Geothermal reservoir evaluation using well testing and analytical modelling

Abstract

Geothermal development is a costly and risky process, which needs extensive studies to enable understanding and successful utilization of the resource. Well testing methods remain important evaluation tools for geothermal reservoirs at all stages of development. Well testing refers e.g. to injection, discharge, build-up, interference and tracer testing. Modelling of geothermal reservoirs is also a useful technique that helps in decision making. Lumped parameter modelling is e.g. a powerful, cost effective alternative to detailed numerical modelling. The main goal of this study was to assess and discuss the role of well testing in evaluation of geothermal resources and for increasing the knowledge on geothermal systems. The work involves presentation of current techniques of well testing, and relevant analytical solutions, analysis and interpretation of temperature and pressure conditions as well as of injection, discharge and tracer test data. Finally, simulation of pressure behaviour due to production was also performed and predictions of reservoir response to future production. The Reykjanes geothermal system, SW Iceland, was selected as a case example for this study

Evaluation of temperature and pressure conditions for wells RN-30 and RN-32 suggest that reservoir temperature and pressure are in the range of 280 – 290 °C and 30 – 100 bar, respectively. Wells RN-30 and RN-32 are characterized by relatively high transmissivity and storativity values which agree with the conceptual model of the Reykjanes system. The estimated negative skin factors of the wells indicate stimulated wells that are in a good connection with the surrounding reservoir. Simple analytical modelling for the representative wells RN-12 and RN-16 indicates that the current reinjection of 15% of the mass production needs to be increased to 50% so as to respond to the current pressure drawdown (around 41 bar), thus increasing the current reservoir pressure by over 6 bar in the next 10 years. Tracer breakthrough and mass recovery show that production wells RN-18, RN-21 and RN-24 are directly connected to injection well RN-33, and that a reinjection rate of 100 l/s into RN-33 can be maintained without serous cooling of these production wells. Drilling of reinjection wells in the region of RN-33 is recommended if they are drilled away from the Reykjanes production zone.