Thermal and fluid processes in a closed-loop geothermal system using CO₂ as a working fluid

Highlights

• The characteristics of water and CO₂ as working fluid in Closed-Loop Geothermal System (CLGS) are compared.

• In CLGS, the heat extraction rate of CO₂ is just a little higher than that of water.

• CO₂ has a higher heat extraction rate, but a lower flow generation temperature than water.

• The liquid with larger heat capacity is more suitable for working fluid of CLGS.

Abstract

A co-axial closed-loop geothermal system is a new method to extract geothermal energy. It uses a continuously closed wellbore and the closed system can avoid many problems of traditional geothermal development methods, such as reservoir blockage and heat transmission fluid leakage. Generally, water as a working fluid will cause a series of problems which are not conducive to the operation and maintenance of the geothermal system. Recently, using CO₂ as a working fluid has attracted considerable attention. CO₂ has a larger mobility and buoyancy that result from its lower density and viscosity. However, a large flow rate will lead to incomplete heating of the working fluid, which will reduce the thermal performance of the geothermal system. In addition, CO₂ has a smaller specific enthalpy, which is not beneficial to the heat extraction of the geothermal system. Therefore, it is necessary to study the advantages and disadvantages of CO₂ and water as working fluid. In this work, a wellbore reservoir coupled model is built to simulate the operation fluid flow and thermal processes in a co-axial closed-loop geothermal system. The difference in productivity between water and CO₂ as working fluids is investigated. The fluid flow and thermal processes of CO₂ and water along the wellbore and the heat-extracting mechanism are analysed. The results show that although CO₂ is more efficient, its output temperature is lower than water. Therefore water is still suitable for geothermal reservoirs with lower temperature. The modelling and analysis methods presented here may provide a theoretical reference for the selection of a working fluid in geothermal engineering developments.

Introduction

Presently, atmospheric contaminations caused by fossil combustion are becoming serious problems around the world. Therefore, new and clean energy sources must be developed. Geothermal energy is a kind of stable and sustainable energy [1,2] that is widely distributed all over the world and is universally acknowledged. Conventionally, geothermal energy is extracted from geological systems characterized by a high temperature, high permeability and large in situ water reserves (hydrothermal systems) [3,4]. However, such conditions are not common for most geothermal sites. Additionally, there are many problems in the development process of hydrothermal geothermal fields, such as reservoir blockages or a lack of abundant recharge [[5], [6], [7]]. Recently, hot dry rock (HDR) geothermal energy as a new type of geothermal resource has drawn considerable attention. HDRs are low permeability, crystalline rocks with temperatures greater than 180 °C within 3–10 km depths. The total amount of HDR energy resources in China is 25 × 10⁶ EJ [8]. However, most HDR reservoirs have very poor permeability and almost no in situ groundwater. Enhanced geothermal systems (EGS) may be a feasible solution. EGS is the engineered reservoirs that have been created to extract economical amounts of heat from low permeability and/or porosity geothermal resources [9]. But the cost of EGS is extremely expensive, and reservoir engineering is difficult to control [10,11]. These problems represent great obstacles to the development of HDR geothermal resources.

During the operation of a hydrothermal geothermal system, cold water is injected into a reservoir, flowing through and heated by the pores and fractures, and then recovered from the production well or directly extract rich in-situ groundwater from the formation without any injection. Those configurations are called an open loop geothermal system (OLGS) because the working fluids have direct contact with the formation. In contrast, Closed-Loop Geothermal System (CLGS) is a relatively isolated system. The working fluid is injected into a continuously closed wellbore and circulated back to the ground in the same wellbore, without direct contact with geothermal reservoirs and in situ water [12]. The advantages of CLGS are: (1) no water circulates in the reservoir, so there is no need to consider possible water contaminants and the permeability of the reservoir; (2) there is no risk of groundwater pollution and scale deposit plugging in the reservoir; and (3) reservoir stimulation projects are not needed, and the additional costs and risks are reduced.

