1. Introduction
Underground coal gasification (UCG) is an advanced method of clean coal utilization by chemical mining, which is characterized by good safety, less investment, high economic performance, etc. [
1]. Despite the great efforts that have been devoted into this aspect, in terms of laboratory investigation and engineering practice, large-scale implementation of commercial production is still limited by certain reasons, including uncertain safety issues, environmental pollution, guaranteed equipment, etc. [
2]. Among such reasons, fracture propagation in the confining rock of a cavity is a serious concern that requires controlling the safety of gas production [
3,
4,
5].
Due to the high combustion pressure of coal during UCG, the confining rock of a UCG cavity is under the coupling effect of earth stress, thermal expansion, and gasifying pressure. Evidence from laboratory and numerical modelling suggests that continuum expansion of UCG cavities induce stratum movement, thereby leading to ground settlement, gas leakage, cavity sealing, etc. [
6,
7]. Therefore, researcher’s interests have been aroused in terms of the mechanical characteristics, permeability performance, and fracture propagation of the confining rock under the high temperature and stress field. For instance, from the view of cavity expansion, Luo and Wang [
8], Akbarzadeh and Chalaturnyk [
9], Janoszek et al. [
10] investigated the confining rock during coal combustion, suggesting obvious changes in the mechanical properties of rock at high temperature and pore pressure. The permeability of near-field rock of a UCG cavity, as announced by Otto and Kempka [
11], Wałowski [
12], Madiutomo et al. [
13], Ding et al. [
14], experiences significant change during cavity expansion, while that of far-field rock is influenced to a limited degree. Meng et al. [
5], Duan et al. [
15], Jin et al. [
16] investigated the fracture characteristics, evolution law and influence tendency induced by pore pressure, thermal damage, etc. All these works convince us that the fracture networks in the confining rock of a UCG cavity is developed during the cavity combustion. Especially, comparison between the UCG and normal coal mining made by [
17,
18] suggests that the speed of fracture propagation during UCG is faster than that under normal mining condition and these fracture networks provide a channel for gas leakage and water infiltration. Škvareková et al. [
19] noted that gas or tar leakage from the cavity to subsurface is one of the main risks during UCG operation. During such leakage, the fracture network in the roof rock contributes the majority of the effort by fluid channel [
13,
20,
21,
22]. Therefore, the fracture network in the confining rock of UCG poses a threaten to the environmental safety and the UCG production.
In the terms of assessing the safety of UCG cavities with consideration to fracture networks, determination of the fracture zone is a difficult issue that needs to be solved. Recently, Li et al. [
23] established a controlling equation with consideration to thermal–mechanical coupling action. According to numerical modelling, the fracture evolution of the overlying rock during UCG was investigated and it was found that a wide damage area was formed during the cavity expansion. Qin [
24] distinguished the difference of water diversion fissures of coal mining and UCG and found that a “butterfly”-shaped fractured zone was formed in the UCG confining rock, while that of coal mining is approximately in the shape of an arch, and the height of fracture zone in UCG is approximately three times of that in coal mining. Lu et al. [
25] studied the crack evolution of overlying rock according to physical and numerical modelling and the fracture zone exhibited a typical “three-zone” structure similar to that of coal mining. Lin [
26] analysed the pilot test of UCG in Inner Mongolia, China, and the height of the water conduction fracture reached 65 m, that is, the possible height of the aquifer. Consequently, the corresponding UCG cavity was possibly subjected to a water hazard. Liu [
27] studied the stability of roof layers of a UCG cavity based on multiphysical modelling, and the stress distribution in the roof rock layers was observed to be changeable due to high temperature, and a larger roof movement was measured than that of coal mining. Falshtynskyi et al. [
28] investigated the substantiating parameters of the formation of stratification cavities in the roof rocks, and a model was developed to calculate the volumes of cavities with consideration to the mechanical behaviour of roof rock.
Despite the above successes in the study of mechanical characteristics, permeability performance, fracture propagation of the confining rock, and the fracture zone determination, the evolution law of fracture zone and its influence factors are still unclear. In addition, the amount of studies illustrated that the fracture zone around a UCG cavity is related to geological and operating conditions [
5,
29,
30,
31,
32,
33,
34]. However, observation by Huang et al. [
35], Zhang et al. [
36], Wu and Jiang [
37] through physical and numerical modelling suggests that the formation of a fracture zone that evolves during UCG is obviously different from that of coal mining. Additionally, quantitative description of the fracture zone distribution in coal mining is still a difficult task for the research community. A prediction model based on this aspect must depend on the empirical formulation with incomplete influence factors [
38,
39,
40]. All of the above indicates that the existing mechanisms and models available are not proper for UCG cavities.
