chapter 2 chapter 2 volumetric gas reservoir engineering volumetric gas reservoir engineering 1

Chapter 2 Chapter 2

Volumetric gas Reservoir EngineeringVolumetric gas Reservoir Engineering

Gas PVTGas PVT

Gas is one of a few substances whose state, as Gas is one of a few substances whose state, as defined by pressure, volume and temperature defined by pressure, volume and temperature (PVT)(PVT)

One other such substance is saturated steam.One other such substance is saturated steam.

The equation of state for gasThe equation of state for gas Classical ideal gas lawClassical ideal gas law ClassicalClassical non-ideal gas law non-ideal gas law

Cubic Equations of State Cubic Equations of State van der Waals equation of state van der Waals equation of state Redlich-Kwong equation of state Redlich-Kwong equation of state Soave modification of Redlich-Kwong Soave modification of Redlich-Kwong Peng-Robinson equation of statePeng-Robinson equation of state Elliott, Suresh, Donohue equation of state Elliott, Suresh, Donohue equation of state Non-cubic Equations of State Non-cubic Equations of State Dieterici equation of state Dieterici equation of state

Virial Equations of State Virial Equations of State Virial Equations of StateVirial Equations of State The BWRS equation of state The BWRS equation of state

Other Equations of State of Interest Other Equations of State of Interest Stiffened equation of state Stiffened equation of state Ultrarelativistic equation of state Ultrarelativistic equation of state Ideal Bose equation of stateIdeal Bose equation of state

The equation of state for an ideal gasThe equation of state for an ideal gas

(Field units used in the industry)(Field units used in the industry) p [=] psia; V[=] ftp [=] psia; V[=] ft33; T [=] ; T [=] OOR absolute temperatureR absolute temperature

n [=] lbn [=] lbmm moles; n=the number of lb moles; n=the number of lbmm moles, one lb moles, one lbmm mole is mole is

the molecular weight of the gas expressed in pounds.the molecular weight of the gas expressed in pounds. R = the universal gas constant R = the universal gas constant

[=] 10.732 psia∙ ft[=] 10.732 psia∙ ft33 / (lb / (lbmm mole∙ mole∙00R)R)

Eq (1.13) results from the combined efforts of Boyle, Charles, Eq (1.13) results from the combined efforts of Boyle, Charles, Avogadro and Gay Lussac.Avogadro and Gay Lussac.

)13.1(nRTpV

In other book1 Darcy = ０ .986927×10-8 cm2

1 Darcy = ０ .986927×10-

Note: In this text 1 Darcy = 1.0133×10-8 cm2

or 1 Darcy 10-8 cm2

or 1 Darcy

10-12 m2 = 1

1 Darcy

Non-ideal gas lawNon-ideal gas law

Where z = z-factor =gas deviation factorWhere z = z-factor =gas deviation factor =supercompressibility factor=supercompressibility factor = compressibility factor= compressibility factor

)15.1(nzRTpV

PandTatgasofmolesnofvolumeIdeal

PandTatgasofmolesnofvolumeActual

),,( ncompositioTPfz )1( airgravityspecificncompositio g

Determination of z-factorDetermination of z-factor

There are three ways to determine z-factor :There are three ways to determine z-factor :

(a)Experimental determination(a)Experimental determination

(b)The z-factor correlation of standing and(b)The z-factor correlation of standing and

katz katz

(c)Direct calculation of z-factor(c)Direct calculation of z-factor

(a) Experimental determination(a) Experimental determination n moles of gas n moles of gas

p=1atm; T=reservoir temperature; => V=Vp=1atm; T=reservoir temperature; => V=V00

pV=nzRTpV=nzRT z=1 for p=1 atm z=1 for p=1 atm =>14.7 V=>14.7 V00=nRT=nRT

n moles of gas n moles of gas

p>1atm; T=reservoir temperature; => V=Vp>1atm; T=reservoir temperature; => V=V pV=nzRTpV=nzRT pV=z(14.7 VpV=z(14.7 V00))

By varying p and measuring V, the isothermal z(p) function can beBy varying p and measuring V, the isothermal z(p) function can be readily by obtained.readily by obtained.

