Mixed-Dimensional Approach for Compositional Multiphase Flow in High-Enthalpy Fractured Geothermal Reservoirs

Abstract

We present a novel multiphase compositional model for geothermal reservoir simulation that incorporates phase separation phenomena and explicitly represents fractures. Mass and heat transfer in porous media typically use phase saturation as an independent variable, but this requires procedures for variable substitution during phase transitions. In contrast, the overall composition formulation avoids such substitutions by maintaining persistent equations and variables in every cell. For brine and steam systems with high enthalpy, we adopt this formulation, using enthalpy as the state variable instead of temperature. The model is introduced in a novel fractional flow form, which enhances numerical efficiency. Fractures are modeled as two-dimensional objects within the surrounding three-dimensional porous medium, offering a mixed-dimensional extension of the multiphase flow model. This allows robust simulation of fluid flow, heat transfer, and phase separation, including the interaction between fractures and the porous medium. Our work makes two key contributions. First, we extend a compositional model to its mixed-dimensional version, enabling numerical simulations of fractured media in complex fracture networks. Second, we introduce an efficient interpolation scheme for H2O-NaCl brine in pressure-enthalpy-composition (PHZ) space, ensuring accurate representation of complex thermodynamic properties. We demonstrate the model's capabilities through simulations of geothermal flow in challenging fracture network geometries.