Therefore, in order to understand the evolution of fracture zones in the confining rock of a UCG cavity in terms of the mechanism and prediction model of the fracture zone, this paper implemented a series of numerical models associated with the thermal–mechanical coupling effect. The influencing factors of geological and operating conditions were taken into consideration, and the mechanism of fracture evolution and the fracture zone in terms of the shear fracture and tensile fracture were analysed according to the numerical results. According to the characteristics of the evolution of fracture propagation, a prediction model was proposed. A comparison between the calculation results and the orthogonal test data verifies the good capacity of this prediction model. The study in this paper provides a new methodology for predicting the fracture zone of a UCG cavity and a safety assessment approach for further study.
4. Evolution of Fracture Zone
The evolution of the fracture zone in the roof layers and the base board of the cavity are calculated with variable influence factors.
Figure 6a–f is the calculated fracture height in the roof layers and base board with variable levels of rock grade, overburden pressure, grade of coal seam, ratio of cavity width to height, thickness of coal seam, and gasification pressure, respectively.
As plotted in
Figure 6, the fracture in the roof layers change obviously with different influence factors, resulting in shear fracture, tensile fracture, or contrast. Specifically, the higher grade (harder) decreases the height of shear fracture (
Figure 6), while the tensile fracture in the roof has a limited change. Once the height of the shear fracture is smaller than that of the tensile fracture, the pattern of the confining fracture is changed from a simply supported beam to a caving arch, as mentioned in
Figure 4b,c. Such phenomenon can also be seen in
Figure 6f. The higher gasification pressure is able to decrease the shear fracture and lead to a change in pattern. To the contrary, as displayed in
Figure 6b,d,e, the promoted overburden pressure, ratio of cavity width to height, and the thickness of coal seam increase the shear fracture height, thereby changing the broken pattern from a caving arch to a simply supported beam. Interestingly, as shown in
Figure 6c, the grade of coal seam has almost no influence on the height of fracture in both the roof layers and base board. Obviously, the fracture depth in the base board is mainly induced by tensile fracture, and the fractures in the coal seam are small relative to those in the roof and base board.
Additionally,
Figure 6 shows a limitation of minimal fracture height or depth in both shear fracture and tensile fracture, which ranges from about 3.5 m to 5 m. As demonstrated in
Figure 5, such a limitation is mainly due to the thermal damage.
It is notable that, as demonstrated in
Figure 4c, both the shear fracture and tensile fracture compose the channel of fluid conductivity into a cavity. Therefore, despite the pattern change during the cavity expansion with different factors, the outline of the fracture dimensions in
Figure 6 should be treated as the available depth of water conductivity and gas emission.
6. Conclusions
This paper targets fracture evolution in the confining rock during UCG cavity expansion. According to the expansion process of a cavity, a numerical model considering different factors in terms of geological and operating conditions was implemented for analysis on the cavity expansion and fracture zone evolution. This paper clarified the mechanism of fracture propagation during a UCG cavity, distinguished the influence factors on the fracture propagation, and proposed a prediction model for the fracture height and depth in roof and baseboard, respectively. Conclusions of this paper can be remarked as:
- (i)
With the expansion of a UCG cavity, the near-field rock is initially broken. Then, a caving arch is formed due to cracks in the roof rock. Subsequently, shear fracture propagated upward due to the failure of caving arch at two springers.
- (ii)
The grade of confining rock, depth, dimension of the cavity, and gasifying pressure are the controlling factors in the fracture evolution, especially the fracture height in roof layers.
- (iii)
Comparison of the orthogonal test and prediction results suggests that the proposed model has a good ability to estimate the fracture zone for the assessment of safety of the UCG cavity.
This paper provides an approach to the prediction of fracturing distribution, not only in the fracture pattern, but also the height and depth in both roof and baseboard, which is the basis for safety assessment of environmental risks and site selection. Therefore, further study should be concentrated at the field verification of the proposed mechanism and model and the methodology of the safety evaluation of water leakage or gas emission through the fracture zone according to the mechanism and model.