experimental study on improving coal permeability with supercritical co2 and low-carbon alcohol...

Procedia Engineering 26 (2011) 2139 – 2145

1877-7058 © 2011 Published by Elsevier Ltd.doi:10.1016/j.proeng.2011.11.2417

Available online at www.sciencedirect.com

Procedia Engineering

Procedia Engineering 00 (2011) 000–000

www.elsevier.com/locate/procedia

First International Symposium on Mine Safety Science and Engineering

Experimental study on improving coal permeability withsupercritical CO2 and low-carbon alcohol solution

YANG Tao1,2, NIE Bai-sheng1,2, ZHAO Cai-hong1,2, LIU Xinna1,2, LI Qian1,2 a1. School of Resource and Safety Engineering, China University of Mining & Technology (Beijing), Beijing 100083,P.R.China;

*

2. State Key Laboratory of Coal Resource and Safe Mining, Beijing 100083, P.R.China;

Abstract

The high-gas and low-permeability are the common problems of China coal mine, which restrain the mining of coal resources safely and efficiently. To solve the low permeability of coal-seam, the variation of permeability was studied experimentally with coal effected by supercritical CO2 and low-carbon alcohol solution (including methanol solution, ethanol solution and isopropyl alcohol solution). The results show the permeability of coal after injecting supercritical CO2 is nearly one order of magnitude over former testing. This is principally resulted from that supercritical CO2 will extract some organic matters of coal substrates. Meanwhile, the permeability of sample effected by low-carbon alcohol solutionis apparently higher than former experiment without solution. Moreover, the bigger volume concentration to certain low carbon alcohol solution, the greater the permeability will increase. As to the three different low-carbon alcohol solution, the isopropyl alcohol solution has a biggest influence on improving coal permeability, whereas the methanol solution has a smallest one.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong.

Keywords: coal; supercritical CO2; low-carbon alcohol solution; permeability

China has the third coal reserves in the word, with 98900 million tons recoverable coal reservesmeanwhile 36.7 trillion tons coal bed gas reserves. However, the structure of the coal seam in China mainly belongs to hyposmosis coal seam, which makes the difficulty for exploiting the coal bed gas [1, 2]

and would be harmful for the prevention of coal gas accident. Therefore, in order to improve the recovery ratio of the coal bed gas and reduce the possibility of the gas accident, it is necessary to study how to increase the permeability of the coal, meanwhile national and international experts have done many

* Corresponding author:Nie Baisheng. Tel.: 8610-82375620.E-mail address:[email protected]

2140 YANG Tao et al. / Procedia Engineering 26 (2011) 2139 – 21452 Yang Tao, Nie Baisheng et al / Procedia Engineering 00 (2011) 000–000

studies in this area. [3-11] Supercritical CO2 extraction technology is a new method which has been widely used in chemical separation field. Generally, the dissolving capacity of the supercritical fluid would increase as the density of the fluid increasing and the critical density of the supercritical CO2 is the largest in the common supercritical fluid, thus the supercritical CO2 has a strong dissolving capacity. [12] What is more, the supercritical CO2 would induce the change of the fracture when reacting on the coal, which is important to improve the permeability of the coal. Meanwhile, the swell mechanism of the low-carbon alcohol solution indicates that when the low-carbon alcohol react on the coal sample, it can extract the ring aromatic compounds and some oxygenated chemicals and few aliphatic series and aromatic compounds that exist as ether link.

This study based on the swell mechanism of the low-carbon alcohol, by means of experiments, we studied the influence of the supercritical CO2 and low-carbon alcohol solution (such as carbinol, alcohol, isopropanol) on the permeability of the coal and relevant law. 1 Experiment methods 1.1 Samples

The coal samples used in these experiments were collected from zichang coal mine in shanxi and dayan coal mine of neimenggu. They are all belong to low- permeability coals. The industrial analysis data of the two kinds of samples is show in table 1.

Table 1 The industrial analysis data of the two kinds of samples

Coal

samplesAad% Mad% Vad% FCad%

zichang 10.68 2.56 30.68 56.08

dayan 17.30 12.50 30.90 39.30

The coal samples shown in Fig.1 were processed into the standard cylinder specimens of φ50×100mm,and both ends surface were polished to meet the laboratory precision.

