Failure characteristics of surface vertical wells for relieved coal gas and their influencing factor

Mining Science and Technology( China)21(2011)83-88Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcFailure characteristics of surface vertical wells for relieved coal gas and theirinfluencing factors in Huainan mining areaXu Hongjie, Sang Shuxun,, Fang Liangcai, Huang Huazhou, Ren Borces and Geoscience, China University of Mining 8 Technology, Key laboratory of CBM Resource and Reservoir-Cenerating ProcessMinistry of Education, Xuzhou 221008, ChinaP Huainan Mining Croup, Huainan 232001,ChinaARTICLE INFOABSTRACTased on data from through-hole and logging, we studied the failure characteristics of surface drainagvells for relieved coal gas in Huainan mining area and its influencing factors. The results show that thAccepted 15 July 2010amaged positions of drainage wells are mainly located at the thick clay layer in the low alluvium andthe lithological interface in the upper section of bedrock in west mining area, The failure depth of casinis 244-670 m and concentrates at about 270-460 m deep. These damaged positions are mainly locatein the bending zone according to three zones of rock layers in the vertical section above the roof dividedenerally, the casing begins to deform or damage before the face line about 30-150 m Special formationstructure and rock mass properties are the direct causes of the casing failure, high mining height and fastadvancingental reasons for rock mass damage. However, the borehole configurationand spacing to the casing failure are not very clear.Copyright o 2011, China University of Mining Technology. All rights reserved.1. Introductionstudied initially by Liang [ 13]. Sun et al. reported that a sheardeformation fracture model based on the rock strata shear slipThe technology of gas drainage from gob area and mining effect was set up, and the key strata effect was researched [14subsidence area by surface vertical well has become a new tech- Based on the analysis of survey data of destroyed drainage wellnical stream of coal bed methane CBm)exploitation in mining area this paper discusses three aspects about the formation structure[ 1-8 This technology has been applied successfully in many and rock mass structure, high production intensity of coal andmines, such as Huaibei, Teifa, Pingdingshan and Tangshan mining technology, and their influence on borehole damageBureau(3, 6, 9, 10]. But it was unsuccessful at some working faces in CBM surface well drainage technology means that relieved gaXieqiao, Guqiao and Zhangbei mine of Huainan city. The stability can be extracted by boreholes drilled from the surface into operand production capacity of some boreholes are restricted by their ating mines prior to mining. As mining advances under a borehole,different levels of damages. Some measures have been adopted to the gas-bearing strata that surround the well will be relieved layerstrengthen their stability, such as improving the well structure, by layer from mined coal seam to the overlying coal mass andadjusting the location of wells, but the desired results often not establish preferential pathways for the released gas to flow towardsbeen achieved. The main factors which influence the damage of the wells and be extracted through a vertical slotted pipe by pumpsvertical wells are the mining height, gob area, thickness of the in surface pump station[25, 15-17].alluvium, formation structure, lithology and construction quality.etc[]. A lot of studies have been made on this problem, but it is still 2. Drainage engineering background in Huainan mining areaot solved. Liu et al. analysed the horizontal displacement andvertical displacement from the surface subsidence's point of view 2.1. Geology backgroundbased on the overlying strata as homogeneous medium [12].Thegas drainage effects of gob wells and well location have beenHuainan mining area is located in the central and northern partof Anhui Province. It has 9 producing mines currently, all subject togas outbursts and characterized by soft coal seams with loworresponding author. Tel: +86 516 83592011中国煤化工 as a close coal groupE-mau address: sangshuxunegm1674-5264/s- see front matter Copyright e 2011, China University of Mining Technology AllCNMHGdoi:10.1016mstc2010.12013X Hongjie et aL/ Mining Science and Technology( China)21(2011)83-88the water-bearing group of the Cenozoic unconsolidated layers and23Mudstonethe Permian sandstone aquifers, and they are most isolated-veinconfined aquifers.22g…] SiltstoneUppe18-22. 