A method of determining the permeability coefficient of coal seam based on the permeability of loade

International Journal of Mining Science and Technology 24(2014)637-641Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstA method of determining the permeability coefficient of coal seam basedon the permeability of loaded coalLi Bo ,D,C, Wei Jianping Wang Kai a. Li Peng Wang KeARTICLE INFOABSTRACTArticle histoThis study developed the equipment for thermo-fluid-solid coupling of methane-containing coal, and2 November 2013nvestigated the seepage character of loaded coal under different working conditions. Regarding thein revised form 5 February 201415 April 2014effective pressure as a variable, the variation characteristics of the gas permeability of loaded methonline 19 August 2014ane-containing coal has been studied under the conditions of different confining pressures and pore presKeywordsmethane-containing coal has been established m Ip between effective stress and permeability of loadedconsidering the adsorption of deformation, amount ofpore gas compression and temperature variation. The results show that the permeability of coal samplesCoaldecreases along with the increasing effective stress Based on the Darcy law, the correlation equationetween the effective stress and permeability coefficient of coal seam has been established by combiningLoaded coalthe permeability coefficient of loaded coal and effective stress. On the basis of experimental data, thisequation is used for calculation, and the results are in accordance with the measured gas permeabilitficient of coal seam In conclusion this method can be accurate and convenient to determine thegas permeability coefficient of coal seam, and provide evidence for forecasting that of the deep coal seare 2014 Published by Elsevier B V on behalf of China University of Mining Technology1 Introductionpermeability in our country, the inaccurate results would beobtained if the change of drilling flow rate cannot be accuratelyThe permeability coefficient of gas indicates not only the diffi- determined with the big borehole flow attenuation coefficient afterculty of gas flow in coal seam, but also the outburst hazard of coal discharging a quantity of gas from pressure drillingand gas and the feasibility of gas extraction. In order to provideA lot of studies on the gas permeability coefficient have beenccurate technical parameters for mine design and gas prevention reported at home and abroad. The marconi pressure method knownwhich ensures the mine safety production, the change of coal seam as the secondary pressure method has been reported [1 Its advanpermeability should be well-known. At present, the permeability tages lie in the simple testing work, short time and the small comcoefficient of coal seam can be determined by laboratory test puting workload, but it considers that only the free gas in coal seamnd/or field measurement Laboratory measurement method is rel- participates in the flow which contradicts that the gas is mainlytively simple with low costs of manpower, material resources and adsorbed in coal seam. The Klinefelter pressure method known astime, but it usually exhibits big difference with the real value. It is the single pressure method has been reported but the influencemainly due to the lack of the fissure of coal during the preparation on the gas pressure rise curve by the amount of discharged methaneorocess of particle coal samples in laboratory, and the coal seam has not been taken into consideration during the period from compermeability coefficient caused cannot be truly reflected For the pleting drilling to hole sealing [2]. The time factor is also eliminatedfield measurement, it mainly adopts in-situ test method on the in the gas source radius formula for calculation.basis of radial unsteady flow theory. Although the actual situationThe foundation for determining the permeability coefficient hason site can be better reflected by this way it costs a lot of man- been established by the proposed radial flow methods [ 1, 3 thepower and material resources. Additionally, due to the poor borehole radial flow method has been widely used in our countryowing to its precise measurement results, practicality and less lim-itations from the cons,of coal seam. although this testCorrespondingr.Tel.:+8613839117169method is compl中国煤化工 veen ab and time(B. Li).numeroCNMHGeam permeaBilityhttp://dx.doiorg/10.1016/j.ijmst.2014.04.0022095-2686 2014 Published by Elsevier B V on behalf of China University of Mining TechnologyB Li et al/International Journal of Mining Science and Technology 24(2014)637-641efficient is analysed