Adaptability Evaluation of Coal-bed Methane Well Completion Methods Based on Multi-objective Decision-making Method

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1 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods Based o Multi-objective Decisio-makig Method Gag Yag 1 ad Zhimig Wag 2 Abstract: Coalbed methae (CBM) is a importat atural gas resource, ad appropriate well completio method is very importat to icrease productivity. At preset there are may CBM well completio methods, icludig fractured vertical well, ope-hole cavity well, U-shape well, V-shape well ad piate horizotal well. Aim at the diversity of CBM well completio methods, multi-objective decisio-makig method is used to evaluate these completio methods ad select the best completio to maximize ecoomic beefit. Firstly implemets productio predictio ad ecoomic evaluatio for each completio method, ad the chooses multi-objective decisio-makig method to evaluate five idexes: cumulative gas productio, et preset value, ivest recovery period, iteral rate of retur ad risk factor, thus the most appropriate completio method ca be achieved. With this evaluatio method applied to Ordos Basi, umerical simulatio results show that piate horizotal well predomiate i the begiig of exploitatio compared with other completio methods, ad its advatages dwidle due to limited reservoir supply capacity i the latter period. I spite of its huge ivestmet ad high risk, with high productivity, its NPV far overweighs other completio methods, so the piate horizotal well is the most suitable completio method. The adaptability evaluatio method proposed provides favorable guidace for o-site CBM well completio method desig, which has a great sigificace to improve Chia s CBM developmet effect. Keywords: Well completio methods, Productio predictio, Ecoomic evaluatio, Multi-objective decisio-makig method. 1 MOE Key Laboratory of Petroleum Egieerig, Chia Uiversity of Petroleum, Beijig, Chia. 2 Correspodig author, Zhimig Wag, MOE Key Laboratory of Petroleum Egieerig, Chia Uiversity of Petroleum (Beijig), 18 Fuxue Road, Chagpig District, Beijig, , Chia. wellcompletio@126.com.

2 154 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , Itroductio CBM is ot oly a kid of importat atural gas resource, but also a oticeable catastrophe gas which threates coal mie safety ad may lead to climate warmig. Reasoable completio method will yield a triple divided which plays a key role i improvig clea atural gas supplies, prevetig gas accidet fudametally ad alleviatig greehouse effect. At preset the commoest methods of CBM well completio at home ad aboard iclude fractured vertical well, ope hole cavity well, U-shape well, V-shape well ad piate horizotal well. As we all kow, it is very importat to select appropriate well completio methods to improve the etire effect [Wag ad Zhag (2011)]. Five evaluatio idexes: cumulative gas productio, et preset value, dyamic payback period, iteral rate of retur ad risk factor, combied with multi-objective decisio-makig method, have bee cosidered i the process of CBM well completio evaluatio i this paper. The commoest methods of multi-objective decisiomakig are fuzzy sythesis decisio-makig method, gray system theory, TOPSIS method, etc. I priciple, the methods above metioed calculate the deviatio betwee certai well completio method ad the ideal optimizatio scheme or the worst scheme. The more certai well completio scheme deviates from the worst scheme or approximates to the optimum scheme is cosidered to be the better oe. While differet solvig processes are used i differet decisio-makig methods, so the fial evaluatio results are differet accordig to differet evaluatio methods. To achieve accurate evaluatio results ad reduce defects, this paper proposes the weighted average of ormalized membership degrees (correlatio degree, close degree) from differet multi-objective decisio-makig methods i order to get the comprehesive membership degree. Higher value idicates better scheme. Differet well completio methods are sorted by comprehesive membership degree value, ad the the most appropriate well completio method will be optimized i Baode miig area. 2 CBM well productio predictio ad ecoomic evaluatio This paper aims to select the optimum well completio method of Baode miig area i Easter margi of Ordos Basi. 2.1 Well completio program desig To optimize the type of well i Baode miig area, 5 well completio schemes are desiged separately. Casig programs ad well completio parameters are show respectively i Fig.1 ad Table 1. Well patter area is 5.76 km 2 ; the reservoir

