| Abstract |
Tracer and geochemistry measurements in fractured Hot Dry Rock (HDR) geothermal reservoirs are usually performed after a fracture connection has been established and constant, nearly equal inlet and outlet flow rates have been achieved. However, during hydraulic fracturing experiments designed to create a low-impedance fracture connection between two wells, the inlet and outlet flow rate s can be dramatically different and can vary during the test, forcing us to revise the common analytical methods for interpreting tracer response curves and geochemistry behavior. This study presents trace r and geochemistry data from several hydraulic fractur in g experiments at the Fenton Hill, NM, HDR geothermal reservoir. Tracers have been injected at various times during these tests: 1 ) initially, before any flow communication existed between the wells; 21 shortly after a flow connection was establish e d ; and 3) after the outlet flow had increased to its steady state value. An idealized flow model consisting of a combination of main frac t u r e flow paths and fluid leakoff into secondary permeability explains the differ e n t tracer response curves for these cases, and allows us to predict the fracture volume of the main paths. The geochemistry during these experiments supports our previously developed models post u l at in g the existence of a high concentration indigenous ìpore fluid. ì Also, the quartz and Na-K-Ca qeothermometers have been used successfully to identify the temperatures and depths at which fluid traveled while in the reservoir. The quartz geothermometer is somewhat more reliable because at these high temperatures (about 250îC) the inject e d fluid can come to equilibrium with quartz in there servoir. The Na-K-Ca geothermometer relies on obtaining a sample of the indigenous pore fluid, and thus is somewhat susceptible to problems of dilution with the injection fluid . |