Research ArticleExperimental Study and Percolation Analysis on SeepageCharacteristics of Fractured Coal and Sandstone Based onReal-Time Micro-CT

Zhaoxing Lv ,1 Qianqian Ji ,2 and Weijie Ren 3

1Key Laboratory of In-Situ Property-Improving Mining of Ministry of Education, Taiyuan University of Technology,Taiyuan 030024, China2College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan 030024, China3The 2nd Engineering Co., Ltd. of China Railway 12th Bureau Group, Taiyuan 030032, China

Correspondence should be addressed to Zhaoxing Lv; lvzhaoxing@tyut.edu.cn

Received 2 September 2020; Revised 28 September 2020; Accepted 22 October 2020; Published 5 November 2020

Academic Editor: Yanlin Zhao

Copyright © 2020 Zhaoxing Lv et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Sandstone and coal are the twomost common types of reservoirs in nature. The permeability of sandstone in oil-bearing formationscontrols its oil and gas production; the permeability of the coal seam containing gas has a crucial influence on the gas drainageefficiency. One of the main factors affecting rock permeability is the spatial distribution and connectivity of pores and fissures inthe rock. In this paper, a small-sized sample with a diameter of 5mm and a height of 10mm was used for the test. The rocksamples under different stress states were scanned in real-time during the seepage testing. Based on 2D images, a 3D digitalsample was reconstructed. We extracted the pores and fissures from the 3D digital sample, studied the size and distribution ofthe largest cluster in the sample, and revealed the influence of confining pressure and seepage pressure on the percolationprobability and permeability of the sample. The research results show that brittle sandstone and plastic coal, two types of rockswith completely different properties of mechanics, have obvious differences in the spatial distribution of the largest clusters.Under the same stress state, in brittle sandstone-like rocks, the connectivity of the fissures is the primary factor affectingpermeability, and the pores are the auxiliary factor; for plastic rocks such as coal, the situation is just the opposite, pores are theprimary factor affecting permeability, and fissures are the auxiliary factor. The research results answer the question: Hydraulicfracturing technology can increase the oil and gas production of sandstone reservoirs but cannot increase the drainage efficiencyof coalbed methane.

1. Introduction

Rock is a natural porous medium material with a large num-ber of randomly distributed pores and fissures inside, poresand fissures constitute the voids in the rock, and the distribu-tion and connectivity of these voids have a vital impact on thepermeability of the rock. Besides, rocks are rich in energyresources, especially sedimentary rocks such as sandstoneand coal. The permeability of these rocks is of great signifi-cance to the extraction of oil, natural gas, and methane.Therefore, the permeability of sandstone and coal and thedistribution of pores and fissures have long been the researchfocus of scholars.

Pores and fissures in rocks provide storage space andtransport passage for fluids, so studying the distributioncharacteristics of pore fissures is a basic work. The currentexperimental methods used to study pores and fissuresmainly include the following: mercury intrusion porosimetry(MIP) [1, 2], gas adsorption (N2/CO2) [3, 4], computerizedtomography (CT) [5–8], and nuclear magnetic resonance(NMR) [9–11]. But these methods have certain limitations;MIP is highly likely to destroy the pore structure due to thehigh-pressure injection of mercury, which may result insome misleading information on the pores [12–14]. The gasadsorption method cannot destroy the pore structure, but itcan only be used for dry rock samples and not for water-

HindawiGeofluidsVolume 2020, Article ID 8832946, 9 pageshttps://doi.org/10.1155/2020/8832946

bearing samples [15], and it is assumed that the shape of thepores is cylindrical [4]; NMR is a fast, nondestructive methodfor characterizing porous media microstructures. However,in terms of microstructure characterization, NMR has a poorrecognition effect on macropores. Moreover, the metal min-erals in the sample may affect the NMR relaxation time T2distribution [16, 17]. Compared with the above methods,micro-CT is a nondestructive and high-precision visualiza-tion scanning technology [5, 18], which has been widely used.Micro-CT not only makes it possible to analyze pores and fis-sures with a spatial resolution of several microns but alsoobtains pore-throat size distribution information such aspore diameter, throat duct length, and pore connectivity[19, 20]. Wang et al. [7, 21] studied the microscopic struc-ture, coal deformation, and water transport behaviors of het-erogeneous coal by using micro-CT. Shi et al. [22] analyzedthe detailed structure of microfractures in different rank coalsthrough micro-CT scanning and determined the effects ofcoal rank on the physical properties of microfractures.

