Simulation of governing processes in superheated and supercritical geothermal systems: mathematical models, numerical methods and field data

Geothermal energy production is based on production of hot water from the subsurface, and exploit the energy in the water for heating and electricity production. To increase the profitability of a geothermal site, the target rock formation should have high temperatures; commonly more than 200 degrees Celsius. Such conditions are most readily available on locations where the temperature increases quickly with depth, for instance in volcanic areas in Iceland. It is an ongoing effort to produce from ever hotter formations. Icelandic energy companies, who are also SiGS partners, have a goal to produce from supercritical conditions, where water has above 374 degrees Celsius, and pressures above 220 bars. Compared with other and better-known subsurface processes, such as production of oil as well as geothermal energy with lower temperature, the high temperatures considered in the SiGS project also introduce different dominant physical processes. For instance, injection of cold fluids during drilling or production may alter properties of existing fractures in the rock or even trigger formation of new fractures. Similarly, temperature changes may disturb the existing chemical balance in the host rock. These processes will modify the flow properties of the rock, and thus both how energy is originally distributed in the rock and how the rock responds to production of the energy. Mathematical modeling and numerical simulations are important tools to study such subsurface processes. In the SiGS project we develop advanced tools that represent changes in chemical equilibrium, as well as transport of energy from the deep subsurface and up to the geothermal fields that are exploited. The models will be employed to study the relative importance of dynamical processes at high temperatures both before and during drilling, as well as under production. As such the project will contributed to enhance the economic viability of geothermal energy.

SiGS targets improved understanding of processes that are key to unlocking near-magma geothermal resources. Production from these low-carbon energy resources have a high economic potential due to their high energy-density, but their utilization in still in an early stage. The high temperatures imply that energy production will face superheated and supercritical conditions, wherein multiple physical processes interact and compete. Initial experience from these systems point to shortcomings in current process understanding, and modeling and simulation capacity, that impede efficient engineering operations. SIGS focus on two mechanisms that are critical to understand superheated and supercritical geothermal systems: 1. Heat transport from deep heat sources towards the geothermal reservoir. 2. Thermal stimulation caused by the introduction of cold fluids during drilling or reinjection. Common for the two mechanisms is a strong dependency on dynamic changes of fracture permeabilities, caused by a combination of thermo-hydro-chemical-mechanical forces. To study the mechanisms, the project combines advanced mathematical modeling and simulation with analysis of data from Icelandic geothermal fields. The modeling applies explicit representation of fractures, using so-called discrete fracture matrix (DFM) models, which provides more accurate repreentation of coupled processes than does traditional tools. The relevant field data includes information from superheated and supercritical coditions accessed by Icelandic Deep Drilling Projects. The project is hosted by the University of Bergen, and also includes the Icelandic GeoSurvey, as well as Norwegian (Equinor) and Icelandic (HS Orka, Reykjavik Energy, Landsvirkjun) industry partners. The complementary background of the academic and industrial partners makes the consortium ideally suited to fulfil the project goals.