Abstract

The development of technically-sound enhanced geothermal systems (EGSs) for geothermal energy extraction is identified as a viable solution for world growing energy demand with immense potential, low carbon dioxide emission and importantly, as an environmentally friendly option for renewable energy production. The use of supercritical carbon dioxide (ScCO2) as the working fluid in EGSs by replacing traditional water-based method is promising due to multiple advantages prevail in ScCO2-injection for underground reservoir stimulation. The evolution of reservoir stimulation using ScCO2 and the understanding of the flow behaviour of a ScCO2-stimulated geothermal reservoir is vital in applying ScCO2-EGSs as a replacement for water-based EGSs. The study is therefore aimed to investigate the flow behaviour of a ScCO2-fractured rock medium at in-situ stress and temperature conditions. A series of permeability tests were conducted for ScCO2 fractured Harcourt granite rock specimens at 90 °C, under varying confining pressures from 5 – 60 MPa using the high-pressure and high-temperature tri-axial set up which can simulate deep geological conditions. Enhanced flow characteristics with a permeability greater than 0.1D were observed in the ScCO2 fractured rock specimen even under high confining pressures of 60 MPa, compared to the intact rock specimens with permeability in the range of 0.01-0.10 µD. Besides, the permeability exhibits a non-linear reduction with increasing confining pressure due to the stress-induced fracture closure. Further, the enhanced permeability of the ScCO2-induced fracture with multiple secondary branches was explained by exploring the CT images of the rock specimens.

1 Introduction

The statistics of Global Resources Outlook (Oberle, 2019), indicates that the doubling of the world population and fourfold increase of global domestic production have caused a significant increase in global greenhouse gas emission. Accordingly, the growth of world's economic and resource consumption needs to be positively shaped-up with negative environmental impacts. Therefore, it is a huge challenge to meet the emerging energy demand while reducing the carbon dioxide emission to the atmosphere, and hence, efforts are taken to diversify the energy production with a significant share from renewable energy resources (ARENA, 2018). Geothermal energy has been considered as a major source of renewable energy with immense potential with minimum environmental impacts. Enhanced geothermal systems (EGSs) are developed to access the heat stored in deep earth by circulating a fluid in a closed-loop. In EGSs, a fluid is injected in high-pressure into the deep earth through a well to open-up natural flow paths in deep rock formations, and the hot fluid which gets heated by extracting the heat from deep, hot rock formations is accessed using a well connected to the flow path. Then, the heated fluid is transported back in a closed loop to the earth's surface for electricity generation. However, it is challenging to develop, operate and achieve desired energy extraction targets from EGSs due to number of problems which occur during well drilling, underground reservoir stimulation, and due to induced-seismicity (Genter et al., 2010; Jefferson W. Tester, 2006; Mills & Humphreys, 2013).

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