by borehole radial flow method, and the purity of 99.999% and the reducing valve adjusts the gas pressureoptimisation of calculation method for permeability coefficient is of air inlet. The measuring system consists of the stress acquisitionproposed [4]. a new method for determining the gas permeability system, the displacement acquisition system, the gas mass flow-coefficient of coal seams has been developed by using the infiltra- meter and the strain collect checkout gear. To ensure the accuracytion rate of particle coal [5. However, it is inconsistent with the and reliability of data, the whole experimental process was con-actual conditions due to lack of the consideration of coal samples ducted on the operating table and controlled by computer prounder loadgram, including the record of data acquisitionThe study investigated the influence of the secondary stress ofcoal and the gas pressure around drill on the permeability coeffi- 2.2. Experimental processcient of coal seam, and found that the permeability coefficient ofcoal gas decreased with increasing distance [6 To enhance theConsidering the effect of confining pressure, the effective stresspermeability coefficient of gas, the means such as water pressure is used as a variation to study the coal permeability by changingblasting crack, loosening blasting, hydraulic flushing and mining the confining pressure and axial compression Based on the exper-protective layer is also studied [7-11. Those studies mainly aimedimental results, the qualitative and quantitative relationshipat optimising the calculation method and improving the perme- between loaded coal permeability and effective stressability coefficient of coal seam, while it needs to measure the gas establishedpressure of coal seam and the gas flow of drilling in field. ThisThe experiments can be divided into two partdetermination process is complicated with a long cycle, so it is difficult to be widely applied in the mines without pressure measure(1) The triaxial loading penetration was studied with the porement conditionspressure of 0.6, 0.9, 1.2 and 1.5 MPa and the confining presThis study utilised the self-developed equipment for thermo-sure of 1, 2, 3 and 4 MPa, respectively. First, the seamingfluid-solid coupling of methane-containing coal, and analysedchuck was pressed to push down the sample until the con-the feature change of the permeability of loaded gas-containingfining pressure increased to a predetermined value. In ordercoal. Then, the qualitative and quantitative relationship betweento avoid the influence of impurity gas in coal, the entire sys-coal permeability and effective pressure was established basedtem was vacuum degassed to be vacuum state, then theon the experimental results. Finally, a new method for determiningmethane was filled into the system and the penetration testthe gas permeability coefficient of coal seams was proposed withwas carried out under this pressure condition. Thus the porethe permeability of loaded coal, which provides the theoreticapressure was adjusted to study its influence on the coal perbasis and prediction of the permeability of deep coal seam.neability under different loading conditions. After increasing the confining pressure to larger load, the pore pressure2. Test of seepage properties of gas-containing coalwas changed according to the test sequence which is usedto simulate the coal loading stress conditions. To confirm2.1. Experimental systemthe influence of effective stress on the coal seam permeability at different loading stages, the same experimental proceWith the self-developed equipment for thermo-fluid -solid coudure was repeatedpling of methane-containing coal, the gas permeation of coal sam-(2)Through changing the confining pressure, the above experiple test can be simulated under different crustal stresses (i.elental process was repeated to study the relationshiponfining pressure and axle load)and pore pressure. As shown inbetween the permeability and effective stress under differ-Fig. 1, this device consists of the loading system, triaxial cell, polent effective stressespressure control systems, data measurement system and temperature control system. It can meet the combination test with differThe process is described as below: (1) put the coal sample in theent confining pressures, axle loads and pore pressures, ensuring a thermo-fluid-solid coupling equipment with the methane-congood sealing function the load is provided by the hydraulic pump taining coal, and check the air tightness and load the scheduThe pore pressure control system consists of the gas bottle, reduc- confining pressure and axle load; in order to avoid the affect ofg valve and pipeline, in which the gas bottle supplies CHa with a impurity gas, the entire system is vacuum degassed more thanAxial hydraulic cylinderAxle load senConstantConfining中国煤化工systemCNMHGFig. 