The effect of gas evacuation on coal permeability test specimens

Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 21, No. 3, pp. 161-164, 1984

Printed in Great Britain. All rights reserved

0148-9062~84 $3.00+ 0.00 Copyright (( 1984 Pergamon Press Lid

Technical Note The Effect of Gas Evacuation on Coal Permeability Test Specimens S. HARPALANI* M. J. McPHERSON*

INTRODUCTION Over the past 30 years, emissions of methane into coal mines have increased significantly. This has occurred because of increased comminution of the coal by mechanized procedures, higher productivity, faster moving faces and a trend towards deeper workings [1]. In order to plan the ventilation requirements of a mine, it is important to have an estimate of the rate at which methane is emitted from source beds and migrates through the strata towards the workings. Numerous computer models have been devised to simulate the release and migration of methane in strata surrounding mine workings, and to assist in the design of methane drainage systems. Most of these models rely upon a knowledge of the permeabilities of the coal and the associated strata. Such flow conductivities depend not only upon the permeability of the undisturbed rock, but also upon the "stress history" of the area; that is, the cycles of stress change that have occurred due to mining. Hence, the permeability of rock in any given location is a function of the rock type, its location with respect to mine workings and time. A number of research centres have carried out permeability-stress tests on rock samples from mining areas or petroleum reservoirs. In many cases, the mechanism of gas migration is complicated by the adsorptive attraction that exists between the gases and the internal surfaces of pores and flow paths. It has often been routine procedure to commence such tests by evacuating the sample until the internal voidage is "cleaned" of water vapour and other adsorbed gases that might otherwise interfere with subsequent permeability measurements [2]. During a sequence of permeability-stress experiments on coal samples at the University of California, Berkeley, it was found that the process of evacuation itself caused a reduction in single-phase permeability. This raises questions concerning the accuracy of previous permeability or adsorption tests that have involved evacuation of carboniferous rock types, and may also have repercussions on the use of high vacuums in methane drainage systems. This paper examines the * D e p a r t m e n t of Materials Science and Mineral Engineering, University of California, Berkeley, C A 94720, U.S.A. 161 R.M.M.S.21/~-D

possible causes of the phenomenon and outlines a method of avoiding it during laboratory testing.

OBSERVATION OF REDUCED PERMEABILITY

FOLLOWING EVACUATION The effect was observed during a series of tests in which the main objective was to determine the variation of permeability with respect to axial and radial loads applied to cylindrical specimens of coal obtained from the mid-Western coalfields of the United States. Such variations exhibit hysteresis effects on permeabilitystress graphs. Reductions in permeability due to applied stress are only partially recoverable when the stress is removed [3]. There is also a time-dependent effect due to the "creep" properties of the coal following any change in applied stress. Figure 1 is a schematic diagram of the apparatus used during the tests. Carefully machined cylinders of coal, 38.1 mm dia and 76.2 mm in length were prepared. The triaxial permeameter cell enabled gas to be passed through each sample. Independent control of the axial stress was maintained by mounting the cell in a stiff testing machine, and of the radial stress by a hydraulic system exerting oil pressure on a 4 mm thick silicone rubber/flexane sheath around the coal, Ancillary monitoring equipment enabled any change in bulk volume of the sample to be measured. The specimen could be evacuated while still in the cell by attaching both inlet and outlet tubes to a vacuum pump. As part of the overall investigation, it was intended to determine the permeability of the samples, first with nitrogen then with methane gas. Owing to the variability of core samples, it was decided to run a nitrogen test followed immediately by a methane test on the same sample. Successive tests on any given sample, even with the same gas, result in decreasing permeabilities because of the effect of cyclic stressing on coal. A series of experiments was performed until the nitrogen gas flow-rate-time curves could be predicted for second and successive cycles on any given sample. It was then required to evacuate the sample to remove all nitrogen and continue with methane. However, as a precautionary measure it was decided to test the technique by a further nitrogen test following the evacuation.

162

HARPALANI and McPHERSON: TECHNICALNOTE ®

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Fig. 1. Schematicdiagram of apparatus [4].

