Evaluation of Gas Evolution Outside the Extraction Section at the Activation of Coal-Bearing Stratum Displacement

The object of research is the processes of gas evolution from undermined sources within the exploited extraction section and beyond its boundaries under the influence of activation of rock displacement. To date, no quantitative dependencies have been established for gas evolution from the worked out (treatment workings) of the carbonaceous stratum when its movement is activated. Difficulties in studying gas evolution in the case under consideration lie in the need to carry out the necessary measurements of methane consumption in the workings and degassing wells of the mine field section and wing over the entire period of time working out the extraction column. The purity of the experiment is ensured by the operation of only one lava in the wing of the mine field. The ventilation scheme of the extraction section and the location of the measurement points in the mine workings make it possible to establish methane emission zones in the worked out space of the exploited and stopped lavas. The paper presents an analysis of the change, currently little studied, of gas evolution outside the exploited extraction section into the worked out space of stopped lavas. The main factors affecting the change in the level of gas evolution are determined, both within the extraction section and beyond its borders.<br><br>The methodology of the experiments is assumed that the level of gas evolution from the undermined coal-bearing stratum is determined by coal mining and the degree of development of treatment works in the extraction section. On the basis of experimental data, the dependencies of gas evolution on coal production and the degree of development of treatment works within the extraction section are established. Out of the extraction section, gas evolution from the worked out space of stopped lavas occurs when the development of the minefield wing is sufficiently developed and is determined by the intensity of coal mining in the exploited section. When the Earth’s surface and the coal-bearing stratum are fully developed, the gas evolution from the mined-out space of the stopped lavas during the initial period of operation of the next lava can be several times higher than the methane emission within the existing extraction section.<br><br>Studies have shown that the mechanism of the process of gas evolution within the extraction section and beyond its boundaries proceeds under the influence of various influencing factors.


Introduction
The relationship of gas evolution from the undermined coal-bearing stratum with the activation of rock movement was established relatively recently [1,2]. For this reason, the quantitative dependencies of this type of gas evolution on influencing factors have not yet been established. The relevance of the issue under consideration is evidenced by the absence of any recommendations in normative documents [3][4][5] on the safe development of gasbearing coal seams in the case of manifestation of methane emission during activation of the shift of the undermined coal-bearing stratum. Thus, the object of research is the processes of gas evolution from undermined sources within the boundaries of the exploited extraction section and beyond its boundaries under the influence of activation of rock displacement. The aim of research is to establish the main influencing factors determining the gas evolution within the exploited extraction section and beyond its boundaries under the influence of the activation of the shift of the undermined coal-bearing stratum.

Methods of research
The difficulties in studying gas evolution in this case are the need for lengthy measurements of methane consumption in mine workings and degassing systems of the mine field section and wing over the entire period of mining of the extraction column. For the purity of the experiment, only one lava should be operated in the wing of the mine field. The ventilation scheme of the extraction section and the location of the measurement points in the mine workings made it possible to establish methane emission zones in the worked out space of the exploited and stopped lavas [6,7].
Such conditions of the experiments corresponded to the 9th western lava during mining of an anthracite layer  2 v with a thickness of 0.9 m of Izvestia newspaper mine of the Donbassanthracite State Enterprise (SE) (Khru stalny, Luhansk Region, Ukraine). Prior to the start of its operation, treatment work in the wing of the mine field was not carried out for two months. The necessary observations were carried out from the start of operation of TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 2/3(52), 2020 ISSN 2664-9969 the extraction section to the termination of the cleaning operations for coal mining for 394 days.
When planning the experimental methods, let's proceed from the generally accepted position that the level of gas evolution from the undermined coal-bearing stratumwill be determined by coal mining and the degree of development of treatment works in the extraction section. To establish these dependencies in certain periods of operation, lava was recorded (Table 1): -duration of these periods; -total A ∑ and average A daily coal production; -average gas evolution within the extraction section I a and from the worked out space of stopped lavas I ws ; -total amount of gas evolution within the extraction section I a ∑ and from the worked out space of stopped lavas I ws ∑ ; -removal of the L coal face from the split furnace.
The different duration of the observation periods in the initial period of the lava operation (May-July) was caused by a change in the number of workings along which the outgoing ventilation air jets were diverted beyond the extraction section. In the future, experimental data recorded over time periods equal to a month were used.

