• Energy Research

AbstractThe influence of deep and regional geological structures is becoming increasingly important in superhot geothermal systems due to their proximity to the transition between brittleness and ductility. In the Los Humeros geothermal field in Mexico, where subsurface fluids reach temperatures of over 350 °C, the surface structures resulting from the collapse of calderas have so far only been interpreted at the local scale. The aim of this work is to place the recent tectonic and volcano-tectonic geomorphologic evolution and structures in the Los Humeros volcanic area in a regional context. NE- and NW-striking dominant structures resulting from a morpho-structural analysis on a regional scale are confirmed by negative and positive anomalies, respectively, after Butterworth filtering of gravity field data with different wavelengths over a local area of about 1000 km2. By analyzing the slip and dilation trends of the observed directions, we show the relevance of the regional context for reservoir exploration. The magnitudes of the principal stresses we estimate indicate a trans-tensional fault regime, a combination of strike-slip and normal faulting. The structures derived from the gravity and morpho-structural analyses, which are parallel to the maximum horizontal stress, have the highest potential for tensile and shear failure. Therefore, the corresponding negative gravity anomalies could be related to fracture porosity. Consequently, we hypothesize that these structures near the transition between brittleness and ductility control fluid flow in the Los Humeros geothermal field.

AbstractThe influence of deep and regional geological structures is becoming increasingly important in superhot geothermal systems due to their proximity to the transition between brittleness and ductility. In the Los Humeros geothermal field in Mexico, where subsurface fluids reach temperatures of over 350 °C, the surface structures resulting from the collapse of calderas have so far only been interpreted at the local scale. The aim of this work is to place the recent tectonic and volcano-tectonic geomorphologic evolution and structures in the Los Humeros volcanic area in a regional context. NE- and NW-striking dominant structures resulting from a morpho-structural analysis on a regional scale are confirmed by negative and positive anomalies, respectively, after Butterworth filtering of gravity field data with different wavelengths over a local area of about 1000 km2. By analyzing the slip and dilation trends of the observed directions, we show the relevance of the regional context for reservoir exploration. The magnitudes of the principal stresses we estimate indicate a trans-tensional fault regime, a combination of strike-slip and normal faulting. The structures derived from the gravity and morpho-structural analyses, which are parallel to the maximum horizontal stress, have the highest potential for tensile and shear failure. Therefore, the corresponding negative gravity anomalies could be related to fracture porosity. Consequently, we hypothesize that these structures near the transition between brittleness and ductility control fluid flow in the Los Humeros geothermal field.

Fracture network is a crucial element to address in any model of the thermo-hydro-mechanical behaviour of a reservoir rock. This study aims to provide quantified datasets and a further understanding of the critical parameters of the fracture network pattern in crystalline rocks. In the Northern Upper Rhine Graben, such rock units are targeted for multiple energy applications, from deep geothermal heat extraction to heat storage. Eleven outcrops were investigated with a combined LiDAR and 2D profiles analysis to extract faults and fracture network geometrical parameters, including length distribution, orientation, connectivity, and topology. These properties are used to decipher the structural architecture and estimate the flow properties of crystalline units. Fracture networks show a multi-scale power-law behaviour for length distribution. Fracture topology and orientation are mainly driven by both fault networks and lithology. Fracture apertures and permeability tensors were then calculated for two application case studies, including the stress field effect on aperture. Obtained permeabilities are in the range of those observed in the sub-surface in currently exploited reservoirs. The dataset provided in this study is thus suitable to be implemented in the modelling during the exploration stage of industrial applications involving fractured crystalline reservoirs.

Fracture network is a crucial element to address in any model of the thermo-hydro-mechanical behaviour of a reservoir rock. This study aims to provide quantified datasets and a further understanding of the critical parameters of the fracture network pattern in crystalline rocks. In the Northern Upper Rhine Graben, such rock units are targeted for multiple energy applications, from deep geothermal heat extraction to heat storage. Eleven outcrops were investigated with a combined LiDAR and 2D profiles analysis to extract faults and fracture network geometrical parameters, including length distribution, orientation, connectivity, and topology. These properties are used to decipher the structural architecture and estimate the flow properties of crystalline units. Fracture networks show a multi-scale power-law behaviour for length distribution. Fracture topology and orientation are mainly driven by both fault networks and lithology. Fracture apertures and permeability tensors were then calculated for two application case studies, including the stress field effect on aperture. Obtained permeabilities are in the range of those observed in the sub-surface in currently exploited reservoirs. The dataset provided in this study is thus suitable to be implemented in the modelling during the exploration stage of industrial applications involving fractured crystalline reservoirs.

Borehole heat exchanger (BHE) arrays represent a key technology for the future provision of sustainable building heating and cooling energy. They are either used as pure geothermal systems only extracting heating energy from the subsurface or they are also used to store excess heat from solar thermal collectors or waste heat from cooling applications in summer. The diversity of the systems makes it difficult to identify the optimal system in terms of emission reduction and economic efficiency. In this study, we assess the most relevant BHE system layouts for heating-only as well as combined heating and cooling purposes using dynamic simulations of the overall heating system in combination with an enviro-economic analysis method. The assessment routine is used in a multi-objective optimization approach to minimize the different system layouts' emission factor (EF) and their levelized cost of energy (LCOE). In order to cope with the high computational cost of the required long-term considerations, an artificial neural network (ANN) has been used to generate a proxy model in an intermediate step of the multi-objective optimization procedure. This approach delivers reliable optimization results, which reveal, that the lowest emissions for heating and cooling systems are realized by solar-assisted layouts. Comparison with a fossil-based reference layout shows that the most economical BHE layout accomplishes a 60% reduction in the EF with a moderate increase in the LCOE of only 13%. If, however, emission penalty costs are taken into account, the evaluated layouts also become economically advantageous compared to fossil-based systems.

Borehole heat exchanger (BHE) arrays represent a key technology for the future provision of sustainable building heating and cooling energy. They are either used as pure geothermal systems only extracting heating energy from the subsurface or they are also used to store excess heat from solar thermal collectors or waste heat from cooling applications in summer. The diversity of the systems makes it difficult to identify the optimal system in terms of emission reduction and economic efficiency. In this study, we assess the most relevant BHE system layouts for heating-only as well as combined heating and cooling purposes using dynamic simulations of the overall heating system in combination with an enviro-economic analysis method. The assessment routine is used in a multi-objective optimization approach to minimize the different system layouts' emission factor (EF) and their levelized cost of energy (LCOE). In order to cope with the high computational cost of the required long-term considerations, an artificial neural network (ANN) has been used to generate a proxy model in an intermediate step of the multi-objective optimization procedure. This approach delivers reliable optimization results, which reveal, that the lowest emissions for heating and cooling systems are realized by solar-assisted layouts. Comparison with a fossil-based reference layout shows that the most economical BHE layout accomplishes a 60% reduction in the EF with a moderate increase in the LCOE of only 13%. If, however, emission penalty costs are taken into account, the evaluated layouts also become economically advantageous compared to fossil-based systems.