Effect of Coal Rank on CO2 Adsorption Induced Coal Matrix Swelling

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Effect of coal rank on CO2 adsorption induced coal matrix swelling with different CO2 properties and reservoir depths Ashani Savinda Ranathunga, M. S.A. Perera, Pathegama Gamage Ranjith, T.D. Rathnaweera, and Xiaogang Zhang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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Energy & Fuels

Cover Page

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Manuscript Title:

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Effect of coal rank on CO2 adsorption induced coal matrix swelling with different CO2 properties

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and reservoir depths

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Authors’ names:

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A.S. Ranathunga1, M.S.A. Perera2,3*, P.G. Ranjith1, T.D. Rathnaweera1 and X.G. Zhang1

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1

Deep Earth Energy Laboratory, Department of Civil Engineering, Monash University, Building 60,

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Melbourne, Victoria, 3800, Australia.

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2

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Melbourne, Victoria, 3010, Australia.

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3

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

Department of Infrastructure Engineering, The University of Melbourne, Building 176,

Adjuct Researcher, Deep Earth Energy Laboratory, Department of Civil Engineering, Monash

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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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B209, Building 175, Melbourne, Victoria,

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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]

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1

ACS Paragon Plus Environment

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Abstract

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Although the greater adsorption potential of carbon dioxide (CO2) in coal is an appealing fact in

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relation to the long-term safe storage of CO2 in coal seams, the resulting coal structure

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modification, particularly through coal matrix swelling, adds many uncertainties to the process. To

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date, many studies have been initiated, particularly on the effects of injecting CO2 and reservoir

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properties on this swelling process and the associated reservoir permeability depletion. These

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influences are largely dependent on the maturity of the coal mass and its structure, including the

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cleat system. However, minor attention has been given to date to the effect of coal rank on CO2

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adsorption-induced coal matrix swelling and was therefore, investigated in the present study. The

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volumetric strain of the Australian brown coal samples for both CO2 and N2 under various

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confinements (tri-axial) and injections was measured at 35 0C constant temperature to investigate

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the influence of CO2 properties and reservoir depth on CO2 adsorption-induced swelling in coal and

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was compared with the results in literature to obtain the effect of coal rank.

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Based on the experimental evaluation of coal matrix swelling under various CO2 and

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reservoir conditions, super-critical CO2 adsorption leads to greater coal matrix swelling in coal, and

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the degree of swelling is dependent on reservoir depth and coal maturity. This coal matrix swelling

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reduces with increasing reservoir depth, due to the associated reduction in CO2 sorption capacity

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into coal. However, this also depends on the pore pressure conditions, and lower effective stresses

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leading to greater swelling reduction, regardless of coal rank. The potential of N2 to recover the

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swelled areas was tested by permeating the coal mass with N2 at different pressures and for different

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durations (24, 48 and 72 hours). The results show a greater potential for recovery at lower effective

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stresses for any coal type and for longer durations of N2 flooding.

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Keywords: CO2 phase and pressure, coal matrix swelling, coal rank, N2 flooding, reservoir depth

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1. Introduction

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The storage of CO2 in deep un-mineable coal seams is currently identified as a potential approach to

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the minimization of anthropogenic CO2 in the atmosphere. Numerous research studies

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therefore been commenced on this method, particularly related to the alteration of coal seam

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properties (hydro-mechanical properties) upon CO2 injection. According to these studies, CO2 has

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greater potential to be adsorbed into the coal mass compared to the existing CH4 in the coal matrix,

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which confirms the greater storage potential for CO2 in underground coal reservoirs 1.

1-19

have

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Generally, CO2 enhanced coal bed methane recovery (CO2-ECBM) is carried out in deep

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coal seams, where CO2 is in its super-critical state (beyond the critical temperature of 31.80C and

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critical pressure of 7.38 MPa) due to the high temperatures and pressure conditions at such depths

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20

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inherent chemical and physical characteristics (liquid-type densities and viscosities) 21. In the CO2

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adsorption process in coal, CO2 first flows through the macro-pores (butt and face cleats) and

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adsorbs inside the fracture walls. Then it slowly diffuses into the micro-pores (pores in the coal

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matrix) and adsorbs into the micro pores 2. However, during this CO2 adsorption into the coal

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matrix process, a strain is induced between the coal matrix and the adsorbing CO2 layer, which is

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commonly known as coal matrix swelling 3, 4, 22.

. Interestingly, super-critical CO2 has greater adsorption potential than sub-critical CO2 due to its

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Coal matrix swelling reduces the pore spaces available for gas movement, resulting in

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reduced overall permeability 23. This is evident in field-scale projects, such as the San Juan basin,

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USA and Ishikari basin, Japan, which showed around 50 to 70% reduction of CO2 injection

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capacity within the first six months to one year 24, 25. Further, according to Botnen et al. 26, the initial

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CO2 permeability of the Williston basin lignite coal seam in North Dakota was reduced by 10 times

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within the first year as a result of coal matrix swelling. In addition, Gale

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process can create significant stress on the cap rock, which may lead to a cap rock failure and hence

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possible CO2 back-migration into the atmosphere. Therefore, it is clear that the effect of coal matrix

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swelling on coal’s flow and strength characteristics is of crucial importance. 3


22

stated that this swelling

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Day et al. 5, Hol et al. 8, Karacan 9, Pan et al.

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, Perera et al.

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, Siriwardane et al.

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, and

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Zutshi and Harpalani 29 have studied this coal matrix swelling effect using tri-axial experiments on

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high-rank coal, and Jasinge

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rank coal. However, the studies carried out for low rank coal used only low injecting pressures and

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examined only gas and liquid state of CO2. Although Anggara et al.

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experiments on crushed lignite from Indonesia for super-critical CO2 (up to 10 MPa) and using coal

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blocks of 30 x 10 x 10 mm3

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swelling. The incorporation of this confining pressure effect is important for the correct

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identification of the swelling effect in field stress environments. Therefore, a knowledge gap exists

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on the effects of coal matrix swelling on permeability under super-critical conditions, especially for

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low-rank brown coal in confined environments.

