A review of fluid flow and heat transfer in the CO2-EGS

LIU Yan-guang, ZHU Xi, YUE Gao-fan, LIN Wen-jing, HE Yu-jiang, WANG Gui-ling*

1 The Institute of Hydrogeology and Environmental Geology, CAGS, Shijiazhuang 050061, China.

2 Geothermal Resources Research Center of China Geological Survey, Shijiazhuang 050061, China.

Abstract: Carbon dioxide enhanced geothermal system (CO2-EGS) now is an emerging research field that is attracting an increasing research interest with broad application prospects based on its low-carbon, economical and renewable features. The fluid flow and heat transfer is the core of CO2-EGS research. In this paper, further research focus is pointed out after summarizing the latest research progress in this field based on the explanation and the advantages of CO2-EGS development process in the hope of providing reference for researchers engaged in this field.

Keywords: Enhanced geothermal system; Carbon dioxide working medium; Fluid-thermal coupling; Numerical simulation

Introduction

Carbon dioxide enhanced geothermal system(CO2-EGS) is an engineering system for developing the huge amount of heat energy by using supercritical CO2(pressure>7.382 MPa,temperature>31.04 ℃) as the working medium to extract geothermal energy from the hot dry rock(HDR) crushed artificially (Pruess K, 2006;WANG Ji-yang, 2009). It is also a new way to deal with global climate change for it achieves the purpose of CO2recycling and geological storage(Brown D W, 2000; Xu T et al. 2010). Preliminary studies indicate, CO2-EGS is becoming a new hot research field worldwide for its broad application prospects thanks to its low carbon, economical and renewable features (Brown D W, 2000; Fouillac C et al. 2004; Pruess K, 2006; Pruess K et al. 2007,2008, 2010; Xu et al. 2010; Atrens A D et al.2008a, b, 2011; Wan Y et al. 2011; Pan L et al.2011a, b, c; Petro M et al. 2012; Na Jin et al. 2012;Hsieh J C et al. 2014; WANG Fu-gang et al. 2013).

CO2-EGS theory develops from relevant experience and theories of the researches and engineering practice in the fields including water-EGS, CO2geological storage, hydrothermal geothermal energy development and exploitation of oil and gas (LI Ke-wen and QIN Tong-luo, 1990;Brown D, 1995; JIANG Pei-xue et al. 1996;ZHOU Zhi-fang and WANG Jin-guo, 2004; ZHAO Yang-sheng et al. 2004; MA Yong-sheng et al.2005; XU Shi-guang and GUO Yuan-sheng, 2009;Xu T et al. 2009; LING Lu-lu and ZHANG Ke-ni,2013). While most of the theories can be applied directly to the CO2-EGS, further research is needed in terms of a number of specific issues of CO2-EGS. Currently, research mainly focuses on CO2-EGS engineering, fluid flow, heat transfer,fluid-rock chemical reactions and heat extraction performance (Brown D W, 2000; Pruess K, 2006,2007, 2008; XU T et al. 2009, 2010; Spycher N and Pruess K, 2010; ZHAO Yan-sheng et al. 2010;WAN Y et al. 2011; HU Li-tang et al. 2012; Pan L and Oldenburg C M, 2013; Hsieh J C et al. 2014).

The core of CO2-EGS research is the fluid flow and heat transfer process, both of which are within the same geological rock space with the issues of interference and mutual coupling (Pruess K, 2006).Therefore, this paper mainly elaborates on the background of CO2-EGS as well as the research progress and development trend of its fluid flow and heat transfer.

1 Current research progress and development trend at home and abroad

1.1 Background of CO2-EGS

CO2-EGS is based on the research on HDR.Studies have shown that HDR is widely distributed and has huge reserves. It is conservatively estimated that the energy reserves in HDR in the crust (with the depth from 3 km to 10 km) equals to 30 times of that in oil, natural gas and coal globally (XU Tian-fu et al. 2012). The main technology to develop HDR is the enhanced geothermal system. A number of countries and regions including the U.S. (Batchelor A S, 1986;Tester J W et al. 2006), Australia (SI Shi-rong,2009), Japan (Sasaki S, 1998), China (YIN Li-he,2010; WANG Gui-ling, 2012; XU Tian-fu et al.2012; LIN Wen-jing et al. 2013), and Europe(Baria R et al. 1999, 2005; Dezayes C et al. 2005;Azaroual M et al. 2007) have carried out relevant studies.

