1

Two phase relative permeabilities for gas and water in selected European coals

Sevket Durucana, Mustafa Ahsanb, Ji-Quan Shia, Amer Syeda, Anna Korrea aDepartment of Earth Science and Engineering, Royal School of Mines, Imperial College London, London SW7 2BP, UK

bSasol Petroleum International Ltd., 6th Floor 101 Wigmore St, London W1U 1QU, UK

 

Abstract

Gas-water relative permeability behaviour of seven European coals of different ranks was

characterised in order to enhance the scientific understanding of the fundamental processes of

two-phase flow taking place within the macrostructure of coal. Laboratory experiments were

carried out on cylindrical coal samples using the unsteady state method to measure gas-water

relative permeabilities due to its operational simplicity. The impact of factors such as

wettability and overburden pressure on coal relative permeabilities were assessed.

Considerable variation in the shapes of the relative permeability curves for different rank

coals was observed, which was attributed to the heterogeneous nature of coal.

Key words: Relative permeability; coalbed methane; enhanced coalbed methane; laboratory experiments

1. Introduction

Coalbed Methane (CBM) or Enhanced Coalbed Methane (ECBM) production using CO2

injection is initiated through a resource evaluation process involving numerical simulations,

making use of reservoir data that has either been estimated through empirical correlations and

history matching of field data, or derived from laboratory tests on coals from a different basin

altogether. As coal is a highly heterogeneous rock, any discrepancies in its reservoir

characteristics can significantly impact the simulation results for a field site.

When a virgin coalbed methane reservoir is first encountered, the entire cleat network is

normally saturated with water and there are small or insignificant quantities of free gas

present. The presence of water significantly hinders the flow of methane through coal seams

and vice versa. Consequently, the effective permeabilities to both water and methane are

reduced. In order to evaluate the deliverability of coalbed methane wells it is important to

                                                             Corresponding author. Tel.: +44-20-7594-7354; fax: +44-20-7594-7444. E-mail address: s.durucan@imperial.ac.uk

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determine the effective permeability for the reservoir throughout its production life (when

two-phase flow is prevalent), and this effect is described quantitatively in terms of the coal

relative permeabilities to the gas and water phases. Fluid flow through the cleat system also

depends on the distribution of fluids in the cleats, which is related to capillary pressure. A

clear appreciation of the internal pore structure of coal and its interaction with gas and water

is required if one is to understand the mechanisms of two-phase flow in a complex porous

media such as coal.

Water can exist in coal in a variety of forms, including free water in the cleats, chemically

bound water of hydration, and water adsorbed onto the surface of the matrix blocks. For

water-saturated coals, increases in gas relative permeability help to restrict water production

and improve gas flow as the seam becomes progressively dewatered. During this process

whereby water is withdrawn from the cleats, there is a change from water relative

permeability dominating to gas relative permeability becoming more dominant. At the same

time, coals generally possess high irreducible water saturations in the cleats, which can be up

to 80%. Their relative permeability to gas is therefore quite low, and according to Meaney

and Paterson [1], it can be as low as 10% of the absolute permeability in some coals.

Many coals are described as hydrophilic, where water is the preferred wetting phase in the

cleats. Additionally, some of this water may reside in the larger pores of the matrix, rather

than the micropores which are fully gas saturated. This hinders the migration of gas from the

smaller interstices deep inside the matrix, therefore the gas will not become mobile until the

water saturation has fallen significantly below 100%. This point is referred to as the critical

gas saturation, and underlines the reason why substantial volumes of water may need to be

produced from a well before gas flow is detected.

However, it should be noted that the matrix, particularly the small micropores, are coated

with methane, causing the matrix to be gas wet, despite the cleats being water wet and often

possessing a high irreducible water saturation.

The shape of the relative permeability curves is dependent on whether the coal is wetted

preferentially by water or gas, which in turn is a function of the lithotypes that constitute the

coal. For instance, clarain and vitrain tend to prefer gas, while durain and fusain are more

easily wetted by water. Moreover, in conventional gas reservoirs, the rock surfaces tend to be

water-wet like the cleats in coalbeds, whereas in coal seams, the methane is adsorbed onto the

matrix, therefore it may well be methane wet. Consequently, coals could potentially display a

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mixture of water wet, methane wet and intermediate wettability behaviour, depending on the

degree of mineralisation. Indeed it is this heterogeneity of coal that is largely responsible for

the variability in relative permeability curves.

