Thermal Cracking in Westerly Granite Monitored Using Direct Wave Velocity, Coda Wave Interferometry, and Acoustic Emissions

Abstract

To monitor both the permanent (thermal microcracking) and the nonpermanent (thermo‐elastic) effects of temperature on Westerly Granite, we combine acoustic emission monitoring and ultrasonic velocity measurements at ambient pressure during three heating and cooling cycles to a maximum temperature of 450°C. For the velocity measurements we use both P wave direct traveltime and coda wave interferometry techniques, the latter being more sensitive to changes in S wave velocity. During the first cycle, we observe a high acoustic emission rate and large—and mostly permanent—apparent reductions in velocity with temperature (P wave velocity is reduced by 50% of the initial value at 450°C, and 40% upon cooling). Our measurements are indicative of extensive thermal microcracking during the first cycle, predominantly during the heating phase. During the second cycle we observe further—but reduced—microcracking, and less still during the third cycle, where the apparent decrease in velocity with temperature is near reversible (at 450°C, the P wave velocity is decreased by roughly 10% of the initial velocity). Our results, relevant for thermally dynamic environments such as geothermal reservoirs, highlight the value of performing measurements of rock properties under in situ temperature conditions.

Development and Recovery of Stress‐Induced Elastic Anisotropy During Cyclic Loading Experiment on Westerly Granite

“…In contrast to previous experimental studies (Chen et al, 2011;Eslami et al, 2010;Heap & Faulkner, 2008;Heap et al, 2009;Kendrick et al, 2013;Pozdnyakova et al, 2009;Trippetta et al, 2013;Yang et al, 2015) which were mostly conducted under uniaxial conditions, our experimental results obtained under triaxial conditions show that subsequent unloading stage induces a strong recovery of both damage and anisotropy. While damage recovery was recently observed under isostatic stress conditions (Brantut, 2015) or during the cooling stage of thermal cycles (Griffiths et al, 2018), the recovery of anisotropy remained undocumented. Our results suggest that while cracks nucleate or propagate during loading (attested by the occurrence of AEs) increasing the crack lengths and the crack densities, unloading allows recovery of contact along cracks, due to elastic closure and back-sliding phenomena (Basista & Gross, 1998;David et al, 2012;Scholz & Kranz, 1974;Walsh & Brace, 1964) during unloading.…”

Coda wave interferometry during the heating of deep geothermal reservoir rocks

“…Interestingly, the study highlighted different mechanical behaviors between the first and the subsequent cycles. While large and mostly permanent changes in the waveforms were measured during the first cycle, interpreted as an apparent reduction in velocity with temperature, the reductions in wave velocity during the following cycles were lower in amplitude (Griffiths et al 2018).…”

“…At the laboratory scale, seismic methods, both active (ultrasonic wave velocity measurements) and passive (acoustic emission monitoring), provide a range of monitoring techniques to analyze the influence of temperature on a rock sample. In particular, coda wave interferometry (CWI) is a method that has recently been used to monitor thermal microcracking in granite (Griffiths et al 2018). CWI uses scattered elastic waves observed in the late part of the seismogram (i.e., the coda) to track small changes in solid materials such as rocks (e.g., Poupinet et al 1984;Ratdomopurbo and Poupinet 1995;Roberts et al 1992;Snieder 2002).…”

“…This drop in apparent velocity presents a strong non-linear behavior when the sample was heated from 70 to 90 °C, occurring contemporaneously with an increase in the acoustic emission rate, attributed to thermal microcracking. Consistent changes in rock microstructure have been recently monitored within a wider range of temperatures in Westerly Granite using the same CWI method (Griffiths et al 2018). Westerly Granite is an interesting material for laboratory experiments as it has been extensively studied (e.g., Brace et al 1966Brace et al , 1968 and its physical properties are well known.…”

“…The fine crystal size of Westerly Granite, about 1 mm in diameter, is similar to the so-called "two-mica granite" (e.g., Hooijkaas et al 2006) which hosts the deep geothermal reservoir exploited at the Enhanced Geothermal System (EGS) test site in Soultz-sous-Forêts, France. Combining velocity measurements, acoustic emission monitoring, and high-resolution CWI measurements within a new experimental setup, Griffiths et al (2018) monitored the microstructural evolution of Westerly Granite during multiple heating cycles from room temperature to 450 °C. Interestingly, the study highlighted different mechanical behaviors between the first and the subsequent cycles.…”