Mechanism of production increasing of coalbed gas for the pinnate horizontal well and sensitivity ef

ARTICLESChinese Science Bulletin 2005 Vol. 50 Supp. 163--170sides of the main horizontal well hole, which can passMechanism of productionthrough more fracture systems in coal layers, communicate the furthest fracture and increase the controlled areaincreasing of coalbed gas forand coal bed permeability and increase the individual wellgas production. It is a patent technology developed bythe pinnate horizontal wellCDX company in USA in late 1990s, which has been usedin the coal bed gas development and made a good produc-and sensitivity effect oftion increasingIt can provide a theory instruction for using the pinnateparameters on gas production horizontal well that studies its production increasingmechanism and analyzes how it can improve productionZHANG Dongli& WANG Xinhaiefficiency. Through the analysis of the parameter sensitivExploration Production Research Institute, Beijing 100083ity on gas production for the pinnate horizontal well,ageneral law on the coal bed property and gas productionof Geoscience of Yangtze University, Jingzhou 434023, China can be got, which can evaluate production value andCorrespondence should be addressed to Zhang Dongli (email: zhangdI@ adaptability of the pinnate horizontal well for differentpepris. com, zhangdongli_xn@sohu. com)types of coalbedAbstract The mechanism of production increasing forThe mathematical model and numerical model for coalthe pinnate horizontal well is analyzed with the simulation gas production by the vertical well and pinnate horizontaltechnique. It is pointed out that the whole horizontal well. well have been developed, and the numerical simulationbore spreading widely and evenly in coal layers makes the of the two production ways have been realized and thestratum pressure drop evenly and fast, which increases the production of the pinnate horizontal well has been fore-chance of desorption and diffusion and area controlled. This casted.). In this paper, the production increasing mechais the basic reason that the pinnate horizontal well can in- nism and sensitivity effect of parameters on gas produccrease production. How such stratum parameters as perme- tion will be analyzed quantitatively by using the numericalability, Langmuir constants and adsorption time constant simulation techniqueaffect the gas production for the pinnate horizontal well isalso studied with numerical simulations. Either for the verti- 1 The mathematic model for coal gas production bycal well or the pinnate horizontal well, bigger stratum per- pinnate horizontal wellmeability, steeper relative permeability curve, smaller residual gas saturation, higher gas content and stratum pressure I. Flow equation of coalbed gas and water in dual me-are more favorable for gas production increasing. Langmuir diumconstants decide the change of desorption quantity and influence the gas production through changing the extent ofCoal layers are considered as the pore-fracture dualmedium. In the initial state water almost fills the fractureping area. The adsorption time constant only influences the and little dissociated gas exist. A great deal of gas exists intime that gas production arrives at the maximum value for matrix in the adsorbed state Gravity is considered andthe vertical well, but it almost has no influence on the gas capillary pressure is ignored. The gas in the fracture satis-duction for the pinnate horizontaal well because the fies the real gas state equation. Velocity of gas is consid-equivalent adsorption time constant is reducedered as the sum of percolation velocity and gas diffusionKeywords: coalbed gas, pinnate horizontal well, mechanism, nu- velocitymerical simulation, sensitivity effect.DDOI:10.136098zk0037gMost methane exists on the surface of coal matrix in for water phase in fracture,the adsorption state. Generally, water in the coal bed needsto be taken out to decrease the coal bed pressure, so theadsorptive methane gas can be released. Only when theThe gas desorption course in matrix is consideredstratum pressure is less than the critical desorption pres- the pseudo steady state diffusion, which satisfies the firstsure, methane can get desorption. If the coal bed has such law of fickproperties as low pressure, low permeability and low gas dCm(nz Dm Fs E(Ps)-Cm())=1/(VE(P )-Cm(D),saturation, the coal gas production may be very low. Sothe economical benefit,. The pinnate horizontal wellforms through drilling several branch wellbores at both中国煤化工CNMHG1)Zhang, D. L, Wang, X. H, Simulation study of coalbed methane pinnate horizontal well production, Coal Geology Exploration, 2005.(inChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESVe(Pg) is gas concentration in equilibrium with gas pres- 1. 2 Pressure sensitive model of porosity and permeabilsure in the fractureWhen p≥p小v出p)=Vpa)=(1Pi)When p< Pd, satisfying Langmuir equation,Cm(1-)(C(P)-C(Pl),(13)VE(P)=V-PC(Pa)-C(Pse)KThe flux flowing from matrix to fracture is considered as≠pk(14)ne source item in the gas phase flow equationKe-H (pi-Pe"2(P-pPkdCm(r)(6) where al, a2 are pressure sensitive factors, Pk is pressurecorresponding to the lowest permeability in the permeThe flux flowing to every main and branch well section ability-pressure curve, which can be measured from ex-is considered as the affluxion item in gas and phase flow perimentequation, and the gas phase flow equation and water phasepercolation equation in the fracture become1.3 The well bore pressure drop modelKgD sFor the pinnate horizontal well, fluid in stratum can notv+V(8)only flow into branch at first, then flow into the main welldr Zsection, but also flow into the main well section directlyIn the well bore, considering the friction pressure drop andacceleration pressure drop resulting from inflow, thepressure relation between two neighboring well sections in9(94S)=(K,gqthe main well bore and branch well bore can be expressedBws vp(8) as follows:向ByBwwhere, g, w are potentials of gas and water phase, q gP4=pM+0.5(4n,+Ap,i=2,…,ngw are gas and water volumes flowing from the unitpn=PM+0.54(15)volume of stratum in the unit time the unit is 1/swhere Pwfe is the bottom pressure at the outlet of the pin-nate horizontal well. Apwf is the sum of friction pressureq8qwdrop and acceleration pressure drop of two neighboringwell sections, subscript i represents the well section tab.ge, w are gas and water volumes flowing from the certain subscript of the upward well section is 1, subscript of thevolume of stratum in the unit time the unit is m /s in the downward well section is nfinite difference rectangle cells system, representedApwfi=Ap fric, + Ape(16)follows.Ap tric is friction dropdown,g, PIDBkrg(Pg -Pwt)Ag1 Pi f i(17)Pwp2 Dqw=PIDP(10) fi is the friction factor of the well section, D is wellborewhere, Piy is wellbore pressure in each differential section, diameter, V is the average flow velocity of the well sec-ID is well index, expressed astion,Vi=-,P is fluid density of the well sec-PID=2T,Lp(11)n五+Stionhe two phase condition of gas and water,Pgiqgi tWitPiAx, is the length of the well secwhere, Ke is isotropic permeability equivalent to the anitropic medium, Ke=(, K, k,), Lp is the differen- tion. Apacc is the acceleration pressure drop,tial well section length in the transformed space, rb isn(+vCVi-Vi-)(18)equivalent radius in the well cell, rw is equivalent well中国煤化工radius. s is skin factor.Saturation equation isCNMHD2 tD2S.+S=1(12)mV1+p v I DAr-p: Vi4+4VR=0Chinese Science Bulletin Vol. 50 Supp. December 2005ARTICLESwe get Av=i-s 4Nvi Ax+VRi, VRi is inflowDvelocity from the branch wellbore to the main wellbore,Vi-l, Vi is inflow velocity and outflow velocity in the sec3758mtion of wellSince Q, 4.9,Vv AxrD, O. R TD2n6586800meq. (16)can be represented by flow rate(2Q+q1+QR)2△(19)16(4+Qk(2Q+q1+QR)@i is the inflow rate from the neighboring upward section,which is sum of flow rates of all main well sections andNbranch sections of the upward part of the well section,2200meRi is the inflow rate of the main well section from itsFig. 1. structure of pinnate horizontal well groupbranch section, qi is the inflow rate of the well sectionfrom stratum, q=qg+ qvFig. 