Regularities of rocks zonal disintegration and methane emissions periodicity in mine roadways

The article presents the results of studies on the establishment of stable relationships between methane emissions into degassing wells and their spatial location, stress-strain state of rock massif and mining operations process. Vibroacoustic and electrometric of mine control methods have been used to establish zoning and alternation of fractured zones in the rock massif during mining operations. Using the finite element method, zones of different permeability in the roof above the longwall face were determined, and a qualitative transition from compressive deformations to tensile deformations was revealed. Based on data from mine experiments at 157 degassing wells, patterns of repeatability of rocks zonal disintegration and periodicity of gas emissions into mine roadways depending on the distance to the moving longwall face have been determined for the first time. It has been established that the concentration of methane gas in a wells with distance from the longwall face changes according to a damped quasi-periodic dependence with recombinant changes in stress-strain states. For wells aimed at the goafs, the period of the quasi-periodic function increases by 40%, the amplitude decreases to 30%. The obtained dependencies make it possible to increase the efficiency of degassing.


Introduction
Methane is a greenhouse gas.During the 2018 Global Methane Forum, representatives of the Global Methane Initiative partner countries (GMI, 42 Countries) made a statement and confirmed priorities for the next 10 years [1,2].The statement and Global Methane Initiative Terms of Reference [3] state that each country must implement the Action Plan, which aims at GMI's overarching goals including the fight against emissions, the extraction and use of methane.The relevance and need to solve this problem was confirmed in 2022 at the Global Forum on Methane, Climate and Clean Air [4].More than 800 experts and researchers from 66 countries spoke at the forum, with a special focus on methane gas.
More than 60% of the total methane emissions are coming from human-related activities (agriculture, coal mining, municipal solid waste, oil and etc.).China is the world's largest coal producer, accounting for almost 50 percent of the production [5,6].As shown in table 1, Ukraine ranks 14th in the world in coal production (0.84% of world production).At the same time, despite the fact that coal mining at our mines does not exceed one percent of world production, Ukraine ranks sixth in the world in terms of methane emissions into the atmosphere (24.2 million tons of CO2 equivalent).Greenhouse gas control measures are being taken worldwide, but not enough.Among the measures taken, a significant contribution is made by the improvement of mining technologies, which ensure the reduction of methane emissions in mine roadways and its emissions into the atmosphere.Therefore, the development of underground coal mining technologies, which ensure the degassing and utilization of methane gas, requires additional research efforts to establish regularities of methane gas emission into mine roadways.[5,6].
A large amount of methane emissions in mines is caused by an increase in the depth of extraction of coal seams and intensification of mining operations.By means of ventilation it is not always possible to reduce the methane content in the mine atmosphere to the standards established by the "Safety Rules ...".This not only negatively affects the economic performance and safety level of coal mines, but also increases greenhouse gas emissions.The use of coal seam degassing methods and progressive ventilation schemes developed by a team of researchers at the M.S. Poliakov Institute of Geotechnical Mechanics NASU allowed degassing large volumes of rock caving zones, as well as producing more than a thousand tons of coal per day from one mine longwall.However, even the complex application of several methods and means of methane control in many cases is insufficient.This necessitates further improvement of methods for control gas in coal mines.
The research objective: to establish the basic patterns of rocks zonal disintegration and associated processes of gas methane emissions periodic into mine roadways during mining operations.

Methods
Research methods: generalization and analysis of experimental data; mathematical modeling of geomechanical and filtration processes by the finite element method; underground studies of the mine degassing parameters; geophysical studies of rock fracturing; statistical processing of measurement results.