30

and Balan and Gumrah

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conducted similar tri-axial tests on low-

13

conducted swelling

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, they did not consider the effect of confining pressure on coal

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The main purpose of this study is therefore to investigate the influence of CO2 phase

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condition (sub-critical and super-critical) on coal matrix swelling and the corresponding alterations

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in brown coal flow characteristics in various in-situ stress environments using Australian brown

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coal (low rank). The results were then compared with the volumetric strain results for high rank

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coal to obtain the effect of coal rank on this CO2 adsorption induced coal matrix swelling.

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Next, an effort was made to investigate the potential of N2 as a possible catalyst for CO2

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sorption induced swelling recovery in coal. According to Day et al. 5, N2 remains as a free gas

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because, it has much lower adsorption potential than CO2 in the coal matrix. This causes for a

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reduction of the partial pressure of CO2, due to the created imbalance between the sorbed and free

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gas phases 1. Hence, it results in a release of adsorbed CO2 from coal matrix which may partially

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recover the reversible swelling by the physical sorption of CO2 16. These released CO2 will migrate

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to distant locations in the coal seam and will adsorb into the coal matrix

31

. Since N2 flows

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selectively through cleats with high permeability, the CO2 adsorbed in low-permeability regions of

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the coal matrix still remains 16. For an example, according to a study done by Perera et al. 21, the co-

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injection of N2 and CO2 (40% N2 + 60% CO2) has caused for a CO2 breakthrough of around 32% of 4


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the total CO2 injected at the end of 50 years of CH4 production. Hence, the use of N2 as a possible

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catalyst for the enhancement of CO2-adsorption induced permeability reduction is favourable.

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Kiyama et al. 16 and Perera et al. 32 investigated the prescribed potential of N2 to recover the 16

32

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coal matrix swelling in high rank coals. According to Kiyama et al.

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swelling recovery is higher at lower effective stresses due to the higher N2 flow rates as it has the

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ability to desorb more CO2 molecules from the coal mass. Therefore, it would be interesting to

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check whether this phenomenon can be applied to low rank coal and was investigated by

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Ranathunga et al.

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high rank coal where lower effective stresses have higher potential of pressurised N2 to partially

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recover CO2 adsorption-induced coal matrix swelling. However, N2 was flooded only for 24 hours

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during the study by Ranathunga et al.

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flooding duration on the degree of CO2 permeability enhancement. Hence, further attention has

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been paid on the importance of offering more time for alternative N2 permeation for greater

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swelling recovery in this study by flooding N2 for 48 and 72 hours.

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2. Experimental methodology

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Natural brown coal samples (low rank) obtained from the Hazelwood coal mine located at Morwell

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in South Gippsland, Victoria were utilised to investigate the influence of CO2 properties and

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reservoir depth on the CO2 adsorption induced coal matrix swelling in low rank coal. These samples

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had an average moisture content of 57% (on a wet basis), a fixed carbon content around 48% (on a

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dry basis), volatile matters of 50.3% (on a dry basis) and an average bulk density of 1110 kg/m3.

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The samples were cored, cut and end ground to 38 mm in diameter and 76 mm high from large coal

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blocks and the detailed procedure of sample preparation can be found in Ranathunga et al. 34.

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and Perera et al.

, the

using Victorian brown coal (low rank). They observed a similar behavior as in

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and is also important to investigate the influence of N2

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The high-pressure tri-axial test rig available in the Deep Earth Energy Laboratory (DEERL)

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was used conduct permeability tests on the brown coal samples. The experiments were conducted at

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three different confinements (11, 14 and 17 MPa) representing different depths and different CO2

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inlet pressures (6-14 MPa). All these permeations were done under undrained conditions in order to 5


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achieve higher CO2 pressures throughout the specimen. N2 was injected into the coal sample before

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and after CO2 injection at various pressures to identify the alterations created by CO2 injection in

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the coal matrix; this could be done due to the comparatively less adsorptive nature of N2 than CO2.

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After the second N2 injection, CO2 was permeated again to check how the alternative N2 flow may

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affect the volumetric strain of the brown coal sample. A summary of the testing programme is

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showed in Figure 1 and the more details of the testing programme is mentioned in Ranathunga et

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al.33.

136 Reservoir depth

Injecting gas properties

1st N2 injection

1st CO2 injection

2nd N2 injection

2nd CO2 injection

Pc = 11 MPa Pi = 6 ~ 9 MPa

1st N2 injection

1st CO2 injection

2nd N2 injection

2nd CO2 injection

Pc = 14 MPa Pi = 6 ~ 12 MPa

1st N2 injection

1st CO2 injection

2nd N2 injection

2nd CO2 injection

Pc = 17 MPa Pi = 6 ~ 14 MPa

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Figure 1. Experimental program for tri-axial flow studies (here Pc is confining pressure and Pi is the

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injection pressure)

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Further, the influence of N2 permeation duration on swelling recovery was also studied by

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flooding N2 for different time periods: 48 and 72 hours on swelled coal samples under 6 MPa (sub-

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critical) and 8 MPa (super-critical) CO2 pressures.

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During this complete test series, the volumetric strain in the coal sample was recorded using

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an advanced data acquisition system at one second intervals to quantify the CO2 adsorption-induced 6


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coal matrix swelling of the brown coal specimens in various stress environments. The volumetric

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strain was calculated using the volume change data given by a syringe pump (see Eq. [1]) which

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was used to apply the confining pressure. When the sample swells, the excess oil volume inside the

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cell is pumped out towards the syringe pump and when the sample shrinks oil is sent towards the

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sample from the pump.

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Eq. [1] was used to calculate the volumetric strain in the coal sample under various injection

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conditions.

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Volumetric strain of the sample (εv) =

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where, Vinitial is the initial pump volume for the respective test condition (stable pump volume after

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applying the confining pressure), Vt is the pump volume at time t and ∆Vx is the oil volume change

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(pumped in or out from the syringe pump). Therefore, negative volumetric strains represent sample

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shrinkage (Vinitial > Vt) and positive volumetric strains represent sample swelling (Vinitial < Vt). The

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whole test series was conducted at 35 0C (> 31.8 0C is the critical temperature of CO2) constant

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temperature to obtain the super-critical condition of the injected CO2 when the pressure goes

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beyond 7.38 MPa (the critical pressure of CO2). The volumetric strain variations of the coal samples

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for 8 MPa CO2 and N2 injections under 11, 14 and 17 MPa confinements are shown in Figure 2.

