An isenthalpic formulation for simulating geothermal systems with phase changes

Geothermal energy extraction requires detailed characterization of geological, physical, and geochemical properties of the reservoir and fluid properties of the geothermal fluids. For a proper understanding of the involved reservoir processes, robust and efficient numerical thermal simulation tools are essential (Zhu and Okuno, 2014, 2016; Connolly et al., 2021). Simulations of mass and heat transfer in porous media traditionally include temperature as an independent variable and the set of natural variables. This, however, requires variable substitution procedures to handle phase transitions, which add complexity to the implementation (Voskov and Tchelepi, 2012). In contrast, the overall composition formulation, has the advantage of well-defined equations and variables at each cell, so it does not require variable substitution procedures, (Voskov and Tchelepi, 2012). In this formulation, the coupled mass and heat transfer problem is solved at each time step by using pressure, enthalpy, and composition, requiring an isobaric and isenthalpic formulation to solve the local phase equilibrium problem. This study approaches the isenthalpic phase equilibrium problem from a unified perspective and ties it to equations of energy and mass conservation. Zhu, D. and Okuno, R. [2014] A robust algorithm for isenthalpic flash of narrow-boiling fluids. Fluid Phase Equilibria, 379, 26–51. Zhu, D. and Okuno, R. [2016] Multiphase isenthalpic flash integrated with stability analysis. Fluid Phase Equilibria, 423, 203–219. Connolly, M., Pan, H., Imai, M. and Tchelepi, H.A. [2021] Reduced method for rapid multiphase isenthalpic flash in thermal simulation. Chemical Engineering Science, 231, 116150. Voskov, D.V. and Tchelepi, H.A. [2012] Comparison of nonlinear formulations for two-phase multi- component EoS based simulation. Journal of Petroleum Science and Engineering, 82-83, 101–111.