3 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 155 thickess is 10m; the horizotal permeability is 1mD; the vertical permeability is 0.5mD [Jie (2010)]. These five well completio schemes are all based o squared well patter as their mock objects. Figure 1: Five CBM well structure diagram. 2.2 Productio predictio CBM module i Eclipse software is used to simulate sigle well productio of differet well completio schemes i Baode miig area. As mock objects are all based o squared well patter ad iterferece betwee wells are igored, productivity calculatio formula of well patter is equal to the product of sigle well productivity ad the umber of well. Uiform grid is adopted i geological model, ad the grid size is 15m. Wellbore diameter i coal seam sectio is 13.9mm, ad the simulatig time is 15 years. Geological parameters i the simulatio are as follows: the thickess of coal seam is 10m; the porosity is 3%; the coal seam buried depth is 600m; the horizotal permeability is 1mD; the vertical permeability is 0.5mD; reservoir pressure is 4.8MPa; the coal-rock desity is 1.4 t/m 3 ; the iitial water saturatio is 1;

4 156 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 Table 1: Five CBM well completio schemes. Scheme Cotrol Quatity Sigle well parameters area/m Fractured Half-legth of fracture, m 60 vertical well Fracture width, mm 6 Fracture height, m 10 Post-fracturig Permeability, D 30 Ope hole Cave diameter, m 2 cavity well Permeability improved area radius, m 15 Improved permeability, md 30 U-shape well Egieerig well horizotal legth, m 500 (bare hole i horizotal sectio) V-shape well (bare hole i horizotal sectio) Egieerig wells horizotal sectio legth 500m Agle betwee two egieerig wells is Piate horizotal well (bare hole) truk, 8 braches, each side has 4 braches, agle is 45 0, brach spacig is 200m, symmetrical distributed, total footage, 5200m, truk legth 1200m, braches legth: 800, 600, 400, 200m. absorptio time is 10d; Lagmuir volume is 30m 3 /t; Lagmuir pressure is 2.9MPa. Accordig to Fig.2 ad Fig.3, piate horizotal well has maximized the draiage area. I this well completio method, high gas productio capacity exists i the begiig of extractio, exceedig those of other well types over the first three years. Its aual productio is slightly less tha those of other well types i the followig years as bearig volume limited ad gas supply abatig. After log extractio (say 15 years), the fial accumulated gas productio is still slightly higher compared with others. Additioally, a V-shape well is composed of 2 horizotal egieerig wells ad a caved well, which has larger cotact area with reservoir. From aual gas productio ad accumulated gas productio, its gas productivity is just secod to piate horizotal well. Above all, piate horizotal well ad V-shape well predomiate i the begiig of exploitatio; their advatages dwidle due to limited reservoir supply capacity i the latter period.

5 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 157 Figure 2: Aual gas productio-productio time. Figure 3: Accumulated gas productio-productio time. 2.3 Ecoomic evaluatio Geerally speakig, mai ecoomic evaluatio idexes are et preset value (NPV), ivest recovery period (IRP) ad iteral rate of retur (IRR) [Che (2011)]. NPV is defied as the sum of the preset values of the idividual cash flows of

