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THE ALPHA FIELD CASE STUDYJón Steinar GuðmundssonTPG4140 NATURGASS
November 10, 2010
• Deliverability and performances• The Alpha gas field data• Material balance and z-factor• PSS flow equation for gas• Pressure function (pseudopressure) and viscosity• The Alpha gas pressure function (polynomial fit)• The Alpha gas well data• Pressure drop in gas wells• Inflow performance and outflow performance• Summary
Deliverability and PerformancesAnalyse and Synthesis
• Pressure profile from reservoir to wellhead is analysed in terms of several performances.
• Reservoir performance gives reservoir pressure with time, which is used in inflow performance plot.
• Shape of inflow performance plot remains the same with time, unless near-wellbore damage occurs due to production.
• Tubing performance curve plotted together with inflow performance, gives the well production rate where the two curves cross each other.
Deliverability and PerformancesReservoir Performance
pR/z
G
Deliverability and PerformancesInflow Performance
pwf
q
Deliverability and PerformancesTubing Performance
pwf
q
The Alpha Gas Field
F ield data
Areal og reservoir, AR 80 km2
Thickness , hR 25 mAverage reservoir pressure, pR 285 bara
Average reservoir temperature, TR 88 oCWater depth, dW 800 mR eservoir depth, dR 2600‐2650 TVD MS LAverage permeability, k 200 mdPoros ity, φ 25,00 %Water S aturation, S w 20,00 %R ecovery factor, R F 60,00 %Z ‐ factor 0,9334S pes ific gravity 0,60Molecular weight of gas (from hysys ), Mg 17,45Gas viscos ity, μ 1,92E ‐02 cpDrainage radius each well 1000 m
Material Balance for Natural Gas FieldsVolumetric Balance
Gas volumes taken at standard conditions (s.c.)Constant volume reservoir without water influxGi = Gas initially in place (resource, not reserve)G = Gas already produced (Gp in many texts)Use FVF (symbol B) for natural gas
(p/z) = (pi/zi) [1 - G/Gi]
Z-factor Alpha Field Gas
0,88
0,9
0,92
0,94
0,96
0,98
1
1,02
0 50 100 150 200 250 300 350
Pressure, p, bara
Z-fa
ctor
Material Balance for Natural Gas Fieldsp/z Method
p/z
pi/zi
GiG Gabandonment
Material Balance Alpha Gas Field
0
50
100
150
200
250
300
350
0 20 40 60 80 100 120 140 160 180Gas Produced, Gp, GSm3
p/z
Flow Equations for Natural Gas in ReservoirEffect of Pressure
• In flow equations for low-pressure gas, pressure-squared is used, p2
R-p2w.
• When the pressure is intermediate, the so calledpseudopressure m(p) is traditionally used (from numerical integration). Same as pressure function.
• At high-pressure, the gas behaves similar to liquid and the pressure drop is expressed by simple difference, pR-pw, which is the same as for conventional oil.
• What is low-pressure, intermediate pressure and high-pressure depends on the gas composition through thepressure function F(p)={p/(μz)}
PSS Rate Equation for Gas
R
wf
p
p gcs
cs
w
ecs dp
zp
pTT
srr
khq
..
....
1
43ln
2
Pseudopressure and Specific Gravity
0
2000
4000
6000
8000
10000
12000
14000
16000
0 50 100 150 200 250 300 350 400
Pressure,p, bara
P/ g
Z, b
arac
p-1
Methane = 0.554
Gas,s.g =0.6
Gas, s.g = 0.7
Gas, s.g = 0.8
Gas, s.g = 0.9
Gas, s.g = 1.0
110 bara 275 bara
Viscosity
0,00E+00
5,00E-03
1,00E-02
1,50E-02
2,00E-02
2,50E-02
3,00E-02
0 50 100 150 200 250 300 350
Pressure, p, bara
Visc
osity
, μ, m
Pa.s
Pseudo Pressure Alpha Field Gas
y = -0,1263x2 + 78,981x - 98,294R2 = 0,9999
0
4 000
8 000
12 000
16 000
0 50 100 150 200 250 300Pressure, p, bara
p/μ g
z, b
ara/
mPa
.s
High-Pressure Gas and Conventional Oil
wfRcs
cs
w
ecs pp
pTT
srr
khq
..
....
1
43ln
2
wfR ppPIq
PI = Productivity Index
Well data ++
Length of well, L 2600 mWellhead pressure, pwf, (assume 80% of pR ) 228 bara
Wellhead temperature, Twh (assumes 80% of TR ) 70,4 oCWellbore radius 0,1 mTubing diameter 0,15 mNumber of wells 6
G as dens ity at s tandard conditions , R ho 0,74 kg/S m3
Air dens ity at standard conditions , R ho 1,23 kg/S m3z ‐ factor at standard conditions (ideal gas) 1
Temperature at standard conditions , Tsc 15,56 oCP ressure at s tandard conditions 1,01325 baraL ife time of reservoir 20 yearsMolecular weight of air, Mair 28,97 kg/kmol
Pressure Drop in Gas WellsFrictional and Hydrostatic
0ln 21
222
1222
2
L
pp
fdpp
zRTfmMdA
Lg
zRTMpp sinexp12
fag pppp
Inflow Performance and Tubing Performance Alpha Wells
0
50
100
150
200
250
300
350
400
0 20 40 60 80 100 120 140
P roduc tion Rate , q, S m3/s
Press
ure, p, bara
Inflow P erfromence
Tubing P erformence
Inflow Performance and Tubing Performance
0
50
100
150
200
250
300
350
400
0 2 4 6 8 10 12 14
P roduc tion R ate, q, MSm3/d
Press
ure, p
, bara
Inflow Perfomance
Tubingf
Summary
• Concept of deliverability can be used to estimate the productionexpected from a gas field from start-up to abandonment.
• The data presented for the Alpha field, the Alpha wells and the Alpha gas, is typical for a high-quality offshore resource.
• PSS rate equation using the general pressure function can be used for all inflow conditions.
• Established analytical equations for pressure drop in pipes can be used to calculate the outflow performance. (The presented outflow results were however obtained from Hysys calculations.)
• Inflow from high-pressure gas reservoirs (HPHT reservoirs, e.g. Kristin) behave similar to conventional oil reservoirs such that PI can be used.
Reelle gassloven
zTT
ppqq
zTT
ppVV
z
zRTpvznRTpV
sc
scsc
sc
scsc
sc
1
1
1
Tetthet og FVF
Bqq
zp
pTTB
Smm
VVFVFB
zRTpM
nVM
znRTpV
sc
sc
sc
sc
3
3
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