Inversion in the permeability evolution of deforming Westerly granite near the brittle–ductile transition Fluid flow through crustal rocks is controlled by permeability. Underground fluid flow is crucial in many geotechnical endeavors, such as CO2 sequestration, geothermal energy, and oil and gas recovery. Pervasive fluid flow and pore fluid pressure control the strength of a rock and affect seismicity in tectonic and geotechnical settings. Despite its relevance, the evolution of permeability with changing temperature and during deformation remains elusive. In this study, the permeability of Westerly granite at an effective pressure of 100 MPa was measured under conditions near its brittle–ductile transition, between 650 °C and 850 °C, with a strain rate on the order of 2·10–6 s−1. To capture the evolution of permeability with increasing axial strain, the samples were continuously deformed in a Paterson gas-medium triaxial apparatus. The microstructures of the rock were studied after testing. The experiments reveal an inversion in the permeability evolution: an initial decrease in permeability due to compaction and then an increase in permeability shortly before and immediately after failure. The increase in permeability after failure, also present at high temperatures, is attributed to the creation of interconnected fluid pathways along the induced fractures. This systematic increase demonstrates the subordinate role that temperature dilatancy plays in permeability control compared to stress and its related deformation. These new experimental results thus demonstrate that permeability enhancement under brittle–ductile conditions unveils the potential for EGS exploitation in high-temperature rocks. The global demand for cleaner energy, such as geothermal energy, is booming, but the high investment costs associated with subsurface operation still inhibit the realization of projects at the industrial scale (e.g., Finger and Blankenship1 and Rossi et al. 2) To date, only shallow high-temperature regions located in the vicinity of a magma body ( 350 °C, and continuous deformation5,13,14. Knowledge of the permeability evolution under such transient conditions is an important prerequisite for predicting the behavior of a geological reservoir, particularly when production and/or injection takes place. Percolation of pressurized fluids along critically stressed faults can trigger seismicity (e.g., Miller et al.15), in not only enhanced geothermal systems (EGSs)16 but also many natural tectonic settings. In this context, a better understanding of the stress and temperature dependency of permeability is needed.The temperature dependency of permeability has been investigated on only a limited scale, and contrasting trends in the evolution of permeability were obtained depending on the investigated rock type (e.g., igneous, metamorphic, and sedimentary)17. A positive correlation between temperature and permeability was attributed to the formation of microcracks between different minerals due to the different thermal expansion coefficients in several rock types (igneous and metamorphic)18. On the other hand, Shmonov et al.17 observed an initial drop in permeability with increasing temperature, with a later increase above 300 °C. In follow-up studies, they observed a consistent decrease in permeability with increasing temperature17; similarly, Bakker et al.12 showed a permanent reduction in the permeability of limestone with increasing temperature. The observed decrease was attributed to the ductile closure of the initial pore space by dislocation creep of the minerals due to viscous relaxation induced by thermoelastic stresses, pressure solubilization, or an ‘excess’ of thermal expansion of the rock18.Deformation plays a leading-order contribution to the evolution of permeability and the fluid’s percolation behavior. Mitchell and Faulkner19 studied the permeability of Cerro Cristales granodiorite and Westerly granite at room temperature, between 10 and 50 MPa effective pressure, and under an increase in differential stress, recording increases in permeability up to two orders of magnitude. This increase was higher before macroscopic failure of the specimen, mainly due to the formation of microfractures. In contrast to materials that exhibit higher initial porosity, Suri et al.20 investigated permeability changes in Indiana limestone under increasing differential stress and recorded a decrease in permeability due to pore collapse with increasing deformation. Investigations on the effect of brittle and ductile deformation on anhydrite at room temperature revealed an increase of up to two orders of magnitude for both brittle and ductile deformation21. In this study, pore volume changes were monitored during the experiments to infer an increase in permeability with decreasing effective pressure.Several other studies have investigated the influence of high-temperature conditions on permeability evolution. Fischer and Paterson22 measured the permeability of three different rock types (limestone, marble, and sandstone) in a Paterson gas-medium triaxial apparatus at confining pressures up to 300 MPa, pore fluid pressures up to 250 MPa and temperatures exceeding 600 °C. The oscillation method was employed to capture changes in permeability due to increasing differential stresses at different stress intervals. Zhu and Wong23 carried out investigations on sedimentary rocks at room temperature, measuring their permeability during axial deformation, and stated that independent of brittle faulting or cataclastic flow, permeability decreases with an increase in the effective mean stress, which contradicts previously reported increases in permeability, especially before brittle failure and in low-porosity rocks (e.g., Mitchell and Faulkner19 and De Paola et al.21). The decrease in permeability described by Zhu and Wong23 was concluded to be related to a variable behavior between low- and high-porosity rocks when subjected to a stress increase. Low-porosity rocks tend to exhibit an increase in permeability with failure; conversely, the pore space of high-porosity rocks becomes more tortuous due to microcracking23. On the other hand, studies carried out on crystalline rocks clearly show an increase in permeability with increasing deformation19,24,25. For example, Coelho et al.25 carried out permeability measurements of altered basalts in a Paterson apparatus at pressures and temperatures of 100 MPa and 400 °C, respectively, recording the permeability at different steps before and after the peak differential stress point of the samples, highlighting an increase in permeability after sample failure. Zoback and Byerlee24, on the other hand, showed a positive correlation with increasing differential stress by investigating the effect of porosity variation on the permeability evolution of Westerly granite under differential stress.Permeability measurements at high pressures and temperatures are undoubtedly still difficult to