6 158 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 the same etity. As a dyamic evaluatio idex, it mirrors a purpose to gai profit ability iside computatio period, its formula is: NPV = t=0 (C I C o ) t (1 + i 0 ) -t (1) Where C I is cash iflow; C O is cash outflow; I 0 is discout rate; t is the year sequece; is the computatio period. Cash iflows of CBM target zoe durig the decisio-makig process iclude sales reveue ad govermet subsidies. Cash outflows iclude costructio ivestmet, cost of productio ad operatio, taxes ad fees, etc. Amog costructio ivestmet are maily composed of well drillig ad completio project ivestmet, surface egieerig. t=0 idicates the ivestmet ad costructio period, which takes a year; t=1 idicates to start productio. Accordig to real CBM well completio situatio, equatio (1) ca be trasformed as follows: FNPV = t=1 Q(t)P f (1 + i 0 ) -t + t=1 Q(t)r b f (1 + i 0 ) -t C 1 t=1 (C 2 +C 3 + T x (t))(1 + i 0 ) -t Where Q(t) is aual productio of CBM well; P is CBM price; f is the ecoomic rate(geerally, 95%); r b is govermet subsidy rate; t is productio period; C 0 is ivestmet durig the costructio period; C 1 is workig capital; C 2 is operatio cost; T x is taxes; i 0 is discout rate. This article calculate productio uder the coditio of differet well completio methods for 15 years, ad substitutes the simulated productio ad date i Table 2 ito formula (2), the get NPV i differet well completio. Table 2 is the ecoomic evaluatio baselie date of CBM well [Luo (2010)]. See from Fig.4, piate horizotal well has maximized the draiage area. I spite of its huge ivestmet, with high productivity, its NPV far overweighs other well types. IRR: The iteral rate of retur (IRR) of a project is simply the iterest rate that makes the NPV of the project equal to 0, which ca be calculated by cash iflows. Calculatig formula is as follows: t=0 (C I C 0 ) t (1 + IRR) -t = 0. (3) Where C I is cash iflow; C O is cash outflow; IRR is iteral rate of retur; t is the year sequece; is the computatio period. (2)

7 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 159 Figure 4: NPV-productio time. Table 2: Ecoomic evaluatio baselie date of CBM well. Well drillig ad completio cost, Fractured vertical well ,000 Ope hole cavity well completio 130 U-shape well 450 V-shape well 850 Piate horizotal well 2200 Surface ivestmet, 10,000/km Workig capital, 10,000 2% of costructio ivestmet Productio ad operatio cost, 1/m Sale price, 1/m Govermet subsidy, 1/m Commodity rate 95% Value added tax City maiteace ad costructio taxes, educatio fees, other local taxes 5% of sales reveue 10% of payable Value added tax Icome tax 25% Costructio ad productio period 1a, 15a Bechmark yields 12% If IRR is greater tha or equal to idustry base yield or discout rate, the profitability satisfies the miimum requiremet, ad this ca be accepted fiacially. The basic rate of retur i CBM idustry is 12%.

8 160 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 Dyamic IRP: IRP i capital budgetig refers to the period of time required for the retur o a ivestmet to "repay" the sum of the origial ivestmet. The term is also widely used i other types of ivestmet areas, ofte with respect to eergy efficiecy techologies, maiteace, upgrades, or other chages. P t t=0 (C I C 0 ) t (1 + i 0 ) -t = 0. (4) P t = To-1 + PV t-1/pv t Where T 0 is year sequece umber; PV t 1 is the accumulated et cash iflow absolute vale last year; PV t is et cash iflow absolute vale of the year. As IRP is geerally calculated from the early year of costructio, where t=0 idicates the startig time of costructio, for a year, thus IPR is P t Determiig evaluatio idexes Productivity predictio ad ecoomic evaluatio show that the piate horizotal well has obvious advatages i terms of capacity or ecoomic, ad the V-type horizotal well follows. However, coal is brittle ad borehole stability is poor, i the process of drillig ad completio some accidets easily occur like borehole collapse, circulatio loss, resistace durig trippig, slackig off, eve borehole buryig ad other borehole problems ad accidets. Because the coal bed is buried relatively shallowly ad the hole curvature is bigger, ad WOB (weight o bit) is hard to meet the requiremets, drillig colum is proe to fatigue failure while drillig horizotal brach boreholes, which results i the borehole problems. Statistics from 48 multi-brached horizotal wells i souther Qishui Basi showed oly 14 wells were successfully implemeted, ad 14 wells occurred collapse ad buried drillig tools. The legth of buried drillig tools is more tha 3250m. I additio, covetioal piate horizotal well completio process is complex, risky, ad complex completio process is ot coducive to the protectio of coal seams, so ope hole completio method is ofte used to reduce harm caused by the cemetig operatio (it is also horizotal sectio of U ad V type well). Because collapse problem is commo i completio process, collapse risk problem must be cosidered whe evaluatig CBM well completio method. We divide the collapse risk ito the followig five levels, very low, low, medium, high, very high, ad itroduce the risk factor to evaluate the collapsed risk. Five collapse risk level correspods to the risk factors, respectively, 0.1, 0.3, 0.5, 0.7 ad 0.9. Combiig with well drillig ad completio experiece i the field, five risk factors correspods to fractured vertical well, ope hole cavity well, U-shape well, V-shape well ad piate horizotal well, respectively, 0.1, 0.3, 0.5, 0.7 ad 0.9. (5)

