About Possibilities to Improve Current Outburst Hazard Prediction Based on Updated Mechanism of Coal and Gas Outburst

In this paper the problem of coal and gas outburst – the coal pit’s most complex and dangerous induced geodynamic phenomenon – occurrence and development has been considered in details. Based on the theoretical and experimental researches it has been shown that this phenomenon may occur in full strength only with sufficient quantity of total potential energy available in the massif’s face area together with the conditions required for development of each stage of the phenomenon. However, the main condition shall be occurrence and development of the intensive opening and extension of cracks quasiparallel to the seam’s exposed surface and saturation of these cracks with pressurised free gas in the face area.

Updating of the coal and gas outburst process allows to improve current prediction of the mine opening hazards with respect to the coal and gas outbursts.

Saturation of unworked coal with energy before coal and gas outburst

Coal and gas outbursts take place in compliance with a certain rule – energy balance in the rock-coal-gas system (1-4 e. a.). The energy balance is a sum of all types of energy participating in the progress of this phenomenon.

From many researches of this phenomenon the elements required for its occurrence as well as the elements resulting from its progress have been established. The energy of these elements has been recorded in the balance equation.

According to the energetical theory of V. V. Khodot (1), the main and necessary condition for the outburst excitation and development shall be satisfaction of the following inequality:

W + E > F + U     (1)

whereby W is the potential energy of coal, E is the kinetic energy of the rocks, F is the work required to move the coal towards the face, U is the work required to brake the coal in course of the outburst.

In the left part the types of potential energy required for occurrence and progress of the phenomenon are shown, while in the right part are the types of work performed during the outburst, i.e. formula (1) is a particular application of the law of conversion of energy. The energy required for the outburst occurrence is converted into the energy which is resulting from the progress of the phenomenon, which means: the bigger energy has been spent for occurrence of the phenomenon, the bigger energy shall be released as the result of the outburst. In the condition of the outburst occurrence one can consider either one type of the phenomenon’s properties or a number of types, i.e. mechanical, thermal, acoustic and other types of properties.

It should be remarked that condition (1) is given overgeneralized. The phenomenon under consideration is known to have a number of stages in its progress. And occurrence of each stage requires its individual condition.

The gas and coal outbursts may occur when in the mine opening area the coal seam includes tectonical deformation of the coal texture deep enough to condition occurrence of the phenomenon.

For the outburst-prone area of the seam, which is characterized by low strength of coal and high gas content, it is characteristic to have prominent area with the massif elements quaziparallel to the face, significant amount of free gas in the cracks between the massif elements with high pressure of such gas in the mine influence area in the vicinity of the opening (Figure 1 a) (5, 6).

Part of the massif under consideration is a porouse-fractured body of elastic and plastic properties with permanent occurrence of deformations. In the physical body of such texture local distortions shall occur in places where the active force exceeds the passive one.

Obviously, this process (propagation of cracks) goes permanently in the whole mine influence area. Foremost it goes where the massif has been significantly released from overburden and gas pressure while preserving high pressure of gas. At the same time, due to the crack propagation and gas pressure decrease inside the crack, such crack may be compressed back to some extent as effected by a higher force in the adjacent crack, though, generally the cracks will grow as far as the massif is released due to shifting of the face to the mine opening. In the immediate vicinity of the face the massif is degassed and released from overburden and gas pressure to high extent, hence, the active force here is close to zero. That is why the area adjacent to the face is inert. It is damping the effect of active forces applied to the part of massif under consideration at a higher depth.

In the face area of the massif the gas permanently ingresses the mine opening, the cracks are growing oscillatory and, consequently, sorption hysteresis of methane with coal occurs (7). That is why high gradient of gas pressure is preserved in the face area of the massif in the disturbed coal, in the immediate vicinity from the face.

As the result of permanent primary development of the cracks quaziparallel to the crop, the super-multilayer system with pressurized free gas between the layers is formed in the the disturbed bands. Gas pressure gradient grows rapidly to the depth of the massif.

Due to the processes described the face area of the massif is permanently in “live”, dynamic condition with growing forces in the developed systems of cracks. Kinetic energy of the massif displacement and the methane sorption energy are converted into the potential energy of the disturbed band or set of bands in the massif. The term “potentially outburst hazardous band of coal” suits excellently to such band (set of bands).

