"EVALUATION OF MATERIAL BALANCE ANALYSISMETHODS FOR VOLUMETRIC, ABNORMALLY-PRESSURED

GAS RESERVIORS"

A.K. AMBASTHA

this article begins on the next page FFJ C-P T-'73 Evaluation of Material Balance Analysis Methods for Volumetric, Abnormally-pressured Gas Reservoirs A.K. AMBASTHA University of Albert@i Currently Used Material Balance Analysis Methods for Abnormally-pressured Gas tion pe t is o tt Reservoirs s it

All iiiitet-i@ll 1)@tl@tiice @iii@ilvsis methods I)j-oposed lol- VOIL]IIIC(liC. @ibiioi-iii,Liliv pr@,;suied -,its i-esei-voirs arc ht@(i oii @i C(ILI@ttioll Of theforill: p GI, the (pi - P) = (; I........................ ..... .. @ILlihoi-s h@tvc Lised different expressions lor et'fectic .,ystcili (c,) depending oii the (-ii-ive iiiech@lili',Ills iticol-I)ol-@Lt@'(1 ill theii- @in@tlv,;es. The l'ollowin@, (lisclisses the ik@o sets ol ilicillo(i, pi-(iposc(l lot- an@ilvzin,@ proCILICtiOll pe['I'Ol-tll@l[ICC (1@it@i lol @iliv pi-@stii-(@(i L,,is i-eservoil-s. Methods Based on a Knowledge of System Compress@bility Hztiiiii L-i Iiiidii I presciilc(l two methods to

correct @ippll,clit ill ])Iilcc obtztille(l I'l-olli aii extr.@pol@itioil (it' c@ll-lv sti-@ii,,ht is a- lille oil @l -,]-@ipli I'oi, in ahnoriii@illy-pi-c,;SLII-C -CS 1- voil. tisiil IA'.) PI-CSSLII-C@. I-)i 1111LI p,, Lictti@il -,as iii-I)I@ice i@ Colillititc i (cco-(iilit-, to liiiiiiiierlincil',,(I@ iiictl]oll I @is@ ............. wlici-c 11 i, illiti@ll i-eselvoil- pi-esslire, p, is iii avet-@t@l,e reser y to explain the s 0 t I-)Ie@sLil-C @it ;oiiie iiiiic lkhilc i-eservoii- is still holi@ivili,-@ i, @til R plot. I)I-CsSLII-C(i i-csci-voir, tti(i c@ll - 11,- -)... C"i + Introduction C, i., S,.i -- C, S,i + C, S,,i............ Material h@Llatice analysis

of pro(ILICtiOll pci-foi-illance data tol ztii abnormally-pressured gas reservoii- SIIOLIICI ilieorelicallv inC]Lidc C,, S,-,j + C,,, S,,j + c, s@i............. water. i-ock. and -,as conipressibilitv ell'ccts, [@or @t volumetric. @ibnt)i-iiiallv-pressured gas reservoir, @t p//, Ilt-@iph shows two I l@iiiiiiici-liti(Il's(I -iiietho(I 11 also Lises Ikko pi,csstii,es. p, @iiid 1)@,. iii-f)l@icc its: = (; , ("1"............................. I...... I........................................ (6) _ tinctly different slopes. Aii extrapolation h@tseci oii @in carly-tillic ,@'ti-iii(,hi line on a plz vs. G,, Ili-aph foi- @in pressured o@iN reservoir yields a si,,nii'icaiitly OVCFCStill)@ItC(I l,'@IILIC ot' the iiiiti@ll

@ i@ic cot-reclioii f@ictoi- (G,") is: ac ll,ts-in-p[ c. This paper disclisscs sevciiii iiiittci-iiil bal@ince In@tlv 13,,i) S" sis iiicthods proposed toi- voluiiieti-ic, @ihiit)i-til@illv-lireSSLircd B iesci-voirs to obtain reLisoil@ibly @tccLii-@ite c,;tiiiizites ot' the iiiiti;il 13,, S,, i + 13,, (Pi I),) (cl + C',,S,i I)I@ice tlie eii-Iv-tiiiie PI-@@(ItIL'[ ia)ll ])I L-SSLII-C, d,,It,,I.(7) (:)ctober 1993, Volume 32, No. 8 19 - ------ ----- ---- - --------------- strai,,hi lines, the l'ii-,,,t line correspotiL]iii,, to ttic e@irly-tinie dat@l t(' cllllll)llt@' ilct-l@ll ztiid the second Iiiic cori-espondiii- tk) the l@ite-tiine datt, ol' dis-

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Evaluation of Material Balance Analysis Methods for Volumetric,

Abnormally-pressured Gas Reservoirs A.K. AMBASTHA University of Alberta

Abstract Material balance analysis of production performance for an

abnormally-pressured gas reservoir is complicated because of water and rock compressibility effects in addition to gas com-pressibility effects. An extrapolation based on an early swiight line on a p/z vs. Op graph for an abnonnally-pressured gas reser-voir yields a significantly overestimated value of the initial gas-in-place. Several researchers have presented material balance analysis methods to obtain reasonably accurate estimates of the initial gas-in-place for abnormally-pressured gas reservoirs. These methods can be broadly grouped into two sets. The first set of metbods requires a knowledge of compressibilities and analyzes produclion performance data to estimare the initial gas-in-place. The second set Of methods attempts to obtaiQ both an effective system compressibility and the initial gas-in-pl.

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Rill1l:J!!l)q ,mu Far.'ihadl-l J pr":'icnted a gl:Lphical m~tilud to c!-.ti-mall.!' inllial gas-in-pIJCo.!. R..:fcr..:nce t.:J.) lise ... a comlant c" In EqUiltlllll ( I) defillcd by:

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Rallhl.!!I) ... t ilnd r-arshad'-l l lTlo.!tilOlI i'i ba!-.cu 1m all extrapolation of:1 .~U aight line 1111 a graph of r 1 I - c,. I PI - p) J I z v .... Gp to t!!-.li-llIate illitial ga!-.-in-place.

Bcg:I,lIlu ,1IlU Whilehe:Jdl") h:we pre'iclltl!d a rc:.ervoir uepletlun llloucI b.I ... CU un Equation (I). Thc}' trcat effective cumpr..: ...... ibility (c,.! ill Equatlun (I) :J::. pre."sure-depenucnt. Theil e\.plcs!-.iun fOI c, . d .... u mciudc", thl! cftCCt of ~olUlion gil'" in W:LtcL (n their meLhnu. Illatcri:d b:Jl:Jncc cquation i'i applieu incrementally over 690 kPa drop .... in Ic!-.crvoir pleS'iUle tu nliLtch prOl.JucLion hi.