Brown (2000) first proposed the concept of EGS which uses supercritical CO2as the working medium for heat transfer, pointing out that compared with water, the supercritical CO2has many favorable factors, which are further proved by subsequent studies on the fluid flow and heat transfer process of CO2-EGS by many researchers(Fouillac C et al. 2004; Pruess K et al. 2006, 2007,2008, 2010; Atrens A D et al. 2008, 2009, 2011;Wan Y et al. 2011; Petro M et al. 2012; Na Jin et al.2012; Hsieh J C et al. 2012; Pan L et al. 2011a, b,c; Pan F et al. 2013). It is also pointed out that during the CO2-EGS process, CO2is stored after flowing to the surrounding rocks, which increases the carbon tax revenue on CO2emissions and therefore reduces the cost of CO2-EGS (Atrens A D et al. 2008a). Of course, CO2has negative attributes such as low specific heat, but it can be made up by the high speed of flow (Vukalovich M P and Altunin V V, 1968). Overall, CO2has stronger heat extraction capability as the working medium of EGS than water (Pruess K, 2006, 2008;Pruess K and Spycher N, 2010). Therefore,CO2-EGS has become a new hot research field.

1.2 Current research progress of fluid flow and heat transfer process of CO2-EGS

CO2-EGS heat extraction process is essentially the fluid flow and heat transfer process, which means that supercritical CO2is injected into the geothermal reservoirs at a constant pressure,temperature and velocity from the injection well to dispel and replace the existing fluid. Some of CO2is delivered by the production well to the surface after being heated up by the high-temperature reservoirs, and then go back underground upon the discharge of heat energy (Pruess K, 2006). Usually,the wellbore and reservoirs are divided into different sub-systems for the fluid flow and heat transfer process in the large pore of wellbore, pores of reservoir and fractured porous media are different (Pan L, 2011). Geothermal reservoirs are sources of heat energy, the core of EGS system.The coupling process of fluid flow and heat transfer between wellbore and reservoir is one of the research focuses of EGS as such process in wellbore also plays an important role.

Initial studies on CO2-EGS mainly are comparative studies of water-EGS. Brown (2000)for the first time developed a qualitative analysis of the heat and mass flow characteristics of supercritical CO2, pointing out that under the same conditions the heat transfer efficiency and mass flow rate of supercritical CO2are much higher than that of water. Afterwards, Pruess (2006, 2008)conducted a qualitative and quantitative analysis using numerical simulation techniques, as well as the analysis on the temperature and pressure sensitivity of geothermal reservoirs. His analysis indicates that the higher the initial reservoir temperature is, the less advantageous the supercritical CO2is when compared with water;the higher the initial reservoir pressure is, the faster the mass flow rate gets and the more the heat is transferred. WANG Fu-gang et al. (2013) studied the effect of injection temperature of CO2on the heat extraction efficiency of EGS, noting that the heat extraction efficiency decreased when the injection temperature increased from 18 ℃ to 42℃. Pruess and Spycher (2010) simulated threedimensional flow and heat transfer effect and found that temperature of reservoir decreases rapidly due to the fast fluid flow at the bottom of wellbore caused by the high pressure gradient that is generated by high density of cold fluid. Thus,thermal breakthrough (short circuit) is achieved by the flowing of fluid with lower temperature and higher density to the bottom of a production well.The flow and heat transfer process of CO2fluid are much more complicated than that of water for such strong movement of CO2fluid is affected both by temperature and pressure, quite different from water (Pruess K and Spycher N, 2010).

Spycher and Pruess (2010) carried out multiphase flow research on the presence of an aqueous phase (stagnant water of hydraulic fracturing and a small amount of the original retained water) at the early stage of CO2-EGS. CO2was dissolved in water during the initial production,and then water in the reservoir gradually reduced by the undergoing displacement of CO2until only supercritical CO2was left. Afterwards, the permeability increased and jumped once the one-way flow was formed, and together the flow rate increased sharply. This further increases the complexity of fluid flow and heat transfer process of CO2-EGS (Pruess K and Spycher N, 2010).