± ∆

× 100% ∶ ∆ = −

Volumetric strain (%)

2.5

0 Pc = 11 MPa Pc = 14 MPa Pc = 17 MPa

-2.5

Pc = 11 MPa Pc = 14 MPa Pc = 17 MPa

4

2

0

-2

-5 0

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[1]

6

5 Volumetric strain (%)

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10 15 Time (h)

20

0

5

10 15 Time (h)

(b) CO2 injection

(a) N2 injection

7


20

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Figure 2. Volumetric strain variation of brown coal samples for 8 MPa (a) N2 and (b) CO2 injection

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under various confining pressures (here Pc is confining pressure)

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3. Results and Discussion

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3.1.

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According to Figure 2, CO2 flow through low rank brown coal causes clear coal matrix swelling

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(positive volumetric strain) compared to N2, and the matrix swelling gradually increases over time,

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regardless of confinement (see Figure 2). The influence of CO2 phase and pressure on coal matrix

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swelling was therefore studied first.

Effect of sub- and super-critical CO2 adsorption on low-rank coal matrix swelling

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Figure 3 shows the variation of observed volumetric strain developed in the tested low rank

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brown coal with CO2 and N2 flows in various confining stress environments. Note that here CO2

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density is used instead of CO2 pressure to characterize the volumetric strain data, because gas

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density more precisely represents the injecting CO2 properties due to its dependency on both

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temperature and pressure 5. The density of CO2 was calculated for each injection condition (35 0C

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and 6 to 14 MPa injection pressures) using the REFPROP data base

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comparison, the sample volume increase or coal matrix swelling is denoted by positive volumetric

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strains and the sample volume decrease or coal matrix shrinkage is denoted by negative volumetric

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strains in Figure 3.

(a)

Pc = 14 MPa

Volumetric Strain (%)

8

8 12 9 10

6 4

7

8 9 10 1214

6

2

7 7

8

200

400

6 6

0 0

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9

Pc = 17 MPa

10

35

. For the purposes of

(b)

Pc =11 MPa

12 Volumetric Strain (%)

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Pc = 11 MPa

2.0

Pc = 14 MPa Pc = 17 MPa

1.5

6

1.0

6 6

0.5

7

7 7

8

8 8

9

9

10

10

12

14

12

9

0.0 600

4

800

CO₂ density (kg/m3)

8


8 12 N₂ density (kg/m3)

16

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Figure 3. Variation of volumetric strain under 11, 14 and 17 MPa confinements during (a) first CO2

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injection and (b) first N2 injection (here the hollow data points represent the super-critical CO2

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conditions and the data labels denote the respective injection pressures)

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As shown in Figure 3(a), the volumetric strain of coal for CO2 flow increases with

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increasing CO2 density/pressure, regardless of confinement. For example, 6 to 7 MPa and 8 to 9

185

MPa CO2 pressure increments causes around 1.96% and 3.68% volumetric strain increments under

186

11 MPa confining pressure. Importantly, under the same pressure conditions, N2 injection exhibits

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only around 0.08% (6 to 7 MPa) and 0.11% (8 to 9 MPa) volume changes, which are insignificant

188

compared to the CO2 effect. The other important fact is that the increase of volumetric strain under

189

super-critical CO2 permeation is comparatively higher than for low-pressure CO2 permeation. For

190

instance, 8 to 9 MPa (super-critical) CO2 permeation causes coal mass swelling around 1.7% greater

191

than the swelling caused by 6 to 7 MPa (sub-critical) CO2 permeation (see Figure 3) under 11 MPa

192

confining pressure.

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The coal matrix swelling occurs due to the sorbed volume of the adsorbate 36, and hence this 15

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sorbed volume is proportional to the amount of coal matrix swelling

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greater adsorption capacity due to its highly chemically reactive nature

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greater coal matrix swelling. In order to elaborate this CO2 phase effect, the average pore pressure

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(here, the average pore pressure is taken as the mean value of the upstream and the steady-state

198

downstream pressures) conditions for the different CO2 injections under the three confining

199

pressures (11, 14 and 17 MPa) were plotted against the upstream CO2 pressures in Figure 4.

200

According to Figure 4, the average pore pressure has a similar behaviour to strain variation where

201

sample is under super-critical condition at higher CO2 pore pressures for all the confinements. For

202

example, when considering the same pressure conditions discussed before where pressure changes

203

from 6 to 7 MPa (sub-critical) and 8 to 9 MPa super-critical under 11 MPa in situ stress, the average

204

pore pressure of the sample is 5.96 MPa (< 7.38 MPa – critical pressure of CO2) and 7.51 MPa (>

205

7.8 MPa – critical pressure of CO2 (see Figure 4). Hence, the super-critical CO2 throughout the 9


. Super-critical CO2 has a 7

and therefore causes

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brown coal specimen during 8 to 9 MPa super-critical gas injections is the main reason for higher

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swelling than for sub-critical CO2. Similarly, Perera et al.

208

increase for 5 MPa sub-critical CO2 injection and around 50% strain increment for 8 MPa super-

209

critical CO2 flows for high-rank coal (Australian bituminous coal). This shows that the greater coal

210

matrix swelling observed under super-critical CO2 permeation is applicable to any coal seam,

211

regardless of its rank. Therefore, the sorbed volume is proportional to the sorption-induced swelling

212

and larger amount of sorbed volume adsorbed at super-critical conditions allow higher swelling at

213

higher CO2 pressures.

