Design and Analysis of Geothermal Wellbore Energy Conversion System Working on Zero Mass Withdrawal Principle

This project is sponsored by the Department of Energy of the United States and dedicated to development of electricity production from the low-enthalpy geothermal reservoirs. The prime interest are reservoirs that are characterized by low temperature of heat source located in deep saline aquifers with high permeable rock. Usually energy production from these resources are not economical by using a conventional binary power plant approach. The presented PhD work is a study of a new system that utilizes a single-well technology and working on supercritical power cycle (PC). The wellbore energy conversion system is operating with Zero Mass Withdrawal (ZMW) principle, which implies no geo-fluid pumping to the surface facility. This study introduces analyses of three main subsystems of the power unit. The heat extraction subsystem (HES) is located at the reservoir depth. The power generation subsystem (PGS) is represented by power cycle, and the heat rejection subsystem (HRS) contains an air driven condenser as the only part located on the surface. Several working fluids were examined. Based on the thermodynamic study the best working fluid choice is carbon dioxide. The project includes a simplified mathematical model derived from energy balance equations for each subsystem. Dimensionless analysis is performed in order to connect subsystems of different scales and show energy flow from the reservoir to the surface environment. The reservoir prototype is a hot saline aquifer located in Vermilion Parish, LA. The numerical model illustrates application of the ZMW method to the energy production from this reservoir. The maximum net power production is constrained by the power spent on a brine pump, which is a function of frictional losses in the downhole heat exchanger (DHE). The numerical investigation defines the optimal operating brine flow regime for the maximum net power production. One of the qualitative parameters of this design scheme is a thermal breakthrough time of injected cooled brine flowing toward the production side. This parameter is derived using potential flow theory application for several cases of flowing reservoirs, and various brine flow rates. The project contains an economic analysis based on determination of Levelized Cost of Electricity (LCOE). The results are in a good agreement with references and show competitive results for low-enthalpy reservoir exploration in terms of electric power production