Prof. E.A. Kolesnichenko, Prof. I.E. Kolesnichenko (Shakhty Institute of the South-Russia State Technical University); Prof. V.B. Artemiev (OAO SUEK)
Identification of zones with high methane content in coal beds is the main issue to be addressed in the context of methane safety in underground coal mines and commercial-scale production of coal-bed methane.
In contrast to prediction, identification implies perfect knowledge of the objective laws governing the formation of zones varying in natural methane content and indications showing these zones with a high degree of probability.
The main issue addressed in our research is the effect of natural conditions and objective laws governing the formation of methane in coal beds. Sudden outbursts in underground workings and explosions of methane emission have proved the local boundedness of hazardous methane concentrations occurring in coal beds under high pressure. However, geologists are not able to provide any reliable information about such zones because of wide spacing of exploration boreholes. Therefore, as a practical matter, methane content is still a probabilistic observation.
The authors have developed and now offer a new method of identification of local zones with high methane content. This method is aimed at the identification of hazardous methane zones, which need to be degassed for the prevention of unforeseen gas concentrations in underground workings, rather than at the detection of methane presence as such. The method is based on the investigation of objective laws and fundamental laws of chemistry and physics.
It is known that peat accumulation within the boundaries of a field took long time. In this period facies changes and alteration of geodynamic, as well as physical and chemical conditions of the organic matter transformation in the peat field took place. Coal beds, as a reflection of these changes, are of layered structure. Therefore, the authors hypothesized on the objective (genetic) nature of the formation of methane concentrations in peat field layers in the periods of warping movements of the earth crust. Methane accumulations in every zone of a seam also reflect the peculiarities of all processes that have taken place there [1, 2, 3].
The research into facies and geodynamic conditions for the formation of high natural methane concentrations was undertaken with coal seams of the Vorkut Coalfield. It is a large brachysynclinal fold (trough). Operating underground mines extract the following coal seams: Moshchny, Troinoy, Dvoinoy, Pervy, Chetverty and Pyaty (Fig. 1). Moshchny seam splits in its central part towards the periphery into Chetverty and Troinoy seams. The latter splits into Dvoinoy and Pervy seams, while Dvoinoy seam splits into Trety and Vtoroy. Pyaty working seam also splits in its periphery into coal seams of non-workable thickness.
In coal faces, development headings and entries of the Vorkut Coalfield coal seams 267 sudden outbursts of methane and broken coal were registered. The zones of coal seams affected by these sudden outbursts were investigated. In most cases the outbursts were from low-thickness layers, which interlaced thick and stable layers of the seams. The outbursts have resulted in the formation of hollow cavities. The volume of outburst methane indicated abnormally high natural methane content of these zones.
To investigate the conditions for the formation of parting layers between coal seams some objective laws were considered of peat deposition in the process of deposit bed formation. These layers were formed due to geodynamic movements. Peripheral parts of a peat field were covered with water, and the process of peat accumulation was discontinued. As a result, on dry land a continuous zone was formed, which split towards the periphery into several seams. A layer was formed between the split parts of a seam, and this layer differed from the upper and lower coal bands by conditions of peat accumulation and deposition. The thickness of this layer grew due to thinning of the dry-land part towards seam splitting. As a practical matter, the technological boundary of a continuous seam splitting is a layer thickness of 0.5 m. With a lower thickness a parting layer is included in the total thickness of a continuous seam. The number of layers in a seam reflects that of large geotectonic movements. These parting layers contain carbonaceous argillite, and sooty coal interlacing along the strike of the seam. Closer to the seam thinning the sooty layer of coal is replaced with rock.
The structure of coal seams was investigated based on the stratigraphic sequence of the zones of sudden outbursts and on geological sections of working coal-seam areas. The analysis of the data shows that sudden outbursts took place on the coalfield periphery, moreover, in different mine-takes they occurred at the same horizons. In Moshchny coal seam the methane outbursts were registered at - 280, -320 and -710 m horizons. In Troinoy seam, the -430 m horizon presented the highest risk of methane, 115 sudden outbursts were registered at 4 mines (Table 1).
Sudden outbursts were caused by abnormally high natural methane content encounted in the course of underground working drivage in mine districts including a parting layer (Fig. 2).
Four varieties were identified of the structure of parting layers between coal seams in the areas of splitting:
- carbonaceous argillite or argillite;
- soft sooty coal overlain by a dirt parting;
- soft sooty coal overlying the dirt parting;
- soft sooty coal.
Sudden outbursts are typical of the fourth variety of seam structure. The main layers of a seam are strong semilustrous banded coals. In some parts of the seam the layers of soft sooty coal are overlain and underlain by foliated and boudinaged coal layers. This proves the fact that on the peat field surface there were various facies environments during the formation of soft sooty and strong coal layers.
The analysis of facies conditions shows that lenses of low-ash sooty soft coal were formed in the zones with favourable piezometric surface for vegetation. Peat accumulation was discontinued after water table recharge on the adjacent surrounding surface areas. Meanwhile, water flows migrating on the surface of adjacent areas imported ultrafine fractions of organic matter, argillaceous and other mineral impurities.
To compare the methane content of the main and parting layers of Moshchny coal seam at Komsomolskaya Mine some measurements were taken in development headings. The methane content of a parting layer was measured during the washout of the advance cavity, while that of the main layers of the coal seam was measured in the course of coal cutting by a shearer. Geologists estimated the natural methane content of the coal seam within the mine-take boundaries at 19.3 m3/t.
