Elsevier

Engineering Geology

Volume 229, 7 November 2017, Pages 31-44
Engineering Geology

Temperature-dependent mechanical behaviour of Australian Strathbogie granite with different cooling treatments

https://doi.org/10.1016/j.enggeo.2017.09.012Get rights and content

Highlights

  • Increasing the temperature cause reduction of strength and elastic characteristics

  • Brittle to quasi-brittle failure behaviour with increasing temperature

  • Induction and propagation of thermally induced inter and intra-granular cracks

  • Higher influence on mechanical and micro-structural characteristics upon quenching

  • Wider un-stable crack propagation of quenched specimen over slowly cooled specimen

Abstract

Understanding the mechanical behaviour of reservoir rock under different temperatures with different cooling conditions is necessary for safe and effective deep geo-engineering applications, including geothermal energy extraction, deep geological disposal of nuclear waste, deep mining and coal gasification projects. The aim of this study is, therefore, to investigate the effect of increasing temperature (from room temperature to 800 °C) followed by two cooling methods (both rapid and slow) on the mechanical behaviour of Australian Strathbogie granite under uniaxial conditions. Further, a separate experimental program was conducted under continuous heating conditions without cooling the samples to compare the results of cooled samples. In order to investigate the strain developments in granite subjected to heating following slow and rapid cooling, ARAMIS photogrammetry technology was adopted, and the corresponding fracture propagation patterns were investigated using an acoustic emission (AE) system. Optical microscopic imaging technology was used to identify the corresponding micro-structural alterations and crack-formation patterns. According to the results, once the rock mass is subjected to higher thermal stresses, strength and elastic characteristics are significantly reduced, mainly due to thermally-induced damage in terms of both inter-granular and intra-granular cracks. The stress-strain response revealed that the failure mode of granite is changed from brittle to quasi-brittle fracturing with increasing temperature. The following cooling causes the strength and elastic characteristics of the granite to be further decreased through the enhancement of crack density, and the influence of rapid cooling is much greater than that of slow cooling, due to sudden thermal shock. This is evidenced by the AE results, according to which both high pre-heated temperatures and high cooling rates cause much quicker crack initiation and propagation in granite with lesser seismicity in the quasi-brittle region.

Introduction

In recent years, due to the demand for high temperature applications in geological sciences and geotechnical engineering, many research studies have focused on understanding the temperature-dependent mechanical behaviour of reservoir rocks. Investigation of temperature-dependent reservoir behaviour is critical for many deep geological applications, including deep geothermal energy exploitation (enhanced geothermal systems), deep mining, deep geological disposal of nuclear waste and coal gasification projects.

With the presence of heat-producing radioactive isotopes and thus elevated geothermal gradients, granite has become an important reservoir material for enhanced geothermal systems (EGSs) (up to 350 °C) (Barla, 2017; Breede et al., 2013). Traditionally, water is used for fracturing and heat-carrying process in EGSs. During the injection of cold water into hot rock, around the borehole, the hot rock is subjected to sudden temperature changes, resulting in the generation of thermal stresses, and with increasing distance from the borehole, the corresponding cooling rates decrease. These thermal stresses significantly influence the mechanical characteristics of the reservoir rock, and reservoir rocks subjected to different heating and cooling conditions, therefore, exhibit very different mechanical characteristics compared to intact rock, and these effects can critically affect the stability of the borehole. Also, the high-temperature mechanical response of formation rocks (mainly granite) is critical for geological nuclear disposal facilities (Ramspott et al., 1979). These facilities experience significant temperature rises, generally in the range of 200 °C to 1500 °C during the decomposition of radioactive substances (Gibb, 2000). Therefore, for the safe storage of nuclear waste under deep geological locations, the mechanical characteristics of the formation rock under high temperature conditions need to be precisely understood. In addition, under coal gasification process, once coal is burnt by injecting oxygen and steam, upper rock layers experience thermal stresses up to 1300 °C (both slow and rapid cooling conditions), and, therefore thermo-mechanical behaviour of cap rock need to be investigated for the safe and effective operational process (Ocampo et al., 2003). However, it should be noted that the cap rock layers in the coal gasification process are mostly sedimentary layers with few granite formations (Qi et al., 2016).