10

observed around 14% radial strain

214

Averaage pore pressure in the coal sample (MPa)

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Pc = 11 MPa Pc = 14 MPa Pc = 17 MPa critical pressure of CO₂

10

8

6 Sub-critical region 4 5

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Super-critical region

7

9

11

13

15

Upstream CO₂ pressure (MPa)

216

Figure 4. Average pore pressure variation in the coal sample during the first CO2 injection (after 24

217

hours of injection)

218

Further, this higher coal matrix swelling super-critical CO2 causes for greater permeability

219

reduction. In the case of low rank coal, Ranathunga et al. 33 found around 20% greater reduction in

220

coal mass permeability for the same CO2 pressure conditions discussed before (from 6 - 7 MPa to 8

221

- 9 MPa) for similar type of low rank coal under 11 MPa confinement. Hence, around 2% increase

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in swelling causes the reduction of the permeability by 10 times, which confirms the influence of

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adsorbing CO2 properties on coal mass swelling and the associated permeability for low-rank coal. 10


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For high-rank coal, Pan et al. 27, observed around 16% greater reduction of permeability from 1 to 3

225

MPa (sub-critical) to 7 to 13 MPa (super-critical) pore pressure variation when the pressure

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difference between the confining pressure and pore pressure 2 MPa under a swelling increment of

227

around 1.5%. Interestingly, both high and low rank coals display around 10 times reduction of

228

permeability according to the relevant swelling strain increment. However, it should be noted that,

229

the pressure conditions considered are not the same and hence the magnitude of 10 may vary.

230

Nevertheless, both low and high rank coal are subjected to greater flow reductions due to the

231

enhanced swelling during super-critical CO2 flow.

232

When comparing the swelling rate with time, according to Figure 2, CO2 absorption-induced

233

swelling occurs at a maximum rate within the first 7 to 8 hours for 6 MPa CO2 injection under 11

234

MPa confining pressure for the tested brown coal. In comparison with the swelling behaviour

235

reported in the research literature for high-rank coal, Perera et al.

236

strain increment within the first 3 to 4 hours of sub-critical CO2 injection under 10 MPa

237

confinement. Unlike the intact coal samples used in the present study, the fractured black coal used

238

by Perera et al.

239

related highly dense cleat system and that may have caused a quicker occurrence of maximum

240

swelling compared to the intact brown coal. In relation to the other two confinements considered

241

(14 and 17 MPa), the maximum swelling rate was seen within the first 11 to 12 hours for 14 MPa

242

confinement and within the first 17 to 18 hours for 17 MPa confinement. The maximum swelling

243

rates for these in-situ stresses (11, 14 and 17 MPa) are 0.36, 0.1 and 0.03, respectively. According

244

to these values, the swelling rate reduces with increasing confining pressure. Similar behavior was

245

observed by Perera et al.

246

swelling rates of around 0.0006 (~3-4 hours) and 0.00042 (~6-7 hours) for 10 and 15 MPa

247

confinements respectively. According to Hol et al. 8, CO2 adsorption capacity reduces at high

248

effective stresses, which explains the reason for the observed reduced volumetric swelling effect at

249

greater confining pressures. This reduction of coal matrix swelling with increasing confinement is

10

10

observed a maximum radial

has more provision for CO2 permeation through the sample due to the maturity

10

for high ranked Australian bituminous coal and found maximum

11


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250

important in field-scale applications, because this represents the influence of coal seam reservoir

251

depth on its swelling characteristics. This is therefore considered in detail in the following section.

252

3.2.

253

According to Figure 3, strain induced by CO2 sorption clearly reduces with increasing confinement

254

and is also confirmed by the reduction of average pressure variation in Figure 4. However, this

255

confining stress influence on coal matrix swelling is more significant at low confinements (11 to 14

256

MPa) compared to high confinements (14 to 17 MPa). For example, around 1.08% and 5.45% strain

257

reductions can be seen for 6 MPa (sub-critical) and 9 MPa (super-critical) CO2 permeation in the

258

coal mass under 11 to 14 MPa confining stress increment and only around 0.6% and 3.02% for 6

259

MPa (sub-critical) and 9 MPa (super-critical) CO2 permeation in the coal mass under 14 to 17 MPa

260

confining pressure increment. This higher swelling reduction under greater confinements is related

261

to the much-shrunken pore space that the coal mass undergoes at deeper coal seams, which reduces

262

flowrate along the coal mass. This result in lesser amount of sorbed volume of the sample causing a

263

reduction of coal matrix swelling upon higher depths. Further, the higher compressive forces

264

available at higher effective stresses may also restrict the volumetric swelling. According to Hol et

265

al. 8, at higher stress applications, the sorbed volume of CO2 will be expelled which causes a

266

reduction of sorption induced swelling. For example, after a series of experiments conducted on

267

high rank coal using various CO2 pressures (10 – 20 MPa) and in situ stresses (5 – 35 MPa), the

268

expelled amount of CO2 0.09, 0.195, 0.307, 0.372 and 0.420 ml for 5, 10, 20, 30 and 35 MPa in-situ

269

stresses respectively. Interestingly, this expelled CO2 volume is increased with the increasing in-situ

270

stress or depth of coal seam which further elaborates the reason behind the lower strain

271

developments at higher depths. Therefore, the increase in volumetric strain reduction is a combined

272

influence of the effective stress increment available under higher compressive forces and the lower

273

CO2 adsorption capacity. Hol et al.

274

stresses for high-rank coal and therefore the results of this study confirm that regardless of the rank

275

of the coal seam, it is subjected to a lower swelling effect if the depth of the seam is great.

Effect of reservoir depth on coal matrix swelling

8

confirmed this lower swelling effect under greater effective

12


Page 12 of 35

Page 13 of 35

276

Interestingly, this reduction of strains is greater at higher injection pressures for both 11 to

277

14 MPa and 14 to 17 MPa stress increments. For instance, 1.71% increase in strain reduction was

278

observed from 7 to 8 MPa injection pressures for 11 to 14 MPa stress increase while 1.1% increase

279

in strain reduction was observed for the same injection pressures for a 14 to 17 MPa stress increase

280

(see Figure 3). This can be explained by the portion of the sample which undergoes different CO2

281

pressures under the respective conditions. The pore pressure development through the sample

282

length after 24 hours of CO2 injection was calculated considering a linear pressure development

283

trend along the sample from upstream to downstream using the upstream/injection pressure and the

284

downstream pressure at steady-state for the comparison purpose. Figure 5 shows the pressure

285

variation along the sample length calculated by the prescribed method for 7, 8 and 9 MPa CO2

286

injections for the three different confinements.

