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Influences of CO2 injection into deep coal seams: A review Mandadige Samintha Anne Perera Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01740 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Cover Page

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Manuscript Title:

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Influences of CO2 injection into deep coal seams: A review

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Authors’ names:

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M.S.A. Perera1, 2

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1

Department of Infrastructure Engineering, Room 209B, The University of Melbourne,

Building 175, Melbourne, Victoria 3010, Australia.

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2

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Monash University, Building 60, Melbourne, Victoria 3800, Australia.

Adjunct Researcher, Deep Earth Energy Laboratory, Department of Civil Engineering,

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Corresponding author:

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Dr Mandadige Samintha Anne Perera

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Department of Infrastructure Engineering,

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The University of Melbourne,

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Room 209 B, Building 175, Melbourne,

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Victoria 3010, Australia.

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Phone: +61-3-9035 8649

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Fax: +61-3-9035 8649

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E-mail: [email protected] 1 ACS Paragon Plus Environment

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Abstract

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For nearly 20 years, CO2 has been injected into coal seams to enhance the recovery of

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methane in a process known as enhanced coal bed methane (ECBM). However, there is a

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huge complexity associated with this process mainly due to the generating complex coal

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chemico-physical structure re-arrangement. This review paper aims to comprehensively

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discuss two main influencing factors upon CO2 injection in deep coal seams: 1) mobilization

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of hydrocarbon and, 2) coal matrix swelling.

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CO2 injection into deep coal seams may remove available polycyclic aromatic

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hydrocarbons (PAHs) from the coal matrix and mobilize them in the coal seam. The amount

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of hydrocarbon that is mobilized from the coal matrix by the injected CO2 is dependent on

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coal rank, maceral content, type of available some hydrocarbons in the coal mass (both

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dissolving and non-dissolving types) and phase state of the injected CO2 in the seam.

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Supercritical CO2 has greater solvent ability and therefore has ability to extract a greater

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percentage of hydrocarbon from the coal matrix. This mobilization of the organic constituents

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of the coal matrix by the injected CO2 causes many environmental issues. For examples,

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PAHs exist in high-volatile bituminous coal are harmful to biota and environment, even at

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relatively low concentrations. On the other hand, adsorption of the injected CO2 into the coal

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mass causes it to be swelled leading significant alternations in its internal coal mass structure,

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resulting in great modifications in its flow and strength properties. This CO2 adsorption

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induced coal matrix swelling process is reduced with increasing temperature, exhibits

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inverted-U shaped variation with coal rank and largely dependent on the pressure and the

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physical state of the injected CO2, where supercritical CO2 creates much greater swelling

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effect compared to gas/ liquid CO2 due to its higher chemical potential. Potential coal seams

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for CO2 sequestration process are available at extremely deep locations and there is a high

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possibility of phase change from gas/liquid to supercritical state and thus the likely have high

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swelling rates.

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Highlights: CO2 injection, deep coal seams, ECBM, hydrocarbon dissolution,

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swelling

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1. Introduction

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Sequestration of CO2 in coal seams is a process by which a bore hole is drilled and an

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injection probe is inserted (injection well), which then allows CO2 to be pumped into the coal

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seam. Coal seams that are located at extensive depths and uneconomic for coal mining can be

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used for this purpose (1, 2) due to many unique favourable properties of them. Importantly, in

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coal 98% of CO2 is stored in an adsorbed phase (adsorption is the main gas storing

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mechanism in them) and only a small portion of CO2 is stored as free gas and therefore, CO2

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exist in a more stable form in deep coal seams with reduced risk of CO2 back-migration into

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the atmosphere 3. Therefore, theoretically the stored CO2 should permanently and securely

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trapped inside the coal seam forever, given the fact that the coal seam is never mined or

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disturbed. According to the prediction of Gunter et al. 1, the securely holding time of the

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stored CO2 inside a deep coal seam is as long as order of 105-106 years.

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In addition, undisturbed deep coal seams contain huge amount of inbuilt methane.

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According to IEA4, one tonne of such coal may contain up to around 25m3 of methane.

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Therefore, the ability of offsetting the CO2 sequestration cost by a valuable energy by-

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product as methane (CH4) is also an important advantage associated with this process. This

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happens in the way that coal mass has higher affinity to CO2 compared to CH4 and therefore,

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while adsorbing injecting CO2 into the coal matrix, the available CH4 is released from it

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.This phenomenon raised the concept of enhanced coal bed methane recovery. The potential 3 ACS Paragon Plus Environment

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of carbon dioxide sequestration process to enhance the coal bed methane production has been

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clearly explained by Gentzis6 and is shown in Fig 1. According to them, CO2 injection has an

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appreciable capability to enhance the coal bed methane production. However, the ratio

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between storing CO2 to producing methane is coal rank dependent, where it can be around

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2:1 for bituminous coals7 and as high as 10:1 for low ranked lignite8. Importantly, this

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producing methane is generally of high purity and can be easily used for power generation or

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heating. Though, methane also a greenhouse gas, substitution of natural gas like methane for

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traditionally using energy production means such as coal would largely reduce the

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greenhouse gas emission to the atmosphere due to the contained less carbon percentage.

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Therefore coalbed methane offer greener energy producing means.

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Figure 1. Potential of CO2 to enhance the coal bed methane production 6

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Coal seams have potential to store a substantial amount of gases due to its large

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surface area associated 2. This is mainly due to the dual porosity system available in coal that

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composed of micro pores (small pores inside aggregate) and macro pores (pore space among

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the aggregates). This micro pore system act as a storage house for gases as moisture cannot

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reach them. Therefore, a coal seam has potential to store about 3-7 times higher amount of

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gas compared to a conventional reservoir (Harpalani and Schraufnagel 9). Moreover, 4 ACS Paragon Plus Environment

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according to Harpalani and Schraufnagel 9, unlike conventional reservoirs, there is a

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considerable amount of water available in natural coal seams and therefore, its permeability

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widely varies with pressure reduction, where the maximum production point may relevant to

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a very low pressure condition and that can be easily achieved by dewatering.

