Reservoir stimulation and engineering (Task 1.2)

The Swiss Federal Energy Strategy 2050 calls for scientific research to support industry's endeavours to (1) generate electricity from geothermal energy and (2) develop deep geological sites for permanent, safe storage of CO2, such that gas-fired power stations may be operated in an environmentally sustainable way. At present, there are no proven sites for these technologies in Switzerland. This project will therefore support industry's first steps towards these aims, namely the assessment of geothermal and gas-storage resources, the exploration for promising drilling sites and the characterization of potential heat- and gas-storage reservoirs. The project will elaborate exploration guidelines and methodologies to lower the risk of failure of exploration drilling.

The research is organized into four subprojects, with overlapping participation of 17 scientists from the disciplines of geology, geochemistry, structural geology, geophysics and petrophysics at the University of Bern, the University of Lausanne and ETH-Zurich, plus in-kind collaboration from industry personnel. In addition to research expenses, we request from NRP70 the salaries for a research position for each subproject, i.e. 3 doctoral students (3 years each) and 1 post-doctoral researcher (2 years).

Subproject A will analyse drill core from wells in Switzerland's main saline aquifer (Upper Muschelkalk) and reconstruct the geological processes that led to formation or destruction of its porosity and permeability. This conceptual understanding will be used to create a 3D static model of the reservoir properties, to serve as a guide in commercial exploration for sites for geothermal electricity production and CO2 sequestration. The potential for these energy applications will mapped into the model, to provide improved estimates of geo-energy resources and a planning tool for decision-makers at local, canton and federal levels.

Subproject B will focus on a >20 km long, hydrothermally active fault that outcrops in granite on the Grimsel Pass (Aar Massif). As well as being a geothermal system in its own right, this fault is an analogue of water-conducting faults in the basement of the Swiss Molasse Basin (SMB), which are targets of geothermal exploration. Surface outcrops will be mapped and a shallow borehole will be drilled through the fault to permit construction of a 3D static model of its geometric and hydraulic properties. This model will serve to assess the geothermal potential of the northern Alpine margin, it will provide exploration companies with a template for hidden faults in the SMB with which they can interpret seismic surveys and plan hydraulic stimulation, and it will provide a realistic framework for dynamic modelling of deep geothermal reservoirs conducted by a parallel subproject of the NRP70 Umbrella cluster.

Subproject C will conduct a detailed geophysical survey of the Grimsel Pass borehole. Geophysical logging of the borehole will characterize the hydraulics of the Grimsel fault, for input into the 3D static model. Surface-to-borehole seismic experiments will use the Grimsel fault as a test case to determine whether seismic methods can remotely detect permeability in fractured crystalline rocks. The results will be used to elucidate the fundamental relations between seismic attenuation and effective permeability of fractured rocks, using a comprehensive poro-elastic approach involving advanced numerical simulations. Corresponding field workflows for this new methodology will be made available to the exploration industry.

Subproject D will conduct cutting-edge laboratory measurements and experiments on core samples to determine the petrophysical properties of the Upper Muschelkalk and Grimsel Fault reservoirs, and to explore in general the relationship between the effective permeability of fractured rocks and the attenuation of seismic waves and electrical resistivity. Measurements of electrical conductivity, elastic properties, seismic attenuation and permeability will be made in new apparatus capable of reaching deep reservoir conditions (T = 250 °C, Prock = 100 MPa and Pfluid = 50 MPa). The results will be used to support subprojects A, B and C, and the developed methodologies will be made available to industry.

The current suite of THMC models are limited because the wide range of timescales associated with the processes underlying fluid-injection in the earth result in numerically cumbersome calculations at insufficient resolution. The complex and coupled processes associated with fluid-injection (fracture, fluid flow, heat flow, and chemical reactions) require a new generation of numerical algorithms designed for High Performance Computing (HPC). A primary focus of the numerical modeling group at the University of Neuchatel is to develop the next generation of simulators that can model the nucleation, growth, and coalescence of numerical fractures that evolve in response to far-field stresses, and stress perturbations arising from poroelastic / thermoelastic stresses and in response to fracture network itself. This network provides the principal pathways for fluid flow, and thus heat flow, allowing simulations of the injection processes, and for studying the long-term viability of a geothermal resource. Our approach is design algorithms optimized for Graphics Processing Unit (GPU) clusters, with the aim of high-resolution and computationally fast simulations.

The chair "Gaz Naturel" aims at reproducing the thermo-hydro-chemo-bio-mechanical THCBM coupled behaviour of the reservoir materials during CO₂ injection and storage in all time scales, in particular the formulation of constitute model and of the mathematical/numercial model for the coupled behaviour. Its implementation in a FE code and the experimental investigations will be processed for the material characterization and validation of the numerical tool.

Permeability is the decisive reservoir property for the economic success of geothermal operations as it determines the flow rates that can be achieved when circulating fluid to mine heat from the reservoir. However, at the depth of deep geothermal reservoir targets, permeability is typically too low for commercial circulation rates and therefore requires enhancement by (hydraulic) stimulation. There is currently no quantitative theory that can predict by how much reservoir permeability may be enhanced during stimulation at a given site. This uncertainty is one of the major investment risks in the future development of deep geothermal energy in Switzerland. In addition, seismicity induced by hydraulic stimulation is the biggest technical and societal acceptance risk. Our project develops and applies novel numerical simulation techniques for the quantification of permeability enhancement during hydraulic stimulation, its spatial evolution during reservoir development, and the seismicity-related processes of fault slip during stimulation.

GEOTHERM-2 conducts cross-disciplinary research towards the development of Enhanced Geothermal Systems (EGS). It represents the second phase of GEOTHERM (2009-2012), a comprehensive program of basic research on key aspects of Enhanced Geothermal Systems (EGS). GEOTHERM-2 conducts cross-disciplinary research targeted to the development of Enhanced Geothermal Systems (EGS). The research addresses aspects concerning the geomechanical characterization of reservoirs, the numerical simulation of a reservoir creation, the long term effects of geochemical reactions on permeability and heat extraction, the assessment and management of the induced seismicity risk, the social acceptance and comparative assessment of the risks inherent to an EGS project, and the analysis of geothermal energy usage in cities, with the case study of Lausanne.

COTHERM started in 2012 as a synergistic collaboration with four sub-projects to advance our understanding of the fundamental geological, chemical, and physical processes governing the sub-surface structure and dynamics of geothermal systems. Our follow-up project will address fundamental questions regarding a new type of geothermal resources (so-called “supercritical resources”) that were recently discovered at Krafla Volcano, Iceland. Supercritical resources bear the potential to boost geothermal power production by up to an order of magnitude per well.