Monitoring and interpreting deformation from hydraulic fracturing during well stimulation and heat recovery promise to generate insights that can improve performance of enhanced geothermal systems (EGS). Borehole strainmeters have been used to characterize in-situ deformation, but current strainmeter technology is unable to function at temperatures typical of geothermal reservoirs. The objective of this work is to develop strainmeters that can be deployed at high temperatures to meet the needs of applications at EGS sites. The initial motivation is to develop an instrument that could be demonstrated at the Utah FORGE site, with applications elsewhere to follow. The approach has been to conduct simulations to evaluate the expected strain signals, and then design, build, and test prototypes that can measure these signals. A design for a high-temperature split-sleeve strainmeter using optical fiber sensors was developed with the intent that it would be deployed behind casing during well completion. A prototype strainmeter was attached to a pipe simulating a casing and heated in an oven at 200°C, 230°C, and 255°C for more than three months. Bending strain was created by periodically applying loads in two different directions normal to the end of the pipe. The instrument measured resulting strains in the range of +/- 25  with relative magnitudes that are consistent with strains predicted using a numerical simulation of the applied loads. The strains varied slightly with time apparently reflecting minor fluctuations in the mechanical system. The results demonstrate performance for at least three months, but the tests are ongoing and the full functional life of the strainmeter at elevated temperature is still being evaluated.

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