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According IEA reports10, CO2 storage capacity in the most favourable coal basins

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available throughout the world is around 15 Gtonnes and the cost associated with CO2 storage

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in them could be significantly reduced through methane production in the best seams

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available among these seams. According to them, the total cost associated with CO2 capture,

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transport (to 300km) and storage in such best sites is around $30-50/tonne lesser than in less

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factorable seams, though around 20-50 folds greater amount of CO2 can be stored in them.

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According to the prediction of Advanced Resources International11, it can be stored more

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than 220 Gt of CO2 in worldwide deep coal seams for a reasonable cost. Table 1 summarises

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some worldwide CO2-ECBM projects.

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Table 1. Some worldwide CO2-ECBM projects12 Location

Year of commencement

Total CO2 injected (t)

Coal seam depth (m)

Allison unit, New Mexico, USA Recopol, Poland Northern Appalachian basin, USA Qinshui Basin, China Yubari, Japan Central Appalachian basin, USA Pump Canyon, USA APP project, China Qinshui Basin, china

1995

277,000

950

2001 2003

760 20,000

1050-1090 550

2004

192

478

2004 2009

884 907

890 490-670

2009

16,700

910

2011 2014

460 1000

560 >1000


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2. What happens when CO2 is injected to a deep coal seam?

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Though, deep coal seams are appreciable sinks for CO2 injection, it creates numerous issues

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in deep coal seams. Upon injection of CO2 into a deep coal seam, the injected CO2 will flow

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through its natural cleat system and eventually adsorbs into cleat walls and then diffuses into

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the coal matrix with replacing the available methane from the seam. Though the process

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apparently simple, this causes the coal mass pore structure and chemico-physico properties to

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be greatly altered 13. This is because, carbon dioxide not only adsorbs on the coal mass cleat 14, 15.

10

walls, but also dissolves in the coal matrix as an organic liquid

Both of which alter the

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coal matrix physical structure while relaxing and rearranging its macromolecular structure.

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Such processes may also create chemical changes by removing available polycyclic aromatic

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hydrocarbons (PAHs) from the coal matrix and mobilize them in the coal seam (Kolak and

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Burruss, 2006). A precise understanding of this alteration is necessary for an effective CO2

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sequestration project, and such understanding cannot be guaranteed without a comprehensive

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overview of the factors affecting the physical and chemical alteration process in deep coal

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seams, upon CO2 injection. Two main influences are comprehensively discussed in this

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chapter: 1) mobilization of hydrocarbon and, 2) coal matrix swelling, upon CO2 injection.

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2.1. Mobilization of hydrocarbon upon CO2 injection

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According to Larsen (2004), coal mass initial pore structure modification occurs since CO2

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adsorption is much greater than that occurs upon methane adsorption. The situation is more

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critical, when consider its highly chemically reactive supercritical state expectable in deep

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coal seams (preferable coal seams for CO2 sequestration should be existed at very deep

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underground (> 0.8km depth) for a secure and effective CO2 storage process and at such 6 ACS Paragon Plus Environment

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depths pressure and temperature conditions exceed the critical condition of CO2). This

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supercritical CO2 has intermediate physical properties between gas and liquid CO2.

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Supercritical CO2 is found to have greater solvent ability and therefore has ability to mobilize

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or extract a greater percentage of hydrocarbon from the coal matrix. This leads supercritical

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CO2 to have greater sorption ability to coal matrix and thus create a greater internal structure

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modification 16.

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If the coal hydrocarbon extraction process by CO2 is considered, according to Kolak

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and Burruss17, both the types and the quantities of extractable hydrocarbon by CO2,

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significantly depend on coal rank, because chemico-physical structure of coal is highly

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dependent on the degree of coalification it’s undergone. Coal can be basically classified

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according to its rank, which is the degree of coalification undergone during the

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transformation18. The rank of the coal is basically decided by its carbon content, where the

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coal that has undergone greater degree of coalification and highly matured has greater

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amount of carbon and therefore, has a higher rank. Contradictory, the coal with low carbon

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content are lesser matured and therefore belongs to a lower rank. Low rank coals are lighter

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(earthier) and have higher moisture levels, volatile matter content and porosity due to their

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lower level of maturity and also the available low carbon content of these coals leads them to

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have lower energy content compared to well matured high rank coal with more glassy

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appearance, and dense nature and lower moisture content. The variation of coal physical

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properties with its rank is shown in Table.2.

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Table 2. Variation of coal properties with rank 5

Property

Lignite

Sub-Bituminous

Bituminous

Anthracite


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Moisture (db%)

50-70

25-30

5-10

2-5

Carbon (db%)

60-75

75-80

80-90

90-95

Volatile matter (db%)

45-55

40-45

20-40

5-7

True relative density (gcm-3)

0.5-0.72

0.8-1.35

1.25-1.5

1.36-1.8

Heat content (Btu/lb)

4000-8300

8500-13000

11000-15000

13000-15000

Oxygen content (db%)

20-25

15-20

10-15

3-5

1

2

Basically there are four main types of coal rank: lignite, sub bituminous, bituminous

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(can be high, medium and low volatile bituminous) and anthracite according to the low to

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high rank sequence (Fig.2), where the lignite and sub bituminous coals basically belong to

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low rank group and, bituminous and anthracite coal belong to high rank group. In general, the

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deeper the seam the higher the rank of the coal due to higher possibility to undergone a

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greater degree of coalification. However, coal classification should also consider its macerals

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(organic material) content as same rank coal may have different amount of macerals,

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resulting in different properties. According to Dryden et al. (1963), coal can be subdivided

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into three basic groups according to its macerals content: 1) intertinite, 2) liptinite and, 3)

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vitrinite, where vitrinite is the most abundant type in coal.

