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