100%

80%

Sample length (%)

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

60%

40%

20%

0% Pi = 7 & Pc = 11

Pi = 8 & Pc = 11

Pi = 9 & Pc = 11

Pi = 7 & Pc = 14

Pi = 8 & Pc = 14

Pi = 9 & Pc = 14

Pi = 7 & Pc = 17

Pi = 8 & Pc = 17

Pi = 9 & Pc = 17

CO₂ injection conditions

287

super-critical CO₂

sub-critical CO₂

288

Figure 5. CO2 pressure variation along the sample for 7, 8 and 9 MPa CO2 injection under 11, 14

289

and 17 MPa confining pressures (here Pi = injection pressure and Pc = confining pressure)

290 291

According to Figure 5, under the confinements considered here, 7 MPa CO2 injection (
7.38 MPa, the critical pressure of CO2) causes the sample

294

to undergo both sub- and super-critical CO2 adsorption. However, the portion of the sample under 13


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 14 of 35

295

super-critical CO2 adsorption is reduced with increasing confinement. For example, around 63%,

296

50% and 32% of the sample (assuming a linear variation from upstream to downstream) is

297

subjected to super-critical CO2 adsorption under 11, 14 and 17 MPa confinements, respectively. As

298

discussed previously, the coal mass has much reduced pore space under higher confinements and

299

therefore CO2 has less ability to transport through it, which results in reduced super-critical CO2

300

distributed area of the sample under greater confinements. Since super-critical CO2 creates much

301

greater swelling in the coal mass, the reduction of super-critical CO2 adsorbed in the coal mass

302

eventually reduces the degree of swelling. Therefore, when the confinement is increased from 11 to

303

14 MPa (1.71%) and 14 to 17 MPa (1.1%), a reduction in volumetric strain increment (swelling) is

304

shown for 7 to 8 MPa CO2 pressures in Figure 5. When 9 MPa CO2 permeates through brown coal,

305

around 100%, 100% and 87.5% of the sample is in super-critical condition (see Figure 5) under the

306

three confinements considered here. Which creates a strain reduction of 1.49% and 0.98% from 8 to

307

9 MPa CO2 pressure variation under 11 to 14 MPa and 14 to 17 MPa in-situ stress increments.

308

Therefore, as explained above, the greater CO2 sorbed volume in the sample due to the larger

309

proportion of super-critical CO2 creates much higher swelling with super-critical CO2 flooding.

310

In relation to permeability for these conditions, 7 to 8 MPa CO2 pressure increase causes

311

3.2% and 1.8% permeability reduction increments with 11 to 14 MPa and 14 to 17 MPa confining

312

stress increments, respectively, while 5.2% and 2.7% increase of permeability reduction for the

313

similar confinement increase was observed for 8 to 9 MPa super-critical CO2 permeation

314

confirms the influence of the effective stress changes on strain variations and its effect on CO2 flow.

315

Moreover, similar to the CO2 injection, the strain developed in the coal mass during N2

316

injection was also checked under changing confining stress environments and the results are shown

317

in Figure 3. According to the figure, 0.34% and 0.28% strain reductions occur for 6 and 9 MPa CO2

318

permeations when the confining stress increases from 11 to 14 MPa. In addition, the confinement-

319

created strain reductions are greater at greater confinements, producing strain reductions of 0.48%

320

for 6 MPa and 0.44% for 8 MPa with increasing confinement from 14 to 17 MPa. However, unlike 14


33

. This

Page 15 of 35 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

321

in CO2 flow, here the influence of N2 pressure on swelling reduction which occurs with increasing

322

confining pressure is negligible (see Figure 3). This is because, compared to CO2, N2 is a less

323

adsorptive, inert gas 7. Therefore, the expected volumetric strain increase for N2 with increasing

324

pore pressure is minimal. Hence, the strain variations observed in Figure 3 are mainly due to the

325

effective stress variation with in situ stresses.

326

3.3.

327

As discussed in previous sections, CO2 flow through the coal matrix causes considerable matrix

328

rearrangements, creating a negative impact on CO2 sequestration. Although this swelling effect

329

reduced with increasing reservoir depth, the swelling seems to happen at considerably higher rates

330

for super-critical injected CO2, which is the most common CO2 phase expected in potential

331

unmineable coal seams 37. This matrix swelling causes a reduction of injection capacities in many

332

field-scale CO2-ECBM projects, making the projects uneconomical (refer to Section 1). Hence,

333

appropriate precautions to reduce this swelling effect are necessary for reservoir productivity

334

enhancement in CO2-ECBM projects. As explained in section 1, the injection of a stream of N2 into

335

the swelled coal mass creates a considerable improvement in the permeability of coal by partially

336

reversing the CO2-induced coal matrix swelling specially at lower effective stresses. However, the

337

precise identification of this ability of N2 requires a comprehensive overview of the swelling

338

characteristics variation of the coal mass during this remediation process. In order to quantify the

339

ability of N2 flooding to partially recover CO2 adsorption-induced coal matrix swelling, the coal

340

sample was subjected to 24 hours of N2 flooding after each CO2 injection and the CO2 injection was

341

then repeated at the same pressure. For 6 and 8 MPa CO2 injection pressures the samples were also

342

subjected to an additional 48 hours of the third N2 flooding after the second CO2 injection, followed

343

by the third CO2 injection. Similarly, 72 hours of N2 flooding (fourth N2 flooding) for 6 and 8 MPa

344

CO2 injection pressures were carried out after the third CO2 injection followed by the fourth CO2

345

injection.

Potential of N2 to reverse CO2 induced coal matrix swelling

15


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

346

3.3.1. After 24 hours of N2 flooding

347

Figure 6 shows the swelling reduction observed with the second CO2 injection after 24 hours of N2

348

flooding for each CO2 injection pressure under the three different in-situ stresses considered here.

349

As the figure shows, under all three confinements, swelling recovery is gradually increased with

350

increasing injection pressure, confirming that high-pressure N2 flooding is more effective in

351

recovering the reversible swelling areas created by CO2. For example, the swelling recovery

352

percentages observed for 6 and 8 MPa CO2 injection pressures are around 1.3% and 8.9% under 11

353

MPa confinement, and 0.7% and 2.9% under 17 MPa confinement, respectively (see Figure 6).

354

According to Kiyama et al.

