Pore structure characteristics of the relative water-resisting layer on the top of the Ordovician in

International Journal of Mining Science and Technology 24(2014)657-661Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstPore structure characteristics of the relative water-resisting layeron the top of the Ordovician in Longgu Coal MineRong Huren, Bai HaiboState Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining 8 Technology, Xuzhou 221008, ChinaARTICLE INFOABSTRACTArticle histororder to study the permeability and water-resisting ability of the strata on the top of the ordovician inReceived 11 November 201Longgu Coal Mine, this paper tested the permeability and porosity of the strata, investigated the fractureReceived in revised form 15 February 2014Accepted 17 April 2014and pore structure features of the strata, and identified the main channels which govern the permeabilityAvailable online 15 August 2014and water-resisting ability of the strata. The permeability of the upper, central and lower strata shows as20504×10-3-2782762×10-3,41092×103-7.3387×103and2.0891×10-3-32705×103um2respectively, and porosity of that is 0.6786-0.9197%, 0.3109-0.3951% and 0.9829-1.8655%, respectivelyRelative water-resisting layerThe results indicate that: (1) the main channels of the relative water-resisting layer are the pore throatswith a diameter more than 6 um; (2)the major proportion of pore throats in the vertical flow channel andMain channels of seepage20m in ability first increases and then sharply decreases;(3)the fractures occurring from the top tothe permeabpth of the strata were filled and there occurred almost no fracture under the depth of 40 mand(4) the ratio of turning point of the main flow channel in the strata on top of Ordovician can be usedto confirm the thickness of filled water-resisting layerse 2014 Published by Elsevier B V on behalf of China University of Mining TechnologyIntroductionIn this paper, through on-site geological prospecting and corefracture statistics, Ordovician carbonate rocks at the longgu coaBefore the 1990s, many studies indicated that the floors of the Mine was determined to exist, with approximately 60 m of a relaoalfield areas in the north China showed as high-pressure and tively impermeable thick layer. Different depths of core samplingwater-rich features and the whole Ordovician was considered as for laboratory experiments were acquired to test the rock perme-water abundance in the karst aquiferability and pore characteristics of the relatively impermeable topFrom a security perspective, performing mine design or evalua- layer of the Ordovician carbonate rocks, to determine the impertion results in the exploitation of coal resources. Lin and Feng mable layer's major proportion of seepage pore distribution anddivided Fengfeng mining area and the tengpei coalfield Ordovician permeability, pore structure characteristics, as well as to analyzeaquifer based on the karst characteristics [1, 2 ] Liu investigated the the macro effects of filling on the pore structure to provide a basiswestern Yanzhou Ordovician aquifer according to its hydrogeolog- for discriminating Ordovician limestone top from the relativelyical characteristics 3impermeable layer.Through years of research on North Chinas Ordovician, Bai suggested that the Ordovician limestone aquifer is water-rich, firstly2. Pore structureproposed the presence of the impermeable top layer of Ordoviciancarbonate rocks or the existence of a relatively impermeable layer 2.1. Pore structure characteristicsand formation mechanism, which has been successfully applied tothe development of several mining areas [4-11]This study analyzed the pore characteristics, i.e., filling andSeveral studies focused on the oil aspect of pore structure char- crack types, of rock samples from the impermeable layer by usingacteristics on the top Ordovician carbonate rocks [12-15]. UntiloW, little attention has been given to non-reservoir limestonean AutoPore IV 9510 Automatic Pressure Mercury Analyzerdolomite formation and the pore structure characteristie(Mercury Porosimetry) manufactured by the United States(MiThe forms of capillary pressure curves contain single andllar shanes. According to the skeCorresponding author. Tel. +86 15895211960ness of pore distril中国煤化工 es of the pore,theE-mailaddress:hbbai@126.com(HBai).capillary pressureCNMHGCategories. Class ahttp://dx.doiorg/10.1016/j.ijmst.2014.04.0012095-2686 2014 Published by Elsevier B V on behalf of China University of Mining TechnologyH Rong, H Bai/ International Journal of Mining Science and Technology 24(20014)657-661一D1Fig. 1. Capillary pressure