The curve of Fig. 2 shows the time dependence of permeability when the coal sample was subjected to constant load and constant gas pressure. For this test, a hydrostatic stress of 2.068 MPa (300 psi) was maintained with an inlet nitrogen gas pressure of 0.276 MPa (40 psi) gauge. The outlet gas pressure was effectively that of the external atmosphere throughout the tests. The decrease in gas flow rate with time demonstrates the "creep" effect of coal under constant stress and constant gas pressure conditions. In the test illustrated in Fig. 2 the flow rate reached equilibrium after 5½hr. However, following the initial test, the sample was evacuated for 17.5 hr at a pressure of some 5-8/~m. The original gas inlet pressure level was then restored and the nitrogen flow rate determined. The resulting flow rate attained equilibrium at a very low value after 2 hr. This flow rate, represented in Fig. 2 by point A, was well below that expected for a repeated test. This somewhat unexpected result was verified by further tests. In all cases, evacuation appeared to cause a substantial reduction in the permeability of the coal.

POSSIBLE

EXPLANATIONS

Two possibilities were considered in searching for an explanation for the reduction in permeability following evacuation. First, there may have been a partial collapse of the internal pore structure as the gas pressure within the voidage was reduced. In order to test this hypothesis, the bulk volume of the sample was monitored. The reduction of gas flow rate that occurred during the 3 hr "creep" period was accompanied by a corresponding reduction in bulk volume. However, no further fall in bulk volume took place during evacuation. This indicated that it was unlikely that any internal collapse occurred during the evacuation process. The second hypothesis was that some of the volatile materials from within the coal matrix exuded into the pore spaces and capillaries. The fall in gas pressure during evacuation may have caused movement across the envelope of melting point lines that exists on the pressure-temperature phase diagram for the mixture of volatiles within the coal. This would lead to the appear-

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Fig. 2. Creep characteristics and effect of evacuation on coal.

27

HARPALANI and McPHERSON: TECHNICAL NOTE

163

ance of liquid globules within the pores causing partial blockage of the pore spaces and the interconnecting capillaries. Some of the components may pass across their sublimation lines and change phase directly from solid to gas. In either case, it is likely that dislocations would occur on the pore walls. In order to test this hypothesis, a specimen of the coal was examined under a transmission electron microscope. This necessitated placing the specimen in a vacuum chamber in order to avoid interference of the electron beam by gas molecules in the air. Figure 3 is a micrograph obtained at a magnification of 50,000, 9 min after the commencement of evacuation--the earliest time at which a clear image could be obtained of a suitable site. Figure 4 shows the same site after 17 hr of evacuation. A comparison of these photographs in the area circled shows that a diminution of the pore space appears to have taken place. It is suspected that much of the change may have occurred quite rapidly during the initial minutes of evacuation and that the alteration in appearance may represent the tail of a decay process. Nevertheless, the evidence indicates that evacuation did indeed cause partial closure of the pores.

Fig. 3. Coal pore 9 min after evacuation.

MODIFIED PROCEDURE OF EVACUATION Following the electron microscope observations, it was decided to experiment with evacuation at low temperature in an attempt to prevent, or at least reduce significantly, the change of phase of the volatiles within the coal. A fresh, unevacuated core was placed in the triaxial permeameter and a nitrogen flow-rate-time test carried out under the same constant stress and gas pressure conditions as in the previous experiments. The result is shown as the upper curve on Fig. 5. The sample was then removed from the cell and placed in a glass jar packed with styrene pellets on all sides. The jar was placed in liquid nitrogen and the temperature of the coal surface observed on a low tem-

Fig. 4. Coal pore 17 hr after evacuation.

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164

HARPALANI and McPHERSON: TECHNICAL NOTE

perature thermometer. The purpose of the styrene insulation was to prevent damage to the sample from thermal shock. The temperature fell gradually, reaching stability at - 1 5 0 ° C after some 2 hr. Whilst maintaining the temperature of the coal at - 150°C, the jar was connected to a vacuum pump and evacuated to 5/~m for a period of 13hr. This was followed by removing the jar from the liquid nitrogen, attaching it to a gas cylinder and flooding the sample with nitrogen at 0.276 MPa (40 psi). This was the same pressure as at the gas inlet during the flow-rate-time tests. The sample was then replaced in the triaxial permeameter and a second flow-rate-time test carried out. The result, indicated by the lower curve of Fig. 5, is typical of that expected of a second stress cycle in the laboratory. The technique of evacuating at low temperature had avoided, or severely diminished, the reduction in permeability caused by the earlier room-temperature evacuations. On the other hand, the original purpose of evacuating samples prior to permeability or adsorption tests was to clean the pores of adsorbed and gaseous contaminants. Thus, it might be expected that low-temperature evacuation would cause a change in observed permeability. The results obtained in these tests showed no evidence of any deviation from a normal second-cycle curve. The need for evacuation, therefore, becomes questionable. It may be the case that simple forced flushing of a single phase gas through the sample is suflicient to reduce the presence of other contaminant gases to small proportions. This must be dependent upon the relative adsorbtivity of the gases involved. CONCLUSIONS (1) The experimental work described in this paper has indicated that evacuation of coal at normal temperatures causes partial clogging of the flow paths and a consequential reduction in permeability. Evidence has been

