Analysis of microscopic pore structures of rocks before and after water absorption

Mining Science and Technology (China) 21 (2011)287- -293Contents lists available at ScienceDirectMining Science and Technology (China)EI SEVIERjournal homepage: www.elsevier.com/locate/mstcAnalysis of microscopic pore structures of rocks before and after water absorptionLi Dejian ab. Wang Guiliang, Han Liqiang2.b. Liu Peiyu.a.b, He Manchaoab, Yang Guoxingb.d,Tai Qimin", Chen Chengb"State Key Laboratory of Ceomechanics and Deep Underground Engineering Beijing 10083. ChinaInstihnte of Ceorechnial Eginering Chine University of Mining 5 Tchnolgy Bejing 1008. ChinaSichuan Souf University of Science and Technologx Sichuam 610101. ChinaEngineering Research Instute of General Amament Depertment, Belfing 100803. ChinaARTICLE INFOABSTRACT .Article history,Hydrophilic characteristics of rocks are affected by their microscopic pore structures, which clearlyRecceived 2 August 2010change after water absorption. Water absorption tests and scanning electron microscopic (SEM) exper-Received in rerised formiments on rock samples, located at a site in Tibet, China, were carried out. Changes of rock pore structuresbefore and after water absorption were studied with the distribution of pore sizes and fractal charac-Acepted 30 November 2010teristics of pores. The results show that surface porosities, fractal dimensions of pores and the complexityof pore structures increased because the number of new small pores produced increased or the originalKeywords:macropore flow channels were expanded after rocks absorbed water. There were points of inflection onHydrophilic characteristicstheir water absorption curves. After water absorption of other rocks, surface porosities and fractalPore structuresPore size distributiondimensions of pores and complexity of pore structures decreased as the original pore flow channelsPore fractal characteristicsbecame flled. Water absorption curves did not change. Surface porosity and the pore fractal dimensionsof rocks have good linear relationships before and after water absorption.Copyright 。2011. China University of Mining & Technology. All rights reseved.1. IntroductionOur study is based on scanning electron microscopic experi-ments. We analysed the data with a double logarithmic curve ofThe process of water on rock in underground engineering rock absorption and fractal theory. Huang et al. discussed the effectprojects is a kind of physical, chemical and mechanical process. It isof water injection on pore structures [3]. They concluded that porean important factor in reducing the mechanical properties of rock.diameters and pore dispersion signifcantly increases after injec-Many rock masses. especially with high expansive clay minerals, tion. They came to the conclusion that the heterogeneity of sand-can rapidly expand, collapse and soften in a short time afterstone was enhanced after water injection. Liu investigated thewater alabsorption, which results in a significant reduction in the changes of pore structure of sandstone after water wash [4]. Hemechanical propertes of rock (1]. Therefore, studies on the inter- found that the cerment of rock was reduced, pores became largeraction between rock and water have atracted considerable atten- and permeability increased. Li et al. thoroughly analysed thetion. In order to explain the interaction between water and rock, changes of pore structure before and after a long-term water washmany scientists at home and abroad have carried out comprehen- [5]. They believed that the micro-pore structure of a reservoir hadsive studies in the field of water-rock chemical mechanism, water- changed considerably and its heterogeneity was more pronouncedrock mechanical mechanism and softening mechanism of strength than that at an earlier stage. Huang thought that the clay minerals ofafter water absorption. The efect of water on the structure of rockrock were washed away and broken up due to the long-term erosionpores is one of the major factors which led to a signifcant reduction by water, which caused fIm stripping of particle surfaces [6]. Inin mechanical rock properties. This effect forms also an important the end, he showed that the pore radius and permeability hadtheoretical basis for the rock softening mechanism. In order to increased. Wang et al. studied the effect on pore structures