Elsevier

Geothermics

Volume 65, January 2017, Pages 44-59
Geothermics

Mechanical behaviour of Australian Strathbogie granite under in-situ stress and temperature conditions: An application to geothermal energy extraction

https://doi.org/10.1016/j.geothermics.2016.07.002Get rights and content

Highlights

  • Mechanical behaviour of granite rocks at high pressures and temperatures.

  • Extraction of geothermal energy from hot dry rocks.

  • The micro-structure alteration in granite is observed using SEM analysis.

  • An increasing temperature leads to an initial increment in reservoir rock strength and shear parameters followed by reduction.

  • Mohr-Coulomb criteria is not suitable for modelling rocks at high pressures and temperatues.

Abstract

Geothermal heat has now been identified as an effective renewable energy source due to severe environmental impacts created by conventional fossil usage on global climatic change. However, its wide application has been limited due to the lack of knowledge, particularly of the geothermal conditions of reservoir rocks at elevated temperatures and pressures. Such high temperatures and pressures possibly alter the mechanical properties of reservoir rocks due to the associated micro-structural and mineralogical alterations of the rock mass, which are an important attribute for wellbore stability and stimulation of geothermal reservoirs for safe and effective geothermal energy extraction. This study therefore investigates the stress-strain behaviour under in-situ stress and temperature conditions by conducting a series of high-pressure, high-temperature tri-axial experiments on Australian Strathbogie granite under four different confining pressures (10, 30, 60, 90 MPa) and four different temperatures (RT, 100, 200, 300 °C). The effect of temperature on the mechanical behaviour of rock specimens was studied under tri-axial conditions and the corresponding fracture propagation behaviour was observed using an advanced acoustic emission (AE) system. The corresponding micro-structure alteration in granite was observed using SEM analysis. According to the findings, increasing temperature leads to an initial increment in reservoir rock strength and shear parameters followed by reduction, and the trend is aligned with the crack formation pattern of the rock mass. This was further confirmed by the SEM analysis, according to which the rock micro-structure is subject to only minor changes at relatively low temperatures and higher temperatures cause micro-cracks to develop along the rock mass grain boundaries. Furthermore, the conventional Mohr-Coulomb criteria failed to model the stress-strain response of rock under geothermal reservoir conditions, and was therefore modified for the corresponding in-situ conditions.

Introduction

Geothermal heat has been identified as a renewable and reliable energy source, given the severe environmental impacts created by conventional fossil fuels such as coal and oil (Martín-Gamboa et al., 2015, Axelsson, 2010). Exploratory geothermal wells have therefore been drilled to test the availability of geothermal reservoir rocks (rocks at elevated temperatures) and deep granite reservoirs have been found to have adequate temperatures to serve as a geothermal reservoirs (Fox et al., 2013). Such explorations involve finding vast blocks of “hot rocks” with fracture systems. Such rocks can be used to generate electricity. In this process, water is first injected and circulated through the fractures in the geothermal reservoirs and eventually pumped back to the surface as steam. However, the exploration of geothermal resources has become a challenge to engineers and geologists, due to the high temperature and stress environments in geothermal reservoirs. For this reason, laboratory experiments conducted under high pressure and temperature conditions (geothermal conditions) can assist in providing basic predictions. A large number of laboratory experiments has therefore been conducted since 1970 to investigate the influence of high pressure and temperature on rock in relation to geothermal heat extraction (Siratovich et al., 2016), deep geological disposal of nuclear waste (Paquet and François, 1980), deep mining (Wawersik and Hannum, 1980) and geological CO2 storage (Dai et al., 2014).

The mechanical behaviour of reservoir rocks is significantly influenced by elevated temperatures, because they cause the micro-structure of the rock mass to be altered through thermal expansion, the development of new micro-cracks, extending and/or widening the existing micro- cracks and various mineralogical alterations. Rock strength-deformation criteria change with temperature, and some rock mechanical properties, such as compressive strength, tensile strength, elastic modulus, cohesive strength and friction angle, decrease with increasing temperature (Heuze, 1983, Dwivedi et al., 2008). However, such alterations are also affected by the confinement applying on the rock mass, and confining pressure causes the suppression of thermal cracks and the extension/widening of existing micro-cracks. This results in further alteration of the rock mass mechanical properties, resulting in changing failure modes (Mogi, 1966). Studies have identified the transition of rock failure mode from brittle to ductile with increasing confinement. However, this transition is diverse due to the different mineralogical compositions and grain boundaries in different rocks (Klein et al., 2001, Wong, 1982).

Although many studies have been conducted to investigate the temperature-dependent mechanical behaviour of reservoir rocks, most experiments have been conducted in unconfined environments (Singh et al., 2015; Shao et al., 2014). Pre-heating the specimens to the corresponding temperature ranges and testing at room temperatures has been frequently done (Bauer and Johnson, 1979, Xu et al., 2008), but the method is not reliable, as geothermal reservoirs experience continuous thermal stresses. Such limitations have occurred mainly due to the limited number of appropriate test facilities available to simulate the stress and temperature conditions of actual geothermal reservoirs. Although some studies have analytically and numerically evaluated the mechanical properties of rocks under high temperatures and pressures (Chester and Higgs, 1992, Tian et al., 2013, Vásárhelyi et al., 2016), to date none has captured the stress-strain behaviour and failure criteria of rocks over a wide range of confining pressures and temperatures, particularly for Australian granite. However, the thermo-elastic response and appropriate failure criteria are essential inputs for many engineering applications under high pressures and temperatures, including geothermal extraction applications. This study therefore intends to fill this gap, and it is hoped it will make an important contribution to many deep underground applications.

Section snippets

Tested material

Granite is an abundant crystalline rock in the earth with medium to coarse grains which was formed as a result of the slow cooling of magma. Granite therefore has abundant radioactive elements (K, Th, and U). Such elements bring elevated geothermal gradients to deep granite reservoirs, which therefore have ideal conditions as geothermal reservoirs. The mineralogical composition of granite mainly consists of quartz, feldspar, plagioclase and biotite, and small amounts of muscovite, amphibole,

The overall stress-strain response

The influence of temperature on the stress-strain behaviour of the tested granite was considered first and the test results (deviatoric load at failure, deviatoric stress at failure, triaxial compressive strength and elastic modulus) are given in Table 3. Fig. 4 illustrates the relationships between deviatoric stress and axial strain curves for the samples tested at 10, 30, 60 and 90 MPa confining pressures at various temperatures (20, 100, 200, 300 °C). In relation to the stress-strain curves of

Conclusions

A series of tri-axial strength tests was conducted on Australian Strathbogie granite under four different confining pressures (10, 30, 60, 90 MPa) and four different temperatures (RT, 100, 200, 300 °C), simulating various geothermal reservoir conditions. The following conclusions can be drawn:

  • Mechanical behaviour of the tested granite is influenced by both reservoir depth and temperature, and the depth effect is much greater than the temperature effect.

  • Granite located at relatively smaller depths

Acknowledgements

This research project is funded by the Australian Research Council (ARC- DP160104223) and the authors would like to thank all the Deep Earth Energy Laboratory staff at Monash University, Clayton campus, Australia and the Monash Centre for Electron Microscopy (MCEM), who dedicated their time and energy to bring this experimental series to a successful conclusion. The sixth author extends his appreciation to the Deanship of Scientific Research at King Saud University (Saudi Arabia) for funding

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