From the experience of mining the outburst hazardous coal seams in Donbass the cases are known when the outburst occurred in course of drilling 42 mm diameter pilot holes – including the test holes to predict the current outburst hazards. These cases surprised the researchers, because such holes produce minimal influence on the face area of the massif. Applying the above ideas to explain such cases one can conclude as follows. When the tectonically disturbed coal texture in the face area of the massif is saturated with energy due to permanent distortions, cracking quaziparallel to the face and sorption hysteresis of methane, at certain moment a qualitative change (or quantum leap) will take place resulting in a transfer to the new condition of the massif – the outburst readiness. Due to over-saturation with energy it is a sort of “overexcited”, and a minor impact, the smallest hole drilled this case, can lead to the back reaction in the shape of the coal and gas outburst. It is like needle punching of a balloon, which results in inevitable bursting. In this case the outburst can be prevented only by anti-outburst impact on the face area of the massif performed early and from sufficient distance.

The processes going on in the face area of the massif at the outburst preparatory stage are designated with the term “saturation with the outburst energy” (5, 6). The higher is the extent of charging, the bigger number of cracks will open getting saturated with pressurized gas, the better manifestation all the stages of coal and gas outburst will have in case it happens.

The possibility of destruction in the face area of the massif is determined by active force to passive force ration in each individual system of cracks. At the same time the maximum value of this ration shall not necessarily coincide with the active force maximum value. This aspect is visually demonstrated in figure 2, which shows the character of variations in areas I-IV depending on the distance to the face x, parameters which determine gasdynamic stability of the seam’s face area: stresses in the massif σ, gas pressure in the seam Рг, force Fа arising from the pressure applied by free gas on the cracks’ walls and directed towards the mine opening (value average for the closed loop hydrodynamic system of cracks), Fп limit force counteracting the above force and Fа/Fп ration indicated with Rо on the figure. As shown in the figure, the maximum value of Fа can exceed significantly the value of Fа in the adjacent area located closer to the face because the gas pressure in this area is decreasing by a quite small value in the direction of the face, while the surface of cracks’ systems quaziparallel to the face is even increasing. At the same time the counteracting limit force can go down by a significantly greater value due to the more sharp decrease of stresses in such area, thus reducing strength of the coal in the direction of the coal crop, and thinning of the resisting coal layer, and Fа/Fп ration here can be higher in the section where the maximum active force is applied.

If the face is in a quiescent state, i.e. no physical impact has effect on the massif, then a quick coal spall due to the exceedance of the active force over the passive one shall be practically impossible because it would require a significant energy to move a significant mass of degassed coal adjacent to the face. At the same time, if the active force in distance х from the face slightly exceeds the passive one and some small potential energy appears, then it will be immediately converted into the kinetic energy due to the tiny displacement of the coal mass. This will result in immediate reduction of the active force by a small value of its exceedance over the passive force due to increase in the cracks gaping and decrease of the free gas pressure inside them. Besides, gas desorption to these cracks will rise up, gas pressure will be restored, and the active force will exceed the passive one again. But then again the displacement will occur reducing the active force back.

Excitation condition of coal and gas outburst

Beginning of the coal and gas outburst is induced by distortions and distraction at the stope heel in process of punching the working tool in unworked coal. At this moment the gas-stressed condition of the seam is changing sharply. The roadhead is moving from position 1 to position 2 (Figure 1, 2).

Stresses at the seam edge will grow sharply effecting intensive distortion and deterioration thereof until the stresses are lowered down to the value which the partially deteriorated coal can bear. The main part of the distortions is known to happen practically in an instance, in a split second. The stress curve moves to the new position corresponding to another position of the face. Gas pressure in contact with the free surface drops down to zero, while the gas pressure curve in the face area is changing slowly, as the gas is being drained to the mine opening rather slowly. In at least few minutes after the mine opening has been shifted to the new position a very high gas pressure gradient is created. Nearby the face the active force grows rapidly, while the passive force, to the contrary, is decreasing sharply as seen from comparison between figures 1 and 2.

In the seam part adjoining the face of the massif prepared for the outburst the active force will exceed the passive one significantly. Contrast to the preparatory stage, here the potential energy conditioned by this exceedance can be big enough to move a mass of coal over a significant distance, measurable in meters. This results in shifting of a V-layer of coal between the face and some nearby system of cracks (let us call it “the ourbursts initiating crack system), where the maximum Fа/Fп, ration has been reached to initiate the coal and gas outburst.

The process of movement and to a great degree deterioration of coal is accompanied by sharp increase of gas desorption due to formation of the new crop surfaces in the energy saturated coal massif and increase of the massif’s cavitation. Together they condition the following outburst stages: layer by layer detachment of coal, gas fragmentation of the stopes and pieces detached from the massif, generation of gas and coal flow in the mine opening.

Inequity (1) is the necessary condition for excitation and progress of the coal and gas outburst. However, satisfaction of this inequity does not mean that this phenomenon will occur necessarily.