advanced to suggest that this may be due to liquefaction or sublimation of volatile materials at very low pressures. (2) Evacuating coal samples at low temperature ( - 150°C) prevents, or reduces, the fall in permeability. This, however, may be a balance of compensating effects of the removal of contaminant gases and an escape of volatiles from the coal matrix. (3) Forced flushing by a single gas may be effective in cleaning a sample of other contaminant gases, dependent upon the rate and time of flushing, and the force of adsorption of each of the gases involved. The purpose of this Note is to report a phenomenon that affects the accuracy of permeability and adsorption measurements of coal samples. A great deal of experimentation remains to be done, including (i) further and more detailed electron microscope studies of coal samples while undergoing changes of temperature and pressure, and (ii) investigation o f the displacement of original pore gases by a single flushing gas. In the meantime, it is recommended that whenever evacuation of a carbonaceous rock is required as part of specimen preparation, and prior to any form o f rock characterization, then that evacuation should be carried out at low temperature.

Received 24 October 1983.

REFERENCES

1. McPherson M. J. and Hood M. Ventilation planning for underground coal mines. Annual Rept, U.S. Dept of Energy, Contract W-7405-ENG-48 (1981). 2. Somerton W. H., SoylemezogluI. M. and Dudley R. C. Effectof stress on permeability of coal. Final Rept, USBM Contract H0122027 (1974). 3. Gawuga J. Flow of gas through stressed carboniferous strata. Ph.D. thesis, Univ. of Nottingham (1979). 4. Mordecai M. and Morris L. H. An investigationinto the changes of permeability occurring in sandstone when failed under triaxial stress conditions. Proc. 12th Syrup. of Rock Mechanics, pp. 221-238. AIMMPE, New York (1971).

GEOMECHANICS ABSTRACTS General

81A

Properties of Rocks and Soils Texture, structure, composition and density Fracture processes Strength characteristics Deformation characteristics Surface properties Time dependent behaviour Physico-chemical properties Permeability and capillarity Compressibility, swelling and consolidation Dynamic properties Classification and identification

81A

Geology Tectonic processes Environmental effects, weathering and soil formation Earthquake mechanisms and effects Frost action, permafrost and frozen ground Hydrogeology Groundwater Chemical and physical changes due to water Measurement of water pressure and its effects Underground Excavations Mines Tunnels Power plants In-situ stresses and stress around underground openings Surface subsidence and caving Temporary and permanent supports Geological factors of importance in underground excavations Construction methods Groundwater problems Influence of dynamic loads due to explosions or earthquakes

83A 83A 84A 87A 89A 90A 90A 90A 90A 91A 92A 92A 92A 92A 93A 93A 93A

93A 94A 94A 96A 97A 98A 98A 99A

100A 101A

Surface Structures Dams and embankments Foundations Slopes Hydraulic structures Earth retaining structures Base courses and pavements Geological factors of importance in surface structure Construction methods Groundwater problems Influence of dynamic loads due to explosions or earthquakes

101A 101A 104A 106A 11 IA 11 I A 112A

Comminution of Rocks Rock fracture under dynamic stresses Drilling Blasting Crushing and grinding Cutting Hardness, abrasion and wear

112A

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I I 3A 113A

Rock and Soil Improvement Techniques Bolts and anchors Grouting Groundwater control Soil stabilisation Soil compaction Freezing

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Site Investigation and Field Observation Planning, geotechnical and structural mapping Core recovery, logging, probing, boring and sampling Photographic techniques Geophysical techniques Presentation and interpretation of data

118A

Subjects Peripheral to Geomechanics General geology Petroleum engineering Concrete technology Snow and ice mechanics

115A 117A 118A

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EXPLANATION OF ABSTRACT FORMAT The information contained in the abstract entries themselves is described in the following example: 841032 Constitutive relations of coal and coal measure rocks. Volumes 1-3

Barbour, TG; Atkinson, RH; Hon Yim Ko; Hawk, DJ; Gerstle, KH US Bureau of Mines report 0FR54(1)-(3)-83, March 1980, 360P The purpose of this project was to study the constitutive properties of coal under both short- and long-term ioadings and to formulate constitutive relations for coal in forms that could be used in stress analysis . . . .

Avail: NTIS, Springfield, Va, 22161 USA (PB 83-178 236)

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