Anelastic Strain Recovery as a Measure of in Situ Stresses at FORGE

ABSTRACT: Anelastic strain recovery, the process of measuring the time dependent recovered strain after a core is cut at depth was utilized to make a measure of the in-situ properties stresses at depth at the FORGE (Frontier Observatory for Research in Geothermal Energy) site in Milford Utah. Core was collected from a region of well 16B at approximately 4860-4870 ft. Core was instrumented with strain gages within 10 hours of the core being cut. The relaxation of the cores was measured for approximately one month, and the results analyzed, which showed that the principal stresses were slightly off vertical, and magnitudes are close to equal. 1. INTRODUCTION Strain measurements on recovered core hold their root in the formation of the rocks themselves. As rocks are formed and stressed with burial, the grains deform and are locked in place either through cementation or through the compression of their neighbors. When cores are removed from depth the stresses are released and the strain begins to recover. This recovered strain is measured and inverted in order to determine the principal strains, which are then converted to principal stresses though determination of the elastic modulus and application of Hooke's Law. This technique has been applied to samples from the Utah FORGE (Frontier Observatory for Research in Geothermal Energy) site to help constrain the in-situ stress measurements at the site. The core was obtained from the 16B(78)-32 well at the site from the first core run taken above the kickoff point just below 5000 ft (core samples are from between 4865 and 4870 ft). The core is a relatively clean granitoid as shown in Fig. 1. The core was obtained on May 9th, 2023, and was instrumented within approximately 10 hours of the core being cut. 2. BACKGROUND There are a number of ways in which in situ stresses can be determined such as hydraulic methods, i.e. fracture testing, acoustic methods, i.e. seismic waveform analysis, and core methods such as anelastic strain recovery (ASR), or overcoring.