NATURAL GEOCHEMICAL TRACERS FOR INJECTATE FLUIDS AT DIXIE VALLEY

Abstract

Since September 1988 over 334.5 x 109 pounds of flashed brine have been returned to the Dixie Valley geothermal reservoir through three injection areas. As a result of the dual flashing process, the injected fluid is enriched in chloride and the heavy water isotopes and depleted in noncondensable and noble gases relative to the produced fluids. Chloride contents and heavy water isotopes of both produced fluids and injectate have significantly increased over the lifetime of the field, indicating substantial returns of injectate to the production wells. The water isotope data on pre-production reservoir fluids do not support the possibility that the changes in chloride concentration are caused by inflow of nearby indigenous chloride-rich water.

Noble gas concentrations, first obtained in 1997, further support the chloride and water isotope interpretation, indicating that they can be used confidently to quantitatively assess the recycling of injectate fluids. In the Section 7 area there is an apparent continuous recycling of injectate, increasing at 5-7% per year. Currently, the Section 7 wells produce a fluid that is between 60 and 100% recycled injectate, which when combined with the total volume fluid injected provides a very simple estimate of the circulating fracture volume of ~0.12 km3. Assuming these fractures account for ~1% of the overall reservoir, the reservoir volume in the Section 7 area is ~12 km3. In the Section 33 part of the field chloride concentrations increased over the first 4 years of production, suggesting that as much as 60% of the produced fluid was recycled injectate. Since then chloride has steadily decreased, such that currently only 37% of the production fluid can be recycled injectate.