Resource assessment and performance predictions in unconventional geothermal reservoirs

  • Ressourcenbewertung und Leistungsprognosen in unkonventionelle geothermische Reservoirs

Deb, Paromita; Clauser, Christoph (Thesis advisor); Saar, Martin (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021, Kumulative Dissertation


Geothermal energy, extracted from the earth’s natural heat, has the potential to sustainably meet the global energy demand with minimal environmental footprint. With possibilities to exploit energy from different types of geothermal reservoirs ranging from conventional hydrothermal resources to unconventional enhanced and super-hot geothermal systems, geothermal energy promises stable, sustainable, and geographically widespread production possibilities. Despite these advantages and the tremendous energy generating potential, geothermal projects suffer due to high initial costs during the exploration stage. Uncertainties associated with deep and unexplored geology significantly increase the financial risks of geothermal exploration projects, consequently influencing investment decisions. Some of the main geological factors that influence resource assessment particularly in unconventional geothermal fields are related to the limited knowledge of the underlying magmatic system, uncertain rock parameters, and coupled subsurface processes. Each of these factors is investigated in this work at different scales. For regional-scale resource assessment, the knowledge of the underlying heat source is essential to correctly evaluate the heat budget of the field. In this study, I propose a methodology for reconstructing the thermal regime in active volcanic fields characterized by multiple volcanic episodes and magmatic intrusions. Results show that reconstructing the complete temporal evolution, integrating magmatic source information from geochemical and petrological studies, can accurately capture the thermal anomalies observed in the geothermal field. At reservoir scale, I evaluate the impact of uncertain rock parameters for reservoir development and planning. Using stochastic modeling, I quantify and reduce uncertainty on temperature estimation, significantly improving the forecast of geothermal potential of the field. Finally, at laboratory-scale, I investigate the coupled nonlinear processes induced by stimulation in geothermal reservoirs. By performing hydraulic fracturing experiments in granite samples, I evaluate the most sensitive factors that influence the post-stimulation processes in low permeability rocks. The well-controlled laboratory-scale experiments resulted in a benchmark dataset, which was utilized for comparative performance assessment of two state-of-the-art numerical simulators. Results indicate that reliable prediction of pressure response and stimulated volume is possible through accurate modeling of injection system, fluid leak-off, and rock saturation conditions.The contributions presented in this thesis support geoscientists, engineers and investors involved in different types of geological exploration. The workflows are presented using examples of unconventional geothermal case studies. However, all the methodologies and investigations presented in this thesis find general application in geological exploration, including conventional geothermal and hydrocarbon resources. The thermal investigation workflow in active magmatic systems is not only valuable for heat budget evaluation, but also has implications for thermal maturity analysis of organic matter, an important factor in hydrocarbon prospect analysis. The stochastic reservoir modeling workflow provides significantly higher confidence on the predicted temperature and therefore, supports decision makers to perform better risk assessment before large economic investments are made. Lastly, the laboratory-scale investigation of hydraulic fractures and the associated experimental dataset is invaluable for code benchmarking and code comparison studies. Such code verification studies are essential for both industry and academic sectors involved in developing advanced stimulation techniques for enhanced geothermal systems as well as for underground geological storages. In conclusion, all the workflows presented in this thesis are proposed with an overall objective of reducing uncertainty in resource and productivity assessment of geothermal fields.