9 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 161 From Table 3, we ca get that the well completio schemes with high productivity ad ecoomic beefits are of high risk. It is hard to choose the well completio scheme of all kids of best idex, so we eed to use multi-objective decisiomakig method to select the best completio scheme. Completio method Table 3: Evaluatio idex i 5 well completio methods. Accumulated gas productio, 100 millio Net preset value, 10 millio Dyamic ivest recovery period, a Iteral rate of retur Risk coefficiet Fractured vertical well % 0.1 Cavity completio % 0.3 U-shape well % 0.5 V-shape well % 0.7 Piate horizotal well % Determiig the weight coefficiet of evaluatio idex I this paper, etropy method ad deviatio maximizig are used to determie the weight of each idex. 3.1 Normalizig the evaluatio matrix I the evaluatio system, idexes differ i cotet, dimesio ad criteria. Therefore, it is ecessary to traslate differet idexes ito uified stadard, amely, stadardizatio [Wag (2010)]. Suppose there are m evaluatio schemes, evaluatio idexes, thus get a multiobjective evaluatio matrix as show i Table 3: r 11 r 1 R =.. = r m1 r m For profit idex (the higher, the better), the coversatio formula is:. (6) r i j -r j mi r i j = r j max -r j mi (7)

10 162 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 For cost idex (the lower, the better) the coversatio formula is: r j max -r i j r i j = r j max -r j mi (8) Where r j mi is the miimum eigevector for the j idex; r j max is the maximum eigevector for the j idex. Hece the ormalized matrix R: r 11 r R =.. = (9) r m1 r m Etropy method Etropy is a fuctio of matter systematic state. Etropy idicates the degree of disorder i the system. The etropy weight calculated from evaluatio matrix is ot a actual importat coefficiet, but a relative degree coefficiet i the meaig of various idexes, which provides how much useful iformatio o this issue. O the issue of m evaluatio schemes ad evaluatio idexes, the etropy of the evaluatio idex j is: H j = -k m f i j l f i j ( j = 1,2..., ) (10) i=1 r i j Where f i j = m,k = 1 lm whe r i j = 0, settig f i j l f i j = 0. r i j i=1 The etropy weight of idex j is defied as w j = 1-H j m- H j Thus gettig the etropy weight matrixw: (11) w = [0.123, 0.261, 0.187, 0.291, 0.138] (12) 3.3 Deviatio maximizatio O the basis of ormalized matrix R, use deviatio maximizig method to calculate objective weight v j of the idex j: ri j -r 0 v j = m i=1 m i=1 ri j -r 0 (13)

11 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 163 Where r 0 is the optimum vector, r 0 = [r 1,r 2,,r ] Normalize the optimum matrix i the judgmet matrix, r 0 = [1,1,... 1] Differece matrix: r i j -r 0 = (14) Utilize formula (13) to calculate the weight v = [0.14, 0.23, 0.211, 0.246, 0.173] (15) 3.4 Determie weight coefficiet of evaluatio idex Combiig etropy method ad deviatio maximizig method, use weighted mea method to calculate the comprehesive weight coefficiet of idex j. ω = (w j+v j ) 2 i = 1, 2,, 5 (16) ω = [0.131, 0.246, 0.199, 0.269, 0.172] (17) 4 Multi-objective decisio-makig applicatio I this part, three multi-objective decisio-makig methods are used: 4.1 Fuzzy sythesis decisio-makig method Establish stadard superior scheme based o membership matrix as a relative criterio of optimizatio ad compariso. idexes membership of superior scheme is the maximum value of idex membership of all schemes, thus the superior scheme g i = ( g 1,g 2,, g ) = (1,1,,1), the iferior scheme b i = (b 1,b 2,, b )= (0, 0,, 0). Suppose u i is the optimum membership degree of the scheme i S(r i,e) = u i [ (ω j r i j -g i ) p ] 1/p, S(r i,e) is the weighed optimum distace of scheme i. S(r i,l) = (1-u i )[ (ω j r i j -b i ) p ] 1/p, S(r i,l)is the weighed optimum distace of scheme i.