12

13


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3

Anthracite Bituminous Sub bituminous

4 Peat

5

Lignite

Plants 6

7

Figure 2. Coalification process

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Coal matrix is a three dimensional solid network with aromatic units cross-linked by

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alkyl chains and, having subjected to a greater thermal maturation, there is a greater

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possibility to form bitumen or free hydrocarbons from the coal mass organic matter.

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However, high-volatile bituminous ranked coal is found to be contain the maximum amount

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of bitumen as further increasing of rank causes the available bitumen to decompose into

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methane. The amount of hydrocarbon that is mobilized from the coal matrix by the injected

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CO2 is also therefore dependent on coal rank as the amount of available bitumen and free

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hydrocarbon in the coal mass is rank dependent. This implies that the degree of coal matrix

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modification creating by the injected CO2 through mobilizing the coal mass free hydrocarbon

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is clearly rank dependent, where such influence is maximum for high-volatile bituminous 9 ACS Paragon Plus Environment

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ranked coal. Further, coal-CO2 bonding in lignite low ranked coal are much tighter than that

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in higher rank coals due to the greater amount of oxygen exist in low rank coal. This is

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because, among all the functional groups in coal, carboxylic group in coal ha greatest

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preference to adsorb CO2 and this particular function group is abundant in the presence of

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greater oxygen content in low rank coals 19.

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However, rank is not the only influencing factor that controls the amount of

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hydrocarbon extraction from coal bitumen by CO2 and some other factors, such as coal

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maceral content, solubility potential of the existing hydrocarbon in CO2, diffusion rate and

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steric factors also significantly affect. For example, coals with greater amount of lipnite are

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subjected to a greater hydrocarbon mobilization during CO2 injection as liptinite is itself kind

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of a hydrogen-rich maceral

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naphthalenes hydrocarbon is found to have higher solubility in CO2 compared to

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phenanthrene hydrocarbon and therefore, coals with greater amount of naphthalenes

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hydrocarbon are subjected to a greater hydrocarbon mobilization effect with CO2 injection.

20.

If the solubility potential influence is then considered,

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The other important fact is the obstructing of coal matrix by none CO2 dissolving

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hydrocarbons such as asphaltenes, which avoids releasing of soluble hydrocarbon from the

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coal matrix during the CO2 injection. For an example, though n-alkanes are dissolvable

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hydrocarbons in CO2, those are often physically trapped within asphaltenes, polar compounds

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with nitrogen, sulphur, and oxygen, and a prominent constituent of bitumen that insoluble in

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CO2. Since CO2 fails to disturb these strong polar links exist in bitumen, existence of these

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hydrocarbons in the bitumen obstructs meeting of CO2 with soluble hydrocarbon existing in

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bitumen and therefore, negatively affects the hydrocarbon extraction process during CO2

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sequestration in deep coal seams. In addition, according to McHugh and Krukonis 21, existing

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of moisture also significantly contributes to enhance this coal matrix dissolution process in

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CO2 by acting as a cosolvent modifier.


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Mobilization of the organic constituents of the coal matrix by the injected CO2 during

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the CO2 sequestration process may cause many environmental issues. For example, some coal

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types, such as high-volatile bituminous coals contain large amount of hydrocarbons,

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including polycyclic aromatic hydrocarbons (PAHs) that are harmful to biota and found to

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create harmful effects to environment even at relatively low concentrations. According to

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Kolak and Burruss

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phenanthrene homologous series are extensively mobilized PAHs in CO2, the more toxic

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PAHs (e.g., the five-ringed members such as chrysene) have high possibility to remain in the

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coal mass during the CO2- coal mass interaction.

10

17,

though toxic two- and three-ringed members of the naphthalene and

Importantly, coal-CO2 interaction not only causes coal hydrocarbon mobilization, it 22

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also leads to form new carbon structures in the coal matrix. For example, Gathitu et al.

12

conducted a microscopic examination (SEM) and found that interaction of supercritical CO2

13

with coal mass form new carbon structures in coal similar to collinite (a type of vitrinite

14

maceral in coal) (see Fig.3).

15

16 17

18

(a) Raw coal

(b) After interacting with supercritical CO2 at 21 MPa

Figure 3. Coal-CO2 interaction creating new coal carbon structures (1:1000 scale) 22

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2.2. Coal matrix swelling occurs upon CO2 injection 11 ACS Paragon Plus Environment

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Apart from hydrocarbon mobilization, swelling of coal matrix upon CO2 adsorption is

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another critical issue of CO2 injection process into deep coal seams. This coal matrix swelling

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occurs in the way that CO2 dissolution in coal upon adsorption into coal matrix forms some

4

free volume, in which the tight macromolecular structure of the coal mass starts to relax or

5

swell, where this swelling mainly occurs in the coal’s maceral group (vitrites, liptite, and

6

clarite) and particularly vitrites offers the greatest contribution

7

creates much greater influence on coal mass pore structure and its flow-mechanical properties

8

compared to hydrocarbon mobilization and thus will be comprehensively discussed in this

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section.

13.

This coal matrix swelling

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2.2.1. What is coal matrix swelling?

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Adsorption of CO2 molecules into coal matrix causes dissolution of CO2 in the coal matrix

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with creating a free volume inside it, in which the tight macromolecular structure of the coal

14

mass starts to relax with inducing a strain in-between the adsorbing CO2 molecules and the

15

coal matrix surface (Fig.4), which is commonly known as coal matrix swelling

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swelling of the coal matrix result in localized stress and strain variations in a coalbed

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confined under overburden pressure

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percentage of the coal mass total volume, this may significantly reduce the available coal

19

mass pore space for gas movement, because coal generally has extremely low total porosity

20

values

21

significantly changing its permeability and limiting the development and potential field

22

application of CO2 sequestration process in deep coal seams

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findings, coal matrix swelling causing significant volumetric strain in coal has caused quite

24

unpredictable CO2 storage capacities in deep coal seams

25

experimentally measured the change in volumetric strain of high rank and low rank coal

18, 23

. This

24

. Although the amount of swelling is just a small

25-27

. The swelling is found to mainly occur around the wellbore area while

28.

