The mechanism of the flowing ground water impacting on coalbed gas content

ARTICLESChinese Science Bulletin 2005 Vol 50 Supp 118-123gas dissipating through cap rocks under stronger hydro-The mechanism of the flowing hen be cfit di ithe stad ta kes gas arwsy atis researed wasground water impacting onaccumulation mechanism, and coalbed gas explorationand developmentcoalbed gas content1 Flowing groundwater lessening the gas contentThe flowing groundwater affects coalbed gas contentQiN Shengfei, SONG Yan TANG Xiuyi?obviously. In both horizontal and vertical plane, the coalFU Guoyouhas a lower gas content in an area of stronger hydrody1. Research Institute of Petroleum Exploration and Development, Petro- namics, while a higher gas content exists in an area ofChina, Beijing 100083, China;weaker hydrodynamic2. Anhui Science and Technology College, Huainan 232000, ChinaCorrespondence should be addressed to Qin Shengfei (email1. 1 Horizontallyqsf@petrochina.com.cn)The coalbed has a low gas content in the area ofAbstract The hydrogeological condition affects the coal. stronger hydrodynamics, and has a high gas content inbed gas storage dramatically. In an area of stronger hydro- water stagnated area. Take Kaiping syncline as an exampledynamics, the coal has a lower gas content, while a higher gas The Kaiping is a dissymmetry syncline with steep north-content exists in an area of weaker hydrodynamics. Obvi. west limb and gentle southeast limb. Of the outcrops ofously, the flowing groundwater is harmful to coalbed gas both limbs the northwest is higher than the southeast. Thepreservation. But few researches focus on the mechanism ofow the flowing water diminishes the coalbed gas content. groundwater is supplied from the northwest limb anddrainages to the southeast limb(ig. 1). Near the axis ofonly makes coalbed gas content lower, but also fractionates the syncline the groundwater is relatively stagnant. Thethe carbon isotope, this research puts forward an idea that it groundwater hydrodynamics of the northwest limb isis the water solution that diminishes the coalbed gas content, weaker than the southeast limb. The southeast limb has arather than the water-driven action or the gas dissipation better runoff condition than the northwest(Fig. 1)through cap rocks. Only water-soluble action can both fracThe gas contents on both limbsuch different. Thetionate the carbon isotope and lessen the coalbed gas content, coal in the coal mines along the northwest has higher gasand it is an efficient way to take gas away and affect the gas contents than the southeast with the same coal seam andcontentdepth. For example, Haogezhuang coalmine, MajiagouKeywords: coalbed methane, methane carbon isotope, groundwater, coalmine and Tangshan coalmine all distribunorthwest of the syncline. The coalbed gas contents inDo10.1360/98zk0038these coal mines are obviously higher than the Linxi,Lujiatuo and Qianjiating coalmines that locate on the southVery different from normal gases that exist in fractures east of Kaiping Syncline(Fig. 1)and pores in reservoir, coalbed methane is mainly comIn addition, the hydrogeological conditions in theposed of the adsorbed gas and a small amount of free and southern part of Qinshui Basin trend to be stagnant andwater-soluble gas. The special gas storage mechanism are beneficial to coalbed gas preservation, which alsodetermines that the gas-rich controlling factors of coalbed shows that the flowing groundwater is harmful to coalbedmethane are different from the normal gas pools. Hydro- gas preservationsgeological condition is one of the most important control-ing factors in coalbed methane preservation. In wate1.2 Verticallystagnant area it is favorable to the coalbed gas preservaVertically, the stronger the hydrodynamics is, the less19-nuoncoalbed gas content the coal has. In the water stagnatedThe coal has a lower gas content in the area of stronger area, the water flows slowly, and the coal still has highhydrodynamics, and a higher gas content in the areacontentweaker hydrodynamicsbviously, the flowing In North China, the carboniferous coal seam covers digroundwater is not favorable to coalbed gas preservation. rectly on the Ordovician limestone. The coal measure canIt will destroy the coalbed gas reservoir. But few re- easily connect with the strong groundwater runoff belt. Itsearches focus on the mechanism of how the flowing wa- had been accepted that the Carboniferous coal seam has ater diminishes the coalbed gas content. Since the special stronggas storage mechanism of coalbed gas is different from中国煤某化了coal seam is weakerthe normal gas field, the manner and mechanism of water than tCNMHGtaking gas away from the coal are much more complicated. The adcapacity On coal to methane increasesIs it the water-dissolved action or water-driven action, or with the buried depth and decreases with temperature118Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLESJinggezhuang 9.63 m/tcoal mine9.20mtcoal minecoal mine oL可 latuo657m/t557mtgshan oQianjiayingcoal minecoal mine876mtElevation( m)15002500Fig. 1. The planimetric map and the SE section of the Kaiping Syncline. (a) Distribution of gas contents of some coalmines in differentparts of Kaiping Syncline;(b)the section showing the groundwater flow of Kaiping Syncline, l, The groundwater flow; 2, the migrationof coalbed gaGenerally, above the depth of 2000 m underground, the methane to investigate mechanisms of reservoir destrucadsorption capacity to methane increases with depth. In tion of coalbed methane by underground waterNorth China, the buried depth of Carboniferous coalmeasure is bigger than that of Permian. Theoretically, the2. 