07.14 V

(b)The z-factor correlation of standing and katz(b)The z-factor correlation of standing and katz Requirement: Requirement: Knowledge of gas composition or gas gravityKnowledge of gas composition or gas gravity Naturally occurring hydrocarbons: primarily Naturally occurring hydrocarbons: primarily

paraffin series Cparaffin series CnnHH2n+22n+2

Non-hydrocarbon impurities: CONon-hydrocarbon impurities: CO22, N, N22 and H and H22SS

Gas reservoir: lighter members of the paraffin series, CGas reservoir: lighter members of the paraffin series, C11

and Cand C22 > 90% of the volume. > 90% of the volume.

The Standing-Katz CorrelationThe Standing-Katz Correlation

knowing Gas composition (nknowing Gas composition (nii)) Critical pressure (PCritical pressure (Pcici)) Critical temperature (TCritical temperature (Tcici) of each component ) of each component ( Table (1.1) and P.16 ) ( Table (1.1) and P.16 ) Pseudo critical pressure (PPseudo critical pressure (Ppcpc) ) Pseudo critical temperature (TPseudo critical temperature (Tpcpc) for the mixture) for the mixture

Pseudo reduced pressure (PPseudo reduced pressure (Pprpr) ) Pseudo reduced temperature (TPseudo reduced temperature (Tprpr) )

Fig.1.6; p.17 Fig.1.6; p.17 z-factor z-factor

iciipc

pcpr P

).(IsothermalconstT

(b’)The z-factor correlation of standing and katz(b’)The z-factor correlation of standing and katz

For the gas composition is not available and the gas gravity For the gas composition is not available and the gas gravity (air=1) is available.(air=1) is available.

The gas gravity (air=1)The gas gravity (air=1) ( ) ( ) fig.1.7 , p18fig.1.7 , p18

Pseudo critical pressure (PPseudo critical pressure (Ppcpc))

Pseudo critical temperature (TPseudo critical temperature (Tpcpc))

(b’)The z-factor correlation of standing and katz(b’)The z-factor correlation of standing and katz

Pseudo reduced pressure (PPseudo reduced pressure (Pprpr))

Pseudo reduced temperature (TPseudo reduced temperature (Tprpr))

Fig1.6 p.17Fig1.6 p.17

z-factorz-factor The above procedure is valided only if impunity (COThe above procedure is valided only if impunity (CO22,N,N22 and and

HH22S) is less then 5% volume.S) is less then 5% volume.

pcpr P

).(IsothermalconstT

(c) Direct calculation of z-factor(c) Direct calculation of z-factor The Hall-Yarborough equations, developed using the Starling-Carnahan The Hall-Yarborough equations, developed using the Starling-Carnahan

equation of state, areequation of state, are

where Pwhere Pprpr= the pseudo reduced pressure= the pseudo reduced pressure

t=1/Tt=1/Tprpr ; T ; Tprpr=the pseudo reduced temperature =the pseudo reduced temperature y=the “reduced” density which can be obtained as the y=the “reduced” density which can be obtained as the

solution of the equation as followed:solution of the equation as followed:

This non-linear equation can be conveniently solved for y using the simple This non-linear equation can be conveniently solved for y using the simple Newton-Raphson iterative technique.Newton-Raphson iterative technique.

)20.1(06125.0

2)1(2.1

432)1(2.1 )58.476.976.14(

)1(06125.0

yyyyteP t

)21.1(0)4.422.2427.90( )82.218.2(32 tyttt

(c) Direct calculation of z-factor(c) Direct calculation of z-factor The steps involved in applying thus are:The steps involved in applying thus are:

making an initial estimate of ymaking an initial estimate of ykk, where k is an iteration counter (which in , where k is an iteration counter (which in this case is unity, e.q. ythis case is unity, e.q. y11=0.001=0.001

substitute this value in Eq. (1.21);unless the correct value of y has been substitute this value in Eq. (1.21);unless the correct value of y has been initially selected, Eq. (1.21) will have some small, non-zero value Finitially selected, Eq. (1.21) will have some small, non-zero value Fkk. .

