A numerical study on compositional modeling of two-phase fluid flow and heat transfer in vertical wells

Abstract

Geothermal energy is a sustainable and renewable source that can be extracted by circulating a single- or two-phase fluid through a geothermal well system. Two-phase flow and heat transfer models are required for predicting pressure and temperature profiles in oil, gas, and geothermal wells. We model fluid flow, thermodynamics and heat transfer in an idealised vertical well for single-(i.e., water) and two-phase fluid mixtures (CO2-, and air-water) under a bubbly flow regime. First, we calibrated a Peng-Robinson Equation of State (PR-EOS) for CO2-, and air-water systems. Second, we solved continuity, energy and momentum equations and modelled transient conductive heat transfer through the well. Third, we investigated the effects of mass flow rate, transient heat transfer, single- and two-phase fluid on the extracted power, temperature, and pressure profiles of the deep well bore heat exchanger.The results show that the temperature of the hot fluid decreases as it flows to the surface both in the case of adiabatic flow and in cases with heat loss. The mass flow rate controls the fluid temperature drop during its ascent to the surface. The production pressure of a gas-liquid phase system is higher than that of a system with single liquid phase at the same injection pressure, temperature, and mass flow rate. Increasing mass flow rate up to a threshold value leads to an increase in the production pressure. Above the threshold mass flow rate (i.e., 7 kg/s in this study), the production pressure reduces because of the significant increase in the frictional pressure drop. Production temperature, production pressure, and total power increases over time due to the local heating around the production well and reduction of the heat loss.