2. Background of mining and location of wellsShiheziformationsandstoneUp to the end of 2008. about 38 surface wells had been17-1Mediumstructed in Huainan mining area. they are located mainly at 13working faces in Pansan, Panyi, Quqiao, Zhangji, Xieqiao and DingjiDestruction of gas drainage wells mainly concentrate upon thmudstonewestern of Huainan mining area, such as Xieqiao, Guqiao andSiltyZhangbei mine. In this paper, we selected 6 damaged wells locatedmudstoneon three working faces operated in No 11-2 coalbed in Xieqiao,Moderate-fineGuqiao and Zhangbei mine( Fig. 2). The wells that are at panel a andB captureom No. 13-1 coal seam and gob area when No 11-2wercoal seam is mined The wells at panel C are used to remove gasShanxi iShiheziCoal seamformationfrom longwall gob areas after No. 8 coal seam mined.The super-thick alluvium in these three mines chosen for the1-2Coal seampresent study ranges from 300 to 600 m. The thickness of coal seamformatiat panel A is 2.8 m on average, the dip angle 13-16. The coal seamsFlg. 1. Stratigraphic column [11]at panel B and C are 2.47 m thick, the dip angle 3-10 and 2-3respectively. Fully mechanized coal mining technology and cavingmethods had been adopted for all the panels. Fig 3 shows the panelparameters and location of the wells that are evaluated in thisecoverable/locally recoverable coal seams with a total thickness of study. The locations of damaged wells represented by solid circles30m(Fig. 1). The occurrence of coal seam is stable with a dip angle are also shown in this figure. Triple-hdal lines are the entriesof 9o on average. Coal seam No 13-1 is recoverable and 4.72 m thick for each panel.on average, and is subject to high risks of outbursts. No 12-1 coalseam is situated 66. 7 m below No. 13. 1 coal seam. It is also recov- 23. Drilling parameters and configurationerable in a large part of the mining area. The thickness is 1.67 mwith a lower risk of outbursts of coal gas. Moreover No 12. 13-2 andDrilling parameter is another factor to be considered for stability15 are the adjacent coal seams near No 13-1 coal seam, and are all of drainage well. the borehole structure of wells in these threelocally recoverable not recoverable coal seams. Thus, the geological panels includes several parts shown as follows: surface casing isconditions for the protected coal seam mining and relieved gas used from the surface to the bedrock with cementing: the inter-drainage have been constructed. The lithology of overlying strata is mediate casing is placed inside the surface casing from the surfacemainly sandy mudstone, mudstone and fine sandstone. The thick- to the roof of the protected coal seam( No. 13-1 coal seam), and thenness of the Cenozoic layer is varied from 20 m to 400 m with a slotted production casing to the roof of the 11-2 seam. thea gradual thickening from east to west. The main aquifers consist of borehole is drilled to a point a few meters below the floor of theDating)m20000Stried pan/m·Deep PardonDeep Guqiao Deep Anx.Pani DarienDeep panlratonFengta CountyBoundary of coal iklBoundary d col min[·rmcmL 5: Poel kr surfce dinaw we中国煤化工CNMHFig. 2 Distribution of studied panels in the Huainan mining area.X Hongjie et aL/ Mining Science and Technology(China)2I(2011)83-883000mFig 3. A schematic diagram of the drill locations of 12 surface wells.working coal seams. The bottom of this " rat hole"is filled with strata as the face is retreated. Furthermore the casing failure haswooden pillar or a white cement plug is put across the mining coal been recorded to happen for a distance of 30-60 m behind the faceseam Iine, and can be up to 150 m(Table 1).In the case of the objective study, the configuration of relievedgas well can be divided into the following two kinds. The first kindof well configuration is shown in Fig 4. The second kind of borehole 4 Analysis of in fluencing factors of borehole damageonfiguration differs from the first one in that the latter adopts theintermediate casing of o 177.8 mm and protection casing thickness 4.1. Rock mass structure, rock mass mechanics and borehole failureof 11.05 mm changed instead of o 139.7 mm and 10 mm. The holesof the two kinds of well type are all drilled to the level below the4.1.1. Rock