(a)zichang sample (b)dayan sample

Fig.1 coal samples

1 - gas gathering device; 2- product separation device; 3 - confining pressure cylinders; 4-- high pressure CO2 cylinders; 5-CO2 fill

cylinders; 6-displacement experiment reaction axe; 7-CO2 preheating and heat preservation device;8- gas accumulated flowmeter; 9-

temperature sensor

Fig.2 the system structure schematic diagram of SC-CO2 displacement experiment1.2 Experimental equipments 1.2.1 The SC-CO2 displacement experiment

2141YANG Tao et al. / Procedia Engineering 26 (2011) 2139 – 2145Yang Tao, Nie Baisheng et al / Procedia Engineering 00 (2011) 000–000 3

The equipment used in the SC-CO2 displacement experiment is self-developed, and it can be used to test the permeability of coal sample. Fig.2 shows the equipment’s mechanism schematic diagram.

1.2.2 The low-carbon alcohol solution experimental systemThe low-carbon alcohol solution experimental system is a self-developed container. The equipment’s

mechanism schematic diagram is showed in Fig.3.

1-bracket; 2-measureing cup; 3-sample; 4- corrosion resistance diaphragm; 5-gasket; 6- inlet; 7-outlet

Fig.3 The equipment’s mechanism schematic diagram of the low-carbon alcohol solution experimental system

1.2.3 Permeability test systemThe permeability test system is the full-automatic gas permeability test instrument (Ultra-Perm 200)

which was made in US, as Fig.4. The propellant gas used for testing is nitrogen.

Fig.4. Ultra-Perm 200 full-automatic gas permeability test instrument

ULTR-PERMTM 200 uses the advanced precise flowmeter and pressure sensor, which can make the testing for the steady gas flow reach a high level. Combining the automatic data collecting and real-time testing, the flow accuracy, efficiency, data accuracy and graphical display can be improved. The instrument can test the permeability with a range form 0.001md to 30000md, and it compatibles all existing core holders and can be used to test the pressure of the environment and reservoir. The operational software can make the data connect with the real-time data collected by different technologies or methods easily. 1.3 Experiments1.3.1 Supercritical CO2 extracting experiment

As the critical temperature and the critical pressure of CO2 are 31.06℃ and 7.39MPa respectively, the temperature and the pressure in this experiment are set as 50℃ and 8.0MPa respectively.

Connect all the needing instrument and heat the cyclic water area to constant 50℃ using the heater band, meanwhile make sure the thermal insulation effect of the pipelines and agitated reactor in a good sate. Then, open the gas inlet control valve slowly and open the gas outlet control valve of the agitated

3

5

6

2

34 7


reactor, keep the CO2 circulate in the coal sample, close the gas outlet control valve after 5 minutes as soon as the gas comes out the outlet. Keep this sate for 8 hours and make the Supercritical CO2 react withthe coal. 1.3.2 Low carbon alcoholic solution reaction experiment

First of all, make up the methanol, ethanol and isopropanol solution with different volume concentrations: 10%，20%，30%，40%，50%，60%，70%，80%. Then, inject the low carbon alcoholic solution with different volume concentrations into the reaction system which has prepared the coal sample respectively. After each injection, close the inject hole and keep 24 hours and then run off the solution, after that take out the coal sample and seal off with plastic wrap. Likeness, different volumeconcentrations solutions of the methanol, ethanol and isopropanol are used in the experiment successively.1.3.3 Permeability testing experiment

Firstly, connect the full-automatic gas permeability test instrument with power and adjust the pressure regulating valve at the front of the control panel to horizontal position, then open the switch at the front of the control panel and the computer, preheat for 30 minutes.

Before the above experiments, all the coal samples have been tested the original permeability, and after the above experiments, all the coal samples are tested the current permeability again.

In this experiment, the coal samples was loaded from the bottom of the core holder according to the serial number, keep the confining pressure at 400psi, and adjust the given pressure of the nitrogen gas cylinder to 70psi (0.5Mpa). Create a new file in the computer software system for inputting the information and the test condition of the coal sample. Set the sensor zero and adjust the pressure according to the permeability feature of the core (Dense core 0-50psi; middle penetrative core >4psi; high penetrative core 0-2psi). Then, measure the permeability for several times and record the result. 2 Experimental results

The coal samples after the supercritical CO2 extracting and Low carbon alcoholic solution reaction are shown in Fig.5.

(a)Coal samples after the supercritical CO2 extracting (b)the low carbon alcoholic solution reaction

Fig.5. the change of coal sample

2.1 The influence of the supercritical CO2 extracting on the permeabilityThe permeability can be calculated using the Equation:

( )0 0

2 21 2

2 p Q Lk

A p p

µ=

− (1)

Where, k is the permeability, md; p0 is the atmospheric pressure at the measuring point, MPa; Q0 is the flow, cm3/s; μ is the viscosity coefficient of the gas, cPa; L is the length of the sample, cm; p1 is the gas pressure in the inlet, Mpa; A is the cross sectional area of the sample, cm2.

