1

Gas Properties

Page, Table and Eq. references are to Gas Reservoir Engineering,

Lee and Wattenbarger (1996)

2

Objectives – Gas Properties• What properties are important for reservoir

engineering

• Which methods are up-to-date and/or simple

• How to calculate important properties

• How to calculate properties in Excel

3

Standard Conditions(see p. 1)

• Tsc = 60 oF

• psc = 14.65 psia (Texas)

• 1 lb-mole = 380.7 scf

4

Composition, Gas Gravity

• Composition– Mole fraction of

components, Table 1.2, p.4

– Table of component properties, Table 1.1, p.2

• M.W., p. 2, Eq. 1.10

• Gas gravity, g, p. 3, Eq. 1.12

cn

iii MyM

1

29// MMM airg

5

Heating Value

• HV = yi (HV)i

• Use “Gross Heating Value”, BTU/scfC1 (methane) = 1010.0 C2 (ethane) = 1769.6C3 (propane) = 2516.1i-C4, n-C4 (butanes) = 3251.9, 3262.3i-C5,n-C5 (pentanes) = 4000.0, 4008.9C6 (hexanes) = 7755.9

6

Heating Value

• Use “Gross Heating Value”, BTU/scf

CO2 = 0H2S = 637.1N2 = 0He = 0H2 = 324.2

7

z-Factor

• Real Gas, p. 2, Eq. 1.5

• pV = z n R T

• Calculate pseudo critical properties, ppc and Tpc

– Kay’s rule (mole-wtd. averages), p. 3– Sutton’s method, pp. 6, 7– Piper, McCain and Corredor (see paper)

8

A simple chart to estimate pseudo critical pressure, ppc and pseudo critical temperature, Tpc

from gas gravity

(limited to similar gases)

9

z-Factor

• Calculate pseudo reduced ppr and Tpr , p. 2

• Determine z as function of ppr and Tpr – Figs. 1.7 and 1.8, pp. 14, 15

– Read z graphically, or

– Solve for z by iteration, p. 274

)(

)(

RankineT

RankineTT

p

pp

opc

o

prpc

pr

10

Dranchuk, Abu Kassem Z factors

Tpr = 1.05

1.3

1.5

1.752.0

2.53.0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Pseudo reduced pressure, ppr

Z f

acto

r

11

H2S, CO2 Corrections for ppc and Tpc

• Sutton, pp. 6,7

• Wichert and Aziz, pp. 7,8

• Piper, McCain, and Corredor, (see paper & Gasprop6.xls)

12

Density

TRz

Mp

Where,

R = 10.73 M = 29 g

Gravity gradient = / 144 psi/ft

13

Formation volume factor

• Bg is either rcf/scf or RB/Mscf, p.16, 17

14

Compressibility

dp

dcg

1

pcg

1

pd

zd

zpcg

11

Ideal gas:

Real gas:

(see pp. 17, 18)

15

Gas Viscosity

• Almost never measured

• Lee, Gonzalez, Eakin method, p. 18

16

Real Gas Pseudopressure• Reservoir flow potential• Defined by Eq. 7.7, p. 169

p

po

dpz

ppm

2)(

17

Real Gas Pseudopressure

Developed by Al-Hussainy, Ramey, and Crawford, 1966– accounts for gas property variation– p2 plots (Ideal Gas) can be used below 2,000

psia because z is almost constant, pp. 169, 170– always used for pressure transient analysis– should also be used for stabilized flow

18

Real Gas Pseudopressure

p

po

dpz

ppm

2)(

19

Real Gas Pseudopressure

20

Real Gas Pseudopressure

z μ almost constant below 2,000 psi , thus

2122)( p

zdpp

zdp

z

ppm

p

p

p

p oo

21

Worksheet for Gas Properties

Calculates gas properties for a single pressure or for a table of pressures.[note: Gasprop4 and earlier versions had an error when nitrogen was non-zero and should not be used with N2]