2 is the production comparison result of the vertial well and the pinnate horizontal well in the above stra2 Analysis of production increasing mechanism for tum condition. Fig. 2 shows that the overall trend of gaspinnate horizontal wellproduction by the pinnate horizontal well is the same asFor analyzing the production increasing mechanism for that of vertical well. The gas production increases until itthe pinnate horizontal well, we make the simulation using reaches the maximum value, then decreases first steeplythe same stratum parameter for the vertical well and the then smoothly. The equal value of gas production for thepinnate horizontal wellsingle pinnate horizontal well is about 10000 m, while theThe simulated setting area is a circle with a radius of gas production for the single vertical well in the area is no1241.5 m in the vertical well model and a square with a more than 100 m or more less if no any stimulationlength side of 2200 m in the pinnate horizontal well model, measure, which is hardly any production worthboth thickness are 5.8 m, so the controlled areas in plane Fig 3 is a radial profile of the stratum pressure and gasare all 4.84 km and the controlled volumes are all 2807x saturation for the vertical well production. The gas satura10 min the two models. The basic parameter used in tion is influenced by gas desorption(making gas satura-simulation comes from the well Jinshi I in the Fanzhuang tion increase), diffusion(making gas saturation even)andsection in Qinshui basin, shown in the third column of output(making gas saturation decrease). Fig 3 shows thatTable 1. From Langmuir constant and critical desorption at the initial stages, pressure decreases steeply and gaspressure, we get that the gas content in the coal bed is 36 22 saturation increases steeply in the area close to the well,m/m, so the overall gas content in the simulated area is while pressure decreases very little in the area far fromabout 1 billion m. The well group consists of four pinnate well, which makes desorption of gas in matrix insuffihorizontal wells, and its structure is shown in Fig. 1ciently, and gas saturation increases slowly. With theTable 1 Fundamental parameters of coal bed for simulationvalueParametersSourcetheory examplewell Jinshi I coalbed0.026Permeability x y/z(um0.30.1×1005144×103well test(prefrac)Thickness(m)8log+borehole logInitial pressure(MPa)3.445well testCritical desorption pressure(MPa)Langmuir pressure(MPa)0.9673.034experiment29.2Adsorption time constant (h)中国煤化工 experimentPore compressibility(1/MPa)6×102Diffusion factor(m"/d)164×10-2CNMHGexperimentChinese Science Bulletin Vol 50 Supp. December 200565ARTICLES02 Cuma200040000010002000300040005000040080012001600Fig. 2. Comparison of gas rate for vertical well and horizontal well0.123 months1 Pg(20 h)4.25 MPa3. 75 MPa6Sg(400h)1.5 vears3.25MP2.75 MPa1001000100002.25 MPaig. 3. Radial profile of stratum pressure and gas saturation for vertical75 MPawell production25 MPaproduction process going, the pressure drop filler expands, 10 yearsbut the pressure drop changes more slowly in the area0.75MPclose to well, and because of the output and diffusion, theas saturation decreases in the area. while in the area farfrom well, pressure decreases steeply and gas saturationincreases from zero. All this makes gas saturation increase15 yearswavy. In this production mode, the stratum pressure allys distributes like a filler, which is notressure drop way, so gas in matrix does not get desorption effectively and production potential in the area cannFig 4. Stratum pressure change simulated in 1for pinnate horibe utilized fully, which results in the low production effizontal wellsee that the stratum pressure decreases with the wholeig. 4 shows the simulated stratum pressure change in pinnate horizontal well as source and diffuse fromthe 15-years'production course in the area with datum in The whole horizontal well bore can all be takenTable I From top to bottom is the stratum pressure iso-"source"of the stratum pressure drop. In contrast, only agraph of 3 months, 1.5, 5,10 and 15 years. From Fig. 4, at partl bore can he taken as the“ source” in thethe initial stage(3 months), the stratum pressure ranges in verti中国煤化工 uate horizontal well can2-4.76 MPa. After two years, the stratum pressure de- decrCNMHGiciently. The so-calledcreases and ranges in 1-4.76 MPa(initial stratum pres- presslvalmost thesure is 4.76 MPa), which shows that the stratum pressure bed area can be controlled, which makes full use of prodecreases very fast and evenly. From Fig. 4, we can also duction potentialChinese Science