In the study of rock permeability, Heiland [23] usedsandstone as the research object to study the permeabilitycharacteristics during deformation and failure processesand the evolution of rock permeability before and after fail-ure; Chen et al. [24] also used sandstone in a triaxial com-pression test to conduct experimental research on thedeformation characteristics, permeability change law, andacoustic emission characteristics of the confining pressureunloading process, and the test results illustrated a correla-tion between the deformation characteristics, acoustic emis-sion, and permeability of the rock during unloading; Wanget al. [25] used a rock mechanic test system to conduct triax-ial compression tests on sandstone and limestone; theirresults revealed the evolutionary characteristics of permeabil-ity during rock failure and the relationship between rockstrength, deformation, and permeability before and afterfailure.

In terms of rock seepage research based on micro-CT, thecurrent main research idea is to obtain a 3D digital rock sam-ple through CT scanning of the rock sample, then use thenumerical simulation method to study the change law ofthe permeability of the digital rock sample, and finally estab-lish the rock seepage model by comparing the simulationresults with the actual seepage test results [26–28]. Becauseof the lack of seepage experiment equipment that can per-form real-time CT scanning, there are few studies on howthe distribution and connectivity of pores and fissures changewith the increment of confining pressure and seepage pres-sure during the seepage test and what is the relationship

between the connectivity of pores and fissures and rockpermeability.

To solve the above problems, in this paper, we newlydeveloped a set of special experimental equipment, whichcan scan rock samples with micro-CT while conducting seep-age tests, and studied the distribution and connectivity ofpores and fissures in the rock samples, and the relationshipbetween the connectivity of pores and fissures and confiningpressure and seepage pressure based on real-time micro-CTscan.

2. Experiment

2.1. Sample Preparation. The yellow sandstone used in thistest was collected from a quarry in the suburbs of Neijiangcity, Sichuan Province; this sandstone, with medium com-pressive strength, good homogeneity, and few visible naturalfissures, is an ideal material for studying the permeability ofthe brittle rock. The coal samples were retrieved from anunderground mine located in the Qinshui Basin in the south-eastern part of Shanxi Province in China, coated with wax,and transported to the laboratory. The coal categorized asanthracite has low strength, uniform texture, high porosity,and abundant natural microfissures; it is a plastic rock.

The main reasons for choosing these two kinds of rocksas research objects are as follows: The main purpose of thispaper is to study the influence of confining pressure andseepage pressure on 1the permeability of brittle and plasticrocks after failure, to obtain the changing pattern of the con-nectivity of pores and fissures with the increment of stresses.For brittle rocks, sandstone with no obvious bedding meetsthis requirement. At the same time, it contains a large num-ber of uniformly distributed pores, so the yellow sandstoneused in the experiment is an ideal rock for studying seepage[29–31]. For plastic rocks, coal is a kind of rock with obviousplastic deformation characteristics. However, in most typesof coals, joints and bedding are developed and unevenly dis-tributed, which will affect the test results; in order to elimi-nate this effect, we selected this kind of anthracite with highporosity and relatively good homogeneity as the researchobject. The processed coal samples and sandstone sampleswith a diameter of 5mm and a height of 10mm are shownin Figure 1.

2.2. Experimental Apparatus

2.2.1. High-Resolution Micro-CT System. The model of thehigh-resolution micro-CT system used in this test is

Figure 1: Yellow sandstone samples and coal samples.

2 Geofluids

μCT225kvFCB. The system consists of an X-ray source, anamorphous silicon flat-panel detector, and a sample stage.The amorphous silicon flat-panel detector used for imageacquisition is composed of 3200 × 2232 pixels, and the sizeof each pixel is 127μm, as shown in Figure 2. Digital controlis adopted to move the sample stage in the three directions ofX, Y , and Z, which can realize high-precision positioning.

The main parameters of scanning are operating currentof 120μA, working voltage of 100 kV, the exposure time of2 s per image, and the geometric magnification of 36.04; theresolution of each pixel was 5.38μm, which means that thesmallest pore size that can be identified was 5.38μm. Duringthe scanning, the rotation angle of the sample stage was 0.9°

for each projection, the sample stage was rotated 360°, andthe number of projections was 400.

2.2.2. Mini Triaxial Seepage Testing Machine. The testingmachine used in this test is newly developed by our researchteam; it is developed for miniature rock sample (5-7mm indiameter and 10-20mm in height). This machine can be usedfor uniaxial compression experiments, triaxial compressionexperiments, seepage experiments, and real-time CT scan-ning. The stiffness of the testing machine is 0:8 × 104 kN/m,the elastic modulus of the loading device is 206GPa, theshear modulus is 80GPa, and Poisson’s ratio is 0.26. Theelastic modulus of the applicable sample is 0.25-5GPa, Pois-son’s ratio is 0.11-0.25, and the stiffness is ð0:1 − 1Þ × 102kN/m.