Ways to solve the problem of reducing methane emissions into mine workings
Complex interdependent geomechanical and gas-dynamic processes of displacement, unloading, stratification and degassing of rocks and coal seams occur around the mine workings hundreds of meters above and below the mined coal seam.Issues of geomechanics and gas-dynamics of the rock massif require the study of many factors affecting the destruction of rocks, the development of new methods for mathematical modeling of rock deformation processes.In particular, in the framework of the symposium on the deformation characteristics of geomaterials (Seoul, Korea, 2011 [7]) and the conference of the International Association of Computer Methods and Achievements in Geomechanics (Kyoto, Japan, 2014 [8]), international experts in the field of geomechanics showed the results of the work and promising areas of research.The necessity of solving the problems was emphasized: the establishment of complex "load-deformation" reactions in geomaterials; accounting for rock anisotropy; evaluation of experimental data obtained during the monitoring of geotechnical structures; development of methods of physical and mathematical modeling in geomechanics [7], as well as simulation methods, including deformation, damage and destruction of rocks [7][8][9].
The study of underground gas reservoirs performance requires the integration of different fields in sciences, including structural geology, rock physics, geomechanics and gas-dynamics.In foreign publications [7][8][9][10] is to emphasize how rock physics and geomechanics help to obtain an insight into important issues linked to gas reservoirs management in the geological environment and to safety assessment.Researchers [7] (Tsanget et al.) presented an overview on the interaction between mechanical deformation and fluid flow in fractured rocks; in the paper by Fornel et al. proposed an integrated method based on rock physics for improved modelling using four-dimensional seismic data in combination with production data.Researchers [4] (Bernabe & Evansdeals) also offer specific software applications, for example, that implement numerical simulation of pressure dissolution strain by solving the diffusion equation at a solid-liquid interface in a crack.
The scientific principles of technology for active control of the state of the massif have been developed at the M.S. Poliakov Institute of Geotechnical Mechanics NASU, in which an analytical description of the stress-strain state of the rock massif near the mine longwall [11][12][13] is given, including taking into account: free gas in the rock formation, water saturation, cracks and geological disturbances [13,14], extreme deformation of rocks [11,15].A model has been developed for the formation of pore-fractured area in sandstones under the influence of natural and technogenic factors [15,18], which is verified by numerous experiments in mines, as well as for the underground geotechnical systems of geomechanical monitoring [19,20] and radiometric control methods [19].At the same time, the interconnected problems of the rocks massif destruction during mining and forecasting gas emissions in the mine roadways are extremely difficult and do not have a final solution.
Thus, reducing of methane emissions into mine workings involves solving a number of interrelated problems.Firstly, the task of determining the geomechanical features of the displacement, stratification and crack formation processes of an underworked rock massif.Secondly, the task for determining of the gas-dynamic processes parameters during the destruction of an underworked rock massif, including of the pore-fractured area parameters to assess of free gases migration during mining operations.Thirdly, the tasks of developing a technology for extracting methane from rock massif and practical recommendations for determining the rational parameters of degassing systems.The solution to all these problems is based on the regularities of the processes of rock destruction and methane emissions into mine openings during mining operations.

Study of the features of rocks zonal disintegration around mine roadways
To monitor of the rock massif state and rock pressure manifestations, methods of mine geophysics using seismoacoustic, microseismic, vibroacoustic and other instruments were previously tested.To determine of the rock massif fracturing, geophysical methods are also used, which require small financial costs for conducting experiments.The boundaries of zones of increased fracturing in the mine roadways were determined using the vibroacoustic method [21].A method with active excitation of vibrations was used, since fractured rock is usually characterized by an increase in the amplitude of vibrations with frequencies below 400 Hz.The frequencies of forced oscillations are caused by resonant phenomena that are comparable to the length of acoustic half-waves.
Vibroacoustic research was carried out with a measurement interval every six meters (layer m3, depth 1300 m).The boundaries of increased fracturing zones are determined by resonant frequencies, which were determined by the "ISK-1Sh" device.As a result of monitoring along the length of the mine roadways, zoning of increased fracturing areas was determined (figure 1).It has been established that zones of increased fracturing, which extend to greater depths in mine working areas 48-54 m, 108-126 m, 162-168 m (zones 1-2, figure 1), alternate with zones of increased fracturing, which extend to shallower depths in areas 66-102 m, 132-166 m, 180-198 m (zones 4-6, figure 1).
To study fracturing using the electrometric method ("Progress" mine, depth 900 m), portable equipment "SHIIS-3M" was used (operating frequency 22.5 Hz, error does not exceed 2%).Fractures break the continuity of the rock massif, so cracks are distinguished by a sharp change in the electrical resistance of the rock massif sections.The value of electrical resistivity increases sharply in the mine roadway roof, which indicates the presence of rock stratification intense processes.The results of electrometric studies can be most clearly presented on a three-dimensional graph (figure 2).Numerous maxima and minima of electrical resistivity are identified, which determine the alternation of increased and decreased fracturing zones of the rock massif.
The alternation of fractured zones is explained by the geomechanical features of rocks zonal disintegration in the process of mining.An increase in stress in the rock massif first leads to the destruction of rocks in any area and increasing displacements of the mine roadway contour, as a result of which the stresses in this area are reduced.In a nearby section of the rock massif, on the contrary, the stresses increase.These processes lead to alternation of fractured zones with different parameters along the mine roadway.Visually, the process can be described as wave-like.