1. Experiment system structure schematic diagram.B. Li et aL./Inal Journal of Mining Science and Technology 24(2014)637-641be stable within 2 h after closing the vacuum pumps to accomplish 9=712 h until the vacuum reaches 0 Pa, and the vacuum degree shoulds±F+the vacuum degassing: and (2)fill the CHa into the coal sampleswith a predetermined pressure, open the outlet valve after the coal where o is the porosity: o the initial porosity of coal mass; and asamples achieve absorption equilibrium, and start to record the the volume strain of coal massdata after the gas flow becomes stable; after the entire process.Based on the effective stress variation equation proposed in thethe pore pressure should be changed to apply next pressurereference, the effective stress variation equation can be expressed3. Permeability equationge=de+ophirThe deforof expansion, pore compression and the where p=pp is the elastic change stage: =s the viscous changeexternal or internal expansion of temperature which were caused stage: =pmax the limit failure stage: de the total stress, MPa; aeby methane adsorption by unit volume coal could be expressed the effective stress MPa; p the pore pressure, MPa; and Sy theby the following equations [12-14The relations between effective stress and gas permeabilityEp=262n(1+bp2)-ln(1+bp1characteristics of coal seam have been deeply researched by the2) scholars in China, and they obey the exponential equation, whichGF=±B△Tcan be expressed as [17]where p is coal mass apparent density, g/cm: Vm the molakvolume of gas, 22.4 x 10 m/mol: R the universal gas constant, where a1 and B, are regression coefficientslimited adsorption amount, cm /g: b the adsorption equmomiooequal to 8.3143 J/(mol- K): v the poission's ratio of coalThe permeability equation considering adsorption expansionstress, compression stress and internal (external)expansion stressconstant, MPa: k, the volume compressibility coefficient, MPa caused by temperature effect is obtained by the combination ofAp the pressure variation, MPa; E the elastic modulus, MPa; Ep the Eqs. (2)-(5)adsorption expansion deformation; es the compression deformation:ef the external and internal expansion deformation dueto the temperature: AT the absolute temperature variation ask=a,exp-B1o+(+色月p6T-To, K; and B the volume thermal expansion coefficient of coalm3/(m3K)After the experimental data is collected and analysed, Eq(6)is utiAccording to the basic definition of porosity, the dynamic evo- lised to fit the experimental data. Fig. 2 shows the regression resultution model of porosity is established under considerationIt could be known from the correlation that the experimental andadsorption expansion, and pore compression and the effect of ther- fitting results met well. The relational expression between perme-mal expansion [15]ability and effective stress is shown in table 1Exponential form (pore pressure 0.9 MPa)Eftective pressure (MPa)Effective(a) Coal sampleb)Coal sample 2Fig. 2. Relation curves between effective stress and permeabilityTable 1Fitting result of test data of permeability and effective stressPore pressureCoal sample 1k=3674∞{-02876{%-[1-1m(1主母+气)p}k=23390{-02646{%-1-1(1-6士即十)pA}k=1389001101TH中国煤化工0.9929Coal sample 2k=149591{-02494{0-10.9981k=10172c{01850CNMHG0.9533k=08861exp{-020-1-12(-6士母+)p}0.9978B Li et al/International Journal of Mining Science and Technology 24(2014)637-641Table 2predict their permeability coefficients, and the predicted resultsSampling mining parameters.are shown in table 3 as belowItemHebi No 8 mine Zhaogu No. 2 mirThe applied method for testing the seam permeability coeffiAdsorption constant(cm /g)41.492945cient in field is based on the drilling radial flow theory It is differdsorption constant b(MPaent to measure the real coal seam pressure due to the complexGeneral gas constant R8.31438.3143technology and multi-factors, especially when the coal seam gasCoefficient of kinetic viscosity(Pas) 1.08 x 10Density of coal(g/cm'pressure cannot be accurately measured. The error of determina-0.0224tion is much larger when the gas flow quantity from boreholes isCoal temperature (K)295used. To calculate the gas permeability coefficient, the samplesAdsorption of expansion stress(MPa) 1.15parallel to the bedding were chosen based on the radial flow theoryCompressive stress(MPa)to simulate the gas flow under the load conditions. the corre-Temperature expansion stress (MPa) 0. 