Experimental studies of the deformation characteristics of mudstones under high temperature exposure

The article presents the results of laboratory thermomechanical tests of argillite samples. It has been confirmed that the deformation-strength properties of the studied type of geomaterials undergo significant changes under high-temperature influences. Features of changes in the values of deformations of argillite samples at various stages of heating were revealed. It is shown that the research results can be used in modeling thermomechanical processes occurring during underground gasification of coal in a reaction channel, to predict the development of deformation of the host rocks of an underground gas generator.


Introduction
One of the modern and actively developing methods of mining coal seams is the method of underground coal gasification (UCG), which allows for shaftless coal mining with the possibility of obtaining energy and (or) chemical gaseous raw materials with the possibility of efficient and environmentally friendly use in various areas of consumption [1][2][3].In the case of mining coal seams using the UCG method, nearby layers of host rocks are subjected to intense high-temperature effects (according to data from the opening of underground generators, the temperature of the reaction surface can exceed 1000 ºС) [1].As a result of this impact, significant changes occur in the internal structure and mechanical properties of rocks.Therefore, the geomechanical justification of the UCG technology must take into account the influence of deformation and destruction processes occurring in the coal rock mass and caused by the operation of the underground gas generator.For example, for a reliable forecast of the development of deformation processes in rocks surrounding the reaction channel of an underground gas generator, data is required on the nature of changes in the mechanical parameters of the host rocks at various stages of thermal exposure [4].
Mudstones, along with sandstones and limestones, are one of the most common types of rocks associated with coal seams.Research into the influence of various temperature influences on the deformation-strength properties of geomaterials has been carried out all over the world for decades [5 -8], however, the precision measurement of thermal deformations in the high temperature range (> 500 ºС) even in laboratory conditions remains a labor-intensive experimental process, first of all for weak rocks, which include mudstones.Examples of the identified difficulties are the difficulty of producing samples of the correct shape, difficulties in forming a statistically significant sample of samples with a weak degree of variability in physical and mechanical properties, as well as various problems of an instrumental and technical nature.
Therefore, the development of methodological approaches to obtaining reliable experimental dependencies characterizing the dynamics of changes in deformation of argillite samples under hightemperature influences remains a relevant and practically significant task.It is the main goal of this article.

Research objects
To conduct research from the soil of the operating mine "Mine named after.S. M. Kirov" OJSC "SUEK -Kuzbass" (Kemerovo region, Russia) samples of argillite were taken.A photograph of one of the original samples is shown in Figure 1a.
The preparation of argillite samples for testing (parallelepipeds measuring 15x5x5 mm) was carried out using a "Struers -Labotom -15" cutting machine.In this case, the sample was leveled in such a way that the spread of measurement values for each of its geometric dimensions did not exceed 0.05 mm.The edges of the images were polished on an automatic grinding machine "Struers -Tegramin-25" (Fig. 2 b).A photograph of several manufactured mudstone samples is shown in Figure 1b.To obtain reliable results of mechanical and thermal tests, as mentioned above, it is necessary to implement a procedure for selecting argillite samples belonging to the same statistical population, i.e. samples with similar structural features, and therefore similar values of their main deformation-strength characteristics.The obvious connection between the structural damage of geomaterials and the values of their strength and deformation parameters determines the use of non-destructive diagnostic methods to solve this problem, allowing, at least indirectly, to assess the degree of defects [10,11].
One of the actively developed methods of non-destructive diagnostics of materials is laser-ultrasound structuroscopy, whose effectiveness for assessing the damage of various types of rocks has been demonstrated in numerous studies [11,12].Therefore, to comply with the previously put forward requirements for conducting the described experiments, before carrying out thermomechanical tests, the specified non-destructive diagnostics of argillite samples was carried out using the "GEOSKAN -02M" device [11,12].
Based on the results of ultrasonic measurements, argillite samples were selected whose acoustic wave propagation velocities belonged to the same range.Thus, a sample was formed, intended for further testing of samples with similar values of physical and mechanical characteristics.

Thermal dilatometry
Currently, studies of the thermal expansion of material samples, including rocks, at high temperatures are carried out using high-precision dilatometers [8].To carry out these studies in the work described, a DIL 402 C installation from "Netzsch" (Germany) was used.
The general design of a dilatometer is shown in Figure 2.This measuring complex uses an air-cooled silicon carbide furnace, which provides temperature measurements in the range from 20 ºС to 1600 ºС.The temperature of the sample during measurement is controlled using a platinum-platinum-rhodium thermocouple located next to the sample.The experiments were carried out at a constant heating rate of 5 ºC/min.
Figure 3, as an example, presents the temperature dependences of the relative linear elongation (strain) ε(T) and the coefficient of linear thermal expansion (CLTE) (T) obtained as a result of thermal tests of 3 argillite samples.Analysis of the dependences α(T) -curves, which are essentially derivatives of ε(T), also confirms the presence of two stages of deformation that are most sensitive to temperature growth and belong to the intervals from 550 ºС to 650 °С and above 900 °С.

Conclusion
The methodology proposed for preparing and censoring argillite samples allows one to obtain better results in laboratory studies of deformation properties under high-temperature conditions.
The mechanical properties of argillite undergo significant changes under thermal influence.Thus, the dependences ε(T) and α(T) obtained during testing of argillite samples showed a significantly nonlinear nature of the change in these parameters.
The demonstrated features in the temperature dependences of the mechanical parameters of argillite samples subjected to heating indicate the need to use the research results when modeling thermomechanical processes occurring during underground gasification of coal to predict the development of deformation of the host rocks of an underground gas generator.