355

creating a partial pressure reduction in the sample, which contributes to the reduced swelling

356

percentage in the sample (refer to section 1). At higher pressures, N2 will flow at higher rates

357

creating a larger imbalance between sorbed and free gas and eventually the amount of partial

358

recovery of swelling is increased. Further, higher N2 permeations caused for higher shrinkages or

359

volumetric strain reductions of the tested brown coal sample as showed in Figure 3. For example, 6

360

to 9 MPa N2 pressure increment result in a shrinkage of around 0.17%, 0.19% and 0.22% during 11,

361

14 and 17 MPa in situ stresses respectively. This effect may also compensate the swelling of the

362

coal sample when considering the net volumetric strain from second N2 injection to second CO2

363

injection. However, further research is needed to confirm the dominating factor: higher flow rates

364

induced partial pressure reduction or higher specimen shrinkages at higher pressures, for N2

365

injection induced swelling recovery.

16

, N2 has the ability to desorb CO2 molecules from the pore faces by

366

16


Page 16 of 35

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

Volumetric strain reduction/ Swelling recovery (%)

Page 17 of 35

20

18.5

18

16.5

16 14.0

14 12

11.2

10.5

10

8.9

Pc = 11 MPa

8.5

8.0

Pc = 14 MPa

8 6

4.6

4 2

1.31.0 0.7

2.6 1.41.2

Pc = 17 MPa

4.9

2.9

0 6

7

8 9 10 CO₂ injection pressure (MPa)

12

14

367 368

Figure 6. Volumetric strain reduction for second CO2 injection compared to first CO2 injection after

369

N2 flooding for 24 hours

370

However, this recovery rate reduces with increasing reservoir depth. For instance, 0.6% and

371

6% reductions in swelling recovery can be seen for 6 and 8 MPa CO2 floods with increasing

372

confinement from 11 to 17 MPa. This is because, at higher confinements, N2 flow ability through

373

the coal matrix slows down, which eventually gives less opportunity for swelling recovery for N2

374

(see Figure 6).

375

It should be noted that before the second CO2 injection, the sample has already been

376

subjected to swelling during the first CO2 injection. Hence, less matrix rearrangement can be

377

expected due to the second CO2 flow than the first 10. Under higher CO2 pressures, the coal matrix

378

undergoes considerable macro-molecular structural alterations due to the inherent chemical and

379

physical interactions of super-critical CO2 and coal pore walls 10, 17. Hence, the expected swelling at

380

higher injection pressures for the re-injection of CO2 will be less (see Figure 6). In addition, the

381

swelling is also reduced by reservoir depth, which causes much less swelling at higher

382

confinements for the second CO2 injection. Therefore, both N2 flooding and swelling reduction due

383

to the alteration in the coal matrix caused by previous CO2 flows and N2 flows collectively

384

influence the observed strain reductions during the second CO2 flow. 17


Energy & Fuels

385

Figure 7 shows the CO2 permeability increment against strain reduction with the second

386

CO2 flow, which confirms that the observed permeability increments after the N2 flood in

387

Ranathunga et al. 33 are clearly due to the net volumetric strain variations which occurred before the

388

second CO2 injection by the N2 flood. Further, for all three confinements, the CO2 permeability

389

increment against swelling recovery exhibits a perfect linear variation (y = x) with an overall

390

goodness fit of 0.99 (R2). This shows the ability of N2 to be used as a catalyst to improve coal mass

391

permeability by partially recovering CO2 adsorption-induced matrix swelling. 25 CO₂ permeability increase after N₂ flooding (%)

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 35

20

y = 1.046x R² = 0.9895

y = 1.047x R² = 0.9927

15 y = 1.0069x R² = 0.9902 10

5

0 0 Pc = 11 MPa

392 393

5 10 15 Swelling recovery after N₂ flooding (%) Pc = 14 MPa

20

Pc = 17 MPa

Figure 7. CO2 permeability increase vs swelling recovery after N2 flooding

394 395

However, this method is only effective for low-rank coal under low confining stresses,

396

because the higher permeability at lower confining stresses allows higher N2 flowrates in to the coal

397

mass, which eventually reverses the swelling. Therefore, it can be expected that if N2 is injected

398

into the coal matrix for a longer time, the amount of swelling recovery will increase. The following

399

section discusses the swelling recovery by N2 after permeation for further 48 and 72 hours of brown

400

coal specimens.

18


Page 19 of 35 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

401

3.3.2. Permeation of N2 for 48 and 72 hours

402

As described above, the duration of N2 flooding has a considerable influence on swelling recovery,

403

as it offers more opportunity for N2 to be involved in the recovery process. Figure 8 shows the CO2

404

permeability after flooding of N2 for 24, 48 and 72 hours of brown coal specimens at 6 and 8 MPa

405

CO2 injections. According to the figure, an increase in CO2 permeability occurs with increasing N2

406

flooding duration. For example, 6 MPa sub-critical CO2 permeability increases by around 9.4% and

407

13.1% when the N2 flooding duration is increased up to 48 and 72 hours during 11 MPa confining

408

pressure. Similarly, for 11 MPa confinement, 8 MPa super-critical CO2 permeability is increased by

409

10.5% and 17.7% for 48 hours and 72 hours N2 floods compared to 24 hours N2 flooding. As

410

explained previously, the longer durations allow more N2 to enter the coal matrix and a greater

411

amount of CO2 to desorb from the coal mass with the reduction of partial pressure, allowing an

412

increment in swelling recovery. Interestingly, this increment is reduced for higher confinements

413

(refer to Figure 8). For example, around 7.2% and 8.3% decrease compared to 11 MPa confinement

414

can be seen for 14 and 17 MPa confinements when N2 flooding duration is increased to 48 hours for

415

6 MPa CO2 injection. For similar conditions, around 4.9% (14 MPa confinement) and 7.1% (17

416

MPa confinement) reductions in CO2 permeability compared to 11 MPa confinement were observed

417

for 8 MPa CO2 flow after 48 hours of N2 flooding. In addition, the permeability increment is greater

418

for higher injection pressures (refer to Figure 8). For example, when the CO2 injection pressure is

419

increased from 6 to 8 MPa, around 4.6%, 4.7% and 1.2% increases in CO2 permeability increments

420

(compared to 24 hours of N2 permeation) were observed for 11, 14 and 17 MPa confining pressures,

421

respectively after 72 hours of N2 flooding. This can be explained by the volumetric strain variation

422

observed during these various N2 floods through the coal specimens.