Study of changes in the state of rocks pore-fractured area using the finite element method
The environmental friendliness of technologies for underground mining of coal deposits that ensure methane degassing depends on the efficiency of degassing systems.Experience has shown that relatively high efficiency can be achieved by means of degassing with gas removal from the mine.
A study of the operation of degassing systems in more than 20 sections of various mines showed that the efficiency coefficient of roof degassing is low and ranges from 4% to 40-50%, while when degassing goafs it reaches 75% and higher.It was established that the standard efficiency of roof degassing by wells was achieved only in two out of five longwalls.In three mine longwalls it is only 4-11%.The low efficiency of degassing is explained by the fact that after mine longwall passes under the wellheads, they are destroyed and do not work.Obviously, the reasons for the low efficiency of roof degassing by wells lie in the discrepancy between the parameters of degassing systems and geomechanical processes in the rock massif.
When mining a coal seam, the main roof settles.Over time, under the influence of high stresses, a disturbed zone is formed above the mine workings.The geomechanical state of the rock massif in the roof of mine roadway in the zone of action of degassing wells changes in space and time.These changes affect the destruction of the rock massif sections, which, in turn, significantly changes the methane content in degassing wells.
In a mathematical model implemented on the basis of the finite element method [13,15,16], the total deformation of a model element (strain tensor) is described by the sum of nodal displacement vectors and is divided into spherical and deviatoric parts.The deviatoric part of the deformation tensor characterizes the change in the shape of the model elements.The spherical part of the deformation tensor is determined by the value of the minimum principal normal deformations of the model elements, which characterize changes in the volumes of the pore-fractured area, and, consequently, its permeability.
Calculations of the stress-strain state of the rock massif were carried out using the widely tested "GEO-RS" © software package, which implements an elastoplastic model of the medium using the finite element method (developed with the participation of the author [13]).Studies of zones of different permeability in the roof above the longwall face (figure 3) showed that during mining, the pore-fractured area in the rocks before the longwall face (zone I) and after the passage of longwall (zone II) changes significantly.In the zone of support pressure in the range of +20...+150 m in front of the longwall, rocks experience compressive deformations, cracks are closed, and the permeability of rocks for methane gas is low.In the range of -10...+20 m there is a qualitative transition from compression deformations to tensile deformations.The process of transition from compression to expansion of cracks is shown on the example of the main roof (figure 4), where changes in the state of the pore-fractured area occur in gas-saturated rocks.Zones of discontinuities (open cracks) are formed above the goaf in the gas-saturated sandstone of the main roof.
Factors for the possibility of methane transfer in a rock massif are the energy of free gas contained in large pores, cracks, places of closed geological disturbances, and, most importantly, the changing of the rock massif stress-strain state.Such conditions are created after passing the longwall face (figure 4, zone II) in the range of -150...-10 m.
The roof rocks decompress and collapse, cracks expand, and the permeability of rocks to methane gas increases.After the passage of mine longwall, cracks and pores in the rock massif are filled with methane, creating conditions for effective degassing by wells.

Mine studies of the methane emissions periodicity into mine roadways
Experience in the practical operation of degassing wells has shown that intensive gas release into the wells occurs when the mining pillar is adjacent to the mined-out space of previously mined longwall.Roof degassing schemes, in this case, involve drilling wells towards the longwall face in the direction of the mined-out space and in the direction of the mined coal pillar (figure 5).
Increasing or decreasing the distances between wells and changing their directions has a positive or negative impact on the operation of degassing wells and the degassing technological scheme as a whole.The angles of ascent and rotation affect the efficiency of degassing, the stability of the well walls, the time of their operation (especially for wells drilled towards the mine longwall), the number of drilling operations, the involvement of various sources of gas emissions.Increasing the length of the well makes it possible to create large overlaps of area above the coal seam during its degassing and significantly increase the distance between wells when degassing remote layers, as well as extend the service life of the well when drilling it in the direction of the longwall face.The main parameter of degassing is the gas inflow into the wells, which depends on the location of the destruction zones in the main roof at the boundary of the working longwall.
Research has established that methane flow and its concentration are periodic.Analysis of the areas of maximum and minimum values of methane content was studied in 157 degassing wells.The nature of changes in methane concentrations in wells, associated with changes in the stress-strain states of overstressed and unloaded of the rock massif sections around extended underground roadways, is illustrated in figure 6 (a combination of "max-min" graphs in figures 6, a and 6, b).Approximation of periodic data using the least squares method confirmed the wave nature of the increase and decrease in methane content in wells along the length of the mine roadways at different distances from the longwall face.That is, wells located in areas 0-28 m, 63-94 m from the longwall face (figure 6, a) emit, on average, 30% less methane than in similar areas located at distances of 28-63 m and 94-120 m.All these wells were directed towards the goaf.
Wells directed along the axis of the mine roadway (figure 6, b) also have a wave pattern of increasing and decreasing methane concentrations, depending on their location relative to the longwall face.However, a reverse process of gas emissions is observed.Thus, in areas 0-28 m, 63-94 m, where the wells are directed towards the goaf, the minimum methane content was recorded, and in the wells located along the axis of the mine roadway, the maximum methane emissions were observed, and vice versa.
Thus, we see a regular alternation of zones of increased and decreased methane emissions in the space above the mine longwall, which, in turn, repeats the regular alternation of the rock massif disintegration zones during the mining process.This fact once again confirms that the nature of gas emissions into wells along the length of the mine roadway is predominantly influenced by the geomechanical processes occurring in the rock massif.
Based on a large number of mine experiments (more than 150 degassing wells were studied), the patterns of repeatability of zonal disintegration of rocks and the periodicity of gas emissions into mine roadways depending on the distance to the longwall face were obtained for the first time.It has been established that the emission and concentration of methane gas Cg in a wells along the length of the mine roadway l, with distance from the longwall face, changes according to a damped quasi-periodic dependence with additive functional equations and recombinant changes in rocks stress-strain states: ( ) Additive components of the quasi-periodic function: for the conditions of location of wells above the goaf, and for the conditions of location of wells in the roof rocks of a mine roadway, on the contrary, it decreases linearly.The patterns make it possible to determine the zonality of areas of increased gas emissions into mine roadways and to develop rational well layouts to increase the efficiency of degassing and the safety of mining operations.

