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Gas transportation and enhanced coalbed methane recovery processes in deep coal seams: A review Xiaogang Zhang, Pathegama Gamage Ranjith, and Mandadige S.A. Perera Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01720 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016
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Energy & Fuels
Cover Page
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Manuscript Title： ：
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Gas transportation and enhanced coalbed methane recovery processes in deep coal seams: A
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review
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Author’s names
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X.G. Zhang1, P.G. Ranjith1*, M.S.A. Perera1,2
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1
Deep Earth Energy Research Laboratory, Building 60, Monash University, Victoria, 3800,
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Australia.
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3010, Australia
Department of Infrastructure Engineering, Building 176, The University of Melbourne, Victoria
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Corresponding author:
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Prof. Ranjith P.G.*
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ARC Future Fellow, Professor of Geo-mechanics
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Director, Deep Earth Energy Research Laboratory, Building 60
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Monash University, Victoria, 3800, Australia.
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Phone/Fax: 61-3-9905 4982
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E-mail:
[email protected] Page 2 of 59
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Abstract
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Coalbed methane (CBM) is a potential green energy supply to address the worldwide energy
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crisis. However, the recovery of economically-viable amounts of methane requires the
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application of production-enhancement techniques. The greater effectiveness of enhanced
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coalbed methane recovery (ECBM) compared to traditional pressure depletion and hydraulic
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stimulation techniques has been identified in terms of higher CBM recovery with minimal
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pollution risk and the ability to contribute to CO2 sequestration. Gas transport behaviour in coal
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seam is the governing factor for ECBM recovery, which includes sorption/desorption and
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diffusion in the matrix and advective flux in cleats. The interactions among sorption, diffusion
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and flow indicate the complexity and abstruseness of gas transport in coal. Therefore, the
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purpose of this paper is to provide a comprehensive knowledge of the gas transport process in
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deep coal seams, particularly in relation to the ECBM process.
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According to the review, the dual-porosity system in coal provides sorption sites, and
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CO2 has much higher adsorption affinity to coal compared to CH4. Gas adsorption capacity for
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CH4 and CO2 greatly reduces with temperature and with the presence of moisture and increases
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with pressure. However, the adsorption capacity for super-critical CO2 decreases with increasing
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pressure due to changes in the associated CO2 properties. Regarding the diffusion process, CO2
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has the highest diffusivity for its smallest kinetic diameter, and that diffusion capability may
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reduce with the existence of moisture for moisture adsorption-induced coal swelling. Seam
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temperature has a positive influence on gas diffusion due to the enhanced kinetic energy and the 2
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pressure effect on diffusion is still open to debate. Upon sorption/diffusion, gas moves towards
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the cleat system through gas flow, which is controlled by permeability, which is in turn greatly
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altered by the gas adsorption/desorption-induced swelling/shrinkage effects during ECBM
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recovery. With high chemical reactive potential, CO2 creates the greatest coal matrix swelling for
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its higher adsorption capacity. Seam permeability increases with increasing injection pressure,
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due to the associated pore expansion and reduces with enhanced swelling. Coal mass swelling
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reduces with increasing temperature due to the exothermic nature of gas adsorption. Dewatering
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coal seams increases coal permeability through the reduced moisture content that provides more
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sorption places for CO2 adsorption. However, this in turn may cause reduced permeability
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through the enhanced swelling effect.
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Keywords: Coal seams; ECBM; Gas transport; Adsorption; Diffusion; Permeability; Swelling
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1. Introduction
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Coalbed methane refers to CH4 trapped in coal seams that is generated during the coalification
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process. During this process plant material is converted into coal with through a complicated
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combination of chemical and physical processes.1 CBM is a relatively clean-burning natural gas
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compared to fossil fuels such as coal and oil. It is therefore an important energy supply and an
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effective substitute for coal and oil in electricity plants in places with abundant CBM resources
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in the context of the ever-increasing world energy consumption as a result of fast
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industrialization and growing population.2
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Most CBM is present in an absorbed phase in coal seams3 and therefore the primary
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recovery of CBM is through dewatering the coal seam to reduce reservoir pressure and thus to
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liberate the adsorbed CBM. However, the gas recovered by this method is generally less than 50% 3
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of the gas in place.4 This is not efficient and economical, because a significant amount of
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methane is left within the reservoir. Therefore, more advanced production enhancement
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techniques are required. Currently, hydraulic fracturing is a widely-used method to enhance the
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CBM recovery rate, which involves injecting pressurised fracturing fluids into the coal seam to
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enhance the seam permeability by creating a fracture network that eventually enhances the CBM
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recovery rate.