Bulletin Vol 50 Supp. December 2005ARTICLESThrough the analysis above, the branch of the pinnatorizontal well spreads widely and evenly, which makesthe stratum pressure decrease fast and evenly, increasingthe chance of gas desorption and producing area. All thisis the fundamental reason of making production of the3 Sensitivity effect of parameters on gas production0.6for pinnate horizontal welloduction islated to several kinds of mechanisms such as desorptiondiffusion and percolation. Production is influenced bymany factors. In a certain coal bed, parameters relate toeach other. Different geology conditions make up differentarameter groups. In general, the lower the heat evolve-ment extent is. the lower the coal rank is the more favorable the coal bed property is for production. To find outthe effect of each factor on production, each parameter'ssensitivity will be analyzed separately as following.Fig. 5. Relative permeability curves with various features.It was pointed out in the study of the factors influencing the gas rate for the vertical well. /-9that the adsorption time constant is the main factor deciding the desorp-tion speed of the adsorbed gas, and that the shape ofothermal adsorb line controlled by Langmuir constantdecides the gas satiation extent in matrix. The two factorsdecide the property of gas source, and play an importantrole in the gas production. On the other hand, the stratumproperties such as stratum pressure and permeability andxvfluid flow property such as gas and water relative perme-10ability decide the transmission property and gas production. How all these parameters affect the gas productionfor the pinnate horizontal well and if the effects are thesame as that of vertical well, still need to be analyzedquantitatively. Besides, for the pinnate horizontal well, thebranch length and the suitable coal bed thickness alsoneed to be studied. In the following calculation, the parameter used in the norm curve for comparison is the sec-Fig. 6. Relative permeability effect on gas productionond column in Table 1. We change the parameter value inturn for getting influence of each factor studied. Four collation of gas in the coal bed. Only when the stratumsymmetrical main horizontal well bores are adopted, pressure drops down to the critical desorption pressure canwhich are parallel separately to two borders of the rectan- gas get desorption and become free gas. But only whengle area. There are three branches on both sides of each the gas saturation is greater than the residual gas saturamain well bore. The angle between the main well bore and tion, can gas flow out, or else only diffuse. The steeper thebranch is 45 degree and the overall length is 16800 m. permeability curve is, the faster the relative permeabilitySome other parameters are omitted here.of the gas phase increases, the greater the flow production3. 1 Relative permeability effect3.2 Permeability effectFig 5 demonstrates two groups of typical permeabilitycurve,and Fig. 6 shows the response of the two differentThe permeability of the coal bed is around 1x10\curves. Fig. 6 shows that the steeper the permeability in a few fields in China. and all others are less than 1x10curve is, the less the residual gas saturation is, and the um2.中国煤化工 cal layers in the coalcloser to the right the point of intersection of gas and wa- bed. Se the verticalter relative permeability curve are, the more likely theCNMHability,ability. Fig. 7 givproduction is to increase. Through analysis, we get that gas production curves of two groups of permeability,the output of the coal gas is a result of diffusion and per- which shows that the increase of permeability results inChineseBulletin Vol 50 Supp. December 2005ARTICLES1K,k(080.3(103gm2)sorption time constant does not affect the gas rate very2Kk0801(103um)The adsorption time constant decides the property ofthe gas source mainly. For the vertical well, the long ad-sorption time results in that gas cannot meet the need ofthe increase of gas production, so the gas production in-creases relatively slowly. However, for the pinnatehorizontal well, the evenly decrease of pressure in thewhole area makes the desorption of gas easier, and relthe supply and demand contradiction. Therefore, thepinnate horizontal well eliminates