A survey of the literature reveals that very little experimental data has been reported for

coal relative permeability, and there are often large discrepancies between field and

laboratory derived curves. There are still no generally accepted methods in the industry for

laboratory measurement of relative permeability in coal. Similarly, few accepted standards

are available for comparing such data. This is primarily due to the physical properties of coal,

which make it difficult for accurate measurements to be taken. The principal reasons why

relative permeability data are not easily obtainable include: the friable and brittle nature of

coals; the low porosity of the cleat network, which requires the accurate measurement of very

small volumes of water; and the stress dependent nature of coal permeability.

Most of the early work in this field was carried out by Reznik et al. [2] who suggested

laboratory tests for determining the air-water relative permeability behaviour of Pittsburgh

coals. Relative permeabilities were measured at steady state conditions with both increasing

and decreasing water saturations. However, water relative permeability values could not be

measured directly, and had to be inferred from corresponding gas relative permeability data

using Corey’s relationships [3]. Dabbous et al. [4] extended this work by determining gas

relative permeabilities at two different overburden pressures. These techniques were

improved considerably by Puri et al. [5] who formulated a standard procedure for sample

selection, handling, preparation and testing of coals.

In a similar way, Gash [6] conducted both steady state and unsteady state tests using tracer

methods, and found that the two techniques yielded comparable gas-water relative

permeability curves, within the experimental error with which saturations could be

determined. Later on, Gash et al. [7] assessed the effect of cleat orientation and confining

pressure on cleat porosity, permeability and relative permeability for Fruitland coals. An

increase in the confining pressure from 450 psi (3.1 MPa) to 1,000 psi (6.9 MPa) caused the

gas relative permeability to decrease less than the water relative permeability.

Laboratory studies carried out by Meaney and Paterson [1] on coal taken from the Bowen

Basin in Australia indicated that the separation of water and gas in the field due to gravity

resulted in higher effective permeabilities than what was measured in the laboratory. This

suggests that actual relative permeabilities in the field are likely to be higher where there is

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gravity segregation. For such flow systems it may be more appropriate to use straight-line

relative permeability relationships since capillary effects are considered negligible in

segregated flow. Shen et al. (2011) investigated the relative permeabilities to gas and water in

different rank coals selected from South Qinshui Basin, China under various gas/water

saturations through water replacement with methane using an unsteady-state method.

In this study the gas-water relative permeability behaviour of different coal types is

characterised in order to further our understanding of the fundamental processes of two-phase

flow taking place within the macrostructure of coal. New relative permeability curves for a

range of European coals of varying rank are presented and analysed. This is realised

primarily through laboratory tests, where gas-water relative permeability curves are

determined for coals, and the impact of factors such as wettability, absolute permeability and

overburden pressure, on coal relative permeability, are assessed. It is hoped that the results

will provide characterisation data that would enable CBM and ECBM simulators to better

describe in situ reservoir conditions and evaluate the effect of carbon dioxide injection on gas

productivity.

2. Relative permeability measurement using unsteady state method

The two most common experimental techniques used in determining relative permeability

data are the steady state and unsteady state methods. Laboratory experiments presented here

were carried out using the unsteady state method [9] due to its operational simplicity. In this

method, the core is initially saturated with water, which is subsequently displaced by

continuous injection of a gas. Saturations vary throughout the experiment and therefore

equilibrium is never attained. The pressure differential and flow rates of the produced fluids

are monitored as a function of time, and the corresponding relative permeabilities are

deduced using Buckley-Leverett displacement theory [10]. The unsteady state gas flood

attempts to replicate the displacement of water in the cleats by gas desorbed from the matrix.

