Gas Properties

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

5
Heating Value

• HV ? yi (HV)i
• Use Gross Heating Value, BTU/scf
• C1 (methane) 1010.0
• C2 (ethane) 1769.6
• C3 (propane) 2516.1
• i-C4, n-C4 (butanes) 3251.9, 3262.3
• i-C5,n-C5 (pentanes) 4000.0, 4008.9
• C6 (hexanes) 7755.9

6
Heating Value

• Use Gross Heating Value, BTU/scf
• CO2 0
• H2S 637.1
• N2 0
• He 0
• H2 324.2

7
z-Factor

• Real Gas, p. 2, Eq. 1.5
• pV z n R T
• Calculate pseudo critical properties, ppc and Tpc
  
• Kays rule (mole-wtd. averages), p. 3
• Suttons 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

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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
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
(see pp. 17, 18)
Ideal gas
Real gas
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

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
19
Real Gas Pseudopressure
20
Real Gas Pseudopressure
z µ almost constant below 2,000 psi , thus
21
Worksheet for Gas Properties
Gasprop6.xls
22
Separation of Wet Gas
qg1
qg2
qgST
Sep. 2
Sep. 1
S.T.
qo
Reservoir rich (wet) gas only
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
• Bukaceks 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
WaterCondensate 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 Condensates
Constant Composition Expansion
(see SPE Test Case 3, SPE 12278)
Vapor
Liquid
p 3428 (D.P.)
3200
3000
2800 psia
29
Lab Tests for Condensates
Constant 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 Condensates
Constant Volume Expansion, Depletion Study
(see SPE Test Case 3, SPE 12278)
produced gas ? Gp and yi
Vapor
Liquid
p 3428 (D.P.)
3000
3000
2400 psia
2400
31
Lab Tests for Condensates
Constant 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 Condensates
Solubility 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