Input Data Correlated Properties

Sep. Gas Gravity (air = 1.0) 0.746 Pressure Bg Bg cg g m(p) density

Oil/Gas Ratio, STB/MMscf 130.2 (psia) Z Factor (rcf/scf) (rb/Mscf) (1/psi) (cp) (psi2/cp)*10-6 (lb/ft3)Oil Gravity, deg. API 55.8 14.65 0.996 1.168814 208.1592 6.852E-02 0.0105 0 0.0707 H2S,fraction 0 100 0.974 0.167467 29.8249 1.027E-02 0.0106 0.9482 0.4932

CO2, fraction 0 200 0.948 0.081493 14.5135 5.277E-03 0.0108 3.876 1.0135

N2, fraction 0 300 0.922 0.052812 9.4054 3.623E-03 0.0110 8.786 1.5640Res. Gas Gravity (air = 1.0) 1.0862483 400 0.895 0.038454 6.8484 2.802E-03 0.0113 15.69 2.1479Res. Temperature, oF 150 500 0.868 0.029828 5.3122 2.315E-03 0.0117 24.59 2.7691Standard Temp, oF 60 600 0.840 0.024071 4.2870 1.994E-03 0.0121 35.45 3.4313Standard Pres., psia 14.65 700 0.813 0.019958 3.5544 1.766E-03 0.0125 48.25 4.1385

800 0.786 0.016877 3.0057 1.595E-03 0.0131 62.93 4.8940Calculated Critical Values 900 0.759 0.014491 2.5807 1.458E-03 0.0137 79.39 5.6999

Tpc, R 469.04938 1,000 0.733 0.0126 2.2439 1.341E-03 0.0144 97.52 6.5554Tpr 1.2997992 1,100 0.709 0.011078 1.9729 1.235E-03 0.0152 117.16 7.4560Ppc, psia 611.67282 1,200 0.687 0.009841 1.7527 1.132E-03 0.0162 138.1 8.3927

1,300 0.668 0.008834 1.5732 1.029E-03 0.0173 160.2 9.3501Single Pressure 1,400 0.653 0.008011 1.4268 9.269E-04 0.0184 183.1 10.3100

pressure (psia) = 1900 1,500 0.641 0.007339 1.3071 8.276E-04 0.0197 206.7 11.2537Results 1,600 0.632 0.00679 1.2092 7.339E-04 0.0210 230.6 12.1648

Z Factor = 0.6250 1,700 0.627 0.006338 1.1287 6.480E-04 0.0223 254.8 13.0322Bg (rcf/scf) = 0.005653 1,800 0.625 0.005964 1.0621 5.711E-04 0.0237 279.1 13.8498Bg (rbbl/Mscf) = 1.0068 1,900 0.625 0.005653 1.0068 5.034E-04 0.0251 303.4 14.6102cg, (1/psi) = 5.034E-04 2,000 0.627 0.005385 0.9590 4.433E-04 0.0265 327.5 15.3387

g, (cp) = 0.0251 2,100 0.631 0.005161 0.9192 3.918E-04 0.0279 351.5 16.0026

m(p), (psi2/cp)*10-6 = 303.4 2,200 0.637 0.004975 0.8860 3.482E-04 0.0292 375.2 16.6018density, (lb/ft3) = 14.6102 2,300 0.644 0.004813 0.8571 3.105E-04 0.0305 398.7 17.1624

2,400 0.653 0.004673 0.8322 2.783E-04 0.0318 422.0 17.67492,500 0.662 0.004549 0.8102 2.505E-04 0.0330 445.0 18.15532,600 0.673 0.004447 0.7920 2.276E-04 0.0341 467.8 18.57422,700 0.685 0.004359 0.7764 2.085E-04 0.0352 490.3 18.94652,800 0.696 0.004272 0.7608 1.904E-04 0.0363 512.6 19.33432,900 0.708 0.004198 0.7476 1.754E-04 0.0373 534.7 19.67623,000 0.720 0.004126 0.7349 1.616E-04 0.0383 556.6 20.01603,100 0.733 0.004065 0.7239 1.499E-04 0.0393 578.2 20.32113,200 0.746 0.004004 0.7132 1.391E-04 0.0403 599.6 20.62653,300 0.759 0.003952 0.7038 1.299E-04 0.0412 620.8 20.90113,400 0.772 0.003903 0.6951 1.217E-04 0.0421 641.9 21.16163,500 0.786 0.003858 0.6871 1.144E-04 0.0430 662.7 21.40933,600 0.799 0.003816 0.6796 1.078E-04 0.0438 683.3 21.64533,700 0.813 0.003777 0.6726 1.018E-04 0.0446 703.8 21.87093,800 0.826 0.003737 0.6655 9.607E-05 0.0455 724.1 22.10203,900 0.840 0.003702 0.6594 9.115E-05 0.0463 744.3 22.30924,000 0.854 0.00367 0.6535 8.667E-05 0.0471 764.3 22.50814,100 0.869 0.003641 0.6484 8.281E-05 0.0478 784.1 22.68494,200 0.883 0.003612 0.6432 7.902E-05 0.0485 803.8 22.86974,300 0.897 0.003584 0.6382 7.553E-05 0.0492 823.3 23.04824,400 0.911 0.003557 0.6335 7.230E-05 0.0500 842.7 23.22094,500 0.925 0.003532 0.6289 6.931E-05 0.0507 862.0 23.38824,600 0.939 0.003507 0.6246 6.653E-05 0.0514 881.2 23.55044,700 0.953 0.003486 0.6208 6.412E-05 0.0520 900.2 23.69494,800 0.967 0.003463 0.6167 6.165E-05 0.0527 919.1 23.85154,900 0.981 0.003442 0.6131 5.947E-05 0.0533 937.9 23.99395,000 0.996 0.003422 0.6095 5.739E-05 0.0540 956.5 24.13585,100 1.008 0.003397 0.6050 5.496E-05 0.0548 975.1 24.31515,200 1.022 0.003379 0.6018 5.321E-05 0.0554 993.5 24.44405,300 1.037 0.003361 0.5987 5.154E-05 0.0560 1011.8 24.57155,400 1.050 0.003343 0.5954 4.988E-05 0.0567 1030.0 24.70415,500 1.065 0.003328 0.5927 4.845E-05 0.0572 1048.1 24.81985,600 1.079 0.003311 0.5897 4.699E-05 0.0578 1066.1 24.94335,700 1.093 0.003295 0.5869 4.561E-05 0.0584 1084.0 25.06435,800 1.107 0.003282 0.5845 4.443E-05 0.0590 1101.8 25.16855,900 1.121 0.003267 0.5818 4.321E-05 0.0595 1119.5 25.28226,000 1.135 0.003253 0.5793 4.205E-05 0.0601 1137.1 25.39356,100 1.149 0.003239 0.5768 4.094E-05 0.0607 1154.7 25.50266,200 1.164 0.003227 0.5747 4.000E-05 0.0612 1172.1 25.59616,300 1.178 0.003214 0.5724 3.900E-05 0.0617 1189.5 25.69956,400 1.192 0.003201 0.5701 3.805E-05 0.0623 1206.8 25.8008

Gasprop6.xls

22

Separation of Wet Gas

Reservoir – rich (wet) gas only

Sep.2

Sep.1 S.T.

qg2qg1 qgST

qo

23

Raw (or wet) Gas Gravity

• When condensate (oil) production is condensed from gas stream in wellbore/separator:

• Reconstruct gravity with oil molecules in the wet gas, pp.11-15, Eqs. 1.37 and 1.38

• for overall separation, R is total GOR

w = (R g + 4,602 o) / (R + 133,316 o/Mo)

where: R = scf /S.T.Bbl o = oil sp. grav. = 141.5 / (131.5 + oAPI)

g = oil sp. grav. bl Mo = oil M.W. ≈ 5,954/(API – 8.811)

24

Raw (or wet) Gas Production

• Use Eq. 1.47, p. 13 to correct separator gas and S. T. Oil production to Wet gas production from the reservoir

Gp = Gp,dry +133,316 Np o / Mo

Gp = scf o = oil sp. grav.

Np = S.T.Bbl Mo = oil M.W.

25

Water Vapor in Gas

• Not included in composition alalysis

• Bukacek’s Correlation, p. 28,29, Eqs.

1.101-1.103

• lb/MMscf/(350 water S.G.) = Bbl/MMscf

26

Worksheet for including water and condensate vapor

Calculation of water content in gas, using Bukacek's method (see Lee and Wattenbarger, p. 28-29)

Temperature, deg F. = 310 A(T) = 3,704,608 Pressure, psia = 10,000 B(T) = 489.17

Water vapor, lb/MMscf = 860 Water in vapor phase, Bbl/MMscf = 2.46 Water in vapor phase, MMscf/MMscf = 0.018 Gas volume correction factor = 1.018

Condensate in gas phase (see Lee and Wattenbarger, pp. 9-16)Oil gravity, deg. API = 49.4 Oil sp. Grav = 0.782Dry (separator) gas gravity = 0.732 Mol.wt. = 146.7Oil production, STB = 1.000 GOR, scf/STB = 1,996 Gas production, Mscf = 1.996 OGR, Bbl/MMscf = 501.0

Wet gas gravity = 1.8695866Gas equivalent of oil = 0.35615507 (MMscf in 1 MMscf of dry gas)"Wet gas" correction factor = 1.35615507Wet gas production, Mscf = 2.70688552

7040 0.712 5012.48510 1.02 520.2120 1.55 186

7670 0.745590613 5718.68

This calculation corrects the dry gas (separator gas) production to account for the oil in the gas phase. Wet gas volumes should be used in any material balance calculations. (Example 1.8)

To apply this to liquid loading estimates, input the reservoir temperature and static pressure. Then input the surface tubing temperature and pressure. The difference is the condensed water in the liquid phase. This liquid water may cause liquid loading.

Water&Condensate vapor.xls

27

Sampling a Gas Condensate Well

• Must be single phase in reservoir (above dew point), Figs. 10.2, 10.3

• Downhole sample– More difficult– Only valid if single phase in wellbore

• Recombined sampling– Requires accurate field GOR, lab matching– Gas and liquid samples at primary separator– Or, Total separator gas and stock tank oil– (see SPE3, Table 4)

28

Lab Tests for CondensatesConstant Composition Expansion

p = 3428 (D.P.) 3200 3000 2800 psia

Liquid

Vapor

(see SPE Test Case #3, SPE 12278)

29

Lab Tests for CondensatesConstant Composition Expansion

• Expansion at constant composition and reservoir temperature

• Quick and inexpensive

• Two phase volume measured

• Liquid volume sometimes measured– not in Table 6, SPE3

30

Lab Tests for CondensatesConstant Volume Expansion, “Depletion Study”

p = 3428 (D.P.) 3000 3000 2400 psia

Liquid

Vapor

(see SPE Test Case #3, SPE 12278)

2400

“produced gas” Gp and yi

31

Lab Tests for CondensatesConstant Volume Expansion, “Depletion Study”

• Carried out in steps (pressure decrements)• Emulates production from constant pore volume

reservoir (depletion)– liquid does not flow to well, Table 7, SPE3

• Produced gas can be analyzed, Table 8, SPE3• Separator ratios or plant products can be

calculated, Table 9, SPE3

32

Lab Tests for CondensatesSolubility and Swelling Test

• Designed to approximate effect of injected gas on phase behavior and dew point

• Performed in increments of gas injection. – Add injection gas – Measure new dew point with CCE– Measure “swollen volume” at new dew point

• (See SPE3, Tables 10-15)

33

Hydrates• When water hydrates are in gas

• Slushy “ice” plugs lines, valves, etc.– when below “freezing point”, pp.29-31– when pressure drop is too great, pp. 29-31

• Solutions– dry gas with ethylene glycol– avoid large pressure drops– heat gas stream– add methanol to lower freezing point

34

Objectives – Gas Properties• Which methods are up-to-date and/or

simple

• What properties are important for reservoir engineering

• How to calculate important properties

• How to calculate properties in Excel