(3) using the first order Taylor series expansion, a better (3) using the first order Taylor series expansion, a better estimate of y can be determined asestimate of y can be determined as

wherewhere

(4) iterating, using eq. (1.21) and eq. (1.22), until satisfactory (4) iterating, using eq. (1.21) and eq. (1.22), until satisfactory convergence is obtained(5) substitution of the correct value of y inconvergence is obtained(5) substitution of the correct value of y in eq.(1.20)will give the z-factor. eq.(1.20)will give the z-factor. (5) substituting the correct value of y in eq.(1.20)will give the z-factor. (5) substituting the correct value of y in eq.(1.20)will give the z-factor.

)22.1(1

)16.952.1952.29()1(

4441 324

)23.1()4.422.2427.90)(82.218.2( )82.218.1(32 tytttt

The equation of state for real gasThe equation of state for real gas

The equation of Van der WaalsThe equation of Van der Waals (for one lb mole of gas(for one lb mole of gas

where a and b are dependent on the nature of the gas.where a and b are dependent on the nature of the gas. The principal drawback in attempting to use eq. The principal drawback in attempting to use eq.

(1.14) to describe the behavior of real gases (1.14) to describe the behavior of real gases encountered in reservoirs is that the maximum encountered in reservoirs is that the maximum pressure for which the equation is applicable is still pressure for which the equation is applicable is still far below the normal range of reservoir pressures far below the normal range of reservoir pressures

)14.1())((2

Peng-Robinson equation of Peng-Robinson equation of state state

where, ω is the acentric factor of the species R is the universal gas constant.

Peng-Robinson equation of Peng-Robinson equation of statestate

The Peng-Robinson equation was developed in 1976 in order to satisfy the following goals:[3]

1. The parameters should be expressible in terms of the critical properties and the acentric factor.

2. The model should provide reasonable accuracy near the critical point, particularly for calculations of the Compressibility factor and liquid density.

3. The mixing rules should not employ more than a single binary interaction parameter, which should be independent of temperature pressure and composition.

4. The equation should be applicable to all calculations of all fluid properties in natural gas processes.

For the most part the Peng-Robinson equation exhibits performance similar to the Soave equation, although it is generally superior in predicting the liquid densities of many materials, especially nonpolar ones. The departure functions of the Peng-Robinson equation are given on a separate article.

Application of the real gas equation of stateApplication of the real gas equation of state Equation of state of a real gasEquation of state of a real gas This is a PVT relationship to relate surface to reservoir volumes of This is a PVT relationship to relate surface to reservoir volumes of

hydrocarbon.hydrocarbon.(1) the gas expansion factor E,

Real gas equation for n moles of gas at standard conditionsReal gas equation for n moles of gas at standard conditions

Real gas equation for n moles of gas at reservoir conditions Real gas equation for n moles of gas at reservoir conditions

> surface volume/reservoir volume > surface volume/reservoir volume [=] SCF/ft[=] SCF/ft3 3 or STB/bblor STB/bbl

)15.1(nzRTpV

conditionsreservoiratgasofmolesnofvolume

conditionsdardsatgasofmolesnofvolume

VE sc tan

scscscsc RTnzVp sc

scscsc p

nzRTpV p

)1:(7.14

scscsc

sc znotezTzTp

][35.35 zT

ExampleExample

Reservoir condition: Reservoir condition: P=2000psia; T=180P=2000psia; T=1800F=(180+459.6)=639.60R; z=0.865F=(180+459.6)=639.60R; z=0.865 >>

surface volume/reservoirsurface volume/reservoir or SCF/ftor SCF/ft33 or STB/bbl or STB/bbl