1 Occurrence of coalbed methane in coal seamsgas content in Carboniferous coal seam should be higherCoalbed methane in coal seams occurs in threethan Permian coal because of the different depths. Actu- forms -, namely, adsorbed gas in the pore of coal ma-ally, it is not the case. In many places of North China, it is trix, free gas in the cleat and medium-large pore of coalcommon that the gas content in lower main coal seam seam that does not contact with the inner surface of coalCarboniferous)is less than the upper main coal seam" grain, and water-soluble gas dissolved in the water within(Table 1)al seam. These three gases can transform each other.In addition, during coal mining process, the areas with 2.2 Possible mechanisms of coalbed methane escapective groundwater probably have little gas outburstwhile in areas with weaker hydrodynamics, it,'s likely toBased on the three occurrence modes of coalbed methhave gas outburst or have a big amount of gas effusion. ane, it is likely to infer the mechanisms of gas storageWangyingzi coalfield gives an example!7. So the weaker destruction of coalbed methane by underground water.water flow condition is beneficialpoor for the gas preserved in active flowinbecomes free gas because water cannot directly carry2 Possible mechanisms of reservoir destruction of away the adsorbed gas. Therefore, it is likely to dissipatecoalbed methane by underground watercoalbed methane by the following ways: gas diffusion中国煤化工naond waterIt is necessary to understand the occurence of coalbed dissCNMHG1)Qin, Smng, X et al., Hydrodynamic control on stable carbon isotope of coalbed methane(in Chinese with English abstract), NaturalChinese Science Bulletin Vol 50 Supp. December 2005119ARTICLESle 1 Gas content in upper and lower main coal seams in some holes of North ChinaBasinCoal seamLocation of coal seamGas content(m/t)Pupper main coal seamlower main coal seamOrdoseam 5Jishi 1Seam 8lower main coal seam13.8upper main coal seam20.721201Seam 15Seam 15eam 3Seam 15cPcPcPcPcPclower main coal seam13.820.72lower main coal seam13.8ain coal seamlower main coal seam22.39QinshuiSeam 3upper main coal seamSeam 15lower main coal seam21.55upper main coal sean17.1Seam 15Clower main coal seam12.7Seam 3upper main coal seam18.17YangquSeam12Clower main coal seam14.75Seam 15lower main coal scam713Most researchers are used to falling into the thinking fusion causes lower gas content. Additionally, the wapattern of the conventional natural gas when they study ter-driven action will not cause coalbed methane carbonthe coalbed methane. Naturally diffusion of coalbed isotope fractionation. The water cannot selectively dis-methane through cap rocks is firstly considered as a main solve methane carbon isotope because of the interfaceway of coalbed methane losses, for conventional natural between gas and water. However, water-dissolved actiongas mainly dissipates through cap rocks besides through can fractionate methane isotope, and thus cause a lighterfaults.Consequently. the accumulation of conventional carbon isotopic value of rudimental coalbed methanenatural gas into reservoir needs high-quality regional caprocks and direct covers. Secondly, dissipation of coalbedmethane by water-driven action is considered because theinterface of gas and water exists for conventional naturalgas. However, water-dissolved action is seldom considered as a way of coalbed methane losses because the wa-tional natural gasAccording to the geochemical characteristic of coalbed°88methane that flowing groundwater diminishes coalbed gascontent, the gas diffusion through cap rocks and water-driven action may be excluded. For there exists strong9020406080100120140carbon isotope fractionation on coalbed methane under thecondition of flowing water action, the carbon isotope ofamples of coalbed methane and normal coal-formed gascoalbed becomes much lighter. Comparing the methaisotopes between the coalbed methane and the normalcarbon isotope values between the normal coal-formed gas coal-formin large/medium-sized gas field and the co3 Water dissolved actionChina(Fig. 2), and putting the coalbed methane isotope 3. 