Fig.6 shows the comparison of the permeability of the sample before and after the supercritical CO2

extracting with different volume stress change with pore pressure. It can be seen that, with the same volume stress, the changing tendency of the permeability before and after the supercritical CO2 extracting with pore pressure is different. However, after the supercritical CO2 extracting, the permeability increases obviously, the average rate of increase nearly 10 times.


1.0 1.5 2.0 2.5 3.0

1

2

3

4

5

6

7

8 data before the exberiment data after the exberiment

perm

eabi

lity/

10-5

D

pore stress//MPa1.0 1.5 2.0 2.5 3.0

1

2

3

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perm

eabi

lity/

10-5

D

pore stress/MPa1.0 1.5 2.0 2.5 3.0

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6


perm

eabi

lity/

10-5D

pore stress/MPa

(a) Θ =13MPa (b)Θ =15MPa (c)Θ =17MPa

1.0 1.5 2.0 2.5 3.0

1

2

3

4

5

data before the exberiment data after the exberiment

perm

eabi

lity/

10-5

D

pore stress/MPa

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

1

2

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4


perm

eabi

lity/

10-5D

pore stress/MPa2.0 2.5 3.0 3.5 4.0 4

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

data before the exberiment data after the exberiment

perm

eabi

lity/

10-5

D

pore stress/MPa

(d) Θ =19MP (e)Θ =21MPa (f)Θ =23MPaFig.6.Comparison of the permeability of the sample before and after the supercritical CO2 extracting [8]

2.2 The influence of the low carbon alcoholic solution reaction on the permeabilityAs can be seen from Fig.7, the permeability of the coal increases after it reacts with the low carbon

alcoholic solution. It also can be seen that the changing rate of the permeability increases as the solution concentration increasing and the increasing rate is larger when the solution concentration is larger. Take methanol solution for example, when its concentration is 10%, the changing rate is the lowest 8.3 times, as the volume concentration increase the permeability increases too, while when its concentration reaches up to 100%, the changing rate is the largest 51.8 times. Therefore, the low carbon alcoholic solution is effective for increasing the permeability of the coal.

volume concentration /%

the

perm

eabi

lity

chan

ging

rat

e/%

0 20 40 60 80 1000

10

20

30

40

50

60

度


the

perm

eabi

lity

chan

ging

rate

/%

0 20 40 60 80 100

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the

perm

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lity

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rat

e/%

0 20 40 60 80 100

10

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80

(a)methanol (b)ethanol (c)isopropanol

Fig.7.The variation of permeability of the coal2.3 Result analysis

(1) According to the research of Stahl et al. [16] on the dissolution regular of the supercritical CO2, the hydrocarbon and lipoid organic compound with low polarity, such as ester, ether, lactone and epoxide, can be extracted out at the low pressure range 7~10MPa. In this study, the supercritical CO2 extracts the organic matter of the coal matrix, which can produce large amount of holes and fractures, and thus the


increase of the permeability of the coal after the supercritical CO2 extracting is larger than that before the supercritical CO2 extracting at the same experimental condition.

(2) Coal is a kind of complex organic rock consists of organic part and inorganic part. As a kind of organic solvent, the low carbon alcoholic solution has the extracting effect and its water solution has the quality of surfactant which can decrease the surface energy of solids and make the water solution wetting the coal surface. Meanwhile, according to the theory that similarities can be solvable easily in each other, the organic solvent can be used to extract the macromolecules in the coal and thus increase the porosityand the permeability of the coal.3 Conclusions

(1) As a kind of fluid with strong extracting power, the supercritical CO2 is available as the power gas for replacing the gas in the coal. After the supercritical CO2 extracting, the surface of the coal loses bright black and presents dark gray. The extracting experiment improves the development of the holes and the fractures in the coal, but it is hard to see the fractures spread all over the coal. As the development of the holes and the fractures, the permeability of the coal will increases obviously, the average increase nearly 10 times.

(2) The permeability of the coal increases obviously after the low carbon alcoholic solution reaction, and as the solution concentration increases, the increase of the permeability is more distant. The influence of the low carbon alcoholic solution with different volume concentration on the permeability is different:the lowest increase is 8.306 times and the largest increase is 51.830 times, which implies that the low carbon alcoholic solution can enhance the permeability.

(3) This study may have reference value for reforming the air permeability of the coal and may be useful for improving the permeability of the coal seal, increasing the gas drainage efficiency, decreasing the gas risk index of the high gas mine and promoting the efficient exploitation.

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experimental study on improving coal permeability with supercritical co2 and low-carbon alcohol...

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