The CLGS design can be classified as three types: co-axial, multiple string, and U-shaped (Fig. 1) [[13], [14], [15]]. A co-axial closed-loop geothermal well is consisted of two tubes and an annulus existing between inner tube and outer tube. During co-axial CLGS running, low-temperature working fluid is injected through the outer tube from wellhead, heated by the surrounding formation in the process of descending to the well bottom, and then extracted to the ground from inner tube with a good adiabatic performance. Multiple string design is proposed by Hasan et al. [16], the bottom part of the well is full of liquid (e.g., water), while the upper portion is full of gas (e.g., nitrogen) [17]. However, However, geothermal well with multiple strings design always require a large borehole diameter because there are two tubing in the geothermal well, which is difficult to construct. Multiple string designs are usually suitable for shallow geothermal resources. U-shaped well design is the most common and classic CLGS design and it has two vertical wells, which are connected by a horizontal well. Obviously U-shaped well design has larger contact area than other well designs, which can increase the thermal efficiency. However, the cost of two vertical wells and a horizontal well is much higher than other single well design. In this study, we are focus on the co-axial closed loop geothermal system.

The heat transfer process of the OLGS includes heat conduction and convection, but CLGS only includes heat conduction around a geothermal well. Hence, the heat extraction rate of CLGS is lower than that of OLGS. In the past, there has been considerable debate about whether the heat conduction of rock can compensate for the thermal depletion around CLGS, and if the system can maintain a stable production rate for a long term [18,19]. In recent years, many scientists have carried out experiments and numerical simulations to study the CLGS with water as the working fluid. The results indicate that the insulation of the inner tube has an important effect on the performance of the system [[20], [21], [22]]. Due to the limitation of the insulation material of the inner tube, CLGS development has been halted for years. With breakthroughs in insulation materials, CLGS has been paid attention by some researcher recently. At present, many kinds of thermal insulating inner tube have been developed, including insulating coatings on the tube wall, thermal insulating tubing or other developed thermal insulating materials recently. Many relevant companies and research institutions around the world have conducted actual engineering experiments and have obtained satisfactory results [[22], [23], [24]]. In addition, there are many ways to use the low temperature geothermal water from CLGS. A typical application is geothermal heating for the building and agricultural facilities through heat exchanger [[25], [26], [27]]. This way can make full use of the hot water with a temperature of 30–60 °C.

In general, the working fluid of CLGS is water, and many investigators have studied water-based CLGS and obtained results [24,[28], [29], [30]]. However, there are still many problems during water based CLGS operations, such as the corrosion of metal wellbore, a large flow resistance and a low flow rate of the working fluid. In contrast, CO₂ as the working fluid has more advantages than water in this ways. Pruess [31,32] performed fundamental numerical studies to evaluate the heat extraction performances of water and CO₂. The results showed that the heat extraction rate of CO₂ can be 50% higher than that of water in certain thermal conditions in a reservoir. This finding is mainly because CO₂ has a larger expansibility and lower viscosity than water, which may generate large density differences between injection and production wells, provide a buoyancy force to reduce power consumption, and lead to a larger mass flow rate. Zhu et al. have studied the heat transfer of CO₂ in tubes and propose a new parameter to distinguish the regimes [33]. Oldenburg et al. studied a U-shaped CLGS with supercritical CO2 (ScCO₂) as the working fluid, which was considered as a more efficient working fluid than water [34,35]. Since the previous researches on the heat transfer characteristics of carbon dioxide in CLGS are all based on U-shaped geothermal wells, there are few researches on coaxial geothermal wells. Therefore, the research object of this study is determined to be coaxial geothermal wells. In this work, the physical process of flow and heat transfer in wellbore with CO₂ and water as working fluid is analysed and a flow-heat transfer numerical models of CO₂ and water are established respectively. The existence of reservoir around geothermal well is considered in the model. By means of numerical simulation, T2well numerical simulation program is used to carry on the wellbore-reservoir coupling numerical simulation. According to the simulation results, the mechanism of heat extraction of two kinds of fluids is analysed, and the influence of important parameters in CLGS operation is discussed. This work may also provide a theoretical reference for the selection of a heat transmission fluid in geothermal engineering.