V. V. Khodot has laid down two more conditions for occurrence of the outburst (1):

– The coal deterioration rate vр must exceed the gas pressure drop rate in the coal cracks vд.

– By the moment when deterioration has finished the gas pressure must preserve the level higher then resistance of the deteriorated coal to the outburst.

Let us consider all the three coal and gas outburst occurrence and development conditions as stated by V. V. Khodot from the view point of possible application in evaluation of the outburst hazard in the current prediction.

The calculations made as per the manual (7) have shown that normally at the depth over 200-300m below datum during excavation of highly gaseous seams (approximately over 10 m³/t) condition (1) is satisfied. But satisfaction of condition (1) does not allow to declare possibility of the outburst, as two more conditions must be satisfied. However, the first one of these conditions, being notionally symbolic, is inapplicable in practice, because vр and vд are physically incomparable.

The second condition is expressed by V. V. Khodot as:

(2)

where Рг is the gas pressure at the coal deterioration front; m is the mass of the deteriorated coal; S is the cross sectional area of the deteriorated coal stope; g is acceleration of gravity; f is the friction factor for coal movement along its slip surface; α is the coal slip surface tilt angle; v is the acceleration required to detach the coal.

It should be remarked that the parameters included in formula (2), except g and α, can not be known beforehand and, consequently, occurrence of the outburst can not be predicted.

Thus, the conditions obtained by V. V. Khodot do not allow to proceed with the evaluation of the outburst hazard in specific conditions of face advance and, hence, current predication of the outburst hazard.

Besides, the following should be remarked. Energy condition (1) suggests the value of potential energy which must be possessed by the massif for excitation and development of the coal and gas outburst. However, the outburst is a multi-staged process. The first stage is a preparatory one. On this stage the massif is being saturated with energy in the outburst influence area as described in the beginning hereof. The second stage is possible when the energy in the massif is enough to move the part of the massif adjoining the face between the first developed crack system and the new location of the face. The third stage which is a layer by layer detachment of coal from the massif is possible when the super-multilayer system of cracks quaziparallel to the face and filled up with pressurized free gas is sufficiently saturated with energy. The fourth stage which is a fracturing of the stopes and pieces of coal formed in course of the coal layers detachment from the massif shall be realized under sufficient gradient of the internal gas pressure in them. The fifth stage is a formation of the gas and coal flow through the mine opening and its intensity is proportional to the kinetic energy of the outburst products moving through the mine opening. Only the final stage of this phenomenon would not require additional energy because it is a termination of the outburst process due to exhausting of energy from the active face part of the massif.

We know that under conditions being approximately same the coal and gas outbursts may either occur or not occur. This allows to suggest that satisfaction of condition (1) and coal and gas outburst occurrence are quite widely separated events. This issue is quite explainable from the points of the outburst energy saturation process in the face area of the massif as described above. The problem is that for development of the outburst the unworked coal in the mine opening influence area must have not only the required energy but also conditions for the outburst development at each stage of the phenomenon.

If such conditions totally do not exist the phenomena would never occur. Only some stages of the phenomenon provided with energy and respective conditions for realization at the said stages can be observed.

Areas for improving the current predication of outburst hazard

For the current predication of the outburst hazard it would be sufficient to establish timely the possibility of the outburst progress up to its second stage. This stage is possible when the system of intensively developed cracks quaziparallel to the face and filled with pressurized free gas is already existing. Such system must be sufficiently saturated with energy already to make a coal spall happening.

Besides the outburst excitation and development conditions as established by V. V. Khodot, there are also conditions established in many years of researching these phenomena. Three of them are used directly in the functioning method for the current predication of the outburst hazard of a seam.

The first one is specific to the coal seam texture in the mine opening area. The coal and gas outbursts take place when tectonical deformation of the coal texture is there. At the same time the maximum normal thickness of a band or a cluster of the adjacent bands in this texture must be at least of critical value. In Kuzbass coal mines it makes 20 cm, and 10 cm in Pechora basin.

The second one. Strength of a band or a cluster of the adjacent bands in the tectonically deformed coal q as measured by strength meter P-1 (8-patent) must not exceed 75 (measured in conventional units). Normally they are lens shaped structures with earthy and an acinose texture (7).

The third one. The maximum initial rate of outgassing from the 1 m long intervals between test holes g_H shall be at least – 4 l/min.

The seam area should be considered outburst-prone only when all three of the above conditions are suggesting that.

