Numerical study of downhole heat exchanger concept in geothermal energy extraction from saturated and fractured reservoirs

Abstract

Geothermal energy has gained a lot of attention recently due to several favorable aspects such as ubiquitously distributed, renewable, low emission resources while leveraging the advances in the associated technologies such as directional drilling and low enthalpy power generation plant. However, there are still many challenges such as the high initial capital cost of drilling and surface facilities, environmental risk of seismicity due to the induced disequilibrium in the formation, and sustainability of project over designed operational life. Traditional downhole heat exchangers (DHE) could potentially reduce the capital cost and the risk of seismicity, but they are unable to maintain a sustainable geothermal energy production over the operational life due to the rapid cooling down of formation in the vicinity of the wellbore. In this study, a novel DHE design is introduced to enhance the energy production rate as well as sustainability for mainly two types of geothermal reservoirs: saturated geothermal reservoirs and enhanced/engineered geothermal systems (EGS). Modeling of DHE is based on the concept of thermal resistance. A geothermal reservoir simulator is built reusing components of an existing blackoil simulator by adding thermal energy transport equations and fracture representation (discrete fracture network). Several verification and validation tests are carried out. Parametric studies are presented for various configurations of DHE and thermodynamic analysis is carried out for the binary power plant cycle. In addition, the geothermal reservoirs Camerina A and Raton Basin are presented as case studies for saturated geothermal reservoir and EGS, respectively. In saturated geothermal reservoirs, the performance of DHE is improved significantly by exploiting forced convection. For EGS, the overall heat extraction rate is also enhanced by adding DHE.