of rockensure stability in underground engineering. an in-depth study on after water injection| 7]. They thought that the injected water wouldthe characteristics of microscopic pore structures of rock beforeand cause pore channels to be blocked during the process of flowing.after water absorption is of great importance [2].Wang et al. and Ma et al. calculated the fractal dimension of themicro-pore structure before and after water absorption 18.9]. They●Corresponding author. state Key Laboratory of Gcomechanics and Deep analysn pore structures. TheyUnderground Engineering. Bejing 100030 China. Tel: +86 13466736516.came中国煤化工y of the pore strucureE-mail ddre ldjecumtb.edu.cn (L Dejian)and ther water foodingCNMHGdoi:10.1016j.t.2011.02.002r Copyright。2011. China University of Mining & Technology. All rgruoloerveu.288L Dejian et al. 1 Mining Science and Technology (China) 21 (2011)287-293Table 1Basic physical properties of rock samples.NLithologyDepth Dry densityParticlePorosityCharacteristics(m) (g/cm2)’ density (g/cm3) (%)C1-N Mudstone3.21.6638.34 Celadon, dense, no bedding. developing joints, smooth sctions, esential minerals: claymineralsandCalciteC2-N Mudstone0.0682.71538.12 White gray. dense, no bedding, developing joints, smooth sections, essential minerals: clayminerals and calciteCS5-N Mudstone131.52.74538.80 Pale yellow, less dense, low strength, essential minerals: dlay minerals, calcite, quartz, feldsparand micaC4-51 Siltstone25.0.652.73439.66 Brown yellow. loose, obvious bedding. low strength, small particlesC4-52 Sandstone.622.71840.41 Brown yllow. loose, low strength, grainy, consisting mainly of clay cement and a small amount5-s Fine sandstone 153.21.59274041.97Pale elowu lo low serghn ml prtides cstin minly of day cement and asmalNote: Elevation of dill holes was 3853.5 m.absorption (dQ/drt) changed during the whole waterronment. Based on hydraulic experiments with rock samples and absorption process. At the early absorption stage, the waterthe professional image analysis software of lmage-pro plus, we absorption capacity which was in a non-linear fashion increasedquantitatively analysed pore structures by SEM images before and almost consistently over time. The water absorption rate acceler-after water absorption and studied the characteristics of pore ated quickly and then decreased from a large to a small value. Atstructure distribution [10]. The fractal characteristics of structural a later stage, the absorption capacity increased linearly withchanges in pores from SEM images are discussed, combined with absorption time. The water absorption curves slowed down tofractal theory and MATLAB software |11].a constant rate (dQ/dt) and remained unchanged during this period.The double logarithmic absorption curves of our rock samples wereconvex, linear and concave. The sandstone (C4-S1, C4-S2 and C5-S)2. Rock samples and hydraulic experimentscurves are of the convex type, while the curves of the mudstonesThe rock samples in our investigation were taken from differentare linear (C5-N) or concave (C1-N and C2-N).depths at a site in Tibet where the weather was very dry. TheThe absorption capacities and absorption rates of sandstonesgeological condition was that of semi-rock and semi-soil and thewere larger than those of mudstones at the same absorption time.sampling site was in semi-consolidated rock strata. The experi-The saturated absorption time of sandstones is much less than thatmental rocks consisted of sandstones and mudstones which wereof mudstones.low cementation and poor strength. They all collapsed easily whenbrought in contact with water. The basic dimensions of our 3. Analysis of micro-pore structure of rock samples beforerectangular rock samples were 50 mmx 50 mm x 100 mm. The and after absorptionthree mudstones. In order to study the water absorption charac-The pore structure characteristics of the rocks were affected notteristics of these rocks, we used the soft rock hydraulic tester for only by their size, arrangement and compaction of rock particles,rock samples, conceptually designed by professor He Manchao. but also by the spatial patterns of clay minerals. The pore structuresFirst, we measured the size. mass and particle density of the rock changed significantly after absorption. There are many reasons forsamples before the hydraulic experiments. Then we calculated their these