Compressibility Factors for High-Molecular-Weight Reservoir Gases
by R.P. Sutton, Marathon OilCo. SPE Membar
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kBSTRACT This paper examines the effect of high concentrations of fraction in the heptanes-plus natural gases on the calculation of gas compreeaibility (Z) factors. Laboratory meaeured gae compositions and Z factore are ueed to evaluate the accuracy of the Standing-Katz chart . It was determined that the chart itself provides eatiefactory sccuracy; however, Kay’e molaraverage combination rulee or comparable gravity relationships for calculating pseudo-critical pressure and temperature reeult in unsatisfactory Z factore for high molecular @e~Qht gasee. The contribution of thie ~..- reeervoir paper are two-fold. Firet, new pseudo-critical property - gae gravity relationehipe are developed, snd second, alternate for methode calculating pseudo-critical from propertiescomposition are established. By utilizing either of theee methods to :alculate pseudo-critical preeaure and temperature , the overall accuracy of Z factors from the Standing-Katz chart is increased almost three-fold.
resulting pressure drop from flow through pipe, static and reeervoir pressure gradiente in gae wells, Ideally, properties are performance. gae PVT designed to determined etudieefrom laboratory duplicate conditions of interest. However, quite or PVT often experimental data ie unavailable, properties muet be evaluated at conditions different from those examined by the laboratory etudies. In theee casea, PVT properties must be determined from Probably the most widely accepted correlation. mixtures ie the for natural correlation gas Standing-Katz 18 (SK) z factor chart.mixtures “
. .1:-I. ..-l.--.,l= gaSeS KIOrlIlall~ ~v~=~...-r w~+mhr hvdrnca~b~n -%-=..-.._ -. n~gu encountered in the petroleum industry can be grouped into two general categories. Natural gaees in the Eiret category contain relatively high concentration of ethane and propane typically ae the result of a low pressure flash with crude oil, while gasee in the second category aregae-condeneatee and derive their ~igh molecular weight from the quantity of leptanes-plue present. This paper is concerned with latter category of gaeee.
The SK chart wae developed using data for binary . and or met”nanewith pr~p=nz, .Fhqa . . LWCZW, =....-. natural gaees having a wide range of composition.3 None of the gas mixturee had molecular weights in excese of 40. The SK chart is actually amodification and exteneion of a generalized Z factor chart (BH) and is developed by Brown and Holcomb1$2 identical to the BH chart at reduced preesuree less than 4. Above thie value, the BH chart wae found to be consistently inaccurate; therefore, Standing and Katz used data from 16 natural gaa mixtures, along with methane Z factors ae a guide, to extend the chart ~in~~ the SK chart ef ~~to...