1 The water-soluble gas in naturedata from South Sydney Basin into the figure for furthercomparison, we can clearly see that stable carbon isotopesWater-soluble gas, mainly methane, widely exists inof the coalbed methane are generally lighter than that of nature and has great resourcees3-17. Simulative experithe normal coal-formed gas. The methane of coalbed gas ment showed that the water can dissolve considerable gasis rich in 2C. The 2C accumulative effect is dramaticunder pressuWhen the gas is over saturated in wa-If the coalbed methane dissipated through cap rocks, ter,in\中国煤化工 ptured by trap and gasthe carbon isotope of the remained coalbed gas should pool-ter-solublean alsobecome heavier, not lighter. Since CH4 diffuses more beCNMHpool. When the wa-easily thanCH4, diffusion will make the coalbed rich in ter-soluble gas is taken to the ground, thewill beCH4. Therefore, it is unlikely that coalbed methane dif- separated out due to the gas saturation decrease I1Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLESMany researchers think that some normal natural gas varying range, as mentioned above, depending little onpools are formed in the way that the gas firstly releases coal ranks. So taking no account of the maturity, the figurefrom water and then accumulates. Wang Lansheng thinks of gas content vs 8 C is made(Fig. 4). Obviously, thethat it is likely that the gas in Sichuan Basin is degassed gas contents have good correlation with 8CI valuesfrom water 8. The Kela 2 gas field, the main supplying From the relationship shown in Fig 4, we can see that thefield for Gas Transportation Project, is considered to be a lower the gas content is, the lighter the carbon isotope iswater-dissolved origin", 20. In northern West Siberia,Cramer et al. think that the gas is related to the wa-ter-soluble gas!. So the water-soluble gases are ubiquiWest Water-solubletous in nature and have not been sufficiently regarded. To-35 Well Ma8coalbed methane, the free gas in every kind of fracan not only be dissolved, but also be easily takenWell Ma3Wellby water.3. 2 Water-soluble impact on the carbon isotopic value ofWell Ma5... ea-38Well Ma401.Well Ma4The water can fractionate methane carbon isotopeZhang Xiaobao studied the water-soluble gas in TaibeiWell Ma8 Well Ma3 Well Ma2 Ma5 Well Ma401 Well Ma4Depression of Turpan-Hami Basin using the method ofdegassing in thermal vacuum. Compared with the normal Fig 3. Methane carbon isotopic values between wells in Hetianhe gasgas from the same area, the methane carbon isotopes ofwater-soluble gases are heavier than the normal gas2zz.Itshows that the water trends to dissolve CHA. The CHis more likely to dissolve in water than CH4It is found that the gas in Hetianhe gas field in TarimBasin also shows the characteristics of water-soluble gasUnder the same condition, the gas components have dif-ferent solubility: CO2 >CH>N2>C),Hs"5).So thewater dissolves the gases, takes them to the area of lowerpressure, and then releases the gases. This process willeasily enrich CO2. To hydrocarbon gases, this will enrichmethane, for methane has bigger solubility than heavyhydrocarbon60-55-5045-40-35-30-25The gas field distributes from east to west like a belt"C‰)(PDB)The geochemical characteristics of natural gases are dif-ferent between the eastern and the westem part. Compared Fig. 4. Relationship between the gas content and the carbon isotope ofwith the eastern part the natural gases in the western part coalbed methane (Data provided by Wang Hongya and Tang Xiuyi)have more CO2 content, the dry coefficient is higher, andon isotope of methane is heavierConcluded from the above, the hydrodynamic condThe water has obvious fractionation to methane carbon tion has much influence on coalbed gas content. Theotope. So the methane carbon isotope becomes gradually stronger the hydrodynamics is, the less gas content theheavier from eastern to western part of this gas field(Fig. coal seam has, and the lighter carbon isotope of coalbed3). It indicates that the water can take moreCH4 awaymethane is. It is the water solution that fractionates theSimilarly, in coalbed methane, the flowing water can coalbed gas isotope. So the flowing water destroys thealso take more CH4 away and leave more. This coalbed gas content by water-dissolved action, rather thanmakes carbon isotope of free gas lighter in coalbed meth- water-driven actionane. Owing to the accumulative effect, the carbon isotopeThe free and dissolved gases are limited among coalbedof coalbed methane becomes lighter in a large degreegas. Why can the water-dissolved action make the carbonisotopic values of coalbed methane so lighter? It depends3.3 The relationship between gas content and carbon on the special gas storagemethaneAs the source rock maturity increases, the methane 3.4中nf methane carbon isotope gettingcarbon isotope of the normal coal-formed gas becomes ght国煤化工heavier in less extent, commonly less than 10%c. However,CNMH Gof 1?c enrichment inthe carbon isotope of coalbed methane has much bigger methane, the researchers have put forth some possibleChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESmechanisms up to present, the main 3 kinds are: O the lower gas content. Water-dissolved action not only causescarbon isotope exchange between CHA and CO2 24) @2 lower coalbed gas content, but also fractionates coalbeddesorption-diffu-sion-absorption effect 25-29, 8 secon- methane isotope, despite the constant exchange betweendary biogenic gas origAs the coalbed gas explosorbed and free gas and the accumulative effectthe old ideas encountered will be discussed in another of coalbed gas systems in China( Grant No. 2004CB720504)article. A new mechanism will be put forward here to explain the"c enrichment in coalbed methaneReferencesAs mentioned above, coalbed methane is mainly com- 1. Ye, J, Wu, Q, Wang, Z, Controlled characteristics of hydroposed of adsorbed gas and a small amount of free and wa-geological conditions on the coalbed methane migration and accu-ter-soluble gas. 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