The method of current predication of the outburst hazard by the seam texture and initial rate of outgassing from the test holes as based on the regulatory documents allows to establish the areas where manifestation of the outburst hazard is possible. When undercut forward in the direction of the mine opening with holes the structures would release the gas intensively as recorded by the gas flow meters. By the gas flow value the area shall be classified as the outburst hazardous or unhazardous. However, in predictions the uhazardous areas are frequently classified as dangerous ones because the high level of outgassing can be recorded not only due to intensive development of cracks quaziparallel to the face but also due to high gas content and high gas permeability of the massif, where significant pressure of free gas in the mine opening influence area is not built and the outbursts are not threatening. That is why the additional parameters are required to distinguish between two types of areas considered.

A meaningful step to increasing of the current prediction accuracy was made by development of the outburst hazard factor, where instead of the maximum measured initial rate of outgassing in the intervals between the test holes the gradient of corrected initial outgassing rate along the hole length was used (5, 6):

(3)

 

where с is a constant factor, min./l; mв is a thickness of the potentially outbursting band (a cluster of adjacent bands) in the drilling points, м; fв is M. M. Protodyakonov scale of hardness for the given band (a cluster of adjacent bands) in the same points; lg* is a distance to the middle of g*н.max, м hole.

This factor when applied rises the current outburst prediction accuracy about 2 times due to rejection of the practically unhazardous areas which otherwise would be classified as outburst hazardous in the prediction. However, further increase in the prediction accuracy is required to reduce the costs of excessive anti-outburst measures.

In process of the outburst hazard evaluation in the current prediction it is required to use some additional parameter for saturation of the massif with energy. First, here the gas pressure measurements in the areas of intensive cracking as determined by the interval values for the initial outgassing can be applied. This would rise accuracy in determination of the outburst hazardous areas, because the high pressure of gas with high maximum of the initial outgassing in many cases would suggest presence of the developed crack system filled with pressurized gas.

Gas pressure in the seam is one of the outburst hazard determinants. Beginning of the phenomenon’s second stage, which is of highest interest, according to (6), can be expressed as follows:

(4)

 

where Р_X is an excessive gas pressure in the distance of Х from the face; S_X is a cross sectional area of the potentially outburst-prone coal band; П is the band’s perimeter; λ_X is a porosity of the coal band in the face parallel plane in the distance of X from the face; τ_π are stress tangent lines along perimeter П of the band; γ is a density of coal in the band; α is a tilt angle between the mine opening and horizon; X is a distance to the face.

To measure the gas pressure the operational method based on pre-calculation of gas pressure in the measurement chamber by pumping of water into this chamber (9) and the dedicated device (10). Application of this method decreases the time spent for measuring the seam gas pressure by two orders of magnitude down to 15 to 30 min.

The current predication of the outburst hazard of a seam improved this way seems to be rather challenging. First, an encounter with the outburst-prone coal structure in process of bed mining is rather infrequent phenomenon. In absence of such structure the seam area being mined is considered outburst unhazardous. I. e. in such areas only the seam texture monitoring to confirm, that it is still unhazardous, is required. Such monitoring would require continues observation of the face by the gedynamic phenomena mitigation group with taking coal bands strength measurements every 4 m. This would not take much time in the mining work cycle.

Nevertheless, with parallel measurements of initial outgassing rate and gas pressure in the seam the areas with high gas content and just good gas permeability and none of the developed system of specific cracks can be classified as outburst hazardous. Besides, in the described predication of the outburst hazard the above mentioned special energetic condition of the area with intensive cracking is not recorded. The massif will be “armed” for the outburst only when that very system of cracks quaziparallel to the face, permanently moving and filled with pressurized gas is located ahead of the face.

Accordingly, some of the interesting results obtained in measuring of electromagnetic emission (EMI) activity N_ср, which is an average number of EMI pulses per minute occurring in the massif near by the face should be noticed. The results of such measurements have shown that activity of electromagnetic emission together with the standard indicator g_н.max and maximum value of methane content on the mine opening face C_max would signal invasion in the hazardous area, where g_н.max ≥ 6. At the same time the maximum value of EMI activity has been obtained before the maximum value of the initial outgassing, which is measured in the test holes drilled ahead of the face, i. e. 5 to 6 m ahead of the EMI measurement point (Figure 3).

Considering the above, this can be explained by the fact that even before invading the outburst-prone area the instrument would detect EMI pulses, conditioned by high energy condition of the area with cracks quaziparallel to the face in process of intensive development and saturated with free gas, i. e. this way readiness of the area to starting the outburst is registered.

During the massif mass deterioration electromagnetic emission is a rather well known phenomenon (11, 12 e. a.). More or less it occurs during opening and growth of crack in process of the seam’s face area saturation with the outburst’s energy.

It would be reasonable to continue the studies in this direction. If this theory is proved, it will be possible to develop a much better and more accurate method for the current predication of the outburst hazard.