Improved Injection Strategies at Blue Mountain, Nevada Through an Improved Conceptual Model, Tracer Testing, and Injection-production Correlation

Abstract

The Blue Mountain geothermal field, a fault-controlled geothermal system in the Basin and Range, has been in operation since November 2009. The resource experienced significant temperature decline from an initial operational strategy that focused injection into wells down dip of the production zone. To mitigate the temperature decline, production rate in the fastest cooling well was reduced, which slowed the rate of cooling and decreased power output. This paper describes the strategy started in 2013 of mitigating temperature decline field-wide by adopting an injection management strategy based on the results of tracer testing, a new resource model, and well-to-well correlation of injections rates to production cooling. The resource model is based on fluid conduits and compartments created by the intersection of three west-dipping low angle faults. The fault geometries were fit to lost circulation zones identified during drilling, fluid exit/entry points in wells, and geophysical data - seismic reflection profiles, micro-seismicity, horizontal gravity gradient lineaments, and aeromagnetic data. The intersection between two faults formed a damage zone that creates a high permeability conduit defining the primary up-flow zone fed at depth by a piedmont fault delineated by gravity and seismic reflection on the west edge of the field. A fourth fault that bounds the range to the SW, trends NW and is considered to be a relatively impermeable structure due to its orientation in the current tectonic regime. The conceptual fault model was combined with tracer data to map potential flow patterns at depth, characterize reservoir geometry, and model the thermal behavior of the reservoir. This analysis was used to develop the permeability structure for a reservoir simulator that showed a much better history match than previous models. The new model and tracer analysis also provided the insight to design and implement an injection strategy using existing wells that has slowed temperature decline. Further plans to mitigate temperature decline evaluated by reservoir simulations, include producing some of the initial western injectors and stimulating idle wells further to the north and south.