2.1. Coal sample collection and preparation

Large coal blocks representative of coal ranks from High Volatile Bituminous to Anthracite

were collected from opencast and underground coal mines in the United Kingdom, France

and Germany as:

- the Schwalbach seam from the Ensdorf underground colliery in Saarland, Germany

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- the No.1 seam from the Warndt-Luisenthal (W-L) underground colliery in Saarland,

Germany

- the Splint seam from the Watson Head open cast site in Lanarkshire, Scotland

- the Tupton seam from the Carrington Farm open cast site in Derbyshire, UK

- the Dora seam from the Rumeaux underground colliery in Lorraine, France

- the 9ft seam from the Selar open cast site in South Wales, UK

- the 7ft seam from the Tower underground colliery in South Wales, UK

In order to preserve their natural moisture content and prevent oxidation during transport

and storage, the blocks were wrapped in protective plastic sheeting at the mine site and

placed in sealed wooden containers. Samples taken from the coal blocks were later cut to the

appropriate sizes and used in different tests. Before initiating the laboratory relative

permeability measurements the coals were characterised for rank, porosity, absolute

permeability and mechanical/elastic properties as reported in Table 1.

Sample selection procedures outlined by Hyman et al. [11], together with

recommendations for measuring relative permeability by Gash et al. [7], were adopted during

the tests,. Efforts were made to select core samples that were as homogeneous as possible so

that the pressure driving force and fluid properties could be maintained at a constant level

throughout the experiment. However, given the anisotropic and highly heterogeneous nature

of coal, this was often difficult to achieve.

Freshly cut core samples of 50 mm diameter were initially placed in a desiccator to help

eliminate any residual gas from the samples. These were then vacuum dried at 60°C to

remove free water in the cleats which could potentially initiate relative permeability effects.

Care was also taken to minimise damage to the coal structure and the formation of artificially

induced fractures, by not using a conventional oven. After about 24 hours of drying, the cores

were weighed. This was followed by full saturation using degassed water and a vacuum

pump. The cores were then re-weighed after 3 days of saturation to establish the pore volume

and macroporosities.

2.2 Experimental set up and test procedure

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During the measurements, a gas-water separation unit and a backpressure device were

connected in series to the outlet end of the Hassler cell core holder as illustrated in Fig. 1. The

gas-liquid separation tube (Tube 2) was designed especially tall to a height of 1.5 m so as to

accommodate as much gas as possible, yet sufficiently thin to minimise errors whilst reading

fluid levels. The internal diameter and wall thickness of the tubes were 25 mm and 6 mm

respectively. The Hassler cell was designed to withstand stresses of up to 100 MPa. Its end

platens were fitted with seals consisting of Viton O-rings possessing a shore hardness factor

of 90 to minimise deformation due to pressure. The gas-water separator tube was partially

filled with a low density paraffin oil, while the gas and water flow rates were measured by the

main outer tube (Tube 1) and the small upturned syringe respectively.

A single saturated coal specimen was inserted into a rubber core sleeve, which was then

loaded into the oil-filled Hassler cell. A confining pressure of 1,000 psi (6.9 MPa) was

applied to simulate the effect of overburden stress. For a number of coal types, the absolute

and relative permeability tests were repeated at high (6.9 MPa) and low (4.1 MPa) confining

pressures in order to assess the effect of overburden stress on the internal cleat structure and

pore size distribution.

Injection of gas into the Hassler cell causes water to be forced out of the fully saturated

core and simulates a drainage displacement process, as the saturation of the wetting phase

decreases throughout the experiment. As water is produced, it accumulates in the small

syringe forcing the oil column to move upwards. When gas production commences, it pushes

the paraffin oil downwards in the tall tube (Tube 1) and any oil that is displaced is transferred

to the second tube (Tube 2), which serves as an interface across which the backpressure is

transmitted. Due to its non-adsorbing characteristics and smaller molecular size, helium was

used as the injected gas in the experiments.

During the experiments, an overall pressure differential in the range of 50-60 psi (0.34 -

0.41 MPa) was applied across the core, based on an upstream gas injection pressure of 250

psi and a downstream backpressure of approximately 200 psi. The pressure gradient was

selected so as to be large enough to minimise capillary end effects, but also sufficiently small

compared with the total system pressure to render compressibility effects negligible.