423

According to Figure 8, the volumetric strain reduction is increased with N2 flooding time for

424

both 6 and 8 MPa CO2 injections. This observation confirms the higher permeability increments

425

(refer to Figure 8) when N2 flooding time is increased, because the increased coal mass swelling

426

recovery for longer N2 flow durations is subjected to enhanced permeability in the coal matrix. In 19


Energy & Fuels

427

addition, the volumetric strain reduction is gradually reduced with confining pressure, and this

428

reduction is comparatively higher for 8 MPa CO2 injection compared to 6 MPa. This indicates the

429

higher permeability increments shown for lower effective stresses compared to higher effective

430

stresses in Figure 8. The effective stress effects on low-rank coals due to the lower strength and

431

elastic modulus and higher shrinkage compressibilities of low-rank coal which result in a more

432

shrunken pore structure under higher stresses. Hence, although the N2 is flooded for a longer time,

433

the expected swelling recovery is restricted with lower flowrates at higher effective stresses. This

434

warrants more research using time durations for N2 flooding longer than 72 hours for low-rank

435

coals.

436 (a)

0.008 13.1%

CO₂ permeability (µD)

9.4% 0.006

0%

0.004

0%

0.002

2.2% 5.3% 0%

1.1% 2.9%

0 Pc = 11 MPa After 24 hrs of N₂ injection

(b)

Pc = 14 MPa After 48 hrs of N₂ injection


0.012 17.7%

0.01 CO₂ permeability (µD)

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 20 of 35

10.5% 0.008

0%

0.006 0.004 0%

0.002

5.6% 10% 0%

3.4% 4.1%

0 Pc = 11 MPa

437

After 24 hrs of N₂ injection


20



Page 21 of 35 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

438

Figure 8. CO2 permeability after flooding N2 for 24, 48 and 72 hours for (a) 6 MPa CO2 and (b) 8

439

MPa CO2 injections during 11, 14 and 17 MPa confining pressures (here the data labels denote the

440

CO2 permeability increase compared to 24 hours of N2 flooding)

441 442

As discussed in section 1, Kiyama et al. 16 and Perera et al. 32 observed the higher swelling

443

recovery at specially at lower effective stresses for high rank coal similar to the low rank brown

444

coal due to the enhance N2 flowrates at lower effective stresses. Interestingly, Perera et al.

445

observed greater permeability increments in high-rank coals even under higher confining stress after

446

24 hours of N2 flooding. According to Perera et al. 32, the reduced flow rates under higher effective

447

stresses allows more time for N2 to interact with CO2 adsorbed coal and to recover more swelling

448

areas. As mentioned above, high-strength high-rank coals undergo less pore structure shrinkage due

449

to higher stresses than low-rank coals. Therefore, unlike for low rank coals, much higher

450

permeability increments with longer durations of N2 permeations at higher effective stresses can be

451

expected following the above-mentioned phenomena. Therefore, this needs to be confirmed by

452

further research for much longer N2 flooding durations for high rank coal.

32

453

Generally, this method is efficient at low confining stresses for both high and low rank coal.

454

Therefore, future research is needed to investigate the most effective applicability of N2 flooding for

455

the swelling recovery process. Several researchers

456

example, field and laboratory studies have implemented the injection of a CO2/N2 binary mixture to

457

recover coal mass swelling-induced permeability reductions

458

have been conducted on high-rank coals and have ignored low-rank coals. Hence, future research on

459

the effective use of CO2 and N2 mixtures to recover coal matrix swelling and related permeability

460

issues for low-rank coals is necessary.

461

4. Conclusions

462

This study provides a comprehensive understanding of CO2 adsorption-induced coal matrix

463

swelling, particularly for low-rank brown coal. The behaviour of volumetric swelling for different

21, 38-41

have tested various approaches. For

42, 43

21


. However, almost all the studies

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

464

CO2 injections at different confining stresses (depths) was studied and according to the results the

465

following conclusions can be drawn:

466

•

CO2 adsorption-induced coal matrix swelling is greater at higher CO2 pressures, particularly for

467

the super-critical phase condition of CO2 regardless of reservoir depth or the maturity of the

468

coal. This is mainly due to the greater adsorption potential of high pressure CO2, which is

469

significantly accelerated with the phase transition from sub- to super-critical.

470

•

Coal matrix swelling decreases with increasing reservoir depth or confining stress, which is

471

beneficial for field applications of CO2-ECBM, preferably in deep coal formations. However,

472

this also depends on the pore pressure conditions as lower effective stresses (lower

473

confinements and higher injection pressures) lead to greater swelling reduction, regardless of

474

coal maturity or rank.

475

•

regardless of pore pressure and reservoir depth.

476 477

Nitrogen (N2) exhibits comparatively inert behaviour in coal, producing negligible swelling,

•

However, the partial pressure depletion created by N2 in the coal mass has the ability to recover

478

CO2 adsorption-induced swelled areas in coal to some extent under lower effective stresses, and

479

is common for any coal type. Further, this ability to recover swelling can be imporved if N2

480

flooding is done for a sufficiently long period under a lower effective stress conditions, and it is

481

probable that a greater amount of N2 can enter the coal mass under such conditions.

482

Acknowledgement

483

The authors wish to express their appreciation of the funding provided by the Australian Research

484

Council (DE130100124) and the Postgraduate Publication Award (PPA) of Monash University.

485

References

486

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487

Skawiński, R., Considerations referring to coal swelling accompanying the sorption of gases and water. Archives of Mining Sciences 1999, 44, (3), 425-434.

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Mazumder, S.; Wolf, K. H., Differential swelling and permeability change of coal in response to CO2 injection for ECBM. International Journal of Coal Geology 2008, 74, (2), 123-138.

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Pan, Z.; Connell, L. D., A theoretical model for gas adsorption-induced coal swelling.

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Vandamme, M.; Brochard, L.; Lecampion, B.; Coussy, O., Adsorption and strain: The CO2-

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induced swelling of coal. Journal of the Mechanics and Physics of Solids 2010, 58, (10), 1489-

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Day, S.; Fry, R.; Sakurovs, R.; Weir, S., Swelling of Coals by Supercritical Gases and Its Relationship to Sorption. Energy & Fuels 2010, 24, (4), 2777-2783.

