Loss of drilling fluids is a common problem that represents a significant cost to geothermal projects and delays continued development of geothermal resources. Various materials have been identified or developed which, when mixed with drilling fluids, are known to mitigate lost circulation to varying degrees. Several questions remain unresolved with regards to how the different lost circulation materials will behave, and particularly how they thermally degrade, under different conditions and time frames. To address these issues, we conducted several tests of various lost circulation materials previously used in geothermal drilling operations, measuring how, when mixed with fluids and heated, the materials degrade and affect the viscosity of fluids. Degradation tests were run from 20 to 250 ˚C, measuring mass loss, water loss, and gas release during degradation of the materials. Viscosity tests were run showing the apparent viscosities with increasing shear rate and temperature of the material mixtures. Microscopic analysis allowed us to characterize the effect of time and temperature on the materials. Together, the results permit quantification of how thermally induced changes in the materials are likely to affect drilling fluids in-situ and how long they can reasonably act to seal lost circulation zones. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This presentation describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. SAND2022-12722 A

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