8.1276.639865.0

200035.35

iwi ESVOGIP )1(

(2) Real gas density(2) Real gas density

where n=moles; M=molecular weight)where n=moles; M=molecular weight)

at any p and Tat any p and T

For gasFor gas

For airFor air

gasgas

airair

(2) Real gas density(2) Real gas density

At standard conditions zAt standard conditions zairair = z = zgasgas = 1 = 1

in generalin general

(a) If is known, then or , (a) If is known, then or ,

(b) If the gas composition is known, then (b) If the gas composition is known, then

where where

)28.1(97.28

8.0~6.0g

g 97.28 ggasM airggas

iigas MnM

97.28gas

airggas

30763.0)(ft

lbmscair

(3)Isothermal compressibility of a real gas(3)Isothermal compressibility of a real gas

nzRTpV ))(:(1 pfznotenRTzpp

znRTppnRTz

since p

p.24, fig.1.9

Exercise 1.1Exercise 1.1 - Problem - Problem

Exercise1.1 Gas pressure gradient in theExercise1.1 Gas pressure gradient in the

reservoirreservoir (1) Calculate the density of the gas, at (1) Calculate the density of the gas, at

standard conditions, whose standard conditions, whose

composition is listed in the table 1-1.composition is listed in the table 1-1. (2) what is the gas pressure gradient in(2) what is the gas pressure gradient in

the reservoir at 2000psia andthe reservoir at 2000psia and

1800F(z=0.865)1800F(z=0.865)

Exercise 1.1 -- solutionExercise 1.1 -- solution -1 -1 (1) Molecular weight of the gas(1) Molecular weight of the gas

sincesince

or from or from

At standard conditionAt standard condition

iigas MnM 15.19 661.097.28

97.28 gas

airggasair

)(0504.0)(0763.0661.0 33 ftlbmftlbmgas

nzRTpV nMzRTpVM mzRT

)(0505.06.51973.101

15.197.14 3ftlbmRTz

Exercise 1.1 -- solutionExercise 1.1 -- solution -2 -2

(2) gas in the reservoir conditions(2) gas in the reservoir conditions

nzRTpV nMzRTpVM mzRT

)(451.6)1806.459(73.10865.0

15.192000 3ftlbmzRT

Exercise 1.1 -- solutionExercise 1.1 -- solution -3 -3

gDp gdDdp

dp 232.32)

1451.6(

23451.6

3451.6

11451.6

lb f 10448.0

ftpsi0448.0

Fluid Pressure RegimesFluid Pressure Regimes

The total pressure at any depth The total pressure at any depth

= weight of the formation rock = weight of the formation rock

+ weight of fluids (oil, gas or water)+ weight of fluids (oil, gas or water)

~ 1 psi/ft * depth(ft)~ 1 psi/ft * depth(ft)

Fluid Pressure RegimesFluid Pressure Regimes

Density of sandstoneDensity of sandstone

)1003048.0(

2.27.2

1202.168

Pressure gradient for sandstonePressure gradient for sandstone

3084.1682.3222.5

)/(16.1144

1084.168

2ftpsi

Overburden pressureOverburden pressure

Overburden pressure (OP)Overburden pressure (OP) = Fluid pressure (FP) + Grain or matrix pressure (GP)= Fluid pressure (FP) + Grain or matrix pressure (GP)

OP=FP + GPOP=FP + GP

In non-isolated reservoir In non-isolated reservoir PW (wellbore pressure) = FPPW (wellbore pressure) = FP

In isolated reservoir In isolated reservoir PW (wellbore pressure) = FP + GP’PW (wellbore pressure) = FP + GP’ where GP’<=GPwhere GP’<=GP

Normal hydrostatic pressureNormal hydrostatic pressure

In a perfectly normal case , the water pressure at any depthIn a perfectly normal case , the water pressure at any depth Assume :(1) Continuity of water pressure to the surfaceAssume :(1) Continuity of water pressure to the surface (2) Salinity of water does not vary with depth.(2) Salinity of water does not vary with depth. [=] psia [=] psia

psi/ft for pure waterpsi/ft for pure water psi/ft for saline waterpsi/ft for saline water

7.14)( DdD

dPP water

4335.0)( waterdD

Abnormal hydrostatic pressure Abnormal hydrostatic pressure ( No continuity of water to the surface)( No continuity of water to the surface)

