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195

The com posi tion of the rese rvoir gas can be ca lcu la ted given th ecomposition s of th e stock -tank liquid , sep ara tor gas, and stock -tank ventgas. T he p roducing ga s-oil rati os mu st be know n also. The ca lculatio n

simulates laboratory recombin a tio n o f th e liquid and gases in quant itiesindicated by the g as-oil ra t ios .Once the composit ion of th e re serv oir gas has been calcula ted, it s

ph ysical pro pertie s can be calculat ed as fo r a dry gas.The first step is con version of gas-o il ratios in scf /ST B to lb mo le

gas/lb m ole stoc k -tank lqu id. T o acc o mplish th is , th e d e nsity and

Surface Composltlon Known

Recombination of Surface Fluids-Compositions Known

The surf ace liquid and gases mu st be recom b ined in a rder to de te rm inethe properties of the gas in th e reserv oir. T h is reco mb ina tio n is done b ycalcula tion if the co m pos itions of th e surface fl uids are know n.

The key to the analy ss of the p ro perties of a wet gas is tha t thep ropert ies of the surface gas ar e not the sa me as the pro p erti es of thereservoir g a s. Liquid cond enses fr om th e r ese rvo ir g as as it mo v es fr omreservoir conditions to su rfa ce condit ions. Thus th e com p osi tion of thesurf ace gas is q uite differen t fr om the co m po siti on of the reser vo ir gas ;having considera bly fewe r of th e int erm edia te and heavy co mpon e n ts.

W et gases are usua lly se p arated in two- stage se paration sys te rns such

a s Figur e 13- 1. A t th e su rf ace, the we ll stream is separ at ed n to st ock -tank Iquid (conden sate), separa to r gas, an d stock-tank gas. Al i thr ee o fthese fluids m ust be inclu ded in th e rec om bin ation c a lculation

a thr ee-stage separ ator system is used, all th ree gases must beincluded in the recombinati on calculati on.

Prop e rt ie s o f W et Gases7

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First , calculat e th e apparent mol ecular w e ight and de n sity of th e stock-tank liquid.

MSTo = 100.9 lb /lb m ol e, Equation 3- 35PS To = 4 7.11 lb /cu ft , fr om 55.9AP I, Equation s 8-2, 8-1Second , con v ert ga s-oil ratio s to lb mol e gas/lb mol e stock -tank liquid .

e 0 .8372 0.3190 0 .00180.0960 0.1949 0 .0063C a 0 .0455 0.2532 0.02 950 .0060 0.0548 0.01770.0087 0.0909 0 .0403

5 0.0028 0.0362 0 .04175 0 .0022 0.0303 0.0435

0.0014 0.0191 0.0999c., 0.000 2 0.00 16 0 .7193

1.0000 1.0000 1.0000

0.7 9 4113

ap parent m o lecu lar weight o f th e s toc k -tank liquid m u st be calcu la ted.Liqu id density calcu lati ons are di sc u sse d in Chapt e r 11 . Appar entmolecular weight of a liquid is calcula ted fr om co mpo sition exactly as

for a gas, E quation 3-35 Thes e gas-oil rat ios in terms o f lb mol es are use d to combine theco mposit ions of the sep ara tor gas, stoc k -tank gas, a nd stock-tank o il inthe prope r ra tios . An e x am pl e will best illu s trate the pr oce dur e.

E XAMPL E 7 :

PET ROL E UM FLUID S

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