География и геология/7. Техника и технология геологоразведовательных работ.

Student and technical officer Hakim A.R., student Uwarovski V.G.,

k.geol.-min.s. Ivanova R.N.

China Petroleum Engineering & Construction Corporation

Irkutsk National Research Technical University, Russia

Prospects for the use of methane from coal seams

Extraction of methane from coal seams combines technology from the oil and coal industry. For example, during the previous generation of methane was produced for local use from wells drilled into the coal seams [1]. Traditionally, methane extracted from coal mining to reduce the risk of [3]. The production of methane from coal seams in a short time has become an important industry, providing a large amount of pure product era, when concern about pollution and shortage of fuel. Extraction of methane from coal seams, combines technology from the oil and coal industry. For example, in the previous generation of methane has been mined for local use from wells drilled in coal seams.

However, for the production of such a product must be broken and dehydrating layer, using technologies from the oil industry, as its natural parameters (in particular pore size in which it is contained) can not afford cost-effective production (table.1).

Arrangements for the hydraulic fracturing are implemented to raise performance of this kind of gas production to commercial level.

Comparison of the properties of CBM with conventional gas Table 1

Conventional Gas	Coalbed Methane
Darcy flow of gas to wellbore	Darcy flow through fractures
Reservoir and source rock independent	Reservoir and source rock same
Macropore size: 1μ to 1 mm	Micropore size: <5Angstrom to 50 Angstrom

To develop the coalbebs economically, gas content and permeability of the reservoir must meet minimum criteria that may be about 150 scf/ton gas content in thin seams and 1 md permeability. Permeability is the network of natural fractures along with any hydraulic fractures must supply the permeability for commercial flow rates of methanes. Natural fractures occur during coalification from shrinkage of the coal matrix after loss volatiles.

To ensure a sufficient level of economic indicators of product gas contents and permeability of the reservoir must meet minimum criteria, which can be about 0,00245806 m3/ton gas content in thin seams and 1 µm2 permeability. Sufficient permeability is provided by a network of natural fractures and cracks resulting from hydraulic fracturing. The natural cracks of coalification from shrinkage of the coal matrix after a loss of volatile substances. To determine permeability is needed one of the pressure transient tests: drillstem, slug and multi-well interference test.

The layer of coal due to its properties of the layer during its formation includes the gas which is adsorbed and cannot be detected by geophysical methods as in the usual manifold. From this it follows that the volume of gas should be determined by volume calculations based on data by the properties. The amount of adsorbed gas as a product is determined by chemical analysis: ash content, moisture content, gas content, bound carbon content and volatile matter.

The results of chemical (the proximal composition analysis of coal)

sample of a coal deposit in Indonesia

Fig. 1

The gas content varies with coal quality and thermal maturity of organic matter. The coals are divided into lignite, subbituminous, bituminous and anthracite, and classes are divided into 13 groups (Fig. 1). Also carried out a General analysis which shows the elemental composition of oxygen, carbon, hydrogen, sulphur and nitrogen. Based on the calculation of gas in place reserves of coalbed methane by using volumetric methods, CBM located at the rank of sub-bituminous coal. Because, Sub-bituminous coal is contains less sulfur but more moisture - approximately 10 to 45 percent - and volatile matter - up to 45 percent - than other bituminous coal types. It has 35 to 45 percent carbon content; its ash content ranges up to 10 percent. If the ash content is high, so the quality of coal will be better to produce coalbed methane. But coals of the bituminous class are most sought after in the CBM process because most properties are optimum at this rank.

Approximate comparison of coal grades according to the classifications of the former Soviet Union (GOSSTANDART) and the U.S. (ASTM) Table 2.

стандарт классификации углей
ГОСТу 25543-88		ASTM D388-98a
Марка угля	Группа угля	Group and Grade of coal
Бурые	уголь группы 1Б	«лигнит A и B»	lignite A and B coal
	уголь группы 2Б	«суббитуминозный С уголь»	subbituminous coal
	уголь группы 3Б	«суббитуминозный B уголь»	subbituminous coal
смешанные	уголь марки Д	«суббитуминозный А уголь»	subbituminous coal
	угли марок ДГ, Г, ГЖО, ГЖ и частично марки Ж	«битуминозный уголь с высоким содержанием летучих»	high volatile bituminous coals
	угли марок КЖ, К, КО, КСН, КС и остальной части марки Ж	«битуминозный уголь со средним содержанием летучих»	medium volatile bituminous coals
	угли марок ОС, ТС	«битуминозный уголь с низким содержанием летучих»	low volatile bituminous coals
	уголь марки T	«полуантрацит»	semi-anthracite
Антрациты	угли групп PA, 1A и 2A	«полуантрацит» «антрацит»	semi- anthracite anthracite
	уголь группы 3A	«метаантрацит»	meta-anthracite

According to the "American Society for Testing and Materials, 1999; table 1, p. 188" with additions by the authors.

To determine what type of data the coals in the respective classification, used in the territory of Russia was carried out a comparison of these classifications on the basis of published literature data and information of the Internet [2, 4]. Table 2 shows the result of the comparison of Western and Russian coal (for example, markings used in the former USSR). According to the compilers [4] more exact match for the specific coal sample is difficult to establish due to frequent ambiguities regarding the conditions of sampling, chemical analysis and industrial use of coal in the Soviet Union. Thus, in accordance with the current classification of coal in Russia the most perspective for extraction of methane are coal seams of bituminous class (Fig. 1), because of their above mentioned properties.

In recent years, CBM projects have rapidly proliferated. Australia had no CBM production in 1995, but in 2008, 4 billion m3 [141 Bcf] was extracted from its extensive underground coal reserves.1 China had in excess of 1.4 million m3 [49 Bcf] of CBM production in 2006.2 These amounts are small compared with US production in 2007—61 billion m3 [2.15 Tcf].

Increase production of coalbed methane is possible using two methods [1]. Using the method Fire: partial pressure of methane is reduced by introducing inert gas, such as helium, or a gas, which can be adsorbed in the coal seams is weaker than methane, for example nitrogen, is thus maintained full pressure. The second method uses the injection of carbon dioxide to displace methane from coal tar fractions. Carbon dioxide is more strongly adsorbed on the coal than nitrogen and methane in coals. When using CO2, the initial flow rate is smaller than when using N2, but the overall flow field in the future is higher.

Development of commercial process of extraction of methane from coal seams is a possible positive step for the environment worldwide. Methane from coal seams can be considered as a variant of the alternative energy source and when used as a fuel source significantly reduces the CO2 emission into the atmosphere, which directly affects the reduction of the greenhouse effect.

References:

1. Golden S. S. Project "Methane of Kuzbass" – the Kuzbass methane. / Gold S.S. // Bulletin of KuzGTU. - 2010. – №. 6. – P. 37-39.

2. GOST 25543-88, The brown coals, hard Coals and anthracites. Classification by genetic and technological parameters. M. Goskomstat of the USSR, 1988.

3. Puchkov, L.A., Extraction of methane from coal-bearing strata in the fields of operating mines to increase the mining safety. / L.A. Puchkov, S.V. Slastunov, S.K. Bajmukhametov. // Coal. – 2001. – №. 11. –P. 56-60.

4. American Society for Testing and Materials, 1999. // https://www.astm.org/ (Date of access: 16.04.2017)