According to the existing

5, 29, 30

. Mazumder et al.31


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samples due to CO2 sorption. They found that with CO2, the high rank coal showed a

2

maximum volumetric strain of 1.48% and a matrix swelling co-efficient of 1.77x10-4 MPa-1.

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The low rank coal exhibited a higher strain of 1.6% and matrix swelling co-efficient of

4

8.98x10-5 MPa-1. Furthermore, around 50% reduction in CO2 injection capacity has been

5

recorded in the San Juan Basin CO2-ECBM field project within the first two years of CO2

6

injection, where coal matrix swelling was the main causative factor 28. The coal swelling also

7

affects the accuracy of the laboratory test results, such as gas adsorption measurements

8

and according to Reucroft and Sethuraman

9

modification leads to create very long CO2 equilibration times in coal seam (up to 200 hours).

10

Coal matrix swelling therefore certainly causes significant unpredictability in CO2

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sequestration process in deep coal seams.

24

32

, swelling induced coal’s pore structure

12

13 14

Figure 4. CO2 adsorption creating matrix swelling process in coal

15 16

However, coal swelling causes to enhance the coal mass pore pressure through available

17

pore space reduction and therefore amount of predictable swelling in coal is the net effect of

18

coal matrix volumetric expansion due to CO2 adsorption and matrix compression due to pore

19

pressure 30. This coal pore structure alternation occurs with swelling process has been widely 13 ACS Paragon Plus Environment

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researched by worldwide researchers, basically through micro scale analysis as SEM. For

2

example, a clear reduction in Victorian brown coal’s matrix pore space with CO2 adsorption

3

induced coal matrix swelling has been observed by Perera et al.33 and is shown below.

4 5 6 7

0.4 μm

8 2.5 μm

9

0.38 μm

2.0 μm 1.6 μm 0.2 μm

10

11

(a) Natural sample 12 13

(b) Super-critical CO2 saturated sample

Figure 5. Shrinking of coal mass pore structure with swelling 33

14

15

Since coal matrix swelling initiates with the CO2 adsorption process, the degree of the

16

swelling occurred should be directly correlated with the CO2 adsorption capacity in coal. As

17

shown in Fig.6, a proportionally changing coal matrix swelling nature in coal with the CO2

18

adsorption capacity in coal has been observed by many researchers

19

that CO2 adsorption is the causative fact for coal matrix swelling and therefore a greater

20

swelling can be expected under higher degree of CO2 adsorption, greater degree of CO2

21

adsorption also causes to induce micro cracks in the coal mass with offering more locus for

22

adsorption process, which also contribute to enhance the coal matrix swelling process

23

Furthermore, the swelling potential is also varied with the rank of coal as discussed in section

34 30

. In spite of the fact

16, 35

.


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2.2.2 of this review. Therefore it is apparent from these two relationships (i.e. swelling vs.

2

CO2 adsorption capacity (Fig. 6) and swelling vs. coal rank (Fig.9)) that the CO2 adsorption

3

capacity can also be varied with the coal rank.

4

4

Swelling ( %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3 2 1 0 0

5

10

15

CO2 Adsorption Capacity (%)

5 6

Figure 6. The change of coal matrix swelling with absolute CO2 adsorption capacity in coal

7

30

.

8 9

According to Day et al.34, the swelling is therefore can be predicted as a function of

10

CO2 adsorption capacity in coal, regardless of type of gas adsorption and the type of the

11

adsorbing coal mass (Eq.[1]). It is clear from both graph and the equation, that the

12

relationship is not linear. At high gas sorption, the effect on swelling is proportionally greater

13

than at low gas sorption. This could be related to the weakening of the coal mass. However,

14

the relationship between volumetric swelling percentage and the volume of absolute

15

adsorption cannot be simply predicted by equation 01, because this relationship can also be

16

influenced by different other factors, such as temperature, injection pressure, injection time,

17

in-situ stresses as well as the pore fluid properties presented in the seam. Therefore, further


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work to investigate the influence of these factors upon coal matrix swelling and CO2

2

adsorption is warranted.

3

4

Vs = −0.0037 + 0.1596Vabs + 0.0101Vabs

2

[1]

5

6

where, Vs is the volumetric swelling percentage and Vabs

7

adsorption.

is the volume of absolute

8

However, with the adsorption of CO2 into the coal mass its available methane starts to

9

desorb with shrinking the coal mass, which is particularly important in the CO2 enhanced 36

10

coal bed methane production process. According to Harpalani and Chen

11

coal matrix shrinkage occurs during coal mass gas desorption is directly proportional to the

12

desorbed gas volume (Fig.7).

Volumetric Strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

, this amount of

0.0016 0.0012 0.0008 0.0004 0 0

2

4

6

8

Desorbed Volume (ml/g) 13

14

Figure 7. Gas desorption casuing coal matrix shrinkage 36.

15


Page 17 of 38

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Energy & Fuels

1

Based on this plot, Harpalani and Chen 36 suggested the following relation to calculate the gas

2

desorption induced volumetric strain in coal;

3

4

 V p   p 

ε v = C  L  PL +

[2]

5

6

where, ε v is the volumetric strain of the coal matrix, C is a coal mass properties dependent

7

constant, p is the pore pressure, and VL and PL are the Langmuir volume and pressure.