perform26. Therefore, for measurements under continuous deformation, permeability is mostly inferred from continuous porosity measurements (e.g., Violay et al.13). However, the conversion between porosity and permeability can be ambiguous. While the former is a scalar describing the pore volume, the latter can be independent of pore volume or only partially associated with it, and this makes a direct translation between the two unreliable—particularly when more deformation modes coexist, such as brittle and ductile deformation27. Notably, regions characterized by brittle–ductile deformation can exist near the Earth’s surface and may be of potential interest for EGS exploitation (e.g., Watanabe et al.5, Violay et al.13, and Noël et al.27). To obtain a more comprehensive understanding of how permeability evolves throughout deformation and under conditions that feasibly represent shallow, high-temperature rocks with a potential for geothermal exploitation, additional experiments are needed to evaluate the important relationship between high-temperature-induced deformation and the continuously evolving permeability to a higher degree of certainty.In this study, we present effective permeability measurements of fine-grained granite at an effective pressure of 100 MPa, under the assumption of a simple effective pressure law28,29, and a temperature up to 850 °C under continuous axial deformation in a Paterson gas-medium triaxial apparatus30. The carefully conducted experiments show that the permeability enhancement of granite is possible during deformation at strain rates on the order of 2·10−6 s−1. Our results show that at high temperatures, the main mechanism controlling permeability enhancement is the creation of new fluid pathways during deformation, while temperature dilatancy is subordinate. This also confirms that a change in permeability is not necessarily related to a change in porosity.The ‘Results’ section describes the experimental data, the ‘Discussion’ section analyzes the findings and the main implications of this study and outlines future research directions, and the ‘Methods’ section at the end of the manuscript describes the sample preparation and properties and the experimental procedure.For simplicity and clarity, only data for each experimental condition (CPWG5, CPWG19, CPWG14, CPWG11, and CPWG3) are analyzed and discussed in the following sections. Nevertheless, the permeability measurements and mechanical data for all the tested samples are represented in Fig. 1 and Table 1.Figure 1(a) Optical microscopy images of thin sections under cross polarized light of the deformed rock, with the major fault plane marked in green. (b) Stress–strain curves for the different samples at different temperatures show an increase in ductile behavior with increasing temperature. (c) Permeability evolution of each tested specimen from 650 °C to 850 °C. The permeability and differential stress errors are given in Table 1. PDC: permeability decrease by compaction; PIBF: permeability increase before failure; F: Failure; PIAF: permeability increase after failure.Table 1 Experimental results for all the tested samples.Figure 1 shows the thin sections and the permeability and stress evolution as a function of the true axial strain of the tested specimens. The shaded boxes in Fig. 1b,c represent the fast rupture stage of the sample, where the reliability of permeability values is low because of method limitations in measuring permeability at high strain rates. The range of low data reliability is slightly different for each sample due to its strain rate dependency. To account for this variability and for display purposes, a shaded boundary is shown. For consistency, these data are plotted but are not considered in the interpretation of the experimental results. The initial permeability for samples deformed between 700 °C and 800 °C lies between 9·10–19 m2 and 1.2·10–18 m2; for samples measured at 650 °C and 850 °C, the initial permeability is lower, 3.48·10–19 m2 for sample CPWG5 at 650 °C, and approximately 2.2·10–19 m2 for the samples measured at 850 °C, CPWG3 and CPWG12 (Table 1). The overall change in permeability as a function of the true axial strain is represented by an increase of up to half an order of magnitude at both high pressure and temperature (Fig. 1). The change in permeability is, however, not uniform with axial deformation, and its increase is most significant after specimen failure. Furthermore, a general decrease in the maximal differential stress correlates with an increase in the temperature.Postexperiment thin section analysisFigure 1a shows thin sections retrieved after the experiments. The petrographic analyses and the stress–strain curves of the specimens from experiments under temperatures from 700 °C to 850 °C show a shear failure dominated deformation, with a prevalent brittle deformation mode (Fig. 1a,b). With increasing temperature, the single well-localized fracture plane that developed in experiments up to 750 °C transitions to a conjugated system of fractures. With increasing temperature, the conjugated system of fractures widens and becomes broader, indicating that a ductile component in the deformation was present and increasingly important. From 750 °C to 850 °C, the system of fractures gradually became a distributed cluster of cracks instead of a single rupture. In addition to a broad fracture cluster, sample CPWG3, deformed at 850 °C, undoubtedly shows at macroscopic scale a barreling effect, which is typically found in experimentally ductile-deformed rocks (Fig. 1a). Moreover, the peak differential stress decreases with temperature, which is another indicator of ductile deformation.Stress–strain and permeability analysisAt 650 °C, the peak differential stress, at which failure occurs, was not reached because of the technical limitations of the load cell (Fig. 1b). Up to 1% true axial strain, the samples underwent a permeability reduction of almost an order of magnitude (Fig. 1c). With a further increase in true axial strain, permeability increased up to slightly less than half an order of magnitude from its lowest value until the maximal experimental stress was reached.The 700 °C experiment corresponds to a peak differential stress of approximately 800 MPa. The permeability decreased up to an approximately 1% true axial strain and subsequently increased immediately before the peak differential stress, similar to the 650 °C experiment (Fig. 1c). However, this behavior is less pronounced than in the specimen measured at 650 °C. After rupture, a permeability almost half an order of magnitude higher than the lower permeability reached during compaction was measured.The peak differential stress at 750 °C was slightly lower but similar in magnitude to that at 700 °C. The permeability evolution displayed a decrease, as for pre
https://www.nature.com/articles/s41598-021-03435-0
Inversion in the permeability evolution of deforming Westerly granite near the brittle–ductile transition
