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Overview of unconventional gas technologies
Coal Seam Methane

Methane from Coal

An important resource of unconventional natural gas is methane contained in coal, which in turn has significant proven reserves in the world which exceed the reserves of other hydrocarbons. There are a number of industrial technologies for the extraction of methane from coal. Among them [1] there are usually three main technological areas based on the status of coal deposits Abandoned Mine Methane (AMM), characterized as methane extracted from spent, suspended or abandoned mines; Coal Mine Methane (CMM) is considered to be independent if methane utilization is carried out with simultaneous commercial coal mining in coalmines and Coalbed Methane (CBM), when methane is produced from coal seams that were not previously involved in the development. The general scheme of technological directions for the extraction of methane from coal seam methane and its further use is shown in Fig. 1.

Fig. 1. The main technologies for the utilization of coal methane

The increased interest in methane in coal seams and coalmines is associated not only with its energy value, but with the explosion risk in mines during coal mining and the high importance of methane as a source of greenhouse gases. With this in mind, the development and application of modern technologies for the utilization of coal methane are necessary regardless of the prospects for using coal itself, at least in terms of spent mines. These principles are reflected in the creation of an international public-private partnership The Global Methane Initiative (GMI), uniting 45 countries  and several hundred members – where you can find information about the most important projects in this industry [2].

Methane content is highest in underground untouched coal seams, where it may exceed 90%. In addition to methane, coal seams contain other gases, including, for example, carbon dioxide and nitrogen.

Coal Seam Methane/ Resources and Production

Coal is formed from sedimentary rocks under the influence of temperature and pressure during a long process called coalification. The solidification of the initial natural mass occurs gradually, its compression leads to the formation of coal and the discharge of gas and water. Methane in a solid carbon matrix remains in a dissolved or sorbed state (mainly in micropores), as well as in a free state in natural fractures and faults. During coal mining, rock is crushed and gas is released from natural traps and can be disposed of. If the natural methane content in coal (gas saturation) depends primarily on its origin and physical properties (density, porosity, hydrophility, etc.), then the technically and especially economically achievable level of its extraction is determined by a set of fundamentally different indicators - geological characteristics, formation depths, their thickness, gas pressure, methods of influencing the formation, etc. Typically, anthracites are characterized by the highest methane content (more than 5 m³ per ton, sometimes up to 15-20 m³), but they have a high density (about 1.5 g / cm³) and low permeability, which is often unacceptable from a technological point of view of methane extraction [3] . Younger coals - lignites and subbituminous coals have significantly lower gas saturation - two or more times less than that of anthracites. Bituminous coals (density of about 1.25 g / cm³) are most suitable for commercial methane production. This type of coal makes up about half of the total production of all types of coal in the USA [4], where large-scale production of methane from coalbed methane is also carried out.

The above list of factors that determine the methane content in coal, even in this extremely shortened composition, makes it possible to understand how difficult its quantification and the method of aggregate accounting for calculating coal methane reserves can be. Nevertheless, there is a lot of data on the reserves and resources of coal methane in different countries and regions around the world, although most of them have a big difference between the limit values ​​and differ significantly from each other. So in [5], the global resources of coalbed methane are estimated at 256 Tcm of which 180 Tcm are considered as easily accessible. At the same time, three of the regions most endowed with this resource were identified - F. Soviet Union - in which, according to the authors, about 43% of the resources are concentrated, as well as the North American region (33%) and Asia Pacific (19%). The most detailed statistics on Proved Reserves of CBM for the USA are presented in [6]. At the end of 2017 they were estimated at 11.878 Bcm, having decreased almost twofold since their peak in 2007 (21874 Bcm). The resource base of CBM in the world is considered in detail in [7]. In this study, in particular, summary estimates of coalbed methane resources for different countries and regions are given; for example, for the United States resources are estimated at slightly less than 400 Tcf, for China - within 1000 Tcf, for Australia - up to 500 Tcf. The most significant resources are indicated for Russia - almost 6000 Tcf, according to the highest estimate, and more than 2500 Tcf for Canada, also according to the highest estimate. In [8], with references to several sources, fundamentally different estimates of CBM reserves in the world are given. The main regions identified are the USA, Russia, China, Australia and Canada. Among the leaders here are the United States and Russia with approximately equal reserves, slightly exceeding 1700 Tcf. More than 1000 Tcf reserves are also expected in China and Australia. Canada, according to this source, contains a little less than 700 Tcf. Other estimates of reserves and resources of coalbed methane are given in [3, 9-19].

