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When Where and Why: The Geologic Context of Lost Circulation While Drilling in a Crystalline Geothermal Reservoir.

Lost circulation is a common problem in geothermal drilling that can represent 20% of the costs for reservoir development. Here we examine the McGinness Hills Geothermal Field to determine the geologic context of and the most common and effective responses to lost circulation. This field, operated by Ormat Technologies, is located in central Nevada on the eastern flanks of the Toiyabe Range and is the fourth largest geothermal complex in the United States. The regional geology is typical of the Basin and Range region, consisting of Paleozoic metasedimentary strata that are intruded by Jurassic granitic plutons at depths greater than ~300m, which represent the main production zone, and are overlain by Oligocene to Miocene volcanic deposits and Quaternary alluvial fan deposits. Faults in the area predominantly trend NNE and NNW with normal movement; the greatest permeability is found at the intersections of these faults. Deep geothermal wells commonly experience problems with lost circulation during drilling, with several wells well losing thousands of barrels of drilling fluids. We examine the depths, lithologies, proximity to faults, and common responses to lost circulation as the first step in forming a comprehensive strategy to address lost circulation in varying contexts. Lost circulation in McGinness Hills generally occurs at depths greater than 2000 ft, well into the metasedimentary basement, Jurassic intrusions, and production interval. Because the depths of lost circulation at McGinness Hills usually corresponds with the production zone, correcting lost circulation adds the challenge of not permanently damaging permeability, which limits the approaches available to solving the problem. The most common approaches to lost circulation mitigation at McGinness Hills include pumping high viscosity sweeps of lost circulation materials, aerating the drilling fluid, and drilling blind without returns. Further study of this and other fields may demonstrate more in-depth correlations between depths, lithologies, and responses and will contribute to a larger effort to develop a systematic understanding of contextually informed responses to problems with lost circulation.