8 9

Several effective factors have been identified for CO2 injection induced coal matrix

10

swelling. The degree of coal matrix swelling that occurs in any coal mass with CO2

11

adsorption is primarily dependent on: 1) coal seam properties and, 2) injecting CO2

12

properties. Coal seam properties can be varied according to the geological characteristics of

13

the particular coal seam, whereas injecting CO2 properties can be varied according to gas

14

type, injection pressure and phase.

15

2.2.2. Seam properties, influencing the coal matrix swelling

16

Coal rank (degree of maturity), temperature and depth have significant influences on coal

17

matrix swelling process.

18

a) Coal rank

19

As mentioned earlier, gas adsorption into any coal mass mainly occurs along its natural

20

cleat system and therefore the associated swelling also occurs along these cleat walls. This

21

is confirm by the greater degree of swelling occurs in coal mass perpendicular to its

22

bedding planes compared to the along the bedding planes

15, 37, 38.

Though this may partially 17

ACS Paragon Plus Environment

Energy & Fuels

1

relates with the anisotropic nature of coal structure that is more highly cross-linked along the

2

bedding plane 39, the nature of the CO2 adsorption (along the cleat walls) also highly affects. This

3

gas adsorption nature along the coal mass cleat system exhibits the direct influence creating

4

by coal mass cleat system on it swelling process, where coal with high cleat density subject

5

to a greater degree of swelling. The natural cleat system in coal form during the

6

coalification process

7

compared to low rank coals as they have been subjected to a greater degree of maturation 16,

8

29

9

rank coal 5. However, according to Lama and Bodzinoy

40

and therefore high ranked coals have properly inbuilt cleat system

and therefore, a higher cleat density can be expected in high rank coal compared to low 41

though cleat density increases

10

with increasing rank from sub-bituminous to bituminous coal, the cleat density starts to

11

decrease afterward up to anthracite coal (Fig.8), and therefore, accordingly the coal matrix

12

swelling ability change with offering a highest coal matrix swelling ability in bituminous coal

13

29

.

14

2.5

Cleat Density (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

2 1.5 1 0.5 0 0

15 16

Anthracite 5 10

15

Bituminous25 20

30

35

Sub bituminous 40 45

Increasing coal rank

Figure 8. Variation of coal’s cleat density with its rank 41

17


Page 19 of 38

42

1

Walker et al.

have observed a similar shape of coal matrix swelling variation with coal

2

rank, in which the amount of swelling increases with increasing coal rank from lignite to

3

bituminous coal and decreases afterward up to anthracite (Fig.9).

4

0.16

Swelling (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.12 0.08 0.04 0 70 Lignite 75

80Bituminous 85

90 Anthracite 95

Coal Rank

5 6

Figure 9. Variation of coal matrix swelling with its rank 42

7

8

This trend has been confirmed by Laxminarayan et al.

43

with obtaining volumetric

9

strain data of sub-bituminous to medium volatile bituminous coal at 0.6 MPa, which clearly

10

exhibited an increasing trend of swelling with increasing rank from sub-bituminous to

11

medium volatile bituminous, probably due to the associated increased cleat density (Fig.8).

12

In 2009, Durucan and Shi

13

coal rank for both CO2 and CH4 adsorption using a range of coal samples with different

14

rank (56.4 – 90.9% fixed carbon content).

44

also showed an increasing trend of coal matrix swelling with

15 16

b) Temperature

17

The adsorbed gas molecules in the coal matrix tend to release at increased temperatures due

18

to the associated increase in their kinetic energy

45

and therefore, increasing of temperature 19

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Page 20 of 38

1

generally causes to reduce the gas adsorption capacity in coal matrix, resulting in lesser

2

degree of swelling under high pressure condition. According to Bae and Bhatia

3

Ranathunga et al. 47, this temperature induced sorption capacity reduction is much greater for

4

super-critical CO2 compared to gas CO2 (Fig.10).

46

and

5

The greater reduction of sorption capacity of CO2 with temperature at higher pressure

6

is related with the physical properties of CO2, including its viscosity and compressibility. In

7

general, gas viscosity increases with temperature and contradictory, liquid viscosity reduces

8

with increases temperature. This is due to the existing much closer and strongly bonded

9

molecules in liquids compared to gas, where these bonding start to get weaken with

10

increasing kinetic energy of the liquid molecules under increased temperature, resulting in

11

reduced viscosity in liquid, where the effect increases with increasing temperature. However,

12

the situation is quite different under gas condition, in which increased kinetic energy of freely

13

moving gas molecules causes to enhance intermolecular collisions frequency and momentum

14

transfer rate both of which causes to reduce the gas phase viscosity with increasing

15

temperature

16

temperature have been shown by many researchers

17

reduction trend of supercritical CO2 viscosity with increasing temperature and a viscosity

18

increasing trend of gaseous CO2 with increasing temperature. The reduction of supercritical

19

CO2 viscosity and associated increased compressibility causes to enhance the slip flow ability

20

of CO2 molecules through the coal mass pores that eventually reduces the contact time with

21

the coal matrix and therefore reduces the adsorption capacity.

48, 49

. These unique behaviours of gas and liquid viscosity upon increased 4, 50

, where Kestin et al. 4 showed a clear

22

However, increasing of temperature may also cause to create thermal cracks in the

23

coal matrix, under such situation due to increased locus for gas adsorption, the adsorption

24

capacity may enhance with increasing temperature

25

conditions. In the meantime, under such high temperature conditions, coal matrix may subject

45, 47

, especially at very high temperature


Page 21 of 38

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Energy & Fuels

1

to thermal expansion with reducing the pore space for gas movement and therefore reducing

2

the sorption capacity inside the coal matrix. However, the expectable temperature condition

3

of potential coal seams for CO2 sequestration process lies within a moderate level and

4

therefore only a small influences creating by thermal cracks and thermal expansions on their

5

adsorption capacity can be expected. This is because, potential coal seams for CO2

6

sequestration exist in between 0.8 km to 2 km depths

7

gradient is in the range of 10.9-36.2 oC/km 52. For examples, only 50 oC, 120 oC, 52 oC, and

8

48 oC temperatures have been recorded in the 2 km deep Black Warrior Basin, Alabama 51, 3

9

km deep Altmark natural gas field in Germany

10

5, 51,

and the general geothermal

53

, and 1km deep San Juan Basin, New

Mexico 54 and 1 km deep Campine Basin, Belgium 55.