the disadvantageresulting from the long adsorption time. On the other hand,the expression of adsorption time constant is T=S78nDm,which shows that the adsorption time constant is related to0diffusion distance and is in the direct proportion to fracture space. The pinnate horizontal well is like many artificial fractures, which increases the fracture density andect on gas productionreduces the fracture space and equivalent adsorption timeAnd because these fractures"affect the adsorption timethe increase of the gas rate. It can be concluded that in the much more than the real fracture in stratum, the effect oftwo-phase flow predominated by percolation in the coal the adsorption time of stratum itself on the gas rate of thebed, the permeability of the coal bed is an important factor pinnate horizontal well is almost covered up, so there isthat influences the gas rate, which is the same as the gen- little difference among several curves in Fig. 8practical prmeability through fracturing is one efficient way to in- 3.4 Langmuir constant effectcrease gas production. In a sense, the main well bore andWe have known that the Langmuir constant decides thebranch spreading widely in the coal bed are like many change range of adsorption quantity through changing theartificial fractures, which is equal to increase equivalent extent of steep of the isothermal adsorption line in thepermeability and increase gas production greatlypressure drop area, so affects the gas production. Fig. 103. 3 Adsorption time constant effectshows that it is the same for the pinnate horizontal wellFrom the three isothermal adsorption curves in Fig. 9The adsorption time constant decides the time that gas curve 2 is the steepest, curve I takes the second place,production reaches the maximum value for the vertical curve 3 is the gentlest. So in Fig. 10, the correspondingwell. The shorter the adsorption time constant is, the ear- gas production of curve 2 is the highest, curve I takes thelier the gas production reaches the maximum value, but second place, curve 3 is the lowest. In conclusion, thethe faster the gas rate decreases. Fig 8 shows that the ad- isothermal adsorption curve can be used to instruct deter-30000Pressure drop areaE10000=29200.P1=0.51.200h=29200.P1=09672.720h=19200P=09673.2000h中国煤化工CNMHGEin. 8. Adsorption time constant effect on gas productionFig 9. Isothermic adsorption curves with different Langmuir constant.Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLES3.6 Stratum pressure effectIf it can be said that the gas content reflects the prop-erty of gas quantity, which also reflects if there is an am-ple supply of gas, the stratum pressure influences the difficulty of exploitation of gas from the energy angle. If thegas content and gas saturation is the same, the higher stratum pressure makes exploitation easier. In Fig. 12, bothLangmuir constant and critical desorption pressure areEal to 3 mPa, so the initialthe initial stratum pressure is 3. 15 and 3.445 MPaseparately. Fig. 12 shows that the higher stratum pressureis more favorable to output of coal gasP,(3.445MPa),P4(3MPa)P,(3.15 MPa), P,(3 MPa)00200300mining the value of the bottom hole pressure By using thesteep part of the isothermal adsorption curve, gas can beoutput more adequately.3.5 Gas saturation effectThe high stratum pressure desorption pressure ratio andgas saturation are important to weigh if a coal bed is favorable to be exploited. Different critical desorption pres-sures result in different gas saturation in the same stratum100200300400500pressure condition. The gas saturation reflects the differ-ence of gas content.In Fig. 11, the initial stratum pressure of both curvesFig. 12. Stratum pressure effect on gas productionare 3.445 MPa, but the critical desorption pressure is 3.7 Branch length effect3.345 and 3 MPa separately. When using the LangmuirFor the general horizontal well, there exists a rationalconstant in column 2 in Fig. 1, the gas saturation is 99.5% horizontal section length if friction and acceleration lossesand 96.9% separately So Fig. ll shows that the higher the are considered. For the pinnate horizontal well, there mayas saturation is, the higher the gas production isbeof each mode mainly depends on the area in which the995%stratum pressure is lower than the critical desorptionpressure. Because of the big controlled area of the pinnatehorizontal well. the border influence has to be consideredFig. 