Modelling Production from Supercritical Geothermal Reservoirs

Abstract

As the opportunities for developing further conventional geothermal resources become increasingly limited, the development of deep supercritical reservoirs is attracting more interest. If the associated exploration, modelling and engineering challenges can be addressed, supercritical reservoirs represent a potentially large and untapped source of high-enthalpy fluid for power generation. To investigate production from supercritical reservoirs, a simple idealized two-dimensional numerical model has been developed, using a supercritical version of TOUGH2 simulator. The model is 7 km deep and produces fluid from 4 – 5 km, a reasonable drillable depth. Numerical experiments indicated that steady-state supercritical conditions at this depth will be found only when permeability below it is low, of the order of 0.1 mD. Simulations of production showed that reservoirs with higher porosity will give higher mass flows but lower flowing enthalpy, as expected. They also showed that, as in conventional reservoirs, mass flows in the well decline over the production period. However, unlike conventional liquid dominated reservoirs, the enthalpy of fluid produced from the supercritical reservoir also declines over time, from around 3100 kJ/kg to 2500 kJ/kg over 30 years. This is a result of the low permeability, which means the upflow zone is relatively narrow. As a result, cold fluid is drawn in more quickly over the production period. This also suggests that drilling for supercritical resources will need to target the upflow zone very accurately and that only high enthalpy (low permeability) geothermal systems should be considered.