Measurements were taken in underground workings before and after sudden outbursts of methane. The methane content of the parting layer ranged from 15.2 to 103.8 m3/t averaging to 34,3 m3/t. In these underground workings the average value for the upper and lower layers of the coal seam was estimated at 7.8 m3/t (Fig. 3). Closer to a place of a sudden outburst the methane content of the upper and lower layers showed lower values. Methane emission was registered at a 7-m distance from the cavity. In other underground workings of the same coal seam the methane content of the parting layer ranged from 7.9 to 40.7 m3/t. averaging to 28 m3/t. For the rest of layers the average methane content was 7.4 m3/t., while for the seam it ranged from 11.3 to 12.9 m3/t.
Apparently, the difference in conditions of peat-field deposits formation must have affected the characteristics of coal material. Zoning of high methane content in the coal seam stems from the pattern of the conditions for the formation of coal material with a distinct type of molecular and permolecular structures and a low energy state.
Coal belongs to a solid body, and the formation of its chemical structure is governed by the laws of chemistry and solid-state physics. All kinds of chemical combinations, including minerals are formed due to stable arrangement of a relatively small number of the primary structure units, i.e., atoms. Wave mechanics is rather instrumental for the investigation of all physical parameters characterizing the properties of a material. It implies the principle of fundamental digital form, which is a physical basis of the theory concerning the constitution of matter.
In similar conditions the same atoms always form one and the same material with the identical structure. However, with the changing of conditions for the synthesis of the same composition a great number of options is available for the structure of this material. For instance, gels of a similar composition have a distinct porous structure. If this fundamental conceptualization is applied to the formation of a peat field one can come to a conclusion that the knowledge about facies environment of its formation is a key to the investigation of the coal material structure and properties with a pin-point accuracy. Knowledge on the patterns and laws governing the formation of different facies can be used for the identification of mining districts with high natural methane content.
The results of measurements show that, in contrast to the lower and upper layers of stronger coal, the parting layer, was formed in different conditions, and it is characterized by different genetic lines of transformation. Zones of high methane content in this layer are lens-shaped occurrences of low-ash coal material separated from adjacent ones by coal shale or argillite. Ash content in the zones of outbursts is lower, while around these areas it is 2-fold higher than that of the lower and upper layers. The most probable conditions for peat accumulation of such lens-shaped occurrences are facies of relic shallow lakes located between beds of running rivers from time to time recharged with fresh sea water or interstitial (pressure) waters. The authors have arrived to this conclusion based on the fact that in marginal zones some boundinaged layers of coal and rock can be met, which are formed in running waters. Sylvestral plants, which wood material consist mainly of lignocellulose tissues and ferns, also contributed to peat accumulation. The components of the vegetable material differ in concentrations of С, Н, О, N and S.
The authors offer a hypothesis [1, 3] that different natural methane concentrations are formed in the course of decomposition and synthesis of organic materials in alkaline and acid media. Higher values of methane concentrations are typical of the gellified phase of coal material, which underwent colloidal transformation in alkaline medium.
Synthesized molecules of peat even in similar conditions (acid or alkaline) from closely located zones differ in sets and number of aromatic rings and functional groups.
In alkali conditions, when ferns occur in water, non-split fats and waxes took part in the formation of peat molecule side groups. Therefore, side groups still contain significant amount of acids, decomposed cellulose and hemicellulose. As a result of the analysis of synthesized parts the authors have developed possible patterns of the structure of peat molecules, which after coalification will have peculiar energy bonds, physical and mechanical properties and methane content. In such conditions, due to polymerization process the molecules included in peat composition are linked as a long chain macromolecule. Its segments may turn relative to other segments over C – C ordinary linkage. Protein and fat molecules tend to straighten and therefore the side branches of the main chain of aromatic rings will become longer. Such a branched isometry of molecules has an effect on the formation of the friable structure of coal with poor physical and mechanical properties. Side groups of macromolecules present the main source of methane. The estimated maximum pressure of methane within the coal seam is 19.8 MPa.
In acid environment, significant amounts of fats and waxes are decomposed and exported with water. Synthesized molecules of peat will be more rigid, as lignin molecules still contain manу hydroxyl and carboxyl groups. The length of functional groups of a peat molecule is small. The amount of added groups of radicals with reactive oxygen is greater than that typical of alkali conditions. Macromolecules are less flexible due to a great number of polar substitutes. Its shape is nearly linear. With a great number of short functional groups and radicals it provides for close-packed arrangement and lacing of macromolecules. Later, it will make stronger the coal material formed in acid environment. However, due to a small number of functional groups the methane content of this material will be low.
Thus, irrespective of all significant changes going within the structure of the organic material from the moment of peat accumulation until the coal seam extraction, coal material has not changed in terms of its molecular and permolecular properties and porosity. Peat recovered to a varying degree is characterized by dissimilar physical and mechanical properties. With the concentration of coal material of alkali genesis in a layer of a coal seam a high methane-content zone is formed with low energy parameters within which boundaries the coal material can be broken by the accumulated gas with the advance of underground working.
1.E.A. Kolesnichenko Genetic theory of formation and prediction of coal-seam zones liable to sudden outbursts. Ugol Magazine, 2000 No 9.- pp. 51-53.(in Russian)
2. I.E. Kolesnichenko, E.A. Kolesnichenko. Geological and genetic grounds for methane explosion prevention in underground mines. Gornaya Promyshlennost Magazine, – 2005 г. No 1, pp. 42-45 (in Russian)
3. E.A. Kolesnichenko. I.E. Kolesnichenko. Sudden outbursts and methane emission: prediction and prevention. Rostov-on-Don, Logos Publishers. 2005. – p. 248 (in Russian)