11

When consider all of these facts, in overall, a reduction trend of CO2 adsorption

12

capacity in coal with increasing temperature can be expected in the potential deep coal seams.

13

Since, CO2 adsorption induced coal matrix swelling is directly proportional to its sorption

14

capacity in coal matrix, a greater reduction of swelling effect with increasing temperature can

15

be expected in supercritical, the highly expectable nature of CO2 in deep coal seams,

16

compared to gas CO2

17

increasing temperature has been shown by Qu 55 through his numerical simulation work.

45

. A clear reduction of CO2 adsorption induced swelling effect with

18


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

1

Figure 10. Variation of coal matrix swelling with temperature 56

2

3

c) Depth

4

Increasing of depth causes the effective stresses applying on the coal mas to be increased that

5

causes the coal pore space to be shrunk, resulting in reduced adsorption capacity

6

However, deeper seams normally have higher coal rank due to associated greater degree of

7

buried pressure and temperature in them and also highly possible the greater coalification

8

times

9

fully understanding of the coal seam depth effect on its swelling ability with CO2 adsorption.

10

However, to date no such detail study has been reported except the study of Jasinge et al. 60,

11

which shows an increment of swelling effect up to 6 MPa confinement and reduction of that

12

with further increasing of confinement (Fig.11). Though the latter part of the curves, the

13

reduction of swelling effect with confinement is due to the associated reduced pore space and

14

adsorption ability exits under high confinements, the initial swelling enhancement with

15

increasing confinement (6 to 8 MPa confining pressure increment) is questionable and

16

therefore needs further studies prior to coming to a final conclusion on depth effect on coal

17

matrix swelling.

59.

57, 58.

Therefore, the combined effects of all of these are required to be considered for a


1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

3.4 MPa CO2 Injection Pressure 3.0 MPa CO2 Injection Pressure 2.4 MPa CO2 Injection Pressure

0

2

4 6 8 Confining Pressure (MPa)

10

12

1

Figure 11. Variation of coal matrix swelling with depth 60

2

3

2.2.3. Injecting gas properties, influencing the coal matrix swelling

4

a) Gas type

5

As shown in Fig.12, CO2 has greater adsorption capacity in coal compared many other gas as

6

N2 and CH4 that leads to create a greater swelling effect compared other gases in coal.

7

According to the existing findings on low pore pressure conditions (< 5 MPa), coal mass

8

swelling due to CO2 adsorption is more than 5 times higher than CH4 and 10 times that of N2

9

61-64

.

0.025

Swelling Strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Swelling Percentage (%)

Page 23 of 38

CO2 CH4

0.02

N2

0.015 0.01 0.005 0 0

2

4

6

Adsorbing Gas Pressure (MPa) 10 23 ACS Paragon Plus Environment

Energy & Fuels

Figure 12. Effect of adsorbing gas type on coal matrix swelling 64

1

2

3 4

This greater adsorption capacity of CO2 in coal has been shown for high gas adsorption pressures by Perera et al. 26 and Ranathunga, et al. 64 is shown in Fig.13.

5

1 Swelling (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

0.1 0.01 0.001 0.0001

0.00001 2

4

6 8 Gas Pressure (MPa)

10

12

6 7

Figure 13. Gas type effect on coal matrix swelling at high gas adsorption pressures 65

8 9 10

b) CO2 Injection pressure and phase

11

As shown in Fig. 14, CO2 adsorption capacity in coal increases with increasing injection

12

pressure due to associated pore space expansion and corresponding potential adsorption area

13

increment in coal matrix

14

coal matrix swelling

15

increasing injection pressure and has been clearly shown by many researchers. Increment of

16

coal matrix swelling with increasing injection pressure has been shown by Jasinge et al. 27 for

17

up to 1.7 MPa low injection pressure conditions using lignite coal, Vishal et al.66 for up to 2

46

. Since this adsorption capacity is proportional to the amount of

34

, a similar increment in coal matrix swelling can be expected with


Page 25 of 38

1

MPa low injection pressure conditions using bituminous coal, Pekot and Reeves

2

5.5 MPa higher injection pressure conditions using a model study.

67

for up to

3 1.4

0.7

1.2

0.6

1 Jasinghe et al. (2011)

0.8

Vishal et al.(2013)

0.6

Swelling (%)

Swelling (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.5 0.4 0.3

0.4

0.2

0.2

0.1

0

0 1

1.5 2 CO2 Injection Pressure (MPa)

2.5

0

2 4 CO2 Injection Pressure (MPa)

6

4 5

(a) Experimental based

(b) Numerical modelling based

6 7

Figure 14. Variation of CO2 adsorption induced volumetric swelling with CO2 injection

8

pressure for low injection pressures predicted using a) experimental studies

9

numerical studies 67.

27, 66

and b)

10 11

However, preferable coal seams for CO2 sequestration process located in very deep

12

underground (> 8. Km) with high pore pressure conditions (> 8MPa) and therefore, greater

13

injection pressure are required to store CO2 in these seams. Therefore, Ranathunga, Perera,

14

Ranjith, Rathnaweera and Zhang

15

swelling for high injection pressure conditions using bituminous coal and found a similar

16

increment in coal matrix swelling behaviour in bituminous coal for up to 10 MPa CO2

17

injection pressures (Fig.15).