The mini triaxial seepage testing machine consists of anaxial loading device, axial stress sensor, axial stress display,confining pressure input port, seepage pressure input port,and triaxial pressure chamber, as shown in Figure 3.

2.2.3. Seepage Test System for Real-Time CT Scanning. Thesystem can perform a CT scan of the sample in real-timewhile conducting the seepage test and obtain the variationtrend of the connectivity of pores and fissures in the rocksample in different stress fields. This system is shown inFigure 4.

Confining pressure and seepage pressure are loaded witha water pump, and the loading medium is distilled water.When confining pressure needs to be applied, close the valve⑤, open the valve ④, and pressurize with a high-precisionwater pump ①; when seepage pressure needs to be applied,

close the valve④ and open the valve⑤. In this way, the sys-tem can switch freely between the two pressure lines. Toensure the stability of the pressure on the line, an accumula-tor is connected to each line. During the seepage test, themini triaxial seepage testing machine ⑥ is fixed on the sam-ple stage of the micro-CT to perform real-time scanning.

2.3. Experimental Method. One of the research objects in thispaper is the seepage characteristics of rock samples after fail-ure. Therefore, before the seepage test, an increasing axialload is applied to the sample under prefixed confining pres-sure until the sample reaches the strength limit and fails.Then, keep the axial strain constant, unload the confiningpressure, start reloading the confining pressure and seepagepressure, measure the permeability of the rock sample, andperform real-time CT scanning.

3D digital rock samples can be reconstructed based on2D grayscale images obtained by CT scanning; in digital rocksamples, the higher the gray value, the greater the density ofthe pixel. To study the connectivity of pores and fissuresunder different stress conditions, it is first necessary to iden-tify and extract the pores and fissures; the most commonlyused method is to binarize the digital sample. In this process,the selection of threshold is a very important step, whichdetermines the accuracy of identifying pores and fissures.Here, the method of segmentation of the DTM threshold isused [32]. Figure 5 is a comparison of sandstone imagesbefore and after binary processing.

According to the method above, a spatial skeleton com-posed of pores is reproduced. In the skeleton, all of the adja-cent pores formed a connected cluster, and there are lots ofclusters in the 3D digital sample. We called the cluster thatcontains the largest number of pore pixels “the largest

Figure 2: High-resolution micro-CT system.

Rock sample①

①

Seepage pressure input②

②

Axial stress display③

③

Confining pressure input⑥

⑥

⑤

⑤

Axial stress sensor⑦

⑦

Axial loading deviceTriaxial pressure chamber④

④

Figure 3: Mini triaxial seepage testing machine: ① rock sample, ②seepage pressure input, ③ axial stress display, ④ triaxial pressurechamber, ⑤ confining pressure input, ⑥ axial stress sensor, and⑦ axial loading device.

3Geofluids

cluster.” As the porosity of the sample increases, the numberof pore pixels contained in the largest cluster will increase[33]. The percolation probability of the 3D digital samplecan be expressed by the following formula:

P = MN

, ð1Þ

where P is the percolation probability. M is the number ofpore pixels contained in the largest cluster. N is the totalnumber of pixels in the 3D digital sample.

3. Experimental Results and Analyses

The permeability of the rock is affected by confining pressureand seepage pressure. In order to figure out the influence ofthese two types of pressures on the permeability changes ofthe rock, the seepage tests and real-time micro-CT scanningare performed on sandstone and coal, which have differentmechanical properties.

3.1. Influence of Confining Pressure on SandstonePermeability. Firstly, the confining pressure of 2.5MPa andthe seepage pressure of 2MPa are applied to the sandstonesample, and then, the axial pressure is slowly loaded untilthe sample fails. The fractured sandstone sample is shownin Figure 6. Keep the seepage pressure unchanged at 2MPaand start to increase the confining pressure step by step. Mea-sure the flux of water discharged from the outlet under each

confining pressure and perform real-time micro-CT scan-ning. The test results are shown in Figure 7.

It can be seen from Figure 7 that when the seepage pres-sure keeps constant, the permeability decreases with theincrement of the confining pressure. It indicates that whenthe confining pressure increases, the original fissures in thesample are compressed, and the width of the fissuresdecreases, which ultimately leads to poor permeability. Fit-ting the curve in Figure 7 shows that the relationship betweenpermeability and confining pressure is a power-lawrelationship.