Research results and discussion
As the extraction section was operated (removal of the coal face from the split furnace), production changed significantly (Fig. 1). The rate of increase in the total amount of gas evolution in the mine field wing I c ∑ was significantly higher than the increase in coal production (∑A) before the main roof precipitated when the working face was removed from the cutting furnace to 87 m.  In Fig. 1: 1, 2 -curves, respectively, changes in coal production and gas production; -coal mining during certain periods of exploitation of extraction section; -total gas evolution (to mine workings and degassing wells) in the wing of the mine field in the considered time periods.
Based on the experience of cleaning operations, under the considered conditions, the sludge of the main roof occurs when the working face is removed from the split furnace to a distance of 80-120 m [8][9][10]. The outrunning growth rate of gas evolution in the mine field wing compared to an increase in coal production indicates the presence of a gas evolution source from the undermined coal-bearing strata outside the extraction section. This source is not directly related to the development of treatment works in the extraction section (with the removal of the working face from the split furnace). To confirm this assumption, let's examine the dependences of gas evolution within the extraction section I a ∑ and beyond its boundaries I ws ∑ on the amount of coal mined ∑А (Table 1, Fig. 2). In the initial period of lava exploitation (when the bottomhole moves away from the split furnace to 87 m), the gas evolution to the local workings and degassing wells (straight line 1) was significantly lower compared to the amount of gas evolution after the main roof was precipitated (curve 2). The averaging curve 2 is located above the three experimental points obtained before the main roof precipitation. This situation indicates the influence on gas evolution from the undermined coal-bearing stratum within the extraction section until the main roof is precipitated by only one factor -coal mining.
In Fig. 2: 1, 2 -direct proportional and nonlinear dependences of gas evolution within the extraction section, respectively, before and after the main roof precipitation; 3, 4 -direct changes in the dependence of gas evolution from the worked out space of stopped lavas, respectively, before and after precipitation of the main roof; , -experimental data on gas evolution within the extraction section, respectively, before and after the main roof precipitation; , -experimental data on gas evolution from the worked out space of stopped lavas, respectively, before and after precipitation of the main roof.
During this period (before the main roof was precipitated), an increase in coal production had a significantly greater effect on the growth of gas evolution from the worked out space of stopped lavas. This is clearly seen from the mutual arrangement of lines 1 and 3 (Fig. 2).
The removal of the working face from the split furnace had practically no effect on the deviation of the experimental data from the averaging curve (2) of the dependence ) obtained during the operation of the extraction section after the main roof was upset (Fig. 2). The gas evolution from the worked out space of the stopped lavas was determined mainly by the level of coal production and did not depend on the position of the working face relative to the split furnace. This is confirmed by the close correlation dependence (correlation coefficient r = 0.91) obtained by processing the experimental data for the entire period of operation of the extraction section (line 4).
In addition, it should also be noted that prior to the settlement of the main roof, the gas evolution from the worked out space of the stopped lavas exceeded the gas evolution within the extraction section by 3.2-4.4 times ( Table 1). They depended only on the level of coal mining (Fig. 2,  lines 1, 3). This ratio of gas evolution is due to a large degree of underworking of the carbonaceous stratum and the earth's surface by stopped extraction sections. The ratio of the size of the worked out space (B) of stopped lavas to the depth of mining (N) was more than five, which indicates the complete underworking of the coal-bearing stratum and the earth's surface.
