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.

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地热井中焓的井下测量——分析、实验和数值研究

摘要 通常在地表监测两相地热流体的焓，以了解地热场的性能。然而，由于沿井筒的热损失，地表焓不能反映井下真实的储层条件。此外，仅凭表面焓，无法单独评估来自不同进料区的能量贡献。本研究开发了一种利用地表输入和氯化物浓度确定两相地热井井下焓的技术，包括分析模型、实验工作和数值研究，以便井下流动焓、每个进料区的能量贡献和可以评估沿井筒的热损失。随着地热流体上升到地表并沸腾，氯化物始终处于液相并变得更加浓缩，并且可以利用沿井筒的氯化物浓度变化来计算流动蒸汽分数和焓的变化。针对具有单个进料区或多个进料区的地热井建立了实现这一想法的分析模型，并通过详细的流程图和示例详细阐述了计算程序。分析模型涉及质量平衡、能量平衡以及进料区液体中氯化物浓度可确定的假设。分析模型的输入包括两相质量流率和地表的焓以及地表和井下的氯化物浓度。温度或压力测量用于在多个进料区的模型中应用能量平衡。进行了实验以测试准确测定两相流体中氯化物浓度的可行性，并验证分析模型中的假设。在实验中确定并可视化进料区入口处混合区域中的氯化物浓度分布。同时，通过ANSYS Fluent的数值模拟对相似的混合过程进行了可视化，数值模拟结果与实验结果一致。因此，实验和模拟都支持分析模型中的假设，即可以准确确定进料区液体中的氯化物浓度。