mass structureThe damage patterns of overlying strata, affected by differentfloor of the protective layer. The first kind is applied in No. 1 and 2 at mechanical properties of strata combination and lithology, becomepanel A and C, and panel B uses the second kind of boreholeinto within 8 m above the roof of the mining coar Chave a depth important ways to influence the strata movement, and key factorsconfiguration. But, the other two gob wells at panfor rock mass structure to impact on the stability of relieved CBMwell. Some useful conclusions would be obtained by setting i= Tol3. Failure characteristics of surface vertical drainage wellsTb if To is the alluvium thickness and Tb is the bedrock thickness Bycomparative stratigraphic column, we can find that the rock massstructures at which boreholes damaged are located have a numberBy the end of 2009 about 15 wells, comprising 40% of the total of peculiarities of T s 400 m and To/Tb 2 1. Moreover, the smallerwells, were damaged. The slippage and deformation are the mainamage patterns, and the damages occur mainly in the low part ofthe to and the larger the ratio To/ Tb, the more serous the damagesthe alluvium or the upper bedrock within the bending zoneurrounding the mining area. However, thick claybed in the alluThe western of Huainan mining area where the damaged borewium and bedrock surface are dominant lithology in the view of the holes are located is characterized by thin alluvium and thickformation where the drilling wells are destroyed, and thgraphic column of all damaggeographical distribution of damages is overwhelmingly uneven thickness is between 138.46 and 357.88 m the alluvium thickness isrom the west to the east. The mines in which wells damaged about 330.30-437 15 m and the radio To/ To changes from 1.09 m tomainly are Xieqiao, Guqiao and Zhangbei in the west of the mining 2.84m Fig.5showsa low negative correlation between the ratio andarea On the contrary, most of wells in central or eastern mines are the distance behind face, which indicates that the damaged time ofusually less affected by the damage. The deformation of some the casing is influenced by the relationship of the alluvium andrainage wells generally begins before the face line about 30-40 m, bedrock. In contrast with the bedrock, the alluvium belongs to weakand then the effective drainage life of wells is limited by the formation with loose composition, low shearing strengthen anddamage it suffers due to the displacement that develops within the non-deformability. Therefore, the alluvium has a coordinatesubsidence trend with the bedrock generally. By comparison withSurfacethick bedrock, the thin bedrock cannot support much thick loading311mm srfacerelyingluring mining on condition that the overlying strata fulfillmentioned conditions for thin bedrock but deep alluvium. Asa result the effective drainage life of borehole is limited with thecloser distance of the well location behind the coal face216mm casing4. 1-2. Rock mass mechanicsThe structural units of rockstructural中国煤化工 mechanical property ofuctureOpen hole94NMHthe data ofFig. 4 Type I gas well structure 51-graphic positions of drainage wells where they damaged areX Hongjie et aL/ Mining Science and Technology(China)21(2011)83-88Table 1Statement of casing failure in HuainanMine Well number Hole Working seam To(m) T,(m) i= To/TbDistance behinddepth(m) depth(m)m) face(m)635.67633.9733033037109AlluviumDeformation and slide463.33andy mudstone/siltstonePanel A-28.19636273526283.712424400Deformation and slide46.00951643Panel C-27573743723202137It/ClayDeformation and slide53622Panel B-3948788484306357912067000100.0ote: Ta Alluvium thickness, Tb. Bedrock thickness.founded and analyzed The damages of boreholes happened both in As can be seen from above discussion, once the rock damagethe bedrock section and the alluvium. The failure depth of casing is occurs, then the failure surface(zone )should take place in a weak244-670 m and concentrates at about 270-460 m deep. The mechanical surface at first. Nevertheless, the failure positions of theged positions are mainly located in the bending zone casing are scanty at lithological interface in the alluvium. Theyaccording to three zones of rock layers in the vertical section above existed in thick clay layer of the lower unconsolidated formationthe roof divided, but also classified as being in the