12 164 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 Where p i the expressio of S(r i,e) ad S(r i,l) is the distace parameter. p =1 is hammig distace; p =2 is Euclidea distace. To calculate the optimum value of membership u i, a objective fuctio should be set as follows: F(u i ) = {u i [ (ω j r i j -g i ) p ] 1/p } 2 + {(1-u i )[ (ω j r i j -b i ) p ] 1/p } 2 (18) I order to calculate mif(u i ), amely, the miimum sum of squared weighed optimal distace. Suppose df(u i) du i = 0 to get the optimal calculatig formula of u i. u i = 1 m [(w i r i j g i ) p ] 2/p i=1 1+ m [(w i r i j b i ) p ] 2/p i=1 (i = 1,2,,m) (19) Usually, by usig Euclidea distace, p=2, the above formula ca be simplified as: u i = 1 m (w i r i j g 2 i ) i=1 1+ m (w i r i j b 2 i ) i=1 (i = 1,2,,m) Utilize the above formula to calculate the optimal membership degree; the greater u i idicates the better scheme. Accordigly, the membership degree of differet well completio methods ca be ormalized as: (20) u = [0.223, 0.053, 0.036, 0.162, 0.548] (21) 4.2 Gray system theory Gray correlatio aalysis is actually a kid of method which compares closeess of geometric shape from differet date. I geeral, the closer geometric shape idicates more similar tred ad greater correlatio. Therefore, durig the process of correlatio aalysis, we firstly determie the referrig series, ad the compare the closeess of other referrig series. By doig this ca we compare ad optimize other sequeces [Feg (2010)]. O the base of ormalized matrix R ca we get referrig matrix r 0 = [r 01, r 02,, r 0 ], where r 0 j refers to the optimal value i row j, thus gettig a differece compariso matrix: r 11 -r 01 r 12 -r 02 r 1 -r 0 r 21 -r 01 r 22 -r 02 r 2 -r 0 i j =.... = r m1 -r 01 r m2 -r 02 r m -r (22)

13 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 165 Calculate gray correlatio coefficiet ϕ i j = mi i mi j i j + ρ max i i j + ρ max i max i j j max j i j (23) Where i j = r 0 r i, ρ is resolutio coefficiet, ρ [0,1], geerally, ρ = ϕ = (24) I the course of calculatig correlatio degree, weighed gray correlatio coefficiet matrix is: ψ i = ϕ i j w( j) (25) Where ψ i is gray correlatio value, w( j) is relative weighed value accordig to importace. Calculate ad ormalize the correlatio degree. ψ i = [0.204, 0.148, 0.138, 0.174, 0.335] (26) 4.3 TOPSIS method Techique for Order Preferece by Similarity to Ideal Solutio, or TOPSIS, is raised by Hwag ad Yoo i This method is applied by makig a compariso ad selectio. By settig positive ad egative ideal solutio ad calculatig the distace betwee real solutio ad positive/egative ideal solutio, the solutio which is closest to the positive ideal solutio ad farmost from the egative optimal solutio is regarded as the optimal solutio [Meti Dagdevire (2009)]. Weighig the stadard matrix R (equatio (9)): R = Positive ideal solutio: r + = maxr i j i = 1, 2,, m (27)