Conclusions
The following scientific results were obtained: 1.Among the set of measures taken to combat greenhouse gases, a significant contribution is made by the improvement of mining technologies, which ensure a reduction of methane emissions into mine roadways and atmosphere.To develop of technologies for extracting methane gas from a rock massif and develop practical recommendations for determining the parameters of degassing systems, it is necessary to establish the geomechanical features of the processes of displacement, stratification and cracking of the rock massif, as well as the regularities of the periodicity of methane emission into mine workings.
2. Vibroacoustic and electrometric methods of mine control have been used to establish the geomechanical features of zonal disintegration of rocks during mining operations.Stresses in the rock massif first lead to the destruction of rocks in any area and displacements of the mine roadways contour, as a result of which the stresses in this area are reduced.In a nearby section of the rock massif, on the contrary, the stresses increase.These processes lead to alternation of fractured zones with different parameters along the mine workings.
3. Numerical experiments using the finite element method on changes in the state of the porefractured area determined zones of different permeability in the roof above the longwall face.It has been established that in the support pressure zone in front of the mine longwall, the rocks experience compressive deformations, the cracks are closed, and the permeability to methane gas is low.Above the longwall face there is a qualitative transition from compressive deformations to tensile deformations.Conditions for the possibility of methane transfer in the rock massif are created after the passage of mine longwall.The roof rocks decompress and collapse, cracks expand, and the permeability of rocks to methane gas increases.The cracks and pores are filled with methane gas, creating conditions for effective degassing by wells.
4. Based on a large number of mine experiments (157 degassing wells were studied), the patterns of repeatability of zonal disintegration of rocks and the periodicity of gas emissions into mine roadways depending on the distance to the longwall face were obtained for the first time.It has been established that the emission and concentration of methane gas in a wells along the length of the mine roadway, with distance from the longwall face, changes according to a damped quasi-periodic dependence with recombinant changes in rocks stress-strain states.For wells aimed at the goafs, the period of the function increases by 40%, the amplitude decreases to 30%.The additive component of the quasiperiodic function increases linearly for the conditions of location of wells above the goafs, and for the conditions of location of wells in the roof rocks of a mine roadway, on the contrary, it decreases linearly.There is a regular alternation of zones of increased and decreased methane emissions in the space above the mine longwall, which, in turn, repeats the regular alternation and recombinant change of stressed and unloaded zones in the process of the rock massif disintegration under the influence of mining operations.The obtained dependencies make it possible to increase the efficiency of degassing.

Figure 1 .
Figure 1.Periodicity of increased fracturing zones along the length of the mine roadway, determined by the vibroacoustic method.

Figure 2 .
Figure 2. Zoning and alternation of fractured zones in the rock massif, determined by the electrometric method: 1zones of increased fracturing; 2zones of reduced fracturing.

Figure 3 .
Figure 3. Assessment of the pore-fractured area in the roof of the coal seam (determined by the values of the minimum principal strains 3): 1in the sandy-clayey shale of the immediate roof 2 m above the coal seam ( ); 2in sandy shale 10 m above the coal seam ( ); 3in sandstone 33 m above the coal seam ( ); 4position of the longwall face.

Figure 4 .
Figure 4. Changes in the state of the porefractured area in gas-saturated rocks of the main roof during mining operations: 1sandy-clayey shale; 2sandstone & sandy shale; 3coal layer; 4sandy shale; 5porous sandstone; 6compression of cracks at a distance of 50 m ahead of the longwall face ( ); 7expansion of cracks at a distance of 50 m after passing the longwall face ( ); 8cracks without changes.

Figure 5 .
Figure 5. Mining technology using a system of degassing wells (using the example of mining the m3 coal seam): 1ventilation drift; 2conveyor road; 3system of degassing wells; 4measuring station; 5geological exploration wells; 6position of the longwall face; 7direction of advance of the longwall face.

Table 1 .
Leading countries for methane emissions from coal mining.