Investigation of a new HDR system with horizontal wells and multiple fractures using the coupled wellbore–reservoir simulator TOUGH2MP-WELL/EOS3

Abstract

Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR) systems, in particular, have the largest potential for long-term sustainability and therefore draw a lot of attention. The combination of horizontal wells and the technology of multiple transverse fractures technology is an attractive approach of such HDRs, which is called in this paper as the specific EGS (enhanced geothermal system). The main objective of this paper is to study the heat extraction over a period of 20 years by water circulating in a deep geothermal reservoir using this specific EGS. The wellbore flow module, T2WELL/ECO2N, is implemented in the parallelized simulator TOUGH2MP as a new code TOUGH2MP-WELL/EOS3, which enables coupled wellbore–reservoir simulations. Using this newly developed code, the sensitivity of heat extraction against various parameters of the formation and fractures is assessed. The influences of the fracture geometry and the space of two neighborhood fractures on the geothermal energy performance are analyzed. According to the results in this paper, the injection rate, the wellbore radius and the fracture permeability are three main influence factors for the distribution of total flow through individual fractures. A large injection rate, a small wellbore radius and a large fracture width will cause the short-circuit effect, which reduces the performance of this specific EGS. Compared with the classic HDR system (doublet or triplet vertical wells + single fracture), this specific EGS has a much higher performance and a longer duration of the economic production. Furthermore, the horizontal wells can also be drilled in both directions of the minimum horizontal stress instead of in just one direction, so that the energy performance is doubled and the investment cost per kW energy production is decreased and the advantage of this specific EGS is significantly enlarged.