Thermo-mechanical characterization of the lithosphere: Implications for geothermal resources

Abstract

The two key ingredients needed to commercially exploit a geothermal energy system are (1) sufficiently high subsurface temperatures and (2) presence of rock formations suitable to act as a geothermal reservoir at reachable depths. Subsurface temperatures are controlled by the heat flowing from deep inside the Earth to its surface with the heat transfer in the lithosphere primarily marked by conduction. The thermal structure of the lithosphere itself depends on its thickness, thermal properties, basal heat flow, and mode of heat transfer, which are all products of its geodynamic history. The thermal structure and rheological properties of tectonic plates determine how rocks in the lithosphere behave mechanically, that is whether they break, bend, or flow, thereby affecting geothermal reservoirs and thus the amount of geothermal resources available. In this PhD thesis, large-scale geophysical models are combined with more detailed basin-scale models to construct physics-based thermo-mechanical models of the lithosphere for geothermal exploration. A one-dimensional thermo-mechanical model is used to study the thermal field and lithosphere strength of the Central System and adjacent Tajo and Duero basins of Spain. Brittle basement and sedimentary rocks, underlain by crust with abundant radioactive minerals, appear to be excellent sites for geothermal energy systems. A three-dimensional stochastic temperature model is used to update the thermal and rheological model of the European lithosphere. Misfits of the thermal model with the temperature observations are used to study lithosphere thermal properties and transient or non-conductive heat transfer. These misfits are then minimized by applying an Ensemble Smoother with Multiple Data Assimilation technique (ES-MDA). Based on the new thermal results the rheological model is refined and used to estimate the integrated strength of the European lithosphere. An improved understanding of the thermo-mechanical state of the lithosphere can aid geothermal resource assessments. This study includes two of those geothermal resource assessments: The first study covers the resource base for Enhanced Geothermal Systems (EGS) in Europe, which is estimated by economically constraining geothermal potential estimates by applying a discounted cash-flow model. By applying different well cost scenario's, a more realistic depth-dependent and economically-constrained technical potential is obtained. The results of a sensitivity analysis show that the Levelized Cost of Energy (LCOE) is mostly dependent on reservoir temperature, depth and permeability. The second study covers a global geothermal energy resource assessment of deep sedimentary aquifers for direct heat utilization. Greenhouse heating, spatial heating, and spatial cooling are considered in this assessment. Subsurface temperatures are inferred from geophysical data and a volumetric heat-in-place method is applied to improve current global geothermal resource base estimates for direct heat applications. The amount of thermal energy stored within deep sedimentary aquifers depends on the Earth's heat flow, aquifer volume, and thermal properties. The thermal energy available is assessed by estimating subsurface temperatures up to a depth of three kilometer depending on aquifer thickness. Suitable aquifers underlay large parts of the Earth's land surface and contain an substantial amount of heat that could potentially be used for direct heat applications.