13 gives the cumulative gas production in 10 yearssame pinnate structure and different branchlengths. It shows that the cumulative gas production doesnot increase monotonously with the increase of branchlength. When the branch length increases from 100 to 200m, the cumulative gas rate increases apparently. When thebranch length increases from 200 to 300 m, the increasingamplitude of cumulative gas rate becomes smaller, andwhen the branch length increases from 300 to 400 m, thecumul中国煤化工 as a result of frictionSo the best branch100200300400presslengthCNMHGcan be determinedaround 300 m. But the branch length plays its role com-Fig. 11. Gas saturation effect on gas productionbined with branch arrangement, stratum and border condi-Chinese Science Bulletin Vol 50 Supp. December 2005ARTICLES4 Conclusion and discussionFor the pinnate horizontal, the whole horizontal wellbore spreading widely and evenly in the coal bed can all1800be taken as the" source"of stratum pressure drop, whichmakes the stratum pressure drop evenly and fast, thus in-creases the chance of desorption and diffusion and areacontrolled. In contrast, only a part of the well bore can betaken as the"source"in the vertical well and the so-calledpressure filler cannot drop pressure efficiently. The effi1400cient pressure drop mode makes full use of the productionpotential, and this is the basic reason that the pinnatehorizontal well can increase production1200Either for the vertical well or the pinnate horizontalability gger stratum permeability, steeper relative perme-ell. biBranch length/mcurve, smaller residual gas saturation, higher gascontent and stratum pressure are more favorable for gasFig. 13. Branch length effect on gas productionproduction increasing. Langmuir constants decide thechange range of desorption quantity through changing theion, so it is better to get the rational branch length and extent of steep of isothermal adsorption curves in thearrangement quantitatively by simulationpressure drop area, thus influence the gas production. The3. 8 Coalbed thickness effectadsorption time constant does not influence the gas pro-duction very apparently because the equivalent adsorptionFig. 14 gives the cumulative gas production under dif- time constant is reduced for the pinnate horizontal wellferent thickness of the coalbed. It shows that the increas- Referencesing amplitude of the cumulative gas rate becomes smallerwhen the coalbed become thicker and thicker. It can be 1. Fan, M.Z. Wang, S H, Principal geologic parameters controllingconcluded that the horizontal well and pinnate horizontalcoalbed gas recoverability, Natural Gas Industry (in Chinese), 1996,well are fit for the medium thick continual coal bed. If thecoalbed is too thin, the vertical controlled area by the2. Yang, L. w, Sun, M. Y, The particularity of coalbed gas reservoirhorizontal well will be too small and the advantage of thein China and technology requirement of production, Natural gasndustry (in Chinese), 2001, 6: 17-19horizontal well cannot be utilized fully and economically. 3. Li, w. Y. wang S. Y, Zhao, Q.B. Exploration&Production ofIf the coalbed is too thick, the coal bed far away from theCoalbed Gas in China (in Chinese), Xuzhou: Mining College ofhorizontal well cannot be exploitedChina Press,2003,1-38,161-200,201-219Technology Magazine, 2003, OcU/No5. Zhang, D. L, Wang, X. H, Simulation study of coalbed methanesingle well production, Journal of Jianghan Petroleum Institute(inChinese),2004,1:76-776. Liu, X.P., Guo, C. Z, Jiang, Z.X. et al. The mox600flow in the reservoir with flow in the horizontal wellbore. ActaPetrolei Sinica(in Chinese), 1999, 3: 82-86single sell production, Joumal of Jianghan Petroleum Institute(in8. Kolesar, J. E, Ertekin, T, Obut, S. T, The unsteady state nature ofsorption and diffusion phenomena in the micropore structure ofcoal: Part2-Solution, SPEFE, 1990, 5(3): 89-99. Kuuskraa, V. A, Wicks, D. E, Thurber, J. L, Geologic and reser-中国煤化工 ion from the antrim shale,Fig. 14. Coalbed thickness effect on gas production.CNMHG2005: accepted July 6, 2005Chinese Science Bulletin Vol 50 Supp. December 2005