Logging, testing and monitoring geothermal wells

Abstract

Wells or boreholes are essential components in both geothermal research and utilization as they enable a drastic increase in geothermal energy production beyond natural out-flow as well as providing access deep into the systems, not otherwise possible. Wells also play a vital role in all geothermal reservoir physics (also called reservoir engineering) research, which would be particularly ineffective without the access into geothermal systems provided by wells. During drilling the main reservoir physics research is performed through logging of different parameters as functions of depth, with temperature and pressure being of particular importance. At well completion the most important reservoir physics research is step-rate well-testing, either through injection or production, which enables the first estimates of well and reservoir properties. Reservoir physics research is also conducted in association with well stimulation at the end of drilling. Repeated temperature logging aimed at estimating undisturbed reservoir temperature and pressure is the key research performed during well warm-up. Monitoring of mass and energy output along with logging down-hole pressure transients is the most important reservoir physics research conducted during output testing of geothermal wells along with pressure recovery logging after wells are shut in and interference monitoring. Much of the same reservoir physics research is also conducted in reinjection wells during drilling and following completion. All of the above provides vital information for reservoir assessments of the geothermal resource in question, but the most important data for that purpose actually comes from monitoring of energy production and the consequent response of the geothermal system during long-term (several years) utilization. The reservoir physics data collected during these phases also plays an essential role in the calibration of various reservoir models.