Flow measurements were started once the inlet and outlet pressures had ceased to

fluctuate. Data were recorded more frequently just after gas breakthrough when flow rates

began changing more rapidly. Upstream and downstream pressures were also monitored at

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regular intervals. The use of sensitive Kenmac pressure regulators helped to achieve better

control over the pressures at each end.

Gas flooding was continued until approximately 4 litres of helium gas had been flowed

through the sample. This was done to ensure that the test was terminated only when the water

relative permeability had become negligible and the gas relative permeability was stable.

Once the separation system could no longer hold any more produced gas, the test was

terminated.

If the same core was to be used again for a repeat test, water was pumped through it to

flush out any gas that had accumulated inside. A backpressure was also applied in the

opposite direction to create a surging effect to facilitate the expulsion of this trapped gas. At

the end of each test the cleat porosity was checked by observing the total volume of water

expelled from the core as a result of the gas flood.

3. Relative permeability results and analysis

The Johnson, Bossler and Neumann (JBN) [9] method was used to calculate relative

permeability curves from the unsteady state test data. In order to apply this procedure

successfully, the system was allowed to stabilise over a one-month period prior to the

experiments so as to minimise capillary end effects. Average end points for different coals

are shown in Table 1 (bottom three columns), along with other relevant coal characterisation

data obtained during laboratory work and data analysis. Examples of representative relative

permeability data derived for each of the seven coal types from the laboratory tests are

presented in Fig. 2.

Splint (Fig. 2c) appears to be quite different from the other coals, having an abnormally

high irreducible water saturation and steep relative permeability curves. The samples were

heavily fractured with large visible flow conduits that were responsible for the channelling of

gas and water at high flow rates not ideal for relative permeability experiments. Similar

behaviour has been reported for sub-bituminous coals in the Powder River basin. The fact

that Splint and Selar 9ft (Fig. 2f) have irreducible saturations at opposite ends of the

saturation range typifies the variability that is so common in coals. Selar 9ft is also mildly gas

wet with an average equipotential flow point (cross point saturation) in the range between

0.55 and 0.60, i.e. greater than 0.5. It is the only coal from the set which exhibited this kind of

behaviour as all the others were water wet to differing degrees (Table 1). Nevertheless, such

conclusions are based on empirical correlations and should therefore be treated with caution.

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Tupton (Fig. 2d), Selar 9ft (Fig. 2f) and Tower 7ft (Fig. 2g) coals generally appear to have

more familiar curve shapes and are comparable to those determined by Gash et al. [7]. In

particular, data from Tower 7ft and Selar 9ft, which have a similar rank and elastic properties,

display a greater coherence than Tupton coal, whose data are moderately dispersed.

Some of the coals, notably Schwalbach (Fig. 2a) Warndt-Luisenthal No.1 (Fig. 2b) and

Dora (Fig.2e) exhibit convex shaped gas relative permeability curves and a relatively flatter

water relative permeability profile. All three coals originate from the Saar/Lorraine Basins

shared by France and Germany. In the case of Schwalbach, the extended water leg could be

attributed to its robust mechanical properties and low permeability, hence the much longer

period over which tests had to be conducted. The breakthrough time for Schwalbach samples

ranged between 12 to15 hours while only 30-45 minutes elapsed in the case of Splint.

The convex shape of the curves suggests that gas flow is not occurring completely through

the main cleat pathways. Instead, part of it is passing through the matrix or other units within

the coal structure. Although water saturation is decreasing, the regions within the structure

from which water is being driven out do not contribute significantly to retarding gas flow. On

the other hand, if the curves are concaved upwards or straight lines, then the gas is able to

drive the water more easily.

Post breakthrough water production was very small in each case, giving rise to generally

low water relative permeabilities. Critical gas saturations (Sgcrit) obtained for the seven

European coals tested appear to be spread out over a broad range of saturations but were

generally found to lie in the 15-35 % band, which sits at the upper end of the range reported

in the literature (Table 2). Irreducible water saturations (Swirr) ranged from 15-40 % for all

coal types except Splint. These values are generally lower than those obtained from previous

work (Table 2).