14 166 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 Negative ideal solutio: r = mir i j i = 1, 2,, m Determie the positive ad egative ideal solutio: R + = [0.131, 0.246, 0.199, 0.269, 0.155] (28) R = [0, 0, 0, 0, 0] (29) Calculate the Euclidea distace betwee target ad ideal solutio: S i + = (r i j -r + ) 2 i = 1, 2,, m (30) Si = (r i j -r ) 2 i = 1, 2,, m (31) Calculate Euclidea distace betwee each well completio scheme ad positive/ egative ideal solutio: S + = [0.303, 0.426, 0.421, 0.318, 0.155] (32) S = [0.224, 0.124, 0.082, 0.174, 0.435] (33) Calculate the close degree of each well completio scheme: C i = S i S + i +S i i = 1, 2,, m (34) Normalizatio: C = [0.223, 0.118, 0.086, 0.186, 0.387] (35) 4.4 Comprehesive evaluatio results The above three multi-objective decisio methods all work o the same priciple to calculate the distace betwee real well schemes ad the positive/egative ideal completio scheme. Differet solvig processes are used i differet methods, which may lead to icosistet results. I order to obtai accurate evaluatio results ad weake defects, ormalized membership degrees (gray correlatio degree, close degree) from differet methods are weighed to achieve comprehesive membership degree i this paper. Greater comprehesive membership degree idicates better scheme. I the meatime, differet schemes are sorted accordig to their value. Comprehesive evaluatio results are showed i Table 4. Judgig from the results, it may come to a coclusio that the optimal completio method is piate horizotal well ad fractured vertical well comes secod.

15 Adaptability Evaluatio of Coal-bed Methae Well Completio Methods 167 Completio method Table 4: Comprehesive evaluatio results Fuzzy comprehesive evaluatio Gray system theory TOPSIS Comprehesive membership degree fractured vertical well Cavity completio U-shape well V-shape well piate horizotal well Total sort 5 Coclusios Piate horizotal well ad V-shape well predomiate i the begiig of exploitatio compared with other completio methods, ad its advatages dwidle due to limited reservoir supply capacity i the latter period. I spite of its huge ivestmet ad high risk, with high productivity, its NPV far overweighs other completio methods. This study shows that the multi-objective decisio-makig method is suitable to deal with the CBM well completio adaptability evaluatio problem. After comprehesive evaluatio of every useful idex, we ca get the most appropriate completio method to target CBM field. After applyig this evaluatio method to Baode miig of Ordos Basi, evaluatig results show that the piate horizotal well is the most suitable completio method, followed by fractured vertical well. Adaptability evaluatio method proposed i this paper provides guidace for o-site coal bed methae well completio program desig, which has a great sigificace to improve Chia s CBM overall developmet effect. Ackowledgemet: This study was supported by Basic Research o Drillig & Completio of Critical Wells for Oil & Gas (Grat No.: ) ad the State Sciece ad Techology Major Project (Well Completio Techology ad Equipmet for Coalbed Methae, Grat No.: 2011ZX ). Simultaeously, the reviewers of the origial mauscript are greatly appreciated; their commets ad suggestios helped improve this paper. Refereces Che, Y. H. (2010): Research o coalbed methae ecoomic evaluatio model based o DEA. Chia Uiversity of Miig ad Techology, pp

16 168 Copyright 2013 Tech Sciece Press SL, vol.9, o.3, pp , 2013 Daǧdevire, M.; Yavuz, S.; Kılıç. N. (2009): Weapo selectio usig the AHP ad TOPSIS methods uder fuzzy eviromet. Expert Systems with Applicatios, pp Feg, Y.; Ji, B.; Li. Y. (2010): A Improved Grey Relatio Aalysis Method ad Its Applicatio i Dyamic Descriptio for a Polymer Floodig Pilot of Xigshugag Field, Daqig. SPE , SPE North Africa Techical Coferece ad Exhibitio, Cairo, Egypt, February Jie, M. X. (2010): Coal-bed Methae Exploratio Developmet Prospects of the Easter Margi Ordos basi. Natural gas idustry, vol. 30, o. 6, pp Luo, D. K.; Wu, X. D.; Zhag, B. S. (2010): Techical ad ecoomic evaluatio of coal bed gas resources i Chia, Beijig, Chia: Coal idustry press. Wag, Z. M.; Zhag, J. (2011): Critical Thickess of a Low Permeable Coal Bed for Horizotal Well Productio i Chia. Eergy Sources, Part A: Recovery, Utilizatio, ad Evirometal Effects, vol. 33, o. 4, pp Wag, Z. M. (2010): The Completio Optimizatio Theory ad Applicatio of Complex Well. Beijig: Petroleum Idustry Press.