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Analytical and Experimental Study of Measuring Enthalpy in Geothermal Reservoirs with a Downhole Tool (2015-2017)

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Investigator(s): Xuhua Gao

Wellhead measurements of enthalpy and mass flow rate are routine monitoring procedures in geothermal fields. Due to wellbore heat loss, measurement of surface enthalpy can only provide incomplete information about the wellbore and the reservoir, especially during the early testing phases of a development. Measurement of enthalpy downhole would allow for better understanding of the reservoir condition and so would be of great practical significance.

Enthalpy of single-phase flow can be obtained directly from measurement of temperature. However, geothermal fluid is often in two-phase flow in the wellbore and the reservoir, and flowing steam fraction would be required to determine the total enthalpy of steam and liquid. Measuring or calculating flowing steam fraction downhole is not trivial due to the velocity difference between the gas phase and the liquid phase. Atalay (2008) introduced a method to measure gas velocity and void fraction with fiber optics. Spielman (2003) and Juliusson et al (2006) proposed that resistivity measurement with sufficient resolution could also be used to determining gas velocity and void fraction. In both methods, the flowing steam fraction is calculated from gas velocity, liquid velocity and void fraction. Both methods are only applicable in bubble flows and require other equipment to measure liquid velocity at the same time.

This work developed a modification and enhancement of the method proposed by Gao et al. (2017) in the following aspects:

  • A new model is proposed to calculate steam flow rate from the feed zone in the case where multiple feed zones exist, which allows for estimation of the enthalpy from each feed zone. Multiple feed zones are common in geothermal fields, so this modification enhances the practical application of the approach.
  • Gao et al. (2017) showed only an analytical model, while in this paper experimental study of the downhole tool is included, demonstrating the ability of the tool to measure chloride concentration in two-phase wells.
  • The assumption that the chloride concentration measured at the inlet of the feed zone can represent the exact concentration of the liquid from the feed zone plays an important role in the model and influences the accuracy of the model. Experimental study was conducted to examine the validity of this important assumption.
  • Computational fluid dynamic (CFD) simulation of the feed zone behavior was carried out using ANSYS Fluent and compared to the experimental results.

References:

Gao, X., Cieslewski, G. and Horne, R. "Development of a Downhole Technique for Measuring Enthalpy." Proceedings42th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2017).