3.1 Effect of wettability on coal relative permeability

All the coals were found to be water wet to differing degrees by virtue of their cross point gas

saturations being less than 0.50, except Selar 9ft, which displayed moderate gas wettability.

Splint coal was the most water wet with an average cross point saturation of only 0.21. The

composition of a coal in terms of its mineral matter content and the dominant lithotypes

influences the wettability, which in turn affects the relative permeability. The presence of

more clarain and vitrain bands in Selar 9ft coal (Table 3) may explain its gas wetness.

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Similarly, coal samples from the Lorraine basin are considered to be more gas wet in

comparison to Warndt-Luisenthal No.1 which is strongly water wet. This tendency towards

intermediate gas wettability in the former is characterised by the following features as

demonstrated in Fig. 3.

An increase in gas saturation at the crossover point from 0.35 to 0.42.

An increase in the cross point relative permeability itself from 0.11 to 0.19.

An overall decrease in the gas relative permeability curve and increase in water

relative permeability values.

A decrease in the irreducible water saturation, i.e. the 1-Swirr end point increases from

0.80 to 0.84 on the chart.

It is worth noting that Dora coal is semi-anthracitic in terms of rank but also contains a

significant amount of ash,constituting some 36.7% by weight.

Coal wettability can also be affected by monolayers of adsorbed methane or carbon

dioxide, which may act as surface active polar compounds. Thicker organic materials

deposited onto coal could have an impact as well. In coal seams where methane productivity

is hindered by unfavourable gas-water mobility ratios, the gas relative permeability could be

improved by altering the wettability of the coal artificially using particular agents that

increase the gas relative permeability. However, given the highly stress dependent nature of

coal permeability, the feasibility of such processes would need to analysed in depth.

3.2 Effect of confining pressure on coal relative permeability

The only published studies to date that have reported the effect of confining pressure on

coal relative permeability are by Dabbous et al. [4] and Gash et al. [7]. Their results have so

far been inconclusive. Studies by Gash et al [7] found that an increase in confining pressure

from 3.1 MPa to 6.9 MPa caused the gas relative permeability to decrease less than the water

relative permeability. Whilst the converse could also be true, where water relative

permeability increases by a greater extent than the relative permeability to gas, owing to a

reduction in confining pressure, neither effect was clearly observed during these experiments.

Fig. 4 shows comparisons between relative permeability data obtained at two different

confining pressures for Tupton and Tower 7ft coals respectively. The change in stress from

4.1 MPa to 6.9 MPa causes a small but noticeable shift in the curves towards lower relative

permeabilities. The interpreted end point saturations are also reduced, with the irreducible

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water saturation in particular being higher due to the entrapment of water pockets. There is

also a slight shift towards lower gas saturations, which is confirmed by Dabbous et al. [4]

who measured gas relative permeabilities for Pittsburgh coals at overburden pressures of 1.38

MPa and 4.14 MPa. However, their end points appeared to be independent of confining

pressure and there was even a slight increase in gas relative permeability at the higher

overburden stress level.

Earlier breakthrough was noticed during some of the laboratory tests at higher confining

pressures. However, this behaviour was not observed consistently and may be insignificant

because, although the application of confining stress can cause the cleats to narrow, it might

not be sufficiently large to alter the cleat configuration.

4. Discussion

Research has suggested that when the effect of large cleats dominates, the relative

permeability curves become straighter and narrower, while if the matrix effect is more

predominant then the curves tend to be spread over a wider saturation range and are less

linear. This trend was observed in the results where those samples containing a larger

concentration of fractures parallel to the direction of flow tended to give rise to steeper

curves, resembling straight lines over a narrow saturation range. This was accompanied by an

overall shift towards higher water saturations and corroborates the work of previous

researchers such as Meaney and Paterson [1].

On the other hand, some of the curves obtained so far have displayed a very sharp increase

in gas relative permeability at high gas saturations, while a much shallower decrease in water

relative permeability is observed at lower values. Consequently, the water relative

permeability effectively falls to zero very soon after breakthrough has occurred, confirming a

high irreducible water saturation. This behaviour could represent a possible shift from the

cleat contribution initially dominating to the matrix becoming more prevalent later on.