curve(Upper).Fig 3. Capillary pressure curve(Central).is the permeability type that has a low threshold pressure and iscalled a coarse pore throat. a left deflection graph curve indicatesthat the threshold pressure is relatively low, with a slightly coarse2.3. Central pore structure parameterspore throat median radius. The intermediate section has a gentlerslope, which corresponds to a low pressure value. For class B, per-Fig 3 shows that the central pore throat radius in the capillarymeability is satisfactory, and the threshold pressure is indicative pressure curves of the rock samples is small. The intermediate segof a coarse pore throat. The intermediate segment has germent shows gentle curves and is characterized by low-permeabilcurveswhich correspond to a high pressure value, and the median radiusity rock samples(from Class a to C)is coarse. Class C has poor permeability, with aThe pore distribution of central rock samples exhibits threehigher threshold pressure and smaller pore throat. The upper rightpeaks(Fig 4). The first peak is greater than 62.21 um, and the pro-graph curve is biased, and the intermediate section is steep, whichortion reaches 20%. The second peak ranges from 6.66 to 16.7 um,corresponds to a higher pressure value. The threshold pressure is and the proportion also reaches 20%. The third peak is between0.46 and 1.13 um, and the ratio reaches 15% In the chart of pene-high, but the value of the larger radius and sorting are relatively tration contribution to the value, the cumulative penetration con-tribution of pore throats which have pore diameters larger than6.66 um, reaching 99%, and the type of pore throats comprise themain flow channel. Two peaks are observed in the pore distribution2.2. Upper pore structure parametersof the rock samples, reaching up to 55% of the total pore throats.The capillary pressure curves of the rock samples from theupper relatively impermeable layer shows significantly more steps 2.4. Pore structure parameters of the lower portion(Fig. 1). Almost no transition is observed in the curved intermediate, and no flat stage is found with the characteristic of lowFig 5 shows the pore throat radius in the lower capillary prespermeabilitysure curves of the relatively impermeable layer of rock samples isThe pore distribution of the upper rock samples is not signifi- small. The intermediate segment shows gentle curves and is charcantly concentrated in some areas( Fig. 2). The absence of an obvi- acterized by low-permeability rock samplesous peak denotes a uniform distribution. The cumulateFig. 6 demonstrates a single-peak pore throat located at lesspenetration contribution rate of the pore throats, with diameters than 0. 34 um, with proportion reaching 70%. According to the pen-larger than 7.39 um, has reached 98%, and the type of pore throats etration rate of the value contribution, the pore throats with diamcomprises n, i.e., the principal of flow channel. The proportion of eters larger than 6.58 um and a cumulative penetration rate of 99%total pore throats, with diameters larger than 7.39 um, only comprise the main seepage channel despite occupying only 5% ofaccounts for30‰the total pore throat.Pore throat frequencyPermeability contributionAccumulated valueT中国煤化工0a00102104202173523130CNMHG(a) Pore distribution chart(b) Penetration contribution to the value chartFig. 2. Pore distribution and penetration contribution to the value chart (Upper)H. Rong. H. Bai/ International Journal of Mining Science and Technology 24 (2014)657-661Cumulative frequency046213152426131122622Pore throat diameter (umPore throat diameter (um)Fig 4. Pore distribution and penetration contribution to the value chart(Central)6 um, resulting in a negligible contribution to permeability. However, the pore throats with diameters larger than 6 um have different proportions in total rock samples, and their penetrationcontribution values are all above 98%. The pore throats with diameters larger than 6 um comprise the main flow channel of the relatively impermeable layer. In the vertical distribution, the regularpattern of the pore throat proportion exhibits line variation, withthe upper rock comprising a lower proportion and the middle taking the maximum proportion. With the peak pore distribution ofthe rock samples, the bottom sharply decreases the proportion ofthe pore. Overall, the proportion of the main seepage paths corre-sponding to the pore throats initially increases and then decreasessharply. permeability changes when the proportion of the mainlomax (%Fig. 5. Capillary pressure curve(lower3. Characteristics of the top Ordovician fractured relativelyimpermeable2.5. Vertical