[=] psia[=] psia

Normal hydrostatic pressureNormal hydrostatic pressure

c = 0 c = 0

Abnormal (hydrostatic) pressureAbnormal (hydrostatic) pressure c > 0 → Overpressure (Abnormal high pressure)c > 0 → Overpressure (Abnormal high pressure) c < 0 → Underpressure (Abnormal low pressure)c < 0 → Underpressure (Abnormal low pressure)

dPP water 7.14)(

Conditions causing abnormal fluid pressuresConditions causing abnormal fluid pressures

Conditions causing abnormal fluid pressures in enclosed water Conditions causing abnormal fluid pressures in enclosed water bearing sands includebearing sands include

Temperature change ΔT = +1 → ΔP = +125 psi in a ℉Temperature change ΔT = +1 → ΔP = +125 psi in a ℉sealed fresh water systemsealed fresh water system

Geological changes – uplifting; surface erosionGeological changes – uplifting; surface erosion Osmosis between waters having different salinity, the Osmosis between waters having different salinity, the

sealing shale acting as the semi permeable membrane in sealing shale acting as the semi permeable membrane in this ionic exchange; if the water within the seal is more this ionic exchange; if the water within the seal is more saline than the surrounding water the osmosis will cause saline than the surrounding water the osmosis will cause the abnormal high pressure and vice versa.the abnormal high pressure and vice versa.

Are the water bearing sands abnormally Are the water bearing sands abnormally pressured ?pressured ?

If so, what effect does this have on the extent of any If so, what effect does this have on the extent of any

hydrocarbon accumulations?hydrocarbon accumulations?

Hydrocarbon pressure regimesHydrocarbon pressure regimes

In hydrocarbon pressure regimesIn hydrocarbon pressure regimes

psi/ftpsi/ft

psi/ft psi/ft

45.0)( waterdD

35.0)( oildD

08.0)( gasdD

Pressure Kick – Oil and WaterP(psia)

55005600

OWCD=5500ft

Pw=2265 Po=2315

Pw=2355 Po=2385

Pw=Po=2490Pw=2535

Depth(ft)

psia 2265 155000*45.0)5000 (psia 2315 5655000*35.0)5000 (

psia 2355 155200*45.0)5200 (psia 23856555200*35.0)5200 (

zone oilin 565D*0.35P5655500*0.35-2490Cor

*35.0 2490) 5500 (

psia 2490 155500*45.0) 5500 (psia 2535 155600*45.0)5600 (

][ 15*45.0

ftDatPftDatPftDatPftDatP

CDOWCatorftDatPOWCatorftDatP

ftDatPzonewaterinpsiaDP

pressure kick-gas and water

psiaftDatP

psiftDatP

zonegasinDP

GWCftDatP

psiaGWCftDatP

psiaftDatP

zonewaterinDP

2265)5000(

245020505000*08.0)5000(

2355)5200(

246620505200*08.0)5000(

2050*08.0

20505500*08.02490

2490)5500(

2535)5600(

15*45.0

P(psia)5000

55005600

Pw=2265 Pg=2450

Pg=2466Pw=2355

Pw=Pg=2490Pw=2535

Depth(ft)

D=5500ft

pressure kick-gas, oil and water

psiaftDatp

zoneoilinDp

psiaOWCftDatp

psiftDatp

zonewaterinDp

2445155400*45.0)5400(

24555655400*35.0)5400(

565*35.0

2490)5500(

2535)5600(

15*45.0

psiaftDatp

psiaGOCftDatpGOCftDatp

psiaGOCftDatp

2265)5000(

239619965000*08.0)5000(

2355)5200(

241219965200*08.0)5200(

1996*08.0

2420)5300()5300(

2400155300*45.0)5300(

24205655300*35.0)5300(

P(psia)5000

52005300540055005600

Pw=2265 Pg=2396

Pw=2355 Pg=2412Pw=2400 Po =Pg=2420

Pw=2445 Po=2455Pw= Po=2490

Pw=2535

Depth(ft)

oilOWC

D=5500ft

D=5300ft

Pressure KickPressure Kick

Assumes a normal hydrostatic pressure regime Pω= 0.45 × D + 15Assumes a normal hydrostatic pressure regime Pω= 0.45 × D + 15 In water zoneIn water zone at 5000 ft Pω(at5000) = 5000 × 0.45 + 15 = 2265 psiaat 5000 ft Pω(at5000) = 5000 × 0.45 + 15 = 2265 psia at OWC (5500 ft) Pω(at OWC) = 5500 × 0.45 + 15 = 2490 psiaat OWC (5500 ft) Pω(at OWC) = 5500 × 0.45 + 15 = 2490 psia