Further investigation is necessary to establish if it is realistically possible to obtain separate

cleat and matrix relative permeability curves.

It is worth noting that in Fig. 5 the initial productivity index (q/ΔP)i influences variation of

data along the relative permeability axis while pore volume affects variation in the gas

saturation direction.

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Average relative permeability data for all coals are compared as relative permeability

ratios on the same axes in Fig. 6. Apart from Splint and Selar 9ft, all other lines fall within a

similar range and are bounded by the envelope shown. The gas saturations at which krg/krw =

1, i.e. the equipotential flow point, range between 0.29-0.38 within the envelope. At the same

time, the curves are generally linear for the four UK coals and may be described by a

relationship of the form ln (krg/krw) = aSg + ln b, where a and b are constants. However, coals

from the Saar/Lorraine Basins are more difficult to characterise mathematically due to their

non-linear nature. Whilst the abnormal behaviour of the Splint coal can be accounted for by

its highly fractured nature, the deviation of Selar 9ft is more difficult to explain.

Helium was used during the relative permeability tests and it was assumed that gas did not

adsorb onto the coal during drainage. In reality, however, the presence of adsorptive gases

such as methane and CO2 means that the process would be occurring, albeit much slower and

over a long period, and could therefore affect coal permeability and relative permeability in

the seams.

Since coal is a highly heterogeneous material, the standard deviations of data do not

necessarily indicate measurement error, and therefore it would not be appropriate to use this

by examining final results alone.

4.1 Use of empirical correlations

One of the main difficulties experienced during the experiments was the measurement of

water relative permeabilities directly. This was partly due to the low pore volumes of the core

samples, which meant that there was insufficient production of water to be able to determine

krw values over a wide enough saturation range. This was mitigated to some extent by using

regression extrapolation or Brooks-Corey relationships to compute actual values near the end

points. Whether or not the use of such correlations is truly applicable to coal seams is still an

area of contention. The modified Corey equations are based on flow through capillary tubes

of a particular distribution of pore throat radii. However, the cleat system in coals does not

actually have a geometry resembling capillary tubes.

Boatman (1961): 35.1)( wwrw SSk  and  5.05.025.0 ])()(1)[1( wwwrg SSSk (2.24) 

where  krw  is  the water  relative  permeability,  krg  is  the  gas  relative  permeability,  Sw  is  the water 

saturation and the normalised effective water saturation (Sw*) is given by  

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grwirr

wirrww SS

SSS

1            (2.25) 

Brooks & Corey (1964):  awrw Sk )(  and  )1()1( 2 w

bwrg SSk        (2.26) 

Equations 2.24 to 2.26 were applied to the experimental data for the different coals giving

rise to average values of the modified Corey exponents summarised in Table 4.

4.2 Sources of error in the results

A satisfactory level of consistency was observed between samples tested from the same coal

type as indicated in Fig. 7.

Errors occurred whilst reading the meniscus levels of the gas-liquid separator as well as

due to inaccuracies in pressure readings. The total error involved in measuring the liquid

menisci in the separator tube was approximately 1.0 cm3. Assuming an average volume

measurement of 6 cm3, the percentage error for each test was about 17 %. The error in

reading pressures was of the order of 0.5 psi (3.45 kPa) for both gauges over a pressure

range of 0-600 psi (0-4.14 MPa).

Relative permeability curves for some of the Schwalbach samples were also found to

match remarkably well with curves determined independently using data from the Weiher-1

well, also drilled in the Saar coal Basin [12]. The characteristic convex shape of the gas

relative permeability curve was observed in both cases, thus highlighting a clear resemblance

between the two sets of data as illustrated in Fig. 8. The lines shown for the Schwalbach coal

are only a visual guide and should not be treated as fitted curves.

