Development of a Downhole Tool for Measuring Enthalpy in Geothermal Reservoirs

Abstract

The amount of thermal energy (enthalpy) contained in geothermal fluid is one of the key parameters used to determine the value of a geothermal resource and vital in understanding the performance of existing reservoirs. The enthalpy of a single-phase fluid can be determined from the temperature and pressure of the fluid; however as a hydrostatic pressure decreases geothermal fluid flow often becomes two-phased and enthalpy calculations require knowledge of the steam and water fractions. Current surface-based methods for measuring enthalpy are expensive and complicated while providing incomplete view of downhole enthalpy. Downhole measurement of enthalpy would provide a better understanding of the geothermal resource and allow for improved measurement of energy produced from different fracture zones. We are developing a method and apparatus for measuring the downhole enthalpy of a flowing geothermal fluid in real-time at high-temperature (HT) and pressure. Our method involves measuring the concentration of selected naturally occurring ions found in the liquid phase of the geothermal fluid throughout the wellbore using a novel electrochemical sensor. The change in liquid-phase ion concentration will be used to calculate the proportion of liquid to steam and allow for accurate enthalpy measurements. While benchtop instrumentation exists for routinely measuring ion concentration under ambient conditions, the sensing materials and supporting electronic components in these devices are unable to survive in the harsh environments found downhole. Results of the electrochemical sensor development effort will be discussed.