65

checked the injection pressure effect on coal matrix

18


Page 26 of 38

0.08 0.07 0.06 0.05 0.04 0.03 4

1

6 8 10 Injection Pressure (MPa)

12

2

Figure 15. Variation of CO2 adsorption induced volumetric swelling with CO2 injection

3

pressure for high injection pressures 65

4 5

A careful look into Fig.13 shows another important fact that is CO2 adsorption

6

induced coal matrix swelling is also highly influenced by the injecting CO2 phase condition,

7

where a clear increment in coal matrix swelling with the phase transition to supercritical

8

condition from the subcritical condition at 7.38 MPa can be seen (here test temperature was

9

38 oC > critical temperature of CO2 31.8 oC). This CO2 phase effect on coal matrix swelling 26

10

was the main interest of Perera, Ranjith, Choi and Airey

11

greater degree of swelling after supercritical CO2 adsorption compared sub critical CO2

12

(Fig.16).

study and has shown a clearly

13 0.005 Super-Critical CO2 Adsorption Sub-Critical CO Adsorption 2

0.004

Radial Strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volumetric Swelling (%)

Energy & Fuels

0.003 0.002 0.001 0 0

14

4

9 Time (Hours)

14


Page 27 of 38

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1

Energy & Fuels

Figure 16. Adsorbing CO2 phase condition influence on coal matrix swelling 26

2 3

According to Massarotto et al.

68

, the highly chemically reactive nature of

4

supercritical CO2 leads to create a greater and stronger bonding with coal matrix and

5

therefore creates a greater swelling effect. This has been confirmed by the greater degree of

6

CO2 adsorption observed for supercritical state CO2 compared to sub critical state one by Bae

7

and Bhatia 46.

8

All of these show that CO2 injection into deep coal seams causes them to undergo a

9

significant coal mass physical structure modification, mainly due to the described coal mass

10

hydrocarbon mobilization, new carbon structures formation and coal matrix swelling.

11

Formation of micro cracks in coal matrix upon reaction with supercritical CO2 has been

12

clearly observed (Fig.17) by Gathitu, Chen and McClure

13

analysis and narrowing of coal mass pore space upon CO2 injection has been observed by

14

Ranathunga, Perera, Ranjith, Rathnaweera and Zhang

15

dissolution, new carbon structure formation and coal matrix swelling are the susceptive facts.

16

Such pore structure modifications may eventually end up with critical events as drastic

17

reductions in CO2 injectibility into the coal seam and violent and difficult-to-control gas

18

outbursts.

65,

22

through the conducted SEM

where CO2 induced hydro carbon

19

20


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

(a) Raw dry coal

2

Figure 17. Supercritical CO2 induced micro cracks in bituminous coal (1:1000 scale) 22.

Page 28 of 38

(b) After interacting with supercritical CO2 at 14 MPa

3 4

Overall, it is apparent that coal seams show considerable differences in behaviour

5

from normal porous gas reservoirs upon CO2 injection. Most complexities are associated with

6

strength and permeability reduction, environmental contaminations and efficiency reduction

7

in CO2 sequestration projects due to difficulties in CO2 injection. Although it is found that

8

supercritical CO2 can induce substantial alterations in coal chemico-physical structure, most

9

of the complex issues related to CO2 phase change is still remain unclear and should be

10

subjected to deep exploration in experimental and numerical sense. Studies should be

11

extended to predict the storage capacity of coal seams to understand the permeability

12

variation which occurs with the swelling/shrinkage process and the corresponding influence

13

on gas exchange dynamics and well injectivity. The preliminary estimations of CO2

14

sequestration projects should be done by carefully considering this complex behaviour and

15

these estimates can become complicated additionally due to seam inhomogeneity.

16

The economical aspects associated with CO2 injection is another important factor that

17

should be considered when planning the injection process. Project costs can be relatively high

18

when compared to conventional methods, as the process is incorporated with new

19

technologies. According to Smith et al. 69, the primary components of the CO2 sequestration

20

system are: 1) CO2 capture from flue gas, 2) preparation of CO2 for transportation by

21

compression and drying, 3) transportation of CO2 through pipelines and, 4) injection of CO2

22

into the coal seam. Studies should be extended to minimize the costs associated with this

23

processes. For example, based on current economic factors, Wong et al. 70, proposed that it is

24

better to partially deplete the coal seam of methane before injecting CO2. Also, it is found

25

that relatively significant amount of methane can be extracted with flue gas injection rather 28 ACS Paragon Plus Environment

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Energy & Fuels

1

than pure CO2 injection. This would greatly reduce the cost spent on pure CO2 separation

2

from flue gas. Therefore, it is clear that a comprehensive economical analysis along with a

3

technical assessment is also necessary for a successful CO2 sequestration project.

4 5

3. Conclusions

6

To date, injection of CO2 into deep coal seams has been identified as an effective means of

7

reducing atmospheric CO2 when considering the unique advantages associated with process

8

including, safe storage through adsorption and ability enhance the methane production

9

through it. Injection of CO2 into any coal seam generate various complex issues in them with

10

resulting significant alteration in their chemico-physical structure. Carbon dioxide injection

11

into deep coal seams may remove available polycyclic aromatic hydrocarbons from the coal

12

matrix and mobilize them in the coal seam. The amount of hydrocarbon that is mobilized

13

from the coal matrix by the injected CO2 is dependent on coal rank as the amount of available

14

bitumen and free hydrocarbon in the coal mass is ranked dependent. Clearly, coal-CO2

15

bonding in lignite low ranked coal are much tighter than that in higher rank coals due to the

16

greater amount of oxygen exist in low rank coal.