One of the limitations of the gas-liquid separator is the fact that gas bubbles pass through

the oil-water interface causing it to become irregular and unstable, and therefore resulting in

inaccuracies when reading water volumes. Whilst saturating the cores in water, there was a

tendency for very small pieces of coal to break off; this will have altered the pore volume

calculations. It could also be argued that the relative permeability tests were not truly

representative of reservoir flow conditions because inert gases such as helium or nitrogen

were used instead of methane, which would cause swelling of the coal. However, it was felt

that the gas flow rates were too high for the influence of matrix swelling to be satisfactorily

observable during the time period over which the tests were conducted. Moreover, the need to

isolate the effects of matrix swelling and two-phase flow were considered important at this

stage, so that an initial baseline of results could be generated. Safety issues with regards to

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handling a methane-water mixture at the downstream end were also taken into account for

this decision.

4.3. Mechanism for two-phase flow in coal

Based on the analysis, it was observed that some two-phase flow is active in the outer regions

of the matrix near the cleat boundary. However, this does not mean that there are relative

permeability effects necessarily taking place within the bulk of the matrix block. Water

particles accumulate in the larger macropores forming a coat of water adjacent to the

cleat/matrix boundary. As a result, gas particles diffusing from the central regions of the

matrix are unable to pass into the cleat network until the concentration of water has dropped

sufficiently to allow gas to find a pathway. This phenomenon is described in Fig. 9.

5. Conclusions

Gas-water relative permeability experiments carried out on different ranks of European

coals using an unsteady state method yielded critical gas saturations in the range of 0.15 to

0.35. Irreducible water saturations ranged between 0.15 and 0.40.

Considerable variability in the shapes of the relative permeability curves was also

observed, which is mainly attributed to the heterogeneous nature of coal, both in terms of

lithotype composition and cleat-matrix configuration. Modified Corey equations were used to

fit some of the experimental data. Although end point saturation values lacked consistency,

the results were generally found to be in keeping with the limited work carried out to date.

References

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Conference, Adelaide, Australia;1996; SPE 36986.

[2] Reznik AA, Dabbous MK, Taber JJ, Fulton PF. Air-water relative permeability studies of

Pittsburgh and Pocahontas coals. SPE Trans. AIME 1974; 257:556-61.

[3] Corey AT. The interrelation between gas and oil relative permeabilities. Producers

Monthly 1954; 19 (1): 38-41.

[4] Dabbous MK, Reznik AA, Mody BG, Fulton PF, Taber JJ. Gas-water capillary pressure

in coal at various overburden pressures. SPE Journal 1976; 16 (5): 261-268.

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[5] Puri R, Evanoff JC, Brugler ML. Measurement of coal cleat porosity and relative

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Table 1. Coal characterisation data obtained during the laboratory experiments and data analysis.

Coal Seam

Schwalbach W-L No.1 Splint Tupton Dora Selar 9ft Tower 7ft

Volatile Matter (d.a.f) (%)

43.6 41.6 40.2 35.3 16.5 10.2 9.1

Fixed Carbon (d.a.f.) (%)

56.4 58.4 59.8 64.7 83. 5 89.8 90.9

Vitrinite Reflectance (%)

0.79 0.71 0.55 0.49 0.71 2.41 2.28

Moisture Content (%) 1.54 1.39 13.3 15.7 0.83 2.07 0.86

Coal Rank High Vol.

Bituminous B

High Vol. Bituminous

B

High Vol. Bituminous

B

High Vol. Bituminous

A

Semi-anthracite Anthracite Anthracite

Young’s Modulus, E (GPa) 3.20 – 3.90 2.19 – 2.69 1.80 – 2.30 1.10 – 1.62 2.41 – 2.84

1.75 – 2.58

1.82 – 2.26

Poisson’s Ratio, 0.26 0.42 0.34 0.36 0.38 0.40 0.32

Absolute Permeability (mD)

0.90 0.52 0.73 2.15 5.52 9.51 2.93

Porosity (%) 0.63 1.76 1.80 1.35 1.38 0.96 0.12

Average Critical Gas Saturation 0.35 0.32 0.15 0.23 0.33 0.40 0.25

Irreducible Water Saturation 0.22 0.19 0.68 0.36 0.15 0.17 0.34

Average Cross Point Gas Saturation 0.40 0.35 0.21 0.38 0.42 0.58 0.37

Table 2. Summary of end point relative permeability data from previous studies.