In oil zone Po = 0.35 x D + C In oil zone Po = 0.35 x D + C at D = 5500 ft , Po = 2490 psi at D = 5500 ft , Po = 2490 psi → → C = 2490 – 0.35 × 5500 = 565 psiaC = 2490 – 0.35 × 5500 = 565 psia → → Po = 0.35 × D + 565Po = 0.35 × D + 565 at GOC (5200 ft) Po (at GOC) = 0.35 × 5200 + 565 = 2385 psiaat GOC (5200 ft) Po (at GOC) = 0.35 × 5200 + 565 = 2385 psia

In gas zone Pg = 0.08 D + 1969 (psia)In gas zone Pg = 0.08 D + 1969 (psia) at 5000 ft Pg = 0.08 × 5000 + 1969 = 2369 psia at 5000 ft Pg = 0.08 × 5000 + 1969 = 2369 psia

In gas zone Pg = 0.08 D + CIn gas zone Pg = 0.08 D + C At D = 5500 ft, Pg = Pω = 2490 psiaAt D = 5500 ft, Pg = Pω = 2490 psia 2490 = 0.08 × 5500 + C2490 = 0.08 × 5500 + C C = 2050 psiaC = 2050 psia → → Pg = 0.08 × D + 2050Pg = 0.08 × D + 2050 At D = 5000 ftAt D = 5000 ft Pg = 2450 psiaPg = 2450 psia

GWC error from pressure measurementGWC error from pressure measurement

Pressure = 2500 psia Pressure = 2450 psia Pressure = 2500 psia Pressure = 2450 psia at D = 5000 ft at D = 5000 ftat D = 5000 ft at D = 5000 ft in gas-water reservoir in gas-water reservoirin gas-water reservoir in gas-water reservoir GWC = ? GWC = ?GWC = ? GWC = ? Sol. Sol.Sol. Sol. Pg = 0.08 D + C Pg = 0.08 D + CPg = 0.08 D + C Pg = 0.08 D + C C = 2500 – 0.08 × 5000 C = 2450 – 0.08 × 5000C = 2500 – 0.08 × 5000 C = 2450 – 0.08 × 5000 = 2100 psia = 2050 psia= 2100 psia = 2050 psia → → Pg = 0.08 D + 2100 → Pg = 0.08 D + 2050Pg = 0.08 D + 2100 → Pg = 0.08 D + 2050 Water pressure Pω = 0.45 D + 15 Water pressure Pω = 0.45 D + 15Water pressure Pω = 0.45 D + 15 Water pressure Pω = 0.45 D + 15 At GWC Pg = Pω At GWC Pg = PωAt GWC Pg = Pω At GWC Pg = Pω 0.08 D + 2100 = 0.45 D + 15 0.08 D + 2050 = 0.45 D + 150.08 D + 2100 = 0.45 D + 15 0.08 D + 2050 = 0.45 D + 15 D = 5635 ft (GWC) D = 5500 ft (GWC)D = 5635 ft (GWC) D = 5500 ft (GWC)

Results from Errors in GWC or GOC or OWCResults from Errors in GWC or GOC or OWC

GWC or GOC or OWC location GWC or GOC or OWC location

affecting affecting

volume of hydrocarbon OOIPvolume of hydrocarbon OOIP

affectingaffecting

OOIP or OGIPOOIP or OGIP

affectingaffecting

development plansdevelopment plans

Gas Material Balance: Recovery FactorGas Material Balance: Recovery Factor Material balanceMaterial balance

