Density of Natural Gases

COPYRIGHT,

1942

AND

1943,

BY THE

AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS (INCORPORATED)

Density of Natural Gases By

MARSHALL

B.

STANDING* AND DONALD

L.

KATZ,* MEMBER A.I.M.E.

(New York Meeting, February

data are reported on 16 saturated hydrocarbon vapors at pressures ranging from 1000 to 8220 lb. per sq. in. and at temperatures ranging from 35° to 250°F. These data have been used to extend the compressibility-factor chart for natural gases up to !O,ooo lb. per sq. in. The relatively large quantity of highboiling constituents present in high-pressure vapors in equilibrium with crude oils makes it necessary to include in the analysis of the gas the molecular weight, density, and possibly boiling range of the heptanes and heavier fraction. Relationships have been presented by which the density of gases may be obtained directly from the temperature, pressure, and gas gravity provided the gases have a common source or are similar in composition. DENSITY

INTRODUCTION

The densities of natural gases are necessary in many engineering computations in petroleum production and utiliz~tion. Gas reserves, changes in reservoir pressure, gradients in gas wells, metering of gases, pipeline flow, and compression of gases are typical problems requiring the density of the gas. A decade ago, engineering computations used ideal gas laws with deviations up to 500 lb. per sq. in} Recent discoveries of pools having pressures up to 7500 lb. per sq. in., and installation of pressure-maintenance and recycling plants, have increased the need for data on gas density at high pressures. METHODS OF COMPUTATION

The accepted method of computing the density or specific volume of natural gases Manuscript received at the office of the Institute in PETROLEUM

Dec. 31, 1940. Issued as T.P. 1323 TECHNOLOGY, July 1941.

*

University of Michigan. Ann Arbor, Mich. , References are at the end of the paper.

1941)

is the use of the ideal gas law corrected by a compressibility factor. The method proposed by K ay 7 of correlating compressibility factors for gaseous mixtures on 5

pseudocritical properties appears to be satisfactory for natural gases. 2 The methane-compressibility factor has been shown to deviate systematically from the behavior of natural gases2 and a chart giving these corrected factors is available. A method of computing specific volume of gaseous mixtures from partial molal volumes has been reported by Sage and L acey. 8 C i ete d a t a for th e compu t a t'IOn omp are not yet available for all hydrocarbon gases. Most of the reported data on gas densities have been in the single-phase region removed from the dew point or saturation condition. Since most gases in the reservoir or while on contact with liquid during flow are saturated, the determination of densities under conditions of saturation is particularly important. Although there is no reason to believe that the saturated gases differ in behavior from those considerably removed from their dew points, the usual experimental determinations find difficulty on approaching the dew-point conditions. This paper reports the density of 16 saturated gases in equilibrium with crude oils. The relationships developed to compute the density of gases to high pressure! have been modified and enlarged. A simplified method of obtaining gas density is presented in the form of charts, which apply for specific groups of gases.

MARSHALL B. STANDING AND DONALD L. KATZ

ments over the usual gas analysis. The heptane-plus fractions were collected and weighed as a liquid. The density and molecular weight were measured if sufficient quantities were available. A smooth curve relationship exists between the liquid density and molecular weight of the heptanes plus and was used if the data for either were lacking. It was found that there was a permanent hold-up in the fractionating column packing as high as 0.5 gram. Differences between the weight of gas out of the pycnometer and the weight of the gas accounted for in the analysis varied up to this quantity. Accordingly, the weight of the heptanes and heavier was taken as the difference between the pycnometer weight of the gas and the weight of the hydrocarbons of molecular weight lower than heptane. A typical analysis of the saturated vapor is given in Table I. The absence of isobutane is due to the method of blending a butane-free gas with the Arkansas crude 6

DATA OBTAINED

In the process of obtaining equilibrium data on mixtures of natural gas and crude oils, the density and analysis of the saturated vapor phases were obtained. These data were in the range of 35° to 250°F. and 1000 to 8220 lb. per sq. inch. The vapors in equilibrium with crude oil were transferred, by mercury displacement, at constant temperature and pressure from the equilibrium cell into pycnometers of 2s-ml. capacity. The contents of the pycnometer were discharged into a lowtemperature fractionating column, to obtain the analysis of the gas. In all cases the pycnometer was weighed before and after discharging the gas and any mercury accompanying the gas was collected and weighed. The combined error on weighing is estimated as ±0.04 gram out of 1.35 gram for the gas of lowest density and 11.2 grams for the gas of highest density. The presence of relatively large quantities of heptanes and heavier required refine-

TABLE I.-Typical Analysis of Vapor Pressure, 4310 Lb. per Sq. In. Absolute. Temperature, 250°F.

~~i:< Heptanes

tion

Temperature, Deg. R

Critical Pressure, P"Lb. per Sq. In. Abs.

0.8058 0.0386

0. 0148

344 549 666 766 829 84 6 90 0 914

0.0521

113 2

673 712 617 551 483 485 451 435 360

Mol Frac-

Compound

Critical

0.0117

0.0197 0.0170

0.0280 0.0123

+ heavier ............ ......

Mol Fraction X pT,

Mol Fraction X pp,

Mol Fraction X Mol Weight

277.2

12.89 1. 16 0.5 1 1. 14

13.5 59.0

54 2 .3 27·5 7.2 10.8 8.2 13.6 5.5 6.4 18.7

44 2 .7

640 . 2

28. SO

21.2

7.8 IS. I 14. I

23.7 I I. I

1,000

1.22 2.01

1.06 1. 27 7.24

Heptanes and heavier: molecular weight, 139; specific gravity, 69'60. 0.791. Gas gravity = 28·50 = 0.985 28·96 10 PH = .!'.-- = 43 = 673 pP, 640.2 . _ T 250 460 . Til - pT, = 44 2 .7 = 1.603 Z from F'g. 2

+

PV

=

PV

= WZRT

NZRT

N

=~=

M Calculated density = lb. per cu. ft.