17

If the other influencing factors are considered, coals with greater amount of lipnite are

18

subjected to a greater hydrocarbon mobilization during CO2 injection. Some hydrocarbons

19

such as, naphthalenes is found to have higher solubility in CO2 compared to phenanthrene

20

and thus subject to a greater hydrocarbon mobilization upon CO2 injection. Apart from those,

21

obstructing of coal matrix by none CO2 dissolving hydrocarbons (e.g. asphaltenes) avoids

22

releasing of soluble hydrocarbon from the coal matrix during the CO2 injection. Coal-CO2

23

interaction may also form new carbon structures in the coal matrix with creating a more

24

complex situation in there. Supercritical CO2 has greater solvent ability and therefore has

25

ability to extract a greater percentage of hydrocarbon from the coal matrix.


Energy & Fuels

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Page 30 of 38

1

However, mobilization of the organic constituents of the coal matrix by the injected

2

CO2 may cause many environmental issues. For examples, polycyclic aromatic hydrocarbons

3

(PAHs) exist in high-volatile bituminous coal are harmful to biota and environment, even at

4

relatively low concentrations.

5

Adsorption of the injected CO2 into the coal mass causes it to be swelled leading

6

significant alternations in its internal coal mass structure, resulting in great modifications its

7

flow and strength properties. This CO2 adsorption induced coal matrix swelling process is

8

dependent on both seam and injected CO2 properties, where the swelling found to be reduced

9

with increasing temperature due to the reduced sorption capacity. Further, swelling exhibits

10

inverted-U shaped variation with coal maturity or rank due the corresponding variation of the

11

coal mass properties as moisture, carbon content and pore space.

12

On the other hand, the amount of swelling largely dependent on the pressure and the

13

physical state of the injected CO2, where supercritical CO2 creates much greater swelling

14

effect compared to gas/ liquid CO2 due to its higher chemical potential. Further, high pressure

15

injection of CO2 causes the swelling process to be enhanced due to the higher flow ability of

16

the injected CO2 under reduced effective stress condition at increased injection pressures.

17

However, potential coal seams for CO2 sequestration process are available at extremely deep

18

locations and there is a high possibility of phase change from gas/liquid to supercritical state

19

in the underground environment owing to changes in field conditions. This confirms the

20

likely existing of high swelling rates in deep coal seams with CO2 injection.

21

4. References

22

1.

23

their role in the mitigation of greenhouse gases from an international, national (Canada) and

24

provincial (Alberta) perspective. Appl. Energy 1998, 61, 209-227.

Gunter, W. D.; Wong, S.; Cheel, D. B.; Sjostrom, G., Large carbon dioxide sinks:


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Energy & Fuels

1

2.

Stevens, S. H.; Kuuskraa, V. A.; Gale, J. J.; Beecy, D., CO (sub 2) injection and

2

sequestration in depleted oil and gas fields and deep coal seams; worldwide potential and

3

costs. AAPG Bulletin 2000, 84, (9), 1497-1498.

4

3.

Gray, I. Reservoir engineering in coal seams; SPE 12514: 1987.

5

4.

Kestin, J.; Whitelaw, J. H.; Zien, T. F., The viscocity of carbon dioxide in the

6

neighbourhood of the critical point. Physica 1964, 30, (1), 161-181.

7

5.

8

of coal properties pertinent to carbon dioxide sequestration in coal seams: with special

9

reference to Victorian brown coals. Environmental Earth Sciences 2010, 1-13.

Perera, M. S. A.; Ranjith, P.; Choi, S.; Bouazza, A.; Kodikara, J.; Airey, D., A review

10

6.

Gentzis, T., Subsurface sequestration of carbon dioxide -- an overview from an

11

Alberta (Canada) perspective. International Journal of Coal Geology 2000, 43, (1-4), 287-

12

305.

13

7.

14

carbon dioxide emissions from power plants. In Greenhouse gas control technologies,

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Eliasson, B.; Riemer, P. W. F.; Wokaun, A., Eds. Pergamon Press, Elsevier Science: 1999; pp

16

181-187.

17

8.

18

USGS research on geologic sequestration of CO2. In Second International Forum on

19

Geologic Sequestration of CO2 in Deep, Un-mineable Coal Seams, Washington, 2003.

20

9.

21

recovery from coal seams. Geotechnical and Geological Engineering 1990, 8, (4), 369-384.

22

10.

23

Formations and Abandoned Coal Mines. CRC Press: London, UK, 2012.

24

11.

25

Coal Seq I Forum, Houston, Texas, 2002.

Byrer, C. W.; Guthrie, H. G., Coal deposits: potential geological sink for sequestering

Burruss, R., CO2 adsorption in coals as a function of rank and composition - a task in

Harpalani, S.; Schraufnagel, A., Measurement of parameters impacting methane

He, M.; e Sousa, L. R.; Elsworth, D.; Vargas Jr, E., CO2 Storage in Carboniferous

Stevens, S. H., CO2-ECBM: Insights from USA and International CBM Pilots. In


Energy & Fuels

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12.

Tanaka, Y.; Abe, M.; Sawada, Y.; Tanase, D.; Ito, T.; Kasukawa, T., Tomakomai

2

CCS Demonstration Project in Japan, 2014 Update. Energy Procedia 2014, 63, 6111-6119.

3

13.

4

Hydro-Mechanical Properties: A Review. International Journal of Energy Research 2012,

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36, (10), 1015-1031.

6

14.

7

Coal during CO2 Injection. Energy & Fuels 2003, 17, (6), 1595-1608.

8

15.

9

International Journal of Coal Geology 2004, 57, (1), 63-70.

Perera, M. S. A.; Ranjith, P. G., Carbon Dioxide Sequestration Effects on Coal’s

Karacan, C. Ã. z., Heterogeneous Sorption and Swelling in a Confined and Stressed

Larsen, J. W., The effects of dissolved CO2 on coal structure and properties.

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