XU T et al. (2010), Pruess and Spycher (2010)and many other scholars (WAN Y et al. 2011;WANG Fu-gang et al. 2013) analyzed the feasibility of using CO2as the working medium for EGS from the perspective of fluid-rock chemical reaction, and proposed an initial solution to the technical issues of heat extraction such as thermal breakthrough (also known as the short circuit)within the reservoir and small porosity.

Pruess pointed out that a comprehensive evaluation of CO2-EGS fluid flow and heat transfer needs complete research process, including a numerical simulation study, experimental studies and field tests. Field tests can be only carried out based on sufficient theories for such tests are costly (Pruess K, 2007). Currently, most CO2-EGS studies are numerical simulation studies using TOUGH2 software. Although outstanding achievements have been made, some shortages still exist.For example, due to the short study period, most studies focus on the capacity of CO2as working medium of EGS for heat extraction, few of which concerns CO2flow process mechanism and its control mechanisms (Borgia A et al. 2013). In addition, TOUGH2 is one of the international advanced numerical simulation software most commonly used for CO2fluid flow and heat transfer of the fractured media. TOUGH2 has a unique advantage in CO2-EGS study and application because it can continuously split the rock matrix by use of MINC in terms of the high temperature gradient between EGS rocks and circulating fluid, and adopts the formula concerning the flow relationship when the fluid viscosity and density change according to temperatures and pressures (XU T et al. 2004). But it lacks of experiment verification due to the late development of EGS, especially that CO2-EGS has no actual field tests and the model parameters are mostly empirical data.

Magliocco (2011) conducted a heat extraction experiment from CO2-EGS reservoirs, but he failed to monitor and control the different flow rates, so the study on effect mechanism of the fluid flow rate at the bottom of the reservoir on heat transfer is insufficient. Nevertheless, the experimental apparatus improvement and development played a catalytic role and provided the basis for later systematical experimentation.

Wang Fu-gang et al. (2013) analyzed the effect of CO2injection temperature on heat extraction efficiency and system sustainability by the numerical simulation research on the influence on CO2-EGS heat extraction efficiency of CO2injection temperature based on the CCS demonstration project in Erdos. Based on the geological structure and thermal storage conditions in Quantou formation of Songliao Basin, SHI Yan(2014) studied the multiphase flow dynamicsthermodynamics process within the wellbore and reservoirs under the injection-exploitation temperature and pressure, and discussed the effect of wellbore flow on the geothermal system with CO2as the working medium for heat transfer.

Pan L et al. (2015) managed to carry out the simulation research on coupling flow and heat transfer between the wellbore and reservoir of CO2-EGS using T2Well, allowing the heat transfer between wellbore and the surrounding rock layers and taking into account the fluid inertia, gravity and friction with well wall. The research shows that the heat extraction efficiency of CO2-EGS is greatly influenced by the heat transfer between wellbore and the cap rock, and that the energy consumption of the pump can be significantly reduced due to the thermosiphon system formed by CO2as the working medium.

2 Prospects

At present, international CO2-EGS research is still in its infancy. The number of laboratory experiments is limited, while most of them aim to obtain some of the simulation-related parameters.Still, such experiments have played a fundamental and guiding role in the future researches on CO2-EGS. Actual field experiments and researches on physical simulating flow and heat transfer are still in need (ZHANG Wei et al. 2013).

For the entire CO2-EGS system, the fluid flow and heat transfer of carbon dioxide enhanced geothermal systems are complicated processes featured with multi-coupling occurrence. As CO2-EGS advances, the requirements for the development and engineering practice of EGS are difficult to meet due to the lack of verification of simple numerical simulation study, therefore using the technical method combined numerical simulation and experiments to study the coupling process of fluid flow and heat transfer by identifying the temperature and pressure within geothermal reservoirs will be one of the important research focuses of CO2-EGS.

Acknowledgements

This study is supported by the project of Maps of Groundwater Resources and Environmental Geology of China and Surrounding Areas(12120113014200) and Series Maps of Karst Environment Geology of China and South East Asia (12120114006401, 12120114006301).