changes, four of which are the main reasons. The changes indry density and porosity. The basic physical properties and char- pore structure are caused by the combined effects of several factors.acteristics of rock samples are shown in Table 1.These four factors are expressed by class A, B, c and D:The rock samples were all dried naturally during our study. TheClass A: A large number of new micro-pores were produced onrock walls were sealed. The characteristic hydrophilic curves of rock the surface of rock particles under the function of fluid leaching andsamples gained from the hydraulic experiment are shown in Fig. 1. erosion, which led to a considerable increase in the number ofAs shown in the characteristic hydrophilic curves of rock spec- micro-pores.imens, the capacity for water absorption increases with theClass B: Clay minerals such as kaolinite,lingg rock particles,absorption time, to the point of saturated absorption. The rate of have poor adhesion to these rock particles. They were vulnerable toa 120rb 100C 100100巨1中国煤化工、102030405060010203040Cumulative absorption time ! (min)Cumulative absorption tiMYHCN M H Gr(min)Q-r curvesInQ- curvesmy-mr curvesF脂1. Characeristic hyrophic cuves of rock speimnens.L Dejian et al 1 Mining Science and Technology (China)21 (2011)287- -29328Table.2Surface porosity and R1Of rock samples before and ater water absorption(%)No.Lithology srye 。e logarithmSurface porosityRouelosachm BeforeAterBefore absorptionAlter absorptionabsorption absorptionabsorptiond≤0.2 umd> 0.2 um2 2-20 20-200 >2000.2-22-2020-200>200C1-N MudstoneConcave13.7434.48 63.22 2.30 0.000.0053.56 44.8215.4730.88 63.73 5.39 0.00 0.00 46.56 49.84 3.61 0.00 0.00CS-N MudstoneStraight13.32 .10.7433.02 57.62 9.36 0.00 0.00 0.00 9091 9.09 0.00 0.00C4-52 SandstoneConvex13.7522.960.) 0.00 90.42 9.270.310.0091.038.33 0.64C5-S Fine sandstoneConves16.5217.560.00 58.47 38.602.930.00 84.66 15.34 0.00be broken off from the particles under fluid shear stress. Theya capillary pore, characterized by the fact that fluids can flow inmoved with the fluid and plugged the pore channels. The micro-pore channels if the external force is greater than the capillarypores became flled.force. The ultra- caillay pore, which is more than 500 um inClass C: The grains and cements, flling the pores, were dis- diameter, is characterized by fluids that can flow freely in poresolved, broke and migrated with movement of fluids. The major channels under gravity, The micro capillary pore, less than 0.2 μmchannels for fluid migration expanded and the pores increased in in diameter, is characterized bysize. The connectivity of rocks improved.under normal conditions [14]. Because the thickness of theClass D: Clay minerals with the capacity to expand, especially adsorption flm on pores is more than 0.1 μm, the fuid cannot flowthe montmoillonite and ilite mixed layers (when the mixed layer in pores which are less than 0.2 μum in diameter. Thus, the capillaryratio is relatively high), expanded after absorption. The internal pores and the ultra-capillary pores play an important function inpore space of the rocks decreased, the pore sizes became smaller water absorption, while the micro-capillary pores are regarded asand the movement of water blocked [12].invalid poresWe studied the changes in micro-pore structures before andIn our further investigation, we divided the pores into two types,after absorption with the distribution characteristics and thed to as macropores (>0.2 um) and micropores (<0.2 μum)fractal characteristics of micropores [13]. The analysis of pore In order to analyse the distribution characteristics of pores, wedistribution characteristics was based on micro-scanning electron subdivided the macropores (>0.2 μm) into four intervals, ie:microscopic experiments which were taken before and after rock“0.2- -2 um",“2- -20 um",“20- -200 μm" and“>200 um". The surfaceabsorption. With the SEM images processed by the Image-pro-plus porosity and Rt of rock samples before and after water absorption6.0, we obtained the average diameter (based on the Centroid are shown in Table 2. R2 of rock samples before and after absorptiontheory), and area pores. Then the pore size distribution and pore are shown in Table 3. The distribution of pore diameters of the rockarea distribution were calculated. We define surface porosity as samples before and after