Advances in Tensor Strainmeters for Geothermal Reservoir Applications

Abstract

Direct measurement of deformation in geothermal reservoirs can provide valuable constraints on stimulation and heat extraction processes. It is feasible to characterize deformation with borehole tensor strainmeters, which can measure multiple components of strain at high resolution, but currently available instruments are limited to ambient temperatures. We have developed a tensor strainmeter for geothermal reservoir conditions. It is a thin sleeve containing optical fibers that is clamped around a casing, and we have conducted a suite of experiments to demonstrate performance in the laboratory. One experiment consisted of deploying a sleeve strainmeter on a steel pipe that was subjected to periodic transverse loads while heated to 225–275 °C for nine months. The strainmeter measured strain responses of up to 30 µε each time the transverse load was applied. The magnitude at each sensor varied slightly during the experiment, but there was no degradation in performance due to heating. A second experiment evaluated the sleeve strainmeter design at elevated pressure. Two strainmeters were evaluated by monitoring strain while they were attached to a standard oil and gas wellbore casing (5.5-inch API) in a custom vessel that was pressurized up to 5000 psi. The strainmeters were functional during the test and calibration data collected after the pressurization were essentially the same as calibration data prior to pressurization, indicating the effects of high pressure were negligible. Reservoir conditions at Utah FORGE are approximately 230°C and 3700 psi. Our experiments demonstrated strainmeter functionality for extended periods at temperatures and pressures significantly greater than those at the Utah FORGE EGS reservoir. The sleeve architecture can accommodate a variety of optical fiber sensors, including fiber Bragg gratings (FBGs), Distributed Acoustic Sensing (DAS) or interferometric sensors, establishing a flexible platform for high-resolution tensor strain measurements in extreme subsurface environments.