Source Swirr Sgcrit Cross-point Sg

Reznik et al. (1974) 0.60 0.07 0.15

Jones et al. (1988) 0.80 0.01 0.05

Young (1989) 0.36 0.04 0.38

Puri et al. (1991) - San Juan 0.38 0.01 0.43

Gash (1991) - Unsteady State 0.35 0.32 0.40

Hyman et al. (1992) 0.40 0.18 0.27

Meaney and Paterson (1996) 0.60 0.11 0.31

Shen et al. (2011) 0.47-0.74 0.02-0.36 0.10-0.27

  16

Table 3. Coal petrology data.

Vitrinite Liptinite Inertinite Minerals

Schwalbach 75.4 17.4 5.0 2.2

Warndt-Luisenthal No.1 74.4 15.6 9.0 1.0

Splint NA NA NA NA

Tupton 59.4 14 25.8 0.8

Dora 31.4 0 0 68.6

Selar 9ft 85.6 0 14.2 0.2

Tower 7ft 84.6 0 15.2 0.2

 

Table 4. Corey exponents for coals of different rank.

Coal Type Water Exponent Gas Exponent

Schwalbach 1.23 2.21

Warndt-Luisenthal No.1 4.15 1.73

Splint 3.71 2.12

Tupton 4.90 1.50

Lorraine 3.19 2.79

Selar 9ft 5.10 2.79

Tower 7ft 3.15 1.51  

  17

Fig. 1. The experimental set-up for the relative permeability tests.

  18

   (a) Schwalbach – Sample 4 (b) Warndt-Luisenthal No.1 – Sample 5

  (c) Splint – Sample 2 (d) Tupton – Sample 6

   (e) Dora – Sample 10 (f) Selar 9ft – Sample 5

 

 (g) Tower 7ft – Sample 7

Fig. 2. Examples of relative permeability curves for the seven coals tested.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krgkrw

  19

 

Fig. 3. Effect of wettability on relative permeability curves.

Fig. 4 Effect of confining pressure on relative permeability for Tower 7ft and Tupton coal samples.

 

Fig.5. Cleat and matrix effects on relative permeability behaviour.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krg (Warndt No.1)

krw (Warndt No.1)

krg (Lorraine)

krw (Lorraine)

Tower 7ft (Sample 4)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krg (4.1 MPa)

krw (4.1 MPa)

krg (6.9 MPa)

krw (6.9 MPa)

Tupton (Sample 2)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krg (4.1 MPa)

krw (4.1 MPa)

krg (6.9 MPa)

krw (6.9 MPa)

0.45 0.85Gas Saturation

Rel

ativ

e P

erm

eabi

lity

All gas produced here through the

main cleat system

All gas produced from smaller cleats

or even matrix

0.0

1.0

1.0

  20

 

Fig. 6. Relative permeability ratios for the averaged curves determined in the laboratory.

 

  (a) Splint          (b) Schwalbach 

Fig.7. Relative permeability curves for different samples from the same coal seam.

0.001

0.01

0.1

1

10

100

1000

0.0 0.2 0.4 0.6 0.8 1.0Sg

krg /kr

w

krg (Schwalbach) krg (Lorraine) krg (Tower 7ft)krg (Tupton) krg (Splint) krg (Warndt)krg (Selar 9ft)

Splint (Samples 2 and 3)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krg (Sample 2)

krw (Sample 2)

krg (Sample 3)

krw (Sample 3)

Schwalbach (Samples 1 and 4)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Gas Saturation

Rel

ativ

e P

erm

eabi

lity

krg (Sample 1)

krw (Sample 1)

krg (Sample 4)

krw (Sample 4)

  21

  (a) Weiher‐1         (b) Schwalbach 

Fig. 8. Relative permeability curves for different coals from the Saar Basin.

 

Fig. 9. Schematic description of the two-phase flow near the boundary between cleat and matrix.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Water Saturation

Rel

ativ

e P

erm

eabi

lity

Gas

Water

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Water SaturationR

ela

tive

Pe

rme

ab

ility

Gas

Water

Gas particles in bulk of matrix

Coat of water along cleat/matrix

boundary

Cleat

Matrix Block

Gas particles unable to leave matrix until

water saturation has decreased