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However, the effectiveness of this method is restricted when there are fractures within the
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coal seam as fracking fluids travel through those fractures without fracturing seams and thus
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limits the effectiveness of creating new fractures. Generally, with a properly constructed and
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supervised fracturing project, hydraulic fracturing poses little environmental hazards to the
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environment.5 However, under specific circumstances this method is sometimes not ecologically
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friendly, because some fracking fluids may contain a variety of chemical additives to facilitate
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fracking process, those additives however may environmentally harmful.6, 7 Groundwater could
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possibly contaminated if a wellbore was not properly constructed especially when there is an
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active aquifer closely overlying or beneath the coal seams, the induced fractures would penetrate
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and connect the aquifer and coal seam, which provides channels for unwanted water to enter coal
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seams and wellbores.5, 8 Another side effect of hydraulic fracturing is that coal seam permeability
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may decrease because of the following causes: coal matrix swelling induced by adsorption of
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fracking fluids, and clogging of fractures within coal seams by unrecovered fracking fluids.
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Besides, much works of well drillings are required to achieve sufficient gas flow rate especially
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for seams with very low permeability. Furthermore, the effectiveness of this method is quite
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sensitive to coal seam characteristics and is not economical for thin coal seams.8
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ECBM is another CBM production-enhancement technique, which involves injection of
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CO2 or N2 or a mixture of both into the seam to enhance methane.8 Of the various ECBM
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techniques, CO2-ECBM has the additional benefit of sequestrating large amounts of CO2, which
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therefore has the potential to reduce CO2 accumulation, a major greenhouse gas, in the
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atmosphere. For example, nearly 6.4 billion cubic feet of CO2 was injected in Allison unit of San
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Juan basin.9 Because of these unique advantages, ECBM and related issues have been widely
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studied by scientists around the globe.
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However, gas transportation is an important factor for achieving successful ECBM
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recovery under given operational conditions. Since CBM reservoirs are usually water saturated
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under in-situ conditions methane production begins with reservoir pressure depletion. As
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pressure decreases, methane transportation in the coal mass initiates and proceeds in three main
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steps: 1) the adsorbed gas first detaches itself from the micro-pores in the coal matrix due to
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pressure reduction; 2) the desorbed gas diffuses through the coal matrix and micro-pores to the
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cleat system (fracture network), 3) the gas then enters the coal mass natural fracture network as a
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free gas (see Figure 1).10 This gas flow in the coal matrix and in the cleat system occurs in two
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different ways; the former (in the coal matrix) is believed to be concentration gradient-driven and
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can be described using Fick’s law, while the latter (in fractures) is considered pressure-driven
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and can be explained using Darcy’s Law.11
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Figure 1. Gas transportation in coal (a) Desorption and (b) Diffusion and (c) Darcy flow10 5
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As mentioned earlier, the recovery of CBM by primary pressure depletion is generally
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less than 50%, and according to current findings, this can be raised to around 90% through the
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correct application of ECBM recovery.8 However, the injection of CO2 and/or N2 inevitably
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changes gas transportation behaviour in coal seams, and the possible interaction between gas and
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coal creates more complications for the gas transportation process in coal. For example, when
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CO2 is injected, a series of physical and chemical reactions take place between the CO2 and the
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coal mass. Coal structure changes take place as a result of CO2 dissolution induced
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plasticization,12 which induces large strains between the adsorbed CO2 layer and surface of pores
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in the coal matrix;13 Chemical reactions between injected CO2 and coal functional groups
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emancipate and mobilize polycyclic aromatic hydrocarbons in coal.14 As a result coal
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permeability significantly decreases, creating reduced gas flow rates in the coal mass.15 For N2
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injection, since coal seam adsorbed much less N2 compared with CH4 at the same conditions,
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injection of N2 into coal seam which remains as gas form in coal fracture system can greatly
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reduce partial pressure of methane and thus significantly enhance methane recovery rate.16
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However, early breakthrough of N2 happens easily during N2 injection production for its non-
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sorption characteristic which requires additional process of purifying the produced CBM, and
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hence, the cost of CBM production increases.8 A better understanding of the complete gas
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transport process in ECBM recovery, including gas adsorption/desorption, diffusion and flow in
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coal, is therefore of great importance for the achievement of satisfactory ECBM recovery results.