Once the rock mass experiences temperature differences, thermal stresses are generated, these are generally tensile stresses at the cooling surface and compressive in the interior of the rock (Collin and Rowcliffe, 2002). The highest tensile stresses are experienced on the surface of the rock and decrease with increasing crack depth. With increasing temperature the number of thermally-induced cracks increases, resulting in the increased crack density of the rock mass. According to David et al. (2012), once the Peyratte granite is heated from room temperature to 600 °C, crack density increases from 0.2% to 4.4%. Further, due to the anisotropic expansion of different mineralogical constituents of the rock matrix, thermal cracks are nucleated and significant enhancement of crack density has been reported above 600 °C, which is mainly due to the α to β transition of quartz, which occurs at 573 °C (Ohno, 1995). These induced thermal stresses alter the mechanical response of the reservoir rock, and these influences are highly dependent on the micro-structural characteristics of the rock, including the mineralogical and grain size distributions. Further, the cooling rate has a significant influence on this mechanical response of the rock, and, depending on the cooling rate, the rock can be thermally shocked after a certain critical quenching (Fellner and Supancic, 2002). Therefore, an understanding of the temperature-dependent mechanical behaviour in reservoirs rock together with quenching effects is necessary for safe geological applications.

Since 1970, a large number of laboratory experiments have been conducted to investigate the effect of temperature on the mechanical behaviour of granite. However, most of the experiments have been conducted by pre-heating the specimens to the corresponding temperature ranges and then testing them at room temperature (Chen et al., 2017; Liu and Xu, 2015; Singh et al., 2015). Many deep geotechnical applications experience continuous thermal stresses together with different cooling rates including ‘thermal shocks’. Only a few experiments have been performed under continuous heating using appropriate test facilities (Kumari et al., 2017; Shao et al., 2015). Limited studies have captured the effect of cooling rate on the mechanical behaviour of the rock (Brotóns et al., 2013; Shao et al., 2014), and none of them has captured the overall influence of continuous thermal stresses and the effect of cooling rate on the mechanical behaviour of reservoir rock. This study, therefore, intends to fill this gap, and offer an important contribution to many deep geological applications.

Section snippets

Testing material

For the present study, granite samples were collected from the Strathbogie batholith located 150 km north-east of Melbourne. It is a 1500 km2 large granitic intrusion in Victoria, mainly consisting of S-type granite. Quartz, feldspar, cordierite, garnet, biotite, and tourmaline are the main constituents of these granites (Phillips and Clemens, 2013). Fig. 1 illustrates a close view of a sample of the tested granite with its grain size distribution. The selected Strathbogie granite is a

Effects of heating and cooling on physical characteristics of granite

At the end of the two cooling treatments, samples were carefully inspected to identify the temperature-dependent physical changes and Fig. 5 illustrates the surface of the granite specimens after the slow and rapid cooling treatments. As shown in Fig. 5, the samples heated up to 300 °C (100, 200 and 300 °C) and then cooled to room temperature did not show any significant physical change in terms of colour and visible thermal cracking. A clear change in the colour of the test specimens (from

Mechanisms of thermally-induced damage in rock matrix

The generation of micro-cracks is the initial stage of weakening for any material, and the mineralogical composition and the grain size distribution play dominant roles, because crack propagation in any rock mass initiates through its weakest plane. In a crystalline rock like granite, two main modes of cracking can be identified: inter-granular and intra-granular (Kranz, 1983). Inter-granular cracks are propagated along the grain boundaries of the rock mass and intra-granular cracks are

Conclusions

A series of unconfined compressive strength tests was conducted on coarse-grained Strathbogie granite samples subjected to heating to various temperatures up to 800 °C, followed by slow and rapid cooling treatments to room temperature. According to the experimental results, granite is subject to significant alterations in its mechanical characteristics upon this heating followed by slow/rapid cooling, and the major findings can be summarised as follows:

  • Cooling of highly heated granite (> 400 °C)

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