tools such as platform-independent compiling and automated benchmarking. A comprehensive benchmarking book has been prepared for publication. Benchmarking has been proven to be a valuable tool for cooperation between different developer teams, e.g. for code comparison and validation purposes (DEVOVALEX and CO2 BENCH projects). On one hand, object-orientation (OO) provides a suitable framework for distributed code development; however the parallelization of OO codes still lacks efficiency. High-performance-computin (HPC) efficiency of OO codes is subject to future research.

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In this paper we present an uncertainty analysis of thermo-hydro-mechanical (THM) coupled processes in a typical geothermal reservoir in crystalline rock. Fracture and matrix are treated conceptually as an equivalent porous medium, and the model is applied to available data from the Urach Spa and Falkenberg sites (Germany). The finite element method (FEM) is used for the numerical analysis of fully coupled THM processes, including thermal water flow, advective-diffusive heat transport, and thermoelasticity. Non-linearity in system behavior is introduced via temperature and pressure dependent fluid properties. Reservoir parameters are considered as spatially random variables and their realizations are generated using conditional Gaussian simulation. The related Monte-Carlo analysis of the coupled THM problem is computationally very expensive. To enhance computational efficiency, the parallel FEM based on domain decomposition technology using message passing interface (MPI) is utilized to conduct the numerous simulations. In the numerical analysis we considered two reservoir modes: undisturbed and stimulated. The uncertainty analysis we apply captures both the effects of heterogeneity and hydraulic stimulation near the injection borehole. The results show the influence of parameter ranges on reservoir evolution during long-term heat extraction, taking into account fully coupled thermo-hydro-mechanical processes. We found that the most significant factors in the analysis are permeability and heat capacity. The study demonstrates the importance of taking parameter uncertainties into account for geothermal reservoir evaluation in order to assess the viability of numerical modeling.

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25For carbon capture and storage to successfully contribute to climate mitigation efforts, the 26 captured and stored CO2 must be securely isolated from the atmosphere and oceans for a 27 minimum of 10,000 years. As it is not possible to undertake experiments over such timescales, 28here we investigate natural occurrences of CO2, trapped for 10 4 -10 6 yr to understand the 29 geologic controls on long term storage performance. We present the most comprehensive 30 natural CO2 reservoir dataset compiled to date, containing 76 naturally occurring natural CO2 31 stores, located in a range of geological environments around the world. We use this dataset 32to perform a critical analysis of the controls on long-term CO2 retention in the subsurface. We 33 find no evidence of measureable CO2 migration at 66 sites and hence use these sites as 34 examples of secure CO2 retention over geological timescales. We find unequivocal evidence 35 of CO2 migration to the Earth's surface at only 6 sites, with inconclusive evidence of migration 36 at 4 reservoirs. Our analysis shows that successful CO2 retention is controlled by: thick and 37 multiple caprocks, reservoir depths of >1200m, and high density CO2. Where CO2 has 38 migrated to surface, the pathways by which it has done so are focused along faults, illustrating 39 that CO2 migration via faults is the biggest risk to secure storage. However, we also find that 40 many naturally occurring CO2 reservoirs are fault bound illustrating that faults can also 41 securely retain CO2 over geological timescales. Hence, we conclude that the sealing ability of 42 fault or damage zones to CO2 must be fully characterised during the appraisal process to fully 43 assess the risk of CO2 migration they pose. We propose new engineered storage site selection 44 criteria informed directly from on our observations from naturally occurring CO2 reservoirs. 45These criteria are similar to, but more prescriptive than, existing best-practise guidance for 46 selecting sites for engineered CO2 storage and we believe that if adopted will increase CO2 47 storage security in engineered CO2 stores.

Meeting inter-seasonal fluctuations in electricity production or demand in a system dominated by renewable energy requires the cheap, reliable and accessible storage of energy on a scale that is currently challenging to achieve. Commercially mature compressed air energy storage (CAES) could be applied to porous rocks in sedimentary basins worldwide where legacy data from hydrocarbon exploration are available, and where geographically close to renewable energy sources. Here we present a modeling approach to predict the potential for CAES in porous rocks. By combining these with an extensive geological database we provide a regional assessment of this potential for the UK.

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