3.2. Influence of Confining Pressure on Sandstone PercolationProbability. Under different confining pressures, the real-time micro-CT scans are performed on sandstones to recon-struct 3D digital samples. Based on the digital samples, we

Water pump①

Accumulator②

Pressure meter③

Mini triaxial seepage testing machine⑥

Confining pressure input⑦

Seepage pressure input line⑧

⑧

Valve④,⑤

①

②③

⑥⑤

⑦

④

Figure 4: Seepage test system for real-time CT scanning and its schematic:①water pump,② accumulator,③ pressure meter,④ and⑤ valve,⑥ mini triaxial seepage testing machine, ⑦ confining pressure input line, and ⑧ seepage pressure input line.

(a) Before binary processing (b) After binary processing

Figure 5: Sandstone images before and after binary processing.

Figure 6: The fractured sandstone sample.

4 Geofluids

extract the skeleton of pores and fissures, obtain the largestconnected clusters, and calculate the percolation probability.Figure 8 shows digital samples and the spatial distribution ofthe largest clusters under different confining pressures.

Comparing the fissures in the digital samples and thelargest clusters in Figure 8, it can be seen that the spatial

forms of the fissures and the largest clusters are similar. Asthe confining pressure increases, the small fissures are com-pressed and closed firstly, and the width of the large fissuresbecomes smaller and smaller. The size of the largest clustergradually decreases with the increase of the confining pres-sure. Counting up the size of the largest clusters and

1 2 3 4 5 6 70

0.5

1

1.5

2

2.5

3

3.5

4

Confining pressure (MPa)

Perm

eabi

lity

(mD

) y = 66.764x–3.264

R2 = 0.9936

Figure 7: Dependence of sandstone permeability on confining pressure.

2.5 MPa 3.5 MPa 4 MPa3 MPa

5 MPa 5.5 MPa 6 MPa4.5 MPa

Figure 8: The 3D digital sample and the spatial distributions of the largest clusters.

5Geofluids

calculating the percolation probability under different con-fining pressures, the results are shown in Figure 9.

3.3. Influence of Seepage Pressure on Coal Permeability. Theconfining pressure of 4.5MPa and the seepage pressure of0.5MPa are applied to the coal sample, and then, the axialpressure is slowly loaded until the sample fails. The fracturedcoal sample is shown in Figure 10. Keep the confining pres-sure at 2MPa and start to increase the seepage pressure stepby step. Measure the flux of water discharged from the outletunder each confining pressure, and perform real-time micro-CT scanning. The test results are shown in Figure 11.

Figure 11 shows that as the seepage pressure increases,the permeability of the coal sample first decreases and thenincreases; when the seepage pressure is 2MPa, the permeabil-ity reaches the minimum. The reasons for this change in per-meability are as follows. At the beginning phase of the test,the relatively high confining pressure puts the fissure surfacein a compressed state. Besides, the low strength of the coalmatrix causes the occurrence of significant deformationunder confining pressure, which causes the fissures to closeand the connectivity of fissures becomes worse. Another rea-son for the decrease in permeability is that water can onlypermeate a small portion of the fissure surface when the valueof seepage pressure is very small. In this situation, the out-ward opening force generated by the seepage pressure actingon the fissure surface is very small compared to the closingforce due to the confining pressure, so the fissure cannot beopened further. Therefore, before the seepage pressure is suf-ficiently high enough to open the fissure, the seepage pressurecan be regarded as the axial pressure acting on the end of thesample, which increases the volumetric stress on the sample,resulting in a decrease in permeability. When the seepagepressure reaches 2MPa, the pressure is already high enoughto cause the fissures to slowly open; therefore, the permeabil-ity of the coal sample starts to increase accordingly.

3.4. Influence of Seepage Pressure on Coal PercolationProbability. Under different seepage pressures, the real-timemicro-CT scans are performed on coal samples to recon-struct 3D digital samples. Based on the digital samples, weobtain the skeleton of pores and fissures, the largest con-nected clusters, and calculate the percolation probability.

Figure 12 shows the spatial distribution of the largest clustersunder different seepage pressures.

