Reservoir characterization of super-hot unconventional geothermal systems, Mexico

Super-hot unconventional geothermal systems (> 350 °C) possess enormous potential for geothermal power and heat generation that could be harnessed in the future. Due to their high temperature fluids, they could provide significantly more energy per well than existing conventional wells. However, the development of such systems is challenging and their controlling factors are not fully understood yet. The high temperatures and often aggressive reservoir fluids have led to numerous drilling problems and eventual abandonments of wells in the past. Overcoming these challenges requires a deeper un-derstanding of these reservoirs and the development of innovative exploration and drilling technologies. As part of the GEMex H2020 project, this thesis focuses on the reservoir characterization of super-hot unconventional geothermal systems linked to volcanic settings in Mexico. Two caldera complexes located in the northeastern Trans-Mexican Volcanic Belt, the Acoculco and Los Humeros caldera, were selected as demonstration sites. Sound knowledge of the reservoir units, their properties, and spatial heterogeneities in the subsurface is crucial for the interpretation of geophysical data as well as the parametrization of numerical models, and thus, forms the basis for an economic reservoir as-sessment and management. To improve the geological understanding of the caldera complexes, iden-tify potential reservoirs, and obtain information on the physiochemical and mechanical characteristics of the reservoir rocks, a comprehensive outcrop analogue and wellbore core study was performed. An innovative multi-method approach was used to create a comprehensive rock property database with more than 31000 data entries on 34 petrophysical and mechanical rock parameters, facilitating the application of the data for different disciplines and modeling approaches at different scales (local to super-regional). All relevant key units from the basement to the cap rock of the geothermal fields were characterized and distinct parameter ranges were defined for each unit. The results highlight the geo-logical complexity of volcanic systems, which leads to high variability in rock properties that must be individually considered for each parameter and unit in a 3D reservoir model. Using the Los Humeros caldera as an example, a step-by-step guide for the parametrization of a 3D geothermal model was proposed. Subsequently, processes that affect the rock parameters were investigated. Fluid flow and fluid-rock interactions of the Los Humeros geothermal field are predominantly fracture-controlled and have significantly altered the physiochemical rock properties and their relationships. The investigation of alteration facies indicates that the reservoir rocks were affected by multiple hydrothermal events over time and that the geothermal reservoir is most likely fed by multiple heat sources. A new chemi-cal discrimination approach was proposed for Los Humeros, which helped to relate the subsurface units to the corresponding formations in the outcrops. Based on the new findings, recommendations for the update of the static 3D geological model were defined. Finally, the results of this work were used to create a local density model using gravity data obtained in the central collapse zone of the caldera complex to better characterize the shallow structures of the geothermal reservoir. Different parametrization approaches were tested including assumed average densities, weighted densities and a multimodal density distribution for each model unit, respectively. By using the multimodal density approach the misfit can be significantly reduced allowing for a more pre-cise mapping of the different lithologies in the subsurface. The results highlight the importance of a profound rock property characterization and the correct estimation of the properties at reservoir depth during reservoir characterization