middle or lower near the bedrock surface, such as No. 1 and 2 well on the panel Cinparts of alluvium, or in the upper section of the bedrock based on Zhangbei mine. And what reason is that?stratal configuration. In the 15 failure surfaces, eight damages of The experimental results of previous research shows that thethem are in the alluvium section, six of them are in the bedrock friction coefficient"f of the clayed generally decreases and thesection and one is the bedrock surface. Of these, three are in thick cohesive force"C" reduces firstly and then increases with increasedclay layer(>30 m) near the bedrock surface and one is the surface thick, also, the thin claybed is characterized by brittle failure whenbetween silt and clay in 4 given lithological characters of damaged the strata displacement is small. But along with the increase ofsurfaces, and then one is in sandy mudstone about 10 m thick, thickness, the displacement curve changes gently. when closingothers are the interfaces of different lithological characters in with a certain point of failure of claN atenzed by plastic failuree displacement goes upemendously, and the clayband is charand lower mudstone. The only one of the bedrock surface damage is [ 20-22 It can be seen, when the clay is thin, the shear strength ofthe lithological character between clay and siltstoneit is mainly controlled by the weak lithological interface, andAffected by mining rock mass slippage occurs firstly at thproducing shear failure and tension fracture easily during miningmechanical surface of the overlying strata. Lithologicalandleading to a higher shear range to break the upper and lowerterface, as a sedimentary discontinuity, would unquestiinterface. Therefore, the casing failure is not prone to occur in thinave a relative weak mechanical property. However, not all litho- clay layer for its higher intensity, friction coefficient and shearlogical interfaces will slide for the mining activity. only those who strength. But when the thickness of the clay exceeds a certain valueare less than the horizontal tensile stress in shearing strength of the tensile cracks are developed and the shear surface is flat androck interface will be much easier to slide. In addition, the interface smooth because of the control for the interbedded clay itself, So theat which the rock mass slide all depends on sliding resistance and thick clay layer has a lower shear strength under this situationthe resistance value is decided by undulation, infilling and its [19, 22) This shows that rigid casing are easily sheared in a thickthickness of weak structural surface and the rock mass character- soft layer and cannot be damaged directly at the interface betweentics of the two sides of it[20] An equation can be obtained by different strata in the loose bed of the quaternary systemsetting the sliding resistance of interface toto for its stability statusbefore mining according to Coulomb,'s formula42. Mining height, advancing speed and borehole failureto =C+otg>tThe fracture, bending and subsidence scopes of the overlyingwhere o is the normal pressure on the shearing plane(MPa). o the strata are determined by the mined-out area after mining. Theinternal friction angle of rock or soil and C the cohesive force of rock subsidence space is decided by working face with large miningor soil (MPa).The cohesive force will be reduced to C and the internal frictionangle to y after mined, making the utmost anti-shear force reduceThe well casing would be cut by the interlayer shear zoneresulting from the slippage of rock mass above structural plane due中国煤化工。2to the shear stress that develops with the strata movement as theCNMHGoal face is retreated.nce behind the coal facX Hongjie et aL/ Mining Science and Technology(China)21(2011)83-8height, and the rock movement state is affected by mining speed. 4.3. Relationship of drilling parameters and borehole stabilityMoreover, the method of rock movement depends on the stratig-raphy and type of rock structure in stress condition. The damaged 43. 