Comparing Figure 12 above with Figure 8, we can see thatthe largest clusters in coal and sandstone have a completely dif-ferent spatial form. In the coal sample, the distribution of thelargest cluster is relatively homogeneous, and its structure is likerandomly distributed pores, which is sponge-like, and the exis-tence of fissures is almost invisible. In the sandstone sample, theform of the largest cluster is firmly controlled by the fissures,and its structure is the same as the fissure structure in the sam-ple. This indicates that the main factors controlling the perme-ability of the plastic rock and brittle rock are different; in brittlesandstone-like rocks, the connectivity of the fissures is the pri-mary factor affecting permeability, and the pores are the auxil-iary factor; in plastic rocks such as coal, the situation is just theopposite, pores are the primary factor affecting permeability,and fissures are the auxiliary factor.

A lot of engineering practical experience shows that thehydraulic fracturing technology can significantly increasethe oil and gas production in sandstone reservoirs but cannotincrease the gas drainage efficiency of coalbed. The reasonlies in the fact that the coalbed is a plastic rock layer, andits permeability is mainly controlled by pores rather than fis-sures. The weak coal undergoes large deformation under theaction of the overlying rock, and the fissures generated byhigh-pressure water close again quickly. For brittle rockssuch as sandstone, as mentioned above, the main control fac-tor on its permeability is the fissures rather than the pores;after being fractured, the sandstone has a large number of fis-sures, which significantly increases the permeability.

The variation of percolation probability with seepagepressure is shown in Figure 13.

It can be seen from Figure 13 that the percolation proba-bility increases exponentially with the increment of seepagepressure. Furthermore, by comparing Figures 13 and 11,when the seepage pressure is greater than 2MPa, the percola-tion probability and permeability both increase with theincrement of seepage pressure, and the variation pattern isthe same. However, when the seepage pressure is less than2MPa, the changing trend of percolation probability and thatof permeability is inconsistent; the reason is that the seepagepressure is relatively smaller than the confining pressure; atthis time, its main function is to increase the volumetricstress at the end of the coal sample, resulting in a decreasein permeability.

2 2.5 3 3.5 4Confining pressure (MPa)

y = 0.5877x–0.801

R2 = 0.923

Perc

olat

ion

prob

abili

ty

4.5 5 5.5 6 6.51

1.5

2

2.5

3

3.5

Figure 9: Variation of percolation probability with confining pressure.

Figure 10: The fractured coal sample.

6 Geofluids

4. Conclusions

This paper takes brittle sandstone and plastic coal as theresearch objects and mainly studies the seepage characteris-tics of these two types of rocks with completely different

properties of mechanics. In the seepage test, a miniature sam-ple was used, and a real-time micro-CT scan was performedat the same time. The percolation theory was used to analyzethe size and shape changes of the largest clusters in the rocksamples under different stress states, and the main control

0 0.5 1 1.5 2Seepage pressure (MPa)

Perm

eabi

lity

(mD

)

2.5 3 3.5 4 4.50

0.5

1

1.5

2

2.5

3

Figure 11: Variation of coal permeability with seepage pressure.

0.5 MPa 2 MPa1.5 MPa1 MPa

2.5 MPa 2.5 MPa2.5 MPa2.5 MPa

Figure 12: The spatial distributions of the largest clusters in coal samples.

0 0.5 1 1.5 2Seepage pressure (MPa)

2.5 3 3.5 4 4.5

Perc

olat

ion

prob

abili

ty

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Figure 13: Variation of percolation probability with seepage pressure.

7Geofluids

factors for the permeability of these two types of rocks werediscussed. The main conclusions are as stated below.

(1) Under different confining pressures, the permeabilitydecreases with the increment of confining pressure,and the power-law relationship between the two issatisfied; at the same time, with the increase of con-fining pressure, percolation probability also decreaseswith the increment of confining pressure; the power-law relationship is still satisfied between the two

(2) Under different seepage pressures, the permeabilityfirst increases and then decreases with the incrementof seepage pressure, and the permeability reaches theminimum when the seepage pressure is 2MPa. How-ever, when the seepage pressure is more than 2MPa,percolation probability increases with the incrementof seepage pressure; when the seepage pressure is lessthan 2MPa, there is only a slight increase in the per-colation probability. The changing trend of percola-tion probability and permeability is inconsistent

(3) The spatial distribution of the largest cluster underdifferent stress states indicates that the main factorscontrolling the permeability of the plastic rock andbrittle rock are different; in brittle sandstone-likerocks, the connectivity of the fissures is the primaryfactor affecting permeability, and the pores are theauxiliary factor; in plastic rocks such as coal, poresare the primary factor affecting permeability, and fis-sures are the auxiliary factor.

Data Availability

All data used to support the findings of this study areincluded within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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