Currently, the existing mines are quantitatively accounting for methane released during its production or during natural diffusion. The generalized data of such accounting is carried out by the U.S. Environmental Protection Agency (EPA), which can be seen, for example, in [20]. The EPA has made several important assumptions that allow for the practical accounting of methane: hard coal is mainly produced in underground mines, and soft coal is produced in surface mines; The EPA also determined methane emissions from coal mining activities by multiplying its production data by the emission levels established in The Intergovernmental Panel on Climate Change (IPCC) [21, 22]. In addition, EPA notes that the amount of gas emitted during coal mining depends on two main factors: coal rank and coal depth. Coals with a higher carbon content usually also contain more methane. With a deeper occurrence of coal, pressure on the formation increases, preventing the migration of methane to the surface [21]. In this study, these provisions are supplemented by the following: for targeted coal production, the level of methane emission is not less than the volume of methane production during its targeted production in coal seams; Anthracite and bituminous are assigned to hard coal, and Subbituminous and lignite to soft coal. Taking into account all these provisions, based on the normative levels of methane emissions from coalmines from [22] and statistics on proved reserves of different types of coal in countries [23, 24], similar to the above method (multiplying available reserves, instead of production volumes, by emission levels), the calculations of the potential of coal seam methane utilization in various countries are performed and presented below in Fig. 2.

Fig. 2. Potential for the utilization of coal methane

Since, according to [21], for underground coal mining, emission factors are between 10 and 25 m³/t, and for open-cast mining only between 0.3 and 2 m³/t, the data obtained primarily depend on the ratio of reserves of hard and soft grades of coal. According to the Federal Institute for Geosciences and Natural Resources (BGR) [23], the United States, China, India, Russia, and Australia possess the largest deposits of anthracite and bituminous varieties. Among these countries, Russia and Australia additionally have gigantic reserves of subbituminous and lignite, which significantly affects the final result. It is these five countries that dominate the presented map in terms of their methane utilization potential. On the other hand, with relatively large coal reserves in Germany, Turkey, and New Zealand, soft grades prevail in them, which, taking into account emission factors (lower than hard coal grades), reduces the methane utilization potential when calculating this method. Certainly, this approach has a number of significant flaws, since here, in addition to the assumptions already listed, the actual depth of the coal seams is not taken into account, the actual methane content in the coals of various deposits is not taken into account, nor methane production technologies, etc. Nevertheless, the results of these calculations, based on generally accepted statistical indicators and methane emission standards in coal mining, can serve as an additional guideline in determining the prospects for coal methane production.
Coalbed methane is produced commercially in a few countries — primarily in the USA, Australia, China, Canada, and in relatively small volumes in Russia, India, Poland, Germany, and the United Kingdom [3, 6, 12, 14, 18, 25-27 ], with Coalbed Methane mining predominantly prevailing, i.e. methane in coal deposits, where coal mining itself is not carried out (Fig. 2, 3, 4).

Fig. 3-4. On the left is the production of coalbed methane (Coalbed Methane) by the leading manufacturers in the world, on the right is the production and reserves of Coalbed Methane in the USA, 2009-2018

Source: Based on Data from U.S. Energy Information Administration (August 2019), Canada Energy Regulator, Australia Government/Department of the Environment and Energy, National Bureau of Statistics of China

Over the past 10 years, significant changes have occurred in the production of coal methane. Firstly, production in the USA has significantly decreased. Between 2009 and 2018, the decrease was almost double - from 1914 to 980 Bcf. Obviously, the shale gas boom in the USA negatively affected the production of coal methane. A similar situation has developed in Canada, where over the same ten-year period the production volume decreased from 340 to 184 Bcf. A certain balance to this decline was provided by China and especially Australia. Beginning in 2015, production in Australia has increased many times over, and it was in Australia for the first time that coal methane was liquefied for its subsequent export. According to the latest statistics from [25], only in 2017 and 2018, coal production in Australia increased by 8% and amounted to 38 Bcm or more than 1350 Bcf. Thus, Australia has become the world leader in the production of Coalbed Methane and in all likelihood will remain in this status for a long time.
Utilization issues for Coal Mine Methane in the USA are discussed in detail in [28]. In 2017, the total level of methane emissions from underground coal mining as a result of venting and degasification technologies amounted to about 3,500 Mcm, of which less than 1,000 Mcm were profitably utilized. A publication [29] reports on the utilization of Coal Mine Methane in the UK, where Alkane Energy, which was later acquired by Infinis, was the leading company in this segment.