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This paper reviews the process of CBM recovery and associated factors, including: 1) general
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properties of coal and CBM, 2) gas sorption in coal, 3) gas diffusion in coal and 4) gas flow in
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coal fractures and cleats relevant to ECBM recovery.
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2. General properties of coal and CBM
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2.1 Coal composition and structure
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2.1.1 Coal composition
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Coal can be defined as an accumulated and compacted result of the biological and geological
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alterations that occur in ancient plant materials. Coal is therefore an aggregate substance formed
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with organic and inorganic materials, and the organic materials are primarily ancient plant
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remains and the inorganic materials include mineral matter, mainly clay minerals, quartz,
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carbonates, sulphides, and sulfates, and some other substances in very small proportions.17
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Although carbon, oxygen, hydrogen, sulphur and nitrogen are the main compositions of coal, it
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does not have fixed chemical compositions due to its heterogeneous nature. Table 1 gives a
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summary of composition of various coals from some coal basins worldwide. Carbon takes the
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majority part of coal composition, normally higher than 50%, followed by oxygen, hydrogen,
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nitrogen and sulphur. Some coals in which carbon content is relative low, the amount of oxygen
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increases dramatically. The amount of carbon contained in coal is one of the most decisive
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indicators of coal maturity, coal is thus categorised as into three main ranks according to its
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carbon content as lignite, bituminous and anthracite. Carbon content for lignite, bituminous coals
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and anthracite are generally less than 70%, 80% ~90% and more than 90%, respectively.18 The
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components in coal, excluded moisture, which can be evaporated at high temperature in the
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absence of air are volatile matter. The coalification process is accompanied by an increase in
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carbon content and a reduction of volatile matter. As rank increases, oxygen contents decrease
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drastically at bituminous stages since a markedly depletion of oxygen functionalities take place
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during coalification process from lignite to bituminous.19
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Table 1. Proximate and ultimate analysis of coals from some major coal basins worldwide Coal No.
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Origin
US
Australia
Mexico
South African
Spain
China
Rank
Low
Low
Medium
Medium
High
High
Ash V.M. F.C. C Ultimate H analysis O (wt%, N daf) S Reference
5.58 44.83 45.69 61.20 3.97 27.98 1.01 0.25
4.80 48.80 46.50 65.40 4.50 24.40 0.47 0.44
21.10 23.70 55.20 86.20 5.50 5.90 1.60 0.80
15.00 29.90 55.10 81.50 5.00 10.50 2.10 0.90
14.20 3.60 82.20 94.70 1.60 2.00 1.00 0.70
8.55 7.58 81.38 92.27 3.52 1.92 1.33 0.79
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Proximate analysis (wt%,db)
db: dry basis; daf: dry and ash free basis; V.M.: volatile matter; F.C.: fixed carbon
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2.1.2 Coal structure
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One particular characteristic of the gas transportation process in coal is that it is greatly
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dependent on its microstructure. Unlike other conventional natural gas reservoirs, in which
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mostly gas exists as free gas in the pores of the reservoir rock, almost all the coal seam gas (CSG)
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is physically adsorbed in the pores of the coal matrix.24 Since the coal seam acts as both source
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rock and reservoir rock for CBM, precise understanding of coal’s structure is of great importance
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for gas transportation through it.
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According to Levine25, on conceptual basis, coal porosity is the volume fraction of coal
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occupied by “void spaces”; while on operational basis, coal porosity is the volume fraction of
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coal that can be employed by a certain fluid and the results are different with different kinds of
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fluids. Coal has a dual porosity system which is composed of cleat-porosity and matrix-porosity
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(based on the pore size). As depicted in Figure 2, cleats are natural fractures which exist in any
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coal seam, and face cleats and butt cleats form the cleat system. The former type is continuous 8
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throughout the reservoir while the shorter butt cleat is discontinuous and normally perpendicular
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to the face cleat and terminates at a face cleat. This cleat system makes a significant contribution
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to the flow characteristics of the seam, its permeability and porosity. The cleat-porosity of the
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pore space in the fracture network is shaped by the cleat system, generally has diameter larger
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than 50nm.26 Matrix-porosity varies greatly in terms of pore size since some coals are highly
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heterogeneous, matrix-porosity can be subdivided into macro-pores (diameter >50nm), meso-
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pores ( 2nm