1. Location effects on the drainage well stabilityspeediness for a short time about 30-150 m behind the coal face, It is absolutely vital that reasonable locations of drainage wellscannot be explained by the mechanism of casing failure of oil and to their stability and drainage efficiency. Horizontal displacementnatural gas well. The huge driving force of time effect required for and vertical subsidence generate and transfer to the surface in thecasing failure can be found from these factors concerning the strata center of the gob area firstly after coal mining and periodicmovement, such as miningob area, roof control method, weighting. With advancing, the value-added horizontal displacemining method and advanceLong-wall fully mechanized ment is smaller with larger subsidence velocity but a fewerstopping technology, cavingare all used at panel a, B and displacement than before. However, the overlying strata havC comparing with miningdisplacement,a larger value-added horizontal displacement but a lag time in thisdistrict that is near the air gateway or haulageway than those in the4.2. 1. Mining heightcenter of the face. Based on the fundamental view point of theThe mining height is one of the most important factors affecting nonuniform distribution of strata movement in space and timetus of the overlying strataIt which is not inevitable in advancing. finding the low risis found that the mining height of panel at which the successful where the surface drainage well is damaged by the strata moverainage wells is less than 2. 3 m based on the data of production ment within the framework of small-scale in working face bychanges and geology of the panel in Panyi and Pansan mine. But the scientific methods and deploying the well site reasonably to fullymining heights of Panel A B and C are between 2.8 m and 3. 2 m, reduce the failure resulted from the rock movement are the keys tond that means the mining height is an intrinsic factor for well capturing efficientlystability with the increasing thick of coal seam, the mined-out areareaches much larger than before making the roof displacement and 4.3.2. Styles of wellbore configurationstrata activity in vertical direction increase largely. In addition,The configuration of a gas drainage well is related to drainagemuch stronger vertical displacement and horizontal displacement effect, including two main types, namely, type I and l. Comparedmay be triggered by the weight load. As mining progresses, all wells with 5 wells with type I at panel C in Xieqiao Mine. No, 2 and 3 wellwould subject to tremendous shear stress as a result of the frequentat panel B in Guqiao Mine with type ll show no obvious integrityperiodic weighting, following the increased destruction of over- and stability. There is no direct relationship between the two kindslying strata movement.of casing structure, either from the amount of damaged positions,No. 2 well was destroyed behind the face 30 m with the coal or the distance behind the coal face. although type ll has a thickerthickness of 2.9-3 2 m and No. 3 was destroyed behind the face materials of the casing and an expanded tube diameter for ga00 m with 2.5-2 8 m coal seam thickness at panel B in Guqitransfer than type l, it still cannot resist shear strength of the rockMine. Meanwhile, the fault nearby the no 3 well has no influence movement. Thus, the optimized borehole configuration to a certainon it. So, a large mining height may create an advantage for casing extent can be able to enhance the shear strength of the casing orfailure ahead of time but a relative smaller mining height in the relax the horizontal stress, but its impact on the casing failure is notvicinity of the No. 3 well may decrease the rate of rock movementeand failure time4.2.2. Advancing speed5. ConclusionsCommonly, a great horizontal sliding in the stratified rock canbe caused by a large mining speed in a short time such as 6 m/d at 1)The relieved CBM wells are mainly distributed in Xieqiao,panel a and C or even more than 10 m/d at panel C and destroyGuqiao and Zhangbei mine located in west mining area, andhe drilling well in different parts of the casing beyond its capacitbegin to deform or damage before thability. by contrast, the mining speed of some panels in Panyi an30-150 m. In addition, the damages occur mainly in the thicknsan mine is only 3-5 m/d, so they are more successful.Accelerating the speed of face advance to a certain extent caninterface of hard and soft rock in the upper bedrock. Themake gob area increase rapidly in a relatively short time. Hence,damage scope mainly in the bending zone but not the fracturedthe essence of this is to increase the rate of the overlying stratazone from the three zones about the rock mass divided. thesdeformation and to make well casing subject a combination ofshow that the vertical tensile strength and compression causedhorizontal shear stress and vertical tensile stress with instabilityby the subsidence of the overlying strata are not the majorfactorsccording to the research about the overlying strata movement 2)This paper divides the strata configuration in the Huainan[5. 