Coal Seam Methane/ Mainstream technologies, advantages and disadvantages

As noted above, there are three fundamentally different technological areas for the extraction of methane from coal  Coalbed Methane, Coalmine Methane and Abandoned Mine Methane. Figure 5 shows a simplified diagram of various methods for utilizing mine methane, including mine ventilation and drainage, as well as methane production by drilling vertical and horizontal wells.

Fig. 5. Simplified scheme of methane utilization methods in coalmines and seams

A1. Ventilation air  A2. Coal cutting machine 
A3. Gob area  A4. Ventilation air methane 
A5. Ventilation shaft  A6. VAM oxidizer 
A7. Coal waste heap  A8. Heat energy 
A9. In-situ users

B1. Production well gob gas  
B2. Drainage pipeline B3. Shaft sinking 
B4. Mine koper  B5. Gas purification unit  
B6. Compressor station B7. Gas generator 
B8. Transformer

C1. Vertical well  C2. Horizontal well 
C3. Gas purification unit  C4. Compressor station 
C5. Gas pipeline

Methods of ventilation and degasification systems at underground coalmines are used to extract methane from existing or abandoned underground mines. Both methods are widely used in industrial practice, however, they are mainly designed to ensure the safety of operations in existing mines and reduce methane emissions into the atmosphere. The latter is achieved by the use of special equipment for the preliminary purification of gas and its useful utilization through combustion in thermal installations for heating residential buildings or industrial premises, as well as for the generation of electricity by special generators (Gas Engines). For example, Jenbacher supplies gas engines with a capacity of up to 9.5 MW to the market which are capable of providing local heat and electricity supply with an efficiency of up to 90% [30]. In recent years, more sophisticated methane utilization technologies for producing liquid hydrocarbons based on the Fischer – Tropsch process have been used.
Target production of coalbed methane is achieved by drilling wells and using special methods of production activation. CBM mining technologies are in many ways similar to those of traditional natural gas and, especially, shale gas. Standard equipment is used, similar options for reservoir evaluation and drilling, waste disposal, transportation of finished products, etc. However, there are significant differences. Coal seams are characterized by a certain thickness, depth, gas content, density, porosity, permeability, water saturation and other, often specific, parameters. To ensure the effectiveness of the process requires accounting for each of them. A significant share is occupied by coal deposits with a large depth, low gas content or permeability, which significantly affects the economic performance of the mining process. To extract methane from a coal seam, it is necessary, besides crushing and grinding the rock, to additionally pump out water from the seam to reduce pressure and activate the processes of desorption of methane from coal. Here, usually on top of the the general barriers characteristic of gas production, additional technological problems arise. One of them is associated with the need to prevent methane from entering the water stream, which in the end can dramatically increase the costs for its extraction. If the content of harmful substances in the extracted water exceeds the norm, then cleaning or disposal is necessary. Another problem is related to wellbore stability during horizontal drilling along a coal seam, often leading to wellbore collapse and a sharp decrease in production. In addition, when methane and water are extracted from a coal seam, its properties will change significantly, affecting the gas flow.
To increase the productivity of coal seams, modern methods of intensification are used, which are mainly the injection of various gases or air into the reservoir, directional drilling and hydraulic fracturing. An extended review of the development of coalbed methane mining technologies is given in [15]. Significant advantages besides the actual intensification of methane production can be provided by injecting CO2 into the reservoir as the main greenhouse gas. The effectiveness of the technology is ensured by the regulation of pressure after pumping water from the reservoir and the different ability of gases to adsorb and desorb on the surface of coal. Interesting details of this process can be found in [31, 32]. Important in this matter are the data of The Langmuir Isotherm, which demonstrate the dependence of the gas content in coal on pressure. In [32], such comparative estimates for methane and carbon dioxide are presented, which show, as an example of coals of a particular deposit, a significantly higher adsorption capacity of carbon dioxide compared to methane. Gas is pumped into the reservoir through the injection well after pumping water out of it, although the sequential use of one well is possible [32]. As a result of the injection of carbon dioxide into the reservoir, a number of specific problems arise, in particular, uncontrolled Swelling, Breakthrough of injected gases and waterlogging, which can lead to cessation of production [32]. Thus, the features of the coal seam are not always compatible using proven technologies for intensifying natural gas production. Carbon dioxide injection turned out to be an attractive technology for its disposal, but problematic in terms of methane extraction from coal seams. Certain hopes are assigned to the search for optimal mixtures of gases, such as CO2 and nitrogen or flue waste, however, this additionally complicates the already expensive mining process and reduces its competitiveness. In [32], the main problems of carbon dioxide use were systematized, among which, in addition to the above-mentioned technological ones, economic and organizational ones were distinguished — a convincing demonstration of the technology at a commercial level, financial stimulation of carbon dioxide burial and practical guidance on the exchange of experience were needed.

Other important methods for increasing the efficiency of coal seams are horizontal drilling and hydraulic fracturing. The practical services of these technologies for coalbed methane are offered by the leader in this area, Halliburton [33]. A detailed comparative analysis between vertical and horizontal drilling is given in [15]. The main advantages of vertical wells, for obvious reasons, are classified here - relatively simple equipment, technically simple implementation of the process, low cost, etc. Disadvantages - a requirement for high reservoir permeability, shallow bedding, low productivity of single wells. On the other hand, horizontal wells have a number of undeniable advantages - they provide small flow resistance, enlarge the coal seam, are usually 20 times that of a vertical well, fast fund recovery. The disadvantages of horizontal wells, in addition to traditional and similar shale hydrocarbons (high cost, complexity of equipment and technology) in [15] mention the increased “requirement for mechanical strength of coal is high so as to guarantee stability of well” [34] states that using multi-branched horizontal wells (MBHW), compared with vertical wells, is an effective way to develop CBM in low permeability reservoirs in China.

It should be noted that the ratio of vertical and horizontal wells in the most progressive hydrocarbon production region, the United States, is constantly changing in the direction of greater use of horizontal wells [35]. So, as of September 2019, out of 886 operating wells 776 were horizontal and another 57 directional. It is obvious that directional drilling technologies, including horizontal, will be applied more widely throughout the world, including the extraction of coalbed methane. The use of fracking in the extraction of coal methane is also a common practice. The advantages and disadvantages of this method of intensifying hydrocarbon production are widely known and described in detail, for example, in [15, 36–39]. It is possible that the use of fracking when expanding coal methane production may be more restrained compared to its use for oil or gas production for economic reasons, as well as in connection with the problems of utilization of process water.

Coal Seam Methane/ Major companies

Among the companies engaged in the extraction of coalbed methane, the development and supply of equipment and technologies in various regions of the world, the following should be highlighted: AGL Energy; Anglo Coal; Arrow Energy; Australia Pacific LNG; Baker Hughes; Blue Energy LTD; BP PLC; Carbon Creek Energy; China National Petroleum Company; China United Coalbed; Comet Ridge; Constellation energy; Consol Energy; ConocoPhillips; Dart Energy LTD; Ember Resources; Encana Corp.; Ephindo energy; ExxonMobil; Gazprom GEECL; Great Eastern Energy; Green Dragon Gas; Corporation Ltd G3 Exploration; Halliburton; Infinis; Metgasco LTD; Methane CO; Nexen INC; Oil and Natural Gas Corporation; Origin Energy; Pioneer Natural Resources; Quicksilver Resources INC; Santos LTD; Sinopec; Shell Change in the value of shares of some public companies operating in the field of unconventional gas production, including coalbed methane can be seen in Fig. 6 and 7. As can be seen from the figures, the shares of most of the companies represented are in the growth zone of the NASDAQ Composite index, which reflects the value of shares of the most high-tech companies, which suggests a healthy financial condition of the industry, despite numerous negative statements.

Fig. 6 and 7. Dynamics of changes in the value of shares of some public companies engaged in the extraction of unconventional gas

Source: Yahoo Finance

Coal Seam Methane/ Research and Innovations

Coalbed methane mining is an intensively developing industry, including in scientific and technological aspects included, as evidenced by the significant volume of issued patents and registered patent applications on this subject throughout the world. Below are the results of the analysis of 4990 patent documents on topics related to the extraction and useful utilization of coal seam methane in the form of applications for inventions registered in 36 patent offices in the world for the period between 2009 and 2018, prepared by 1651 applicants from 36 countries. The analysis method can be found in the Research & Analysis section of the Advanced Energy Technologies website.

The top 10 leading patent offices that registered the largest number of applications included offices of China, the USA, Canada, Australia, Europe, Russia, Brazil, Mexico, Japan and WIPO (Fig. 8). The two leading patent offices 
the United States and China  accounted for 43.9% of all patent applications. Almost 90% of inventions were registered in the top 10 offices for the period under review. The most productive in terms of the number of documents (Fig. 9) were residents of the USA (almost 50% of applications), China (about 18%) and the Netherlands (8.9%). It should be noted that among the most popular patent offices in the world appear offices of countries with the greatest potential for coal seam methane utilization the USA, China, Russia, Australia, Canada, except India. Also, there are no residents from this country among the top 10 applicants.

Fig. 8 and 9. Left
Distribution of patent applications among the main patent offices of the world; right  distribution of applicants by residency

Source: Advanced Energy Technologies

The patent offices of the USA and China registered the least number of applications from non-residents – 26 and 32%, respectively, although it was precisely in the offices of these countries that the largest representation of non-residents from different countries was noted. For example, representatives from 20 countries managed to register their documents in the US Patent Office, and 14 in China. In the offices of Canada, Russia and Australia, the number of applications from non-residents was approximately the same in percentage terms – from 85 to 88%.

Figures 10 and 11 show statistical data reflecting the list of technological, economic, and organizational problems that the main patent solutions were directed to, as well as information on the technological orientation of inventions in the form of elements for the phased production and utilization of coalbed methane.

Fig. 10 and 11. On the left
– The main problems of patent applications, on the right – The share of patent applications by elements of the technological chain

Source: Advanced Energy Technologies

LERF – Low efficiency caused by specific features of reservoir; LEPP – Low efficiency of production processes; LEST – Low efficiency of surface treatment; LEG – Low efficiency in general; HCPP – High costs of production processes; HCE – High CAPEX / Equipment; HOOE –High OPEX Operation and expendables; UE – Unaccounted expenses; HCT – High costs of transportation; HCG – High costs in General; ESI – Environmental and social impact; AOP – Administrative and organizational problems; PU – Problem unidentified; MTE – Methane (fromCoal) to electricity;  MTL–Methane (from Coal) to liquid; DR – Drilling; E – Exploration; HF – Hydraulic fracturing; P – Production; IE- Infrastructure & environment; TT – Treatment & transportation

As follows from the data presented, the inventors were most concerned about the problems of Low efficiency of production processes and Low efficiency of surface treatment. A significant number of applications were devoted to problems related to Environmental and social impact. Among the problems of an economic character, the most popular was High OPEX Operation and expendables, which were reflected in more than 1200 documents.

The inventors showed the greatest interest in considering the elements of the phased production and utilization of coal seam methane in the Production technologies (mentioned in 22.2% of patent applications) and Hydraulic fracturing (17.6%). Drilling and Treatment & transportation attracted inventors in approximately the same way - in more than 13% of cases. Of the technologies for the beneficial utilization of the extracted methane, the technology related to the production of liquid fuels - Methane (from Coal) to liquid - was presented more often than others.

The top 10 applicants out of 1651 who have registered their patent applications include 981 documents or about 20% (Table 1). ExxonMobil Upstream Research Company (USA), Shell Internationale Research Maatschappij B.V. (Netherlands) and Halliburton Energy Services, Inc (USA) showed the highest patent activity over this 10-year period of time, registering more than or about 300 patent documents.

Table 1. Top 10 applicants

CompanyUSExxonMobil Upstream Research Company14.1341
CompanyNLShell Internationale Research Maatschappij B.V.11.6325
CompanyUSHalliburton Energy Services, Inc.12.4290
CompanyUSBuker Huges Incorporated12.9151
OrganizationCNCUMT China University of Mining and Technology13.1104
CompanyJPKawasaki Heavy Industries, Ltd.9.195
CompanyUSOren Technologies, LLC11.593
CompanyUSMarathon GTF Technology Ltd11.690
CompanyGBJohnson Matthey Pls12.289
CompanyNLSchlumberger Technology B.V.11.268

Source: Advanced Energy Technologies

In addition to American and Dutch companies, the top 10 included companies and organizations from Japan, China and the UK.

Figures 12 and 13 present the main problems of the patent applications of the top 10 applicants and the interests of the top 10 applicants among the elements of the phased production and utilization of CBM.

Fig. 12. The main problems of patent applications of the top 10 applicants

Source: Advanced Energy Technologies

LERF– Low efficiency caused by specific features of reservoir; LEPP Low efficiency of production processes; LEST Low efficiency of surface treatment; LEG – Low efficiency in general; HCPP - High costs of production processes; HCE High CAPEX / Equipment; HOOE– High OPEX Operation and expendables; UE– Unaccounted expenses; HCT– High costs of transportation; HCG– High costs in General; ESI Environmental and social impact; AOP – Administrative and organizational problems; PUProblem unidentified

Fig. 13. The share of patent applications for elements of the technological chain among the top 10 applicants

Source: Advanced Energy Technologies

 Methane (fromCoal) to electricity; MTL–Methane (from Coal) to liquid; DR – Drilling; E – Exploration; HF  Hydraulic fracturing; P – Production; IE  Infrastructure & environment; TT– Treatment & transportation


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