Measuring Real-time Concentration of Ionic Tracers and pH in Geothermal Reservoirs Using a Ruggedized Downhole Tool

Abstract

Chemical tracers are commonly used to characterize the fracture network and determine the connectivity between the injection and production wells in geothermal reservoirs. Currently, most tracer experiments involve injecting the tracer at the injection well, manually collecting liquid samples at the wellhead of the production well, and sending the samples off for laboratory analysis. While this method provides accurate tracer concentration data at very low levels of detection, it does not provide information regarding the location of the fractures which were conducting the tracer between wellbores. Sandia is developing a high-temperature wireline tool capable of measuring tracer concentrations and pH downhole using electrochemical sensors. The goal of this effort is to collect real-time pH and ionic tracer concentration data at temperatures up to 225 °C and pressures up to 3000 psi. A prototype electrochemical sensor and the initial data obtained will be presented detailing the measurement of iodide tracer concentrations and pH at high temperature and pressure in a recently developed laboratory scale autoclave.