190.2sponding calculation equation was selected according to the actualoal seam gas pressure in two mines, and the results show that thepredicted and the measured values are basically identical althoughSince this experiment is the axial steady state seepage, the inadequacies exist. Although the coal seam gas flow can belower part center of the specimen is defined as the coordinate ori- impacted by the factors such as stress, pore pressure and tempergin, and x-axis and y-axis are respectively the horisontal and axial. ature as well as water and other gases, metamorphic degree, geoRegarding the gas flow in coal seams in accordance with Darcy's logical structures and coal body structure, it is still applicable tolaw, the gas flow could be expressed by the following equation predict the deep coal seam permeabilityg=kp. dp--k OP5 Conclusions(1)With the self-developed equipment for thermo-fluid-solidwhere Po is the atmospheric pressure of measuring points,P-P:A the permeability coefficient; and u the gas dynamic viscoupling of methane-containing coal, the relationship equation between loaded coal permeability and effective stresshas been established based on the adsorption deformationBased on the above equation, it can be obtained as followspore gas compression volume and temperature expansion.The results show that the permeability represents nonlineardecreasing relationship with increasing effective stress(2)On the basis of contrasting and analysing the existing probIn Eq( 9), the relationship between permeability coefficient andlems of the test method for coal seam permeability coeffieffective stress can be obtained by substituting Eq( 6)into Eq. (8)cient, the samples were adopted parallel to the beddingwith the consideration of the adsorption of expansion stress, poreaccording to Darcy law. Regarding the effective stress as agas compression volume and temperature expansionvariable which combined the loaded coal permeability coef-ficient with the effective stress, the relationship equationan exp-B1o1-(a+between effective stress and coal seam permeability coefficient was establishedIn addition, a field test of coal-seam gas-pressure was carriedIn Eq. (9), the coefficient of volume compressibility Ky is out to verify the accuracy of equations based on the AQ10470.000516 MPa the volume heat expansion B is 0.00016 m2007(Direct measurement of the coal mine coal seam gas pres-(m-K); the rest parameters, such as the adsorption of expanssure). Finally, the coal seam permeability coefficient was testedstress, adsorption of compressive stress and temperature expansion and calculated according to the radial flow theory The experimeneffect, are shown in Table 2 [13]tal results show that the field data of coal seam permeability coefficient are basically consistent with the derivate data4. Qualifying equationAcknowledgmentsIn order to verify the accuracy of the equation, appropriateplaces were chosen to conduct the coal seam gas pressure test inThis research is supported by the National Basic Research Pro-Hebi No 8 coal mine and Zhaogu No. 2 coal mine, according to code gram of China(No. 2012CB723103), the ministry of educatiorequirement of AQ1047-2007(Direct measurement of the coal Innovation Team of China(No. IRT1235 ) the State Key Laboratorynine coal seam gas pressure ) The permeability coefficients of each Cultivation Base for Gas Geology and Gas Control of Henan Polcoal seam are respectively determined and analysed after the gas technic University of China(no wS2012A01), the Provincial Openpressure of coal seam becomes stable, as shown in Table 3. Accord-oratory Fund of Minal Materials Key disciplines of China (no.ing to the buried depth, confining pressure and measured gas pres- MEM13-10), and China Postdoctoral Science Foundation (Nosure values of each sampling location in mine Eq (9)is selected to 2014M552003).Comparison between the predicted and measured resultsName of mineBurial depthconfining pressureContent of gas(cm/ Measured permmyH中国煤化工 ity coefficientCNMHG50-78019.3-273660-84000175-00996B Li et al /International Journal of Mining Science and Technology 24(2014)637-641References[9 Shi BM, Yu QX, Wang K Test research on coal seam permeability dynamicseams, Whina un i miXe Fan xG Read r torage in coal seams, 11. Beijing. (10) Lu YY. Ge Z l, Li XH. Chen JE, Liu Y. Investigation of a self-excited pulsed wate[1 Zhou SN, Lin BQ. The theory of gas flow and12]Jthe permeability of the coal2010:39(1):55-69[3] Hu GZ, Wang HT, Tan HX, Fan XG, Yuan ZG. Gas seepage equation of deep [11] Yu HL, Liu X, Huang MD, Wen ZG, Wu FT, Li PF. 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