Production = OGIP (GIIP) - Unproduced gasProduction = OGIP (GIIP) - Unproduced gas (SC) (SC) (SC)(SC) (SC) (SC) Case 1Case 1 ：： no water influx (volumetric no water influx (volumetric depletion reservoirs)depletion reservoirs) Case 2Case 2 ：： water influx (water drive reservoirs) water influx (water drive reservoirs)

Volumetric depletion reservoirs -- 1Volumetric depletion reservoirs -- 1

No water influx into the reservoir from the adjoining aquiferNo water influx into the reservoir from the adjoining aquifer Gas initially in place (GIIP) or Initial gas in placeGas initially in place (GIIP) or Initial gas in place （（ IGIPIGIP ））＝＝ G G ＝＝ Original gas in place Original gas in place （（ OGIPOGIP ）） [=] Standard Condition Volume[=] Standard Condition Volume

Material Balance Material Balance （（ at standard conditionsat standard conditions ）） Production Production ＝＝ GIIP GIIP －－ Unproduced gasUnproduced gas （（ SCSC ））（（ SCSC ）（）（ SCSC ））

Where G/EWhere G/Eii = GIIP in reservoir volume or reservoir volume filled with gas = GIIP in reservoir volume or reservoir volume filled with gas ＝＝ HCPVHCPV

3/][37.35

ftSCFTz

pEwhere

SCFEsVG

)33.1(EE

Volumetric depletion reservoirs -- 2Volumetric depletion reservoirs -- 2

)34.1(1 i

37.35sinft

.:137.35

37.351 constTTnote

)35.1(1

factoreryreGas

depletionduringstageanyateryregasfractionaltheG

Gwhere p

In Eq.In Eq. （（ 1.331.33 ））

HCPVHCPV≠≠const.const. because: because: 1. the connate water in reservoir will expand1. the connate water in reservoir will expand

2. the grain pressure increases as gas2. the grain pressure increases as gas (or fluid) pressure declines(or fluid) pressure declines

?.constE

wherewhere

4.~3.)()(

ppGPdFPd

GPFPOP

)36.1(

EGdHCPVd

HCPVinreductionato

leadswaterofansionsignnegative

volumeporeinitialV

volumewaterconnateinitialV

dpVcdV

pore vol.

dpVcdV

FP=gas pressure

FP=gas pressureFP

initialiti

initialiinitialiti

initialitiinitiali

wciwcf

HCPVSPVV

HCPVPVV

dpVcdpVcHCPVdE

difference

Gwithcomputing

SandpsicpsicFor

1987.01

987.0013.011

2.01010;103

)33.1(

p/z plotp/z plot

From Eq. (1.35) such asFrom Eq. (1.35) such as

A straight line in p/z v.s Gp plot means that the reservoir is A straight line in p/z v.s Gp plot means that the reservoir is

a depletion type a depletion type

)35.1(1

In plotGpsv

Y=a+mx

Abandon pressure pab

Gp/G=RF 1.00

Water drive reservoirsWater drive reservoirs If the reduction in reservoir pressure leads to an expansion of If the reduction in reservoir pressure leads to an expansion of

adjacent aquifer water, and consequent influx into the reservoir, adjacent aquifer water, and consequent influx into the reservoir, the material balance equation must then be modified as:the material balance equation must then be modified as:

Production = GIIP Production = GIIP －－ Unproduced gasUnproduced gas （（ SCSC ）（）（ SCSC ）（）（ SCSC ）） GGp ＝＝ G G －（－（ HCPV-WHCPV-We ）） EE Or Or GGp ＝＝ GG －（－（ G/EG/Ei －－ WWe ）） EE where We= the cumulative amount of water influx resultingwhere We= the cumulative amount of water influx resulting from the pressure drop.from the pressure drop. Assumptions:Assumptions: No difference between surface and reservoir volumes ofNo difference between surface and reservoir volumes of water influxwater influx Neglect the effects of connate water expansion and pore Neglect the effects of connate water expansion and pore volume reduction.volume reduction. No water productionNo water production

Water drive reservoirsWater drive reservoirs With water productionWith water production

where where We*EWe*Ei //GG represents the fraction of the initial hydrocarbon represents the fraction of the initial hydrocarbon

pore volume flooded by water and is, pore volume flooded by water and is,

therefore, always less then unity.therefore, always less then unity.

