An Integrated Geological and Mathematical Framework for the Characterization, Modelling and Simulation of Fractured Geothermal Reservoirs

Geothermal energy is a sustainable and renewable form of energy that is already being exploited in regions were heat in the subsurface is easily accessible, such as near tectonic plate boundaries where volcanism is common. However, in regions where the ground is less 'hot', away from tectonic plate boundaries, exploiting heat from the ground requires accessing geological reservoirs deep in the subsurface (4-5 km or more). In enhanced geothermal systems (EGS), heat from the subsurface is extracted from rocks where fluid flow predominantly occurs in fractures. Cold water is injected into rocks at depth, where the water flows through the fractures and is heated by the surrounding rocks, before the heated water is brought back to the surface for energy purposes. To understand the properties and dynamic behaviour of such reservoirs, there is a need to understand both the capabilities of the rock to exchange heat with fluids, and to understand how the fluids flow through the fractures. This understanding is achieved through a combination of several fields, two of the most central of which are geological reservoir characterisation and mathematical modelling. The geologists characterise the the rocks and the fractures within it, attempting to provide as much information as possible about the reservoir. The mathematicians, on the other hand, make use of this information to model and simulate fluid flow and heat exchange in the reservoir. However, some of the main challenges of this approach are that there is i) an incomplete understanding of which geological parameters provide the most direct route to understand the flow in the fractures, and ii) that the mathematical simulation tools are not fully capable of preserving a necessary level of geological detail in its representation of the reservoir. The main aim of this project has been to overcome these challenges, bringing the geology and mathematics of fractured geothermal reservoirs closer together, developing new and improved workflows. The research in the project has focused on how natural fracture systems can studied and characterized, to develop methods, knowledge and numerical tools that can be used in geothermal energy systems. The project work has lead to several breakthroughs in improving the geological realism in mathematical models of fracture networks. This has involved that we have been able to identify which geological parameters that are critical for correct and improved repersentation of fracture networks in reservoir and flow simulation models, as well as having successfully established modelling and simulation methods that are able to reproduce the network properties of natural fracture networks in a more realistic way than what is presently possible with existing methods. We have developed new numerical methods and software, and new knowledge about how mathematical concepts can be utilized to describe geological fracture networks quantitatively. The most important breakthrough achieved in this project is that we have been able to develop methods for stochastic fracture generation that utilize quantitative information about the connectivity of fracture networks, based on data from naturally occurring fracture networks. The results of the project have been presented in 30 contributions at international conferences and meetings, and has resulted in the publication of 20 peer-reviewed papers in international journals.

The project has (i) developed new insights into the characterization and quantitative description of fracture networks, and (ii) improved the integration of such data into reservoir models and numerical simulations of fluid flow and energy extraction, and (iii) developed new numerical medthods/tools for integration/honouring of quantitative network topology into fracture grid models. Of particular significance is the achievement of developing numerical methods that are able to integrate topological fracture network data into simulation models. For geothermal reservoir characterization and modelling globally, application of the methods developed in this project has the potential to help create more accurate reservoir and simulation models, which in turn can contribute to more accurate and improved modelling and forecasting of energy extraction in geothermal systems.

Fractures represent the main conduits for fluid flow in Enhanced Geothermal Systems (EGS) and characterization of fractures with the aim to understand fluid flow generally involves collection and identification of a range of different fracture data. Based on the data, simulation models are constructed with the aim of identifying the fracture network's impact on reservoir dynamics. Geological modeling and simulation are both crucial for assessing the commercial viability of production strategies for fractured geothermal reservoirs, but the current practice is hindered by the following obstacles: - Despite a global wealth of knowledge of fractured reservoirs, it is unclear which set of fracture parameters that provide the most direct route to realistically predicting flow properties. - Computational efficient simulation tools that also preserve a necessary level of geological detail are still to be developed - The above points can only be addressed by combining advancement within characterization and simulation of fractured media; however, state of the art within the two fields advance separately, and advancement in one only slowly spills into the other. In this project, we will overcome these difficulties by developing an integrated geological and mathematical framework for collection and mutual transfer of data between geological and simulation models for EGS. The main deliverables of the project are: [D1] Optimized geological data types and formats and best-practice workflow for collection of geological data that provide the most direct route to realistic prediction of flow properties. [D2] Simulation models based on optimized geological input that accounts for the topological characteristics of fracture networks. [D3] A fully integrated geological-mathematical framework for reservoir characterization and flow simulation for geothermal basement reservoirs.