23]. the faster the mining speed is applied, the faster horizontalmining area into two types according to the thickness of thedisplacement, and the larger strata displacement. In this case, italluvium and bedrock The strata configuration in west miningmakes easier for inconsistent rock movement and deformationarea is an unstable formation characterized by tb 400 m andsuffered in different parts of the overlying strata, resulting in theTo/Tb> 1. The lower alluvium is an unstable formation to thedrainage well for its dead load during mining, or thick clay layerand other soft layers presented would result in the decreasedof bed separation is lessened and the scope of the roof is reduced shear strength with the increased thickness of these layers. The24. If quickening the mining speed during the working face beingharden lithology near the upper bedrock, with a large horiclosed approach to the drainage well, the time of periodicweighting and pressure-relief effect of pressure transmission willVu中国煤化工 e displacement at thereduce and increase the roof weighting step with an indistinctiveCNMHrata. Thereforstrata behavior. Moreover, the damage of drainage well influencedJbe taken in the wellby the rock movement may decrease for the reduced subsidenceon design and drilling process.X Honglie et aL/Mining Science and Technology3)High coal production intensity is the essential reason for casing 16] u G]. Theoretical research and practice on coal mine methane extraction anfailure. Rapidly increasing of gob area is the result of excessivemining height and too fast advancing speed, and the strata[7 He TC, Li HG, Zhang H]. Gas drainage technologyoccurring behind the face line for a short distance f quickening 18) sang ss. Xu Hl Fang uc. L GL Huang HZ stresoalbed methanehe mining speed during the working face being closed approachto the drainage well, the time of periodic weighting and pressure. [] Liang ZM, Zhang B, Wu HB. An attempt at methane extraction from adjacerrelief effect of pressure transmission will reduce and increase theyer or gob by surface borehole. Safety in Coal Mines 1999: 11: 32-4 [Inroof weighting step with an indistinctive strata behavior, oncondition that there is an unchanged mining height. Moreover. 1101 Song FH Gas drainage technology of goaf with boreholes from surface groundTangshan mine. Coal Science and Technology 2009; 37(4): 43-5 Inthe damage of drainage well influenced by the rock movementmay decrease for the reduced subsidence of the goaf roof.111 Sang Sx Gin Y, Jiang B, Fan BH, Xu L Zhang H. Studies on coalbed methane4)It is the key to reduce the failure resulted fromoment in huainanmovement to find the low risk zone where the surfage [12] Liu Yz, ui XH. Safety analysis of stability of surface gaswell is damaged by thement within the2149-5of small-scale in working face and deploy the well site[13] Liang YP. Practice of methane drainage by surfaceng area. Journal of Mining Safety Engineering 2007; 24(4): 409-13 [ Inreasonably. But nonetheless, the change of borehole configu-ration for the casing failure keeps not obvious.二915(3)304ief coalbed methane drainage in mining anThis work is sponsored by the National High-Tech Research and 6l Waiel as heng ag: from dian r te eB, Frac sere s oInden an d xtremelyDeevelopment Program of China (No. 2007AA06Z220) the Keynck key stratum. Journal of China University of Mining TechnologyScience and Technology Program of Ministry of Education(No2:182-6[17] Karacan CO Forecasting gob gas venthole preReferences118] Yuan L Theory of pressure-relieved gas extraction and technique systemintegrated coal production and gas extracton. Journal of China Coal Societyextraction. In: The 6th international workshop on CBM/CMM in Chind 2006: Ii9) Sun Cz. Zhou RG. The strmeseiuctural effect of rock mass deformation and failure[2] Yuan L Key technique of safe mining in low permeability and methane-rich[20] Li ZQ. Applied rock mechanics. 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