Insulating Geothermal Well Casings from Thermal Stress with Nitrogen Gas or Nitrogen Foam

Abstract

Fenton Hill Hot Dry Rock Geothermal Site Well EE-3 was designed and completed to function as a geothermal production well. As fracturing and completion of the reservoir with the injection well, EE-2, progressed it become evident that it would be desirable to fracture inEE-3 as well to obtain a flow connection between the wells. Unfortunately, the 9 5/8" of production casing in EE-3 had been pretensioned to 885,000 lbs to accommodate its intended service as a hot water production well. Cool down of the casing was thus limited to only 11°C (20°F) to keep the stress in the top joints of the casing below the minimum yield stress, or else the pretension had to be released. Before incurring the risk and expense required to release the tension, fracturing experiments were performed to evaluate the use of gaseous nitrogen and 75% quality nitrogen gel foam as insulating media in an annular wellbore configuration, i.e. the nitrogen gas or foam was placed in the annular gap between the tubing string and casing.