Initiating Geothermal Well Discharge With Coiled Tubing

Abstract

Geothermal wells, with shallow feedzones and with subsurface conditions close to boiling, will often discharge naturally or with help from an artificial or natural gas cap. However, for deep-feeding geothermal wells in under-pressured reservoirs, the requirements to initiate discharge using a gas cap increase substantially. Furthermore, wellhead pressure of more than 60 bar may be needed in order to achieve boiling when the well is opened. This method increases the potential to cause casing and cement damage through rapid heating. To reduce the thermal shock loading on the cemented casing by heating the well in a controlled fashion, injection of gas through coiled tubing (gas lift) can be employed to initiate flow. This method reduces the density of the overburden of cooler water to the extent that it will flow to surface, even when the water level is several hundred meters below the wellhead. The cool fluids are gradually replaced with hot fluids from the deep feedzones and the wellbore is heated in a controlled manner until the well can self-discharge. This paper presents a case study of a gas lift to initiate well discharge using coiled tubing and nitrogen gas injection. The project planning and equipment specifications are discussed. The data collected during the stimulation is compared with predictions which were made using correlations from empirical data.