absorption is shown in Fig. 2. The ditri-a ratio of pore areas to SEM images areas, R1 as a ratio of quantities bution of pore areas of rock samples before and after absorption isof pores in a range of diameters to total number of pores and Rzas shown in Fig. 3.d ratio of areas of pores in a range of different diameter to totalThe interrelationship between pore size distribution and waterareas of pores.absorption characteristics was analysed by the distribution of poreThe analysisf pore fractal characteristics was also based on the diameter, the distribution of pore area (Figs. 2 and 3), suFaceSEM images. With SEM images processed and analysed by MATLAB porosity, R\ and Rz (Tables 2 and 3) before and after watersoftware, we obtained the fractal dimensions of rock pore struc- absorption. We classifed C1-N and C2-N as a mixed A-C class, C5-Ntures, pre- and post-absorption under the principles of Fractal Box as a B class and C4-S1, C5-S and C4-S2 asa C class.Dimension.3.11. Mixed A-C class (C1-N, C2-N)3.1. Pore distibution characteristics of rock samples before andThe lithology of the rock samples was mudstone. The main poreafter water absorptiontype was intergranular. The surface porosity increased after waterabsorption.According to the classifed standard of capillary pore systems,According to Fig.2, R of small pores(d≤0.2 μum) increased froma pore diameter between 0.2 and 500 um is referred to as 34.48% to 53.56% (C1-N) and from 30.88% to 46.56% (C2-N), And R1Table3Surface porosity and R20f rock samples before and after water absorption(%]Lithology Style of theCurveofrockBeforeAfterAter absorptiond≤02 pm0.2-2 2-20 20- 200 >2000.2-2 2-20 20-200 >20018.4061.88 0.00 0.00C2-N Mudstone172941.66中国煤化工873.90 0.00 0.00C5-N Mudstone13.320.7119.3776.39 0.00 0.00C4-S2 Sandstone000THCNMHG21.08 7787 0.0016.52 .17.56 .1.84 15.87 82.29 0.00 0.000.00 12.91 87.09 0.00290L Dejicn et al /Mining Sience and Tethnology (Chino)21 (011)287 -293a 100001C 100，80 t20山0.1-02 2-20 200 200000.1-0.2 2-20 200 20000一0.1-0.2 2-20 20-2000Dianeter of the pore (um)Diameter of the pore (um)CI-NC2-NCS-Nde■Before absorptionf00[Afer aborpion100[60400LoL0.1-02 2-20 200-20000.1-0.2 2: -20 200 20000.1-0.2 2-20 200-2000Diuneter of the pore (um)C4-S1C4-S2CS-SFR 2. Ditributio of pore diameters of rock samples before and ater water aborption..of the macropores (d > 0.2 μm) decreased after water absorption.We conclude that the rate of water absorption changed quicklyThis indicated that a large number of new micropores wereduring the process of absorption. Hence, the water absorptionhingProsiproduced on the surface of rock particles tproduced on the surace ofrock prticles by leaching erosion and caraceritics of rock samples changed considerably The style ofother forces. A large number of new small pores had no effect on the water absorption curves which were concave had obviouswater absorption except to increase the porosityinfection points.According to Fig 3, Rz of small pores (d≤0.2 um) increasedslighty. This is consistent with the analysis of pore size dstrbution.3.12. Class B (CS-N)But the major change was the increase in the size of macroporesThe lithology of this rock samples was mudstone. The main pore(primarily changing from 0.2- 2 pum to 2- -20 um in diameter) andtype is intergranular. Surface porosity decreased.their areas. The surface porosity of the rock samples increased. ThisAccording to Fig. 2, RI of small pores decreased from 0.71% toshows that the pore size had become larger with the migration ofthe0.00% while that of macropores increased to 100.00%. The surfacefuid and it had a considerable efect on the water absorption of rockporosity decreased after absorption. This indicates that claya 1001800.1-0.22-20 200 200001-02 2-20 200 2000Diamneter of the pore (um)CI-N .C2.NCS.N■Before abopion00 [■After absorption100060f0.1-022-20 200-2000中国煤化工20200Dianeter of the pore二(um)THYHCNMHG'y 3. Dsribution of pore areas of rock samples betore and ater aborption.L Dejian et aL / Mining Sctence and Technology (China)21 (011)287 29391a SEM image (z00m0 100 times)bConverted black and white birnsp C Reconstructed binary imageFlg 4. Corversion proess of image for the fractal calculation (betore absorpion,a SEM imnage (z00m 100 timcs)bConverted black and white bimapC Rcconstructed binary imageFg S. Conversion process of image for the factal alulation (ater absorption)minerals, fling the rock particles, can easily fall off and break 3.1.3. Class C (C4-S1, C4-S2, C5-S)away from the particles under the effect of fluid shear stress. TheyThe lithology of the rock samples was sandstone. The major poremoved with the fluid in the pores afer absorption. Thus, the pore type was intergranular. Surface porosity increased.channels were plugged and the small pores became flled. TheAccording to Fig. 2. the diameter oflarge pores (R of macroporesnumber of small pores was significantly reduced and a small was 10.00% before and after water absorption) became larger thanamount of large diameter pores became smaller. These caused the before. The surface porosity increased after absorption. This indi-number of small pores to be reduced and that of large pores to cates that the grains and cements flling the pores were dissolved,increase. The surface porosity became smaller. The disappearance broke and migrated with the fluid, causing the major fluid migrationof a large number of small pores had no effect on water absorption channels to extend and the pore size to increase. Thus, the surfaceof the rock sample.porosity increased. The expansion of the main channels for fuid hadAccording to Fig. 3. Rz of small pore decreased to 0.00% and the a great impact on the water absorption of rock samples.surface porosity became smaller, consistent with the analysAccording to Fig, 3. R2 of macropores (R2 of small pore waspore size distribution. R2 of macropores changed lttle. This indi- 0.00%) inocregsedinthefE20-200 μn(C4-S1, C5-S)orcates that such a change has only a small effect on the water in the diameters of“>200 um" (C4-S2). It is consistent with theabsorption of the rock sample.analysis of pore size distribution. Such changes can have a greatWe come to the conclusion that the rate of water absorption effect on water absorption of rock samples.changed lttle and the curve characteristics of rock samplesWe conclude that the rate of water absorption changedbarely changed during absorption. Hence, the style of the water considerably as well as the water absorption characteristics of rockabsorption curve had no inflection point, ie. as it became samples. The style of the water absorption curves had cleara straight line.inflection points which made the curves convex.No.ClassithologySryle of the double logrithmicSurface porosity (x)Fratal dimension of surface poreabsorption curveBefore absorption After absorptionBefore absorption After adbsorptionCMudstone13.741661.71C-NConcave中国煤化工1.66C5-Ntraight13.321.0SiltstoneConvex12.06C4-52Sandstone13.75MYHCNMHGC5-SFine sandstone16.5217.561.57292L Dejian e al /Mining Sdence and Tecnology (China) 21 (011)287-29332. Pore fracal carocrisits of rock samples before and oferThis variation provided for surface porosity to increase, for fractalwater absorptiondimensions of surface pores to became larger, for surface porosity3.2.1. SEM image processingto decrease and for fractal dimensions of surface pores to becomesmaller, Meanwhile, surface porosity and the fractal dimensions ofThe SEM images used tor the calculation of facas were graysurface pores showed a good linear relationship before and afterimages (shown in Figs.4a and 5a). The steps calculating the fracalwater absorption, as shown in Fig. 6.Clauonshp berore and afterbox dimension were the fllowing (15]. First of all, the SEM imageswere transformed into black and white bitmaps by Photoshop4. Concdlusionssoftware (shown in Figs. 4b and 5b) Then, the black and whitebitmaps were tasferred to Matlab. Fnally the binary images(shown in Figs. 4c and 5c) and binary data matrices were obtained,Based on our water absorption experiments, we obtained waterabsorption curves of rock samples and analysed the pore structure3.22. Calculation of the box dimensionditribution of six rock samples, including changes in pore distri-Fist, the binary data matrices were cassifed irbution and of pore fractal characteristics.submarix by the MATIAB pogam. Then, suasrsodrsanastneeWe arrive at the following conclusions:number of submatices whcereatainenesemarorder andte1) Water absorption rates of the sandstone rock samples arewepetinendaliaslesencotenettheimare points (pixel0)faster than those of mudstone rock samples. In other wordscoordinatned. Fillythe datawere plttedina double lgritmicsaturated absorpion timneof sndstoepresutn oererwordsthedinate system and the data were ftted by using the method ofmddstone.Thebsonptione Ctsandstonre is muchless than thatofleast squares. The slope should be the box dimension of the images.omudsone The absorptionon capacity of sandstone is lrger than thatfmudstone over the same time for absorption3.2.3. Results of fractal dimensions2)The initial absorption capacity (Q) changed greatly for all rockThe fractal dimensions of rock samples before and after samples. The water absorption rate (dQ/dt) accelerated quickly, butabsorption are shown in Table 4. The suface porosity and fractal decelerated slowly to a constant rate after a certain time. There aredimensions of the mixed A- C dlass with concave curves and the three models of water absorbing characteristic curves, ie. convexC-dlas rock samples with convex .curves all increased atermodels, linear models and concave models. The characteristicabsorption. The surface porosity and the fractal dimensions of the curves of sandstone were convex, while those of mudstone wereB-class rock sample with its straight absorption curves decreasedconcave and straight.after absorption.3)A large number of new small pores in the mixed A- C rocksWhen the surface pore fractal dimensions became larger, it were produced and, as well, pore diameters ofC-class rocks becameindicated that the complexity of pore structures had increased, asenlarged after absorption. R2 of macropores increased considerablywell as the heterogeneity. after absorption. This is the reason that and rapidly. Surtface porosity increased and the water absorptiona large number of new small pores of the mixed A C rocks were curves showed significant changes.produced as well as the enlargement of pore diameters of theA large number of small pores in the B-class rock were flledC-class rock afer absorption. It Caused the diameter of large poresfter absorption. Surface porosity decreased, while the poreto increase, as well as the ratio oflarge pore areas. showingthatthediameters of large pores changed little. These had no effect on thecomplexiy of pore srutures had increarers sa in hatheabsorption characteristic of our rock samples, so that the waterporosites were iced after absorpton,n The frctal dimesionsabsorption curve did not change.orsuriace Pporesbecame larger and the heterogeneity increased4) The surface porosity and fractal dimerr bohtrert nOrCsi and fast dneieres of srtrer porsafter absorption. The curves of water absorption had obvious pointscomplexireororedscelasoss increased after absopion, Theof inflection.complexity of pore structure increased and the curves of waterA large number of small pores of the B-class rock were flledabsorption had clear points of inlection.after absorption, while the pore diameter of large pores changedBoth the surface porosity and fractal dimensions of surface poreslttle This caused the surface porosity to decrease and the structureof B-class rocks decreased after aborpion. The pore structuresof pores to become single. The complexity of the pore structuresbecame single. The complexity of pore structures reduced and thewas reduced. The fractal dimensions of surface pores becamecurves of water absorption barely changed.smaller and the heterogeneity decreased after absorption. The5) The porosity and fractal dimensions of surface pores showedcurves of water absorption showed barely any change.similar variation and good linear relationships before and afterwater absorption.3.2.4. Variation berween sirfae porsity and foctal dimensions ofrock samplesPorosity and fractal dimensions of surface pores, presented inAcknowledgementsTable 4. show similar variation before and after water absorption.Financial support for this work, provided by the Key BasicResearch_ Program of China (Nos. 2010CB226800 and25.●Brnboption●Ata ahoption2007CB202200), National Natural Science Foundation ofChina(No.50490270) and the Inovation Team Development Program of theMinisty of Education of China (No. IRT0656) is gratlullyacknowledged.ReferencesT Ts1) Zho中国煤化工mal of Rock Mechanics andSurdact fBactdl dmasionSurfae fnetul dimaFg6 Relation between planar porosity and surface pore fractal dimension before and21 HYHC N M H Gs ideep mining egineringafter water absorption.lin Chinese]L Dejin et oL/ Mining Scence and Tchnoloay (China)21 (011)287 -293293[3] Huang Ss. 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