Downhole measurement of enthalpy in geothermal wells – An analytical, experimental and numerical study

Abstract

The enthalpy of two-phase geothermal fluids is typically monitored at the surface to understand the performance of a geothermal field. Surface enthalpy, however, cannot reflect real reservoir conditions downhole because of the heat loss along the wellbore. In addition, with only the surface enthalpy, the energy contributions from separate feed zones cannot be evaluated individually. A technique for determining downhole enthalpy in two-phase geothermal wells using surface inputs and chloride concentrations was developed in this study, including analytical models, experimental work and numerical studies, so that the downhole flowing enthalpy, the energy contribution from each feed zone and the heat loss along the wellbore can be evaluated. Chloride always stays in the liquid phase and becomes more concentrated as the geothermal fluid ascends to the surface and boils, and the change in chloride concentration along the wellbore can be utilized to calculate the change in the flowing steam fraction and the enthalpy. Analytical models to implement this idea were established for geothermal wells with a single feed zone or multiple feed zones, and the calculation procedures are elaborated with detailed flow diagrams and examples. The analytical models involve mass balance, energy balance and an assumption that the chloride concentration in the liquid from the feed zone is determinable. Inputs to the analytical model include two-phase mass flow rates and enthalpy at the surface and chloride concentrations at the surface and in the downhole. Temperature or pressure measurements are utilized to apply the energy balance in the model for multiple feed zones. Experiments were conducted to test the feasibility of an accurate determination of chloride concentration in two-phase fluids and validate the assumption in the analytical model. Chloride concentration distribution in the mixing area at the inlet of the feed zone was determined and visualized in the experiments. At the same time, similar mixing processes were visualized by a numerical simulation using ANSYS Fluent, and results from the numerical simulation are consistent with those from the experiments. Therefore, both the experiments and the simulation support the assumption in the analytical model that chloride concentration in the liquid from the feed zone can be determined accurately. It is demonstrated that the proposed method provides a fast and cost-effective way to determine the downhole two-phase flowing enthalpy in geothermal wells with multiple feed zones and estimate flow rates and energy contribution from each individual feed zone.