**1. Introduction**

Underground coal gasification (UCG) is a technology that converts hydrocarbons into syngas in-situ [1,2]. Crucially, UCG combined with carbon capture and sequestration (CCS) may have a special promise [3]. While UCG-CCS has potential promise, some key environmental and engineering issues require resolution. We focus on key rock mechanics problems including excessive subsidence and fugitive gases—each critically impacted by the response of strength and permeability to the imposed high temperatures. Similar to long-wall mining, excavation of coal from depth may cause subsidence, especially over long-term operations and under high crustal stress. It ought to be possible to inject CO2 into former UCG voids which are currently at a depth of more than 800 m [4]. The UCG

**Citation:** Yang, S.-Q.; Tang, J.-Z.; Elsworth, D. Creep Rupture and Permeability Evolution in High Temperature Heat-Treated Sandstone Containing Pre-Existing Twin Flaws. *Energies* **2021**, *14*, 6362. https:// doi.org/10.3390/en14196362

Academic Editor: Nikolaos Koukouzas

Received: 13 August 2021 Accepted: 21 September 2021 Published: 5 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

process is intensely exothermic with the temperatures occasionally exceeding 900 ◦C in the combustion zone. Prior observations indicate that high temperatures significantly transform the physical and chemical properties and resulting mechanical behavior of rocks (e.g., heating and quenching the first caprock may result in cracking or other behavior that compromises the integrity of the storage [5]). In addition, the high permeability of the UCG goaf for geological CCS projects, CO2 storage in artificially developed UCG goaf with high permeability appears to be an attractive proposition [4], provided storage security of the caprocks can be guaranteed.

Heat transfer mechanisms in rocks are complex and nonlinear [6–11]. UCG production boreholes typically experience extreme temperatures of the order of ~900 ◦C which significantly exceeds the critical temperature (*T*c~400–500 ◦C) that induces changes in the mechanical and permeability behavior of typical sandstones [11]. Temperatures from ambient to 400 ◦C correspond to vaporization then escape of adhered water, combined water and structural water. Between 400–600 ◦C, the minerals in the sandstone thermally react and transform, resulting in porosity increase, reduction in permeability and diffusivity, and changes in heat capacity [12]. Reservoir rheology is also sensitive to temperature [6,13,14]. An increase in temperature accelerates creep strains and consequently reduces the time-tofailure [15]. The total axial strain increases with increasing temperature, with instantaneous elastic strain and instantaneous plastic strain increasing slightly as the temperature increases from 25 to 700 ◦C but then increasing substantially as the temperature reaches 1000 ◦C [16]. However, the response of fractured rocks is usually different from that of intact rock—since stress concentrations are present at the ends of pre-existing cracks—promoting the initiation of new cracks and the reduction in rock mass strength [17,18]. Previous investigations [19–22] addressed crack initiation and coalescence for jointed rock masses under short-term loading with key mechanisms of creep contributing to time-dependent deformation and crack propagation [23,24]. Investigation of time-dependent response of jointed rock [21,25–28] have defined key failure mechanisms but few characterizations study the comprehensive impact of thermal effects on time-dependent interaction of thermal cracking across the scales.

The high-temperature treatment of the rock mass during underground coal gasification leads to the formation of fractures and changes in the fluid transmission characteristics of the strata, which is a direct result of the combustion process [29]. Thermal cracks result and increase permeability as the temperature increases from 400–600 ◦C [10]. Prior observations have promoted our understanding of permeability evolution due to shortterm heating and loading demonstrating a linear correlation with volumetric strain [24,30]. However, the timescales of permeability evolution of the goaf in UCG-CCS operations should also be noted. During creep loading the permeability will typically decrease before increasing with the onset of failure at late deformation stages [28,30]. Some observations reveal that the temperature effects are on timescales of permeability evolution and that during the multi-step loading and unloading cycles process the permeability first decreases slightly as temperature increases from 25 to 300 ◦C and then increases with increasing temperature [28]. Heating rate significantly affects thermal damage as evident in that permeability is exponentially correlated with the number of AE events [24].

Consequences of the combined presence of pre-existing fissures, high confining pressures and high temperatures on the evolution of strength and permeability for UCG-CCS operations has not been previously considered. Correspondingly, the following focuses on defining the influence of thermal damage on creep properties and permeability evolution of pre-cracked specimens of red sandstone from micro-scale to laboratory scale. The primary focus is to identify creep parameters and overall permeability evolution in the pre-cracked rock in response to different pre-treatment temperatures in defining characteristics of the evolution of transport properties and mechanical strength.