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Day, S.; Fry, R.; Sakurovs, R., Swelling of moist coal in carbon dioxide and methane.

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Day, S.; Fry, R.; Sakurovs, R., Swelling of Australian coals in supercritical CO2. International

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Karacan, C. Ö., Swelling-induced volumetric strains internal to a stressed coal associated with CO2 sorption. International Journal of Coal Geology 2007, 72, (3), 209-220.

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critical carbon dioxide adsorption-induced coal matrix swelling on the permeability of naturally

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fractured black coal. Energy 2011, 36, (11), 6442-6450.

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11. Balan, H. O.; Gumrah, F., Assessment of shrinkage–swelling influences in coal seams using

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rank-dependent physical coal properties. International Journal of Coal Geology 2009, 77, (1–

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experiments on methane, nitrogen and carbon dioxide on dry Selar Cornish coal. International

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constrained conditions. International Journal of Coal Geology 2011, 85, (1), 56-64.

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27


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

Reservoir depth

Page 28 of 35

Injecting gas properties

1st N2 injection

1st CO2 injection

2nd N2 injection

2nd CO2 injection

Pc = 11 MPa Pi = 6 ~ 9 MPa

1st N2 injection

1st CO2 injection

2nd N2 injection

2nd CO2 injection

Pc = 14 MPa Pi = 6 ~ 12 MPa

1st N2 injection

1st CO2 injection

2nd N2 injection

2nd CO2 injection

Pc = 17 MPa Pi = 6 ~ 14 MPa

Figure 1. Experimental program for tri-axial flow studies (here Pc is confining pressure and Pi is the injection pressure)


Page 29 of 35

6

2.5

0 Pc = 11 MPa Pc = 14 MPa Pc = 17 MPa

-2.5

Volumetric strain (%)

5 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

Energy & Fuels

Pc = 11 MPa Pc = 14 MPa Pc = 17 MPa

4

2

0

-2

-5 0

5

10 15 Time (h)

20

0

(a) N2 injection

5

10 15 Time (h)

20

(b) CO2 injection

Figure 2. Volumetric strain variation of brown coal samples for 8 MPa (a) N2 and (b) CO2 injection under various confining pressures (here Pc is confining pressure)


Energy & Fuels

(b)

Pc =11 MPa

12

Pc = 14 MPa

9

Pc = 17 MPa

10 8

8 12 9 10

6 4

7

8 9 10 1214

6

2

6 6

0 0

7 7

200

Volumetric Strain (%)

(a) 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 30 of 35

Pc = 11 MPa

2.0

Pc = 14 MPa Pc = 17 MPa

1.5

6

1.0

6 6

0.5

7

7 7

8

8 8

9

9

10

10

12

14

12

9

8

0.0 400 600 CO₂ density (kg/m3)

800

4

8 12 N₂ density (kg/m3)

Figure 3. Variation of volumetric strain under 11, 14 and 17 MPa confinements during (a) first CO2 injection and (b) first N2 injection (here the hollow data points represent the supercritical CO2 conditions and the data labels denote the respective injection pressures)


16

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

Averaage pore pressure in the coal sample (MPa)

Page 31 of 35

12

Pc = 11 MPa Pc = 14 MPa Pc = 17 MPa critical pressure of CO₂

10

Super-critical region

8

6 Sub-critical region 4 5

7

9

11

13

15

Upstream CO₂ pressure (MPa)

Figure 4. Average pore pressure variation in the coal sample during the first CO2 injection (after 24 hours of injection)


Energy & Fuels

100%

80%

Sample length (%)

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 32 of 35

60%

40%

20%

0% Pi = 7 & Pc = 11

Pi = 8 & Pc = 11

Pi = 9 & Pc = 11

Pi = 7 & Pc = 14

Pi = 8 & Pc = 14

Pi = 9 & Pc = 14

Pi = 7 & Pc = 17

Pi = 8 & Pc = 17

Pi = 9 & Pc = 17

CO₂ injection conditions super-critical CO₂

sub-critical CO₂

Figure 5. CO2 pressure variation along the sample for 7, 8 and 9 MPa CO2 injection under 11, 14 and 17 MPa confining pressures (here Pi = injection pressure and Pc = confining pressure)


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

Volumetric strain reduction/ Swelling recovery (%)

Page 33 of 35

20

18.5

18

16.5

16 14.0

14 12

11.2

10.5

10

8.9

8.5

Pc = 11 MPa 8.0

Pc = 14 MPa

8 6

4.6

4 2

1.31.0 0.7

2.6 1.41.2

Pc = 17 MPa

4.9

2.9

0 6

7

8 9 10 CO₂ injection pressure (MPa)

12

14

Figure 6. Volumetric strain reduction for second CO2 injection compared to first CO2 injection after N2 flooding for 24 hours


Energy & Fuels

25 CO₂ permeability increase after N₂ flooding (%)

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 34 of 35

20

y = 1.046x R² = 0.9895

y = 1.047x R² = 0.9927

15 y = 1.0069x R² = 0.9902 10

5

0 0

5 10 15 Swelling recovery after N₂ flooding (%) Pc = 11 MPa

Pc = 14 MPa

20

Pc = 17 MPa

Figure 7. CO2 permeability increase vs swelling recovery after N2 flooding


Page 35 of 35

(a)

0.008 13.1%

CO₂ permeability (μD)

9.4% 0.006

0%

0.004

0%

0.002

2.2% 5.3% 0%

1.1% 2.9%


(b)



0.012 17.7%

0.01 CO₂ permeability (μD)

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

10.5% 0.008

0%

0.006

0.004 0%

0.002

5.6% 10% 0%

3.4% 4.1%




Figure 8. CO2 permeability after flooding N2 for 24, 48 and 72 hours for (a) 6 MPa CO2 and (b) 8 MPa CO2 injections during 11, 14 and 17 MPa confining pressures (here the data labels denote the CO2 permeability increase compared to 24 hours of N2 flooding)


Effect of Coal Rank on CO2 Adsorption Induced Coal Matrix Swelling

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