= !!: V

=

= 0.933

pounds divided by mol weight

PM ZRT

= 0.93343 X10 10.71 X 28·50 X 710

Experimental density = 0.277 X 62.43 = 17·29 lb. per cu. ft.

= 1735 lb

.

. per cu.

ft

.

DENSITY OF NATURAL GASES

and varying quantities of natural gasoline G that contained normal butane but no isobutane. Data on the density and properties of 16 saturated vapors in equilibrium with a crude oil are given in Table 2. PSEUDOCRITICAL CONDITIONS OF HEPTANES AND HEAVIER FRACTIONS

Computation of the pseudocritical temperature and pressure of a gas is given in Table 1. Because of the high content and high molecular weight of the heptanes and TABLE

Run

NO,a

vised to be in agreement with Smith and Watson's newly defined boiling points. This mean average boiling point may be converted to the molal average from the knowledge of the initial boiling point of about 200°F. for the heptanes and heavier fraction. The molal average boiling point and gravity give the pseudocritical temperature by Smith and Watson's relationship. The densities and molecular weights of heptanes and heavier of Table 2 have been used to compute the pseudocriticals shown in Fig. I. It should be noted that the

2.-Properties of Saturated Gases

ExperiExperi- mental mental ComDensity. pressibility Gram per C.C. Factor Z ---- - - - - - - - - - - - - - - - - - - ---- - - PresTemper.. Gravity. Mol Per Mol Per MolWt. Density sure, Lb. Cent Cent ature. Air = I per Sq. Deg. C1+ 6%0 C7' F. CR, C,' In. Abs.

A-5 ..... ... A-4L ... ... A-I. .. .... A-21. .. ... A-3 ... B-I. .... ...... C-2 ..... C-I. .... .... C-3I. .. ..... D-1. . .... ...... E-1. ..... . . . . . . . . . . . F-I. .... F-2 .... F-3 ... .... .... G-2 ................ G-3 ...............

1,000 1,600

3. 185 5.270 8,220 2,920 1,010

2.880 5.330 4.33 0 4. 1 95 3.185 4.310 5.530 3.485 4.970

120 120 120 120 120 250 120 120 120 120 120 250 250 250 35 35

0.659 0.671 0·766 0.840 0.899 0 766 0.629 0.736 1.040

0.876 0·736 0·915 0·985 1.070

0·926 1.074

0·9200

0.9158 0.8800 0.8588 0.8440 0.8750 0·9345 0.8895 0.7995 0.8340 0.8920 0.8450 0 8060 o. 7840 0.8230 0.7810

Series A and B ........ . 7.000 cu. Series C. F and G ............... . 3.500 cu. Series D ........................ . 7.000 cu. Series E .................. . 7.000 cu. • Estimated.

a

Natural gas: gravity

=

ft. ft. ft. ft.

gas gas gas gas

131 127 144

0.0444 0.0268 0.0021

0.or80

0.0573 0.0375 0.0243 0.0495

O.753 b

1I3 151 123

0.750' 0 800 o. 788 0.771 0.768 O. 791 0 808 0.765 0.792

130 139 157 128 145'

0.0521

bbl. bbl. bbl. bbl.

0.802

IISb 100b

O.72sb

130 b

0.0588 0.0434 0.0631 per per per per

0.74 2' 0.742' 0.775' 0.788

I lob 110b

0.0090 0.0124 0.0303 0.0400

50 50 25 75

vol. vol. vol. vol.

I

crude crude crude crude

0.0564 0.097 0 224 0.340 0.404 0.154 0.0556 0.203

0·400 0.327 0.261 0.192

0.277 0.349 0.383 0.463

0.870 0.826 0.8ro

0·970 1·358 0.884 0.852 0.777 1.031

0.865 0.879 0.925 0.931 1.031

0.737 1.008

+ 50 vol. gasoline. + 50 vol. gasoline. + 75 vol. gasoline. + 25 vol. gasoline.

0.596; CH" 93.20 per cent; C 2H s, 4.25 per cent; CaH g, r.6r per cent; N 2, 0.43 per cent; C02, 0.51 per cent.

heavier fractions, the usual procedure of using the critical properties of heptanes or octanes for this fraction is no longer reliable. Smith and Watson lO reported a method of obtaining the pseudocritical pressure directly from liquid density and molecular weight. For the pseudo critical temperature, the molal average boiling point must be known in addition to the liquid density. The mean average boiling point 10 may be obtained from the density and molecular weight by a chart similar to that given by Egloff and Nelson (ref. 3, p. 235) and re-

calculated values deviate somewhat from the normal paraffin curve of critical temperatures and pressures as a function of molecular weight. The difference between the calculated pseudocriticals and the criticals of the normal paraffin hydrocarbons is much smaller when using the molecular weight of the heptanes and heavier than when using the liquid density as the correlating factor. In no case would the critical properties of heptanes have been representative of the heptanes and heavier fraction. Since the data given are for saturated

MARSHALL B. STANDING AND DONALD L. KATZ

I43

density of the saturated vapors at 8220 lb. per sq. in. and 120°F. checked the computed density within the error of reading the methane chart.

vapors, it is indicated that the molecular weight, liquid density, and boiling range if possible, should be measured for gases at equilibrium above 1000 lb. or with appreci130 0

-"

(."

~Vo~

0

,,(.">

"'/

~f'\po.

,./ °0

V

v:; ,,~\o /~ ,,0"

0

/' 0

00o

)7

..

.~

~..."o'"

Q.1b

oc.~~":"l--

po.,