Temperature influence on fracture behavior in clay-rich mudstone: A comprehensive experimental study

Abstract

This study investigates the fracture behavior of clay-rich mudstone under varying temperature and pressure conditions, which is crucial for the safety of geological structures. It focuses on three fracture types: pure mode I tensile fractures, pure mode II tensile fractures, and shear fractures, examining specimens at room temperature (RT) and after thermal treatments at 250 and 500 ℃. The findings reveal that increasing temperatures makes the mudstone more brittle, enhancing fracture velocity, toughness, load-bearing capacity, roughness, and the fracture process zone (FPZ) radius. Notably, tensile fractures induced under pure mode II displayed the highest velocities, while shear fractures exhibited the lowest velocities, smoothest surfaces, and greatest resistance to failure. The application of a confining pressure of 4 MPa significantly improved shear fracture toughness by 119.7%, 98.5% and 71.9% at RT, 250 ℃ and 500 ℃, respectively, and reduced roughness by 8.2%, 22.4% and 30.4%. This research offers a novel, comprehensive view of how temperature and pressure impact fractures in mudstone sensitive to temperature due to its high clay content and water affinity. The findings provide valuable insights applicable to geothermal energy, oil and gas exploration, and underground construction, thereby enhancing the understanding of fracture mechanics in geological contexts.

1. Introduction

Rock fractures play a significant role in various geological processes, including extracting natural resources, producing geothermal energy, and storing fluids in subsurface reservoirs [1 −3]. They appear mainly as tension and shear fractures, and can also be artificially induced such as in hydraulic fracturing [4, 5]. Temperature is one of the most important factors influencing rock mass behavior at depth. It is essential for evaluating fractures that enable the transport of underground resources such as oil, gas, and geothermal energy fluids. Therefore, this research focuses on the impact of temperature on mechanical behavior of three common types of fractures: tensile fractures from pure tension, e.g., fractures in the upper part of anticlines (Figure 1(a)) and those created during hydraulic fracturing (Figure 1(b)), tensile fractures resulting from pure shear, like off-fault fractures on the extensional side of dynamic shear ruptures (Figures 1(c) and (d)), and shear fractures from pure shear, such as those formed directly in front of the tips of shear ruptures (Figure 1(e)) and those observed on rock slopes (Figure 1(f)).

The impact of temperature on rock properties has garnered significant attention with advancements in deep earth engineering [6 −11]. Numerous researchers explored how temperature influences intact rock behavior, including its effects on elastic modulus [12], sound wave velocity [13], tensile strength [14], compressive strength [15], and shear strength [16], etc. In addition, many researchers investigated the temperature effect on rock fractures, which is the primary focus of this study. However, most investigations have concentrated on tensile fractures under pure mode I. For example, GUO et al [17] investigated thermally treated granite’s microcracking behavior under this loading mode at temperatures ranging from room temperature to 600 ℃. They observed a microcracking transition between 150 and 200 ℃ and found that mode I fracture toughness remained relatively constant from 50 to 150 ℃, and then decreased sharply between 150 and 400 ℃. This reduction was attributed to an increased fracture process zone (FPZ) size and a decreased inherent KIC. Under mixed mode loading (I/II) and pure mode II, FENG et al [18] found that thermal treatment reduces the peak load, fracture toughness, and fracture energy of sandstone. Sandstone heated to 700 ℃ exhibited a fracture toughness less than half that of room-temperature samples. REZAEE and NOORIAN-BIDGOLI [19] found that rock fracture toughness of tensile cracks under different loading modes increased with temperature up to 200 ℃, after which it decreased. They concluded that temperature and grain size significantly influence fracture behavior, with larger values leading to distorted crack paths. ZHOU et al [20] concluded that higher temperatures change the failure modes of sandstone and granite from brittle to ductile, though at different temperatures. Granite shows a distinct ductile failure at 500 ℃, while sandstone exhibits a minimal ductile behavior at that temperature. As temperatures continue to rise, both rock types experience more complicated crack paths, rougher surfaces, and increased internal damage.