The obtained experimental data on the results of processing the observation results for a separate mining section made it possible to establish the total amount of coal mined ∑А = 141687 t. To it corresponded the amount of gas I a ∑ = 4430.9 thousand m 3 emitted within the extraction section. From the worked out space of stopped lavas for the entire period of exploitation of the excavated section I ws ∑ = 6312.5 thousand m 3 stood out. According to these data, during the operation of the lava (394 days), an average of 360 tons of coal was mined per day, which corresponded to an average gas evolution (7.8 m 3 /min) within the extraction section and 11.1 m 3 /min from the worked out space of stopped lavas. These experimental data made it possible to establish a change in the ratio of gas evolution within the extraction section I a ∑ and I a , and beyond its boundaries I ws ∑ and I ws in certain periods, differing in coal production. The change in coal production during certain periods of lava exploitation ∑А and A was evaluated by their relation to coal mining A st ∑ and A st during the mining of the entire extraction column (Table 1). In a similar way, a change in gas evolution I a ∑ , I ws ∑ and I ws , I a was established, and, in certain periods of lava exploitation, in relation to these parameters to indicators during the mining of the extraction column: The relative change in coal production and gas production during certain periods of operation of the section was determined using the parameters characterizing the mining of the extraction column as a whole: The relative change in gas evolution within the extraction section and from the worked out space of stopped lavas was described by different dependences (Fig. 3). The relative gas evolution within the extraction section before the main roof was precipitated was significantly lower compared to subsequent relative gas evolution values.
For this reason, the experimental values of the relative gas evolution before the removal of the bottom face from the cutting furnace at a distance of 87 m do not coincide with the averaging curve (1) obtained for other periods of operation of the extraction section.
In Fig. 3 st ; , -experimental gas evolution data within the extraction section, respectively, before and after the main roof precipitation; -experimental data on gas evolution outside the extraction section from the worked out space of stopped lavas; r, R -respectively, the correlation coefficient and correlation ratio.
This is an additional confirmation of the influence in the initial period of lava exploitation of two main factors that determine gas evolution within the extraction section -coal mining and the degree of development of treatment works before the main roof settles. After settlement of the main roof, ceteris paribus, the main influence is the level of coal production.
The relative gas evolution outside the extraction section from the worked out space of the stopped lavas was practically independent of the precipitation of the main roof within the exploited area. This is confirmed by statistical processing of experimental data (Fig. 3, b, curve 2).
In this case, all experimental points are uniformly located relative to the averaging curve. This indicates that the gas evolution from the worked out space of the stopped lavas is associated with the intensity of mining of the exploited extraction site at all stages of the development of treatment works.
Studies have shown that the mechanism of the process of gas evolution within the extraction section and beyond its borders proceeds differently under the influence of influencing factors. Confirmation of the distinctive course of gas evolution is a significant scatter of experimental data between the pairs and, and (Fig. 4). Fig. 4 shows: 1 -averaging direct changes I a ∑ 4430 9 . and I ws ∑ 6312 5 . ; 2 -averaging direct changes in relations I a 7 8 . and I ws 11 1 . ; , -experimental data for determining, respectively, averaging lines 1 and 2; r -correlation coefficients; σ -standard deviation. The standard deviations of these data pairs from the averaging lines (1, 2) are respectively 0.048 and 0.81.
The results of the studies allow to improve the requirements of regulatory documents [6,7,11] regarding the creation of safe mining conditions for the gas factor.

Conclusions
1. Gas evolution within the extraction section depends on the level of coal production and the degree of development of treatment works within its boundaries. Out of the extraction area, gas evolution from the worked out space of stopped lavas with sufficient development of treatment operations in the mine field wing is determined by the intensity of coal mining in the exploited area.
2. When the Earth's surface and the coal-bearing stratum are completely undermined, the gas evolution from the worked out space of the stopped lavas during the initial period of operation of the next lava can be several times higher than the methane emission within the existing extraction section.
3. Change in gas evolution within and outside the extraction section during certain periods of lava exploitation takes place according to different dependences, which indicates the presence of distinctive features of the processes of methane evolution within the boundaries of the exploited mining section and the mine field wing.