distribution of pore structure characteristicsThe layers of the Ordovician at Longgu Mine are shaped by shalThe pore throat distribution histogram shows significant differ- low marine deposits. Owing to late weathering and erosion, Ordo-ences in the pore distribution of the rock samples of the relative vician features remain in the formation of Badou, Gezhuang, andimpermeable layer(Fig. 6). The pore distribution of the upper rock lower Majiagou. According to the hydrogeological drilling condisamples is uniform without peak value. The pore distribution of tions, the thickness and composition of the impermeable layerthe central rock samples is multipolar with three peak values at are distributed as follows: the thickness of impermeable layer is62.21, 6.66 to 16.70, and 0.46 to 1.13 um. The pore throats of the 61.9 m, as exposed by the 6-2-2th hole, and the layers are com-the peak is located at less than 0.71 ulllower part of the rock samples are unimodal in distribution, and posed of gray and light gray dolomitic lime rocks and limestonedeveloped by the fissure and filled by calcite. The thickness ofAccording to the curve of contribution to permeability, the pore the impermeable layer is 81.7 m, which is exposed by the 3-lththroat diameter of the relatively impermeable layer is less than hole, and the layers consist of dense light grey limestone, dolomiticL Pore throat frequencyCumulative frequency2口 Permeability contributionAccumulated value10中国煤化工6581Pore throat diameter (um)CNMHG(a) Pore distribution chart(b)Penetration contribution to the value chartFig. 6. Pore distribution and penetration contribution to the value chart (lower ).eLimestone Exposed ordovician4. Deposition on the pore structureDolomiteFig 8 shows that the upper rocks of the relatively impermeablelayer are subjected to intense denudation during deposition, whichresults in the following conditions: first, the pore throats withLimestonediameters larger than 6 um and less than 6 um are more developedthan the original rock; second, porosity and fracture rate of theocks are larger despite being filled. The pore throats with diameters larger than 6 um pores are filled, such that the radius of thesepore throats decreases sharply, resulting in a smaller size than theoriginal rock Pore throats with diameters less than 6 um are moreLimestonedifficult to fill such that the effect of filling is unnoticeable. FillingDolomitesuch a large proportion of pore throat causes the filling of theupper rock samples to become homogenized in pore distributionLimestoneThe central rocks of the relatively impermeable layer are alsosubjected to strong dissolution and filling in the deposition pro-cess. However, the dissolution rate is weaker than that of the upperFig. 7. Schematic distribution of crack growth raterocks because of various factors, such as water power and depth. Asshown in Fig. 9a, the fracture density of the upper relatively imperlimestone, and grey-brown dense uniform brownish gray dolom- mable layer is smaller, filling only occurs in some sections, andites. The thickness of the impermeable layer is 60.58 m, which is fractured rock specimens are not completely filled and remaineddolomites, with a few small cracks and sutures, and calcite-filled 6 um are partially filled, thus comprising a larger proportion thanlight gray limestone, which developed from the fissure. The thick-he upper throats. The dissolution has a stronger effect than fillingness of the impermeable layer is 160.04 m, which is exposed by the in terms of the distribution of pore throat from rock samplesstone and argillaceous limestone. According to the geological and solution during the deposition process, which is weaker than dis-134th hole, and the layers are composed of gray dolomitic lime-The rocks of the impermeable layer are weakly subjected tohydrogeological survey results, the existence of the 60 m relatively central. Thus, the dissolution does not occur in the partial sectionimpermeable layer on the top of Ordovician can be established and the macro is performed on dense rocks, as shown in Fig. 9beven without the influence of fault structurehe pore throat diameter of the main microscopic rock samplesBased on the analysis of drill core and measurements, the rela- is less than 0.34 umtive impermeable layer is divided into two parts: the first with aAn analysis of the pore distribution characteristics, permeabilthickness of 20 m is called the upper fracture filling type and thesecond with a thickness of 40 m is called the medium-lower crackity, and fractured features of rock samples from different depthof the relatively impermeable layer reveals that the action of fillintype. The proportion of the fracture rate of the fissure-filling type, is important to pore distribution. When the large pores of thewhich is larger than the original fracture, is 20% to 50%. However, upper rock samples are filled, the permeability of the rock sampleswhen filled by Al203 mud and calcite, the final fracture rate ranges is weakened. The permeability of rock samples from the centralfrom 2% to 9% The proportion of the fracture rate of the medium- impermeable layer is more enhanced than that of the upper rocklower crack type is only 2-3%, as shown in Fig. 7.samples because the action of filling in the central impermeable中国煤化工CNMHG(a)Central part (30 m)(b) Lower part(50 m)H. Rong. H. Bai/ International Journal of Mining Science and Technology 24 (2014)657-661Pore structure and sedimentation comparison table.Relatively impermeable layer (Porosity(%Permeability(10 Hm) Major proportion of seepage throat (%Filling and dissolution0.6786-091972.0504-2.7827Central (20-32Dissolution-part fillingLower part(32-609829-186542.0891-3.2705Partial dissolutionyer is weaker than that in the upper layer, thus resulting in large Graduate Student Training Project in Jiangsu Province of Chinapore throats. The main characteristics of the lower rock samples(CXZZ13-_0934)are gratefully acknowledgedare small pore throats and low permeability because these samplesare unaffected by the action of filling and dissolutionReferencesshown in Table 1, the thickness of filling segnt of tupper fissure is 20 m, and the thickness of the rock samples with [ 1l Lin ZP Ordovician karst developmentity in Fengfeng mining area inpore throat that suddenly increases relative to the proportion of[2] Fen QY. Knee levon coal layer control law of the ordovician karst developmentthe main flow channel is 20 m. Thus, the boundary of the actionCoal Geol Explor 1990: 1: 40-4.of filling can be determined by the proportion of the pore throat [3 Liu xx, Yu KJ. Hydrogeologic characteristics of ordovician system in Westfor the main flow4 Bai HB, Ma D, Chen ZQ. Mechanical behavior of groundwater seepage in Karstcollapse pillars. Eng Geol 2013: 164: 101-6.5 Conclusions[5] Bai HB, Miao XX. Research on main controlling factors of Ordovician Karstdevelopment in Lu'an Coal Field. J Mining Saf Eng 2008: 25(1): 17-216 Miao Xx, Pu H, Bai HB. Principle of water-resisting key strata and its1)The thickness of the relatively impermeable layer in Longguwhich is composed of gray limestone filled with calcite and2008;37(1):1-4light gray dolomitic limestone, is approximately 20 m, and[7] Bai HB, Mao XB, Wu Y, Chen ZQ. Research on water-reserved mining with highater pressure under large-scale thrust-fault In Ordovician Karst Chinese Jthe seepage rates of rock samples of the relatively imperme-Rock Mech Eng 2009; 28(2): 246-52able layer are lower than 11 18 at 108]Zhou GL Wu Il, Miao ZY, Hu XL, Li X, Shi X, Cai ZD, Shang YK. Effects of process(2)The difference in the pore distribution of rock samples fromared by solid heat carrier wiLonggu Coal is high, the upper distribution is uniform, the [9] Xue GW, Liu HF, Li W. Deformed coal types and pore characteristics incentral distribution is multipolar, and the lower part is uni-eng coalmines in Eastern Weibei coalfields. Int J Mining Sci Technolmodal. Pore throats with diameters greater than 6 um 110) Yang RC, Fan AiP. Han ZZ, Wang XP. Diagenesis and porosity evolution ofmainly comprise the flow channel of the rock samples. Theart of Sulige gas field, Ordos Basin. Int Jmajor proportion of pore throats in the vertical flow channelMining Sci Technol 2012: 22: 311-6first increases and then decreases sharply[11 Zou GG, Peng SP, Zhang H, Hao HB, Han Y. Study of predicting water(3)The ratio of turning point of the main flow channel of the rel-009;38:390-5atively impermeable layer on the top Ordovician in Longgu [12] Wang CS, Bai HB, Miao XX. Experimental research on porosity of OrdovicianMine influences rock boundaries and can be used as a basis2009;38(4):455-6for determining the thickness of the filled impermeable (13) He wX, Yang L Ma CY, Guo W. Advances in exploration and exploitationtechnologies of shale gas. Nat Gas Geosci 2011: 22(3): 477-81[14 Macang HY. Study on pore structure and seepage characteristics inHuhenuoren oilfield Reservoir Eval Dev 2008: 2(4): 1-4[15 Xu WS, Zhao PR, Nie PF Porosity-permeability interrelation of limestone andAcknowledgmentste reservoirs. Oil and Gas Geology 2008: 29(6: 806-11Financial supports for this work provided by the National basicResearch Program of China(2013CB227900)and the Innovation of中国煤化工CNMHG中国煤化工CNMHG