GGG wpe

)41.1(1

Water drive reservoirsWater drive reservoirs

sincesince

)41.1(

in water flux reservoirs

Comparing

i 1 in depletion type reservoir

Water drive reservoirsWater drive reservoirs

In eq.(1.41) the following two parameters to be determinedIn eq.(1.41) the following two parameters to be determined

G and WG and We

History matching or “aquifer fitting” to find WHistory matching or “aquifer fitting” to find We

Aquifer model for an aquifer whose dimensions are of the same Aquifer model for an aquifer whose dimensions are of the same order of magnitude as the reservoir itself.order of magnitude as the reservoir itself.

Where W=the total volume of water and depends primary on theWhere W=the total volume of water and depends primary on the

geometry of the aquifer.geometry of the aquifer.

ΔP=the pressure drop at the original reservoir –aquifer boundaryΔP=the pressure drop at the original reservoir –aquifer boundary

)41.1(

Water drive reservoirsWater drive reservoirs The material balance in such a case would be as shown by plot A The material balance in such a case would be as shown by plot A

in fig1.11, which is not significantly different from the depletion in fig1.11, which is not significantly different from the depletion lineline

For case B & C in fig 1.11For case B & C in fig 1.11 （（ p.30p.30 ）） =>Chapter 9 =>Chapter 9

Bruns et. al methodBruns et. al method This method is to estimate GIIP in a water drive reservoir This method is to estimate GIIP in a water drive reservoir From Eq. (1.40) such asFrom Eq. (1.40) such as

EWGGor

)40.1(

1is plot as function of

Bruns et. al methodBruns et. al method

The result should be a straight line, provided the correct aquifer model has been The result should be a straight line, provided the correct aquifer model has been selected.selected.

The ultimate gas recovery depends both on The ultimate gas recovery depends both on (1) the nature of the aquifer ,and (1) the nature of the aquifer ,and (2) the abandonment pressure.(2) the abandonment pressure.

The principal parameters in gas reservoir engineering:The principal parameters in gas reservoir engineering: (1) the GIIP(1) the GIIP (2) the aquifer model(2) the aquifer model (3) abandonment pressure(3) abandonment pressure (4) the number of producing wells and their mechanical define(4) the number of producing wells and their mechanical define

1is plot as function of

Hydrocarbon phase behaviorHydrocarbon phase behavior

Residual saturation (flow ceases)Liquid H.C deposited in the reservoirRetrograde liquid Condensate

C--------- ＞ D-------------- ＞ E

Re-vaporization of the liquid condensate ?NO!Because H.C remaining in the reservoir increaseComposition of gas reservoir changed Phase envelope shift SE direction Thus, inhibiting re-vaporization.

E--------------- ＞ F

producing Wet gas (at scf)Dry gas

injection

displace the wet gas

Δp smallKeep p above dew pt.

until dry gas break through occurs in the producing wells

Condensate reservoir, pt. c,

Equivalent gas volumeEquivalent gas volume The material balance equation of eq(1.35) such asThe material balance equation of eq(1.35) such as

Assume that a volume of gas in the reservoir was Assume that a volume of gas in the reservoir was produced as gas at the surface.produced as gas at the surface.

If, due to surface separation, small amounts of liquid If, due to surface separation, small amounts of liquid hydrocarbon are produced, the cumulative liquid hydrocarbon are produced, the cumulative liquid volume must be converted into an equivalent gas volume must be converted into an equivalent gas volume and added to the cumulative gas production to volume and added to the cumulative gas production to give the correct value of Gp for use in the material give the correct value of Gp for use in the material balance equation.balance equation.

Equivalent gas volumeEquivalent gas volume If n lbIf n lbmm –mole of liquid have been produced, of molecular –mole of liquid have been produced, of molecular

weight M, then the total mass of liquid is weight M, