Title: Geothermal Reservoir Evaluation Using Well Testing and Analytical Modelling - Case Example: Reykjanes Geothermal System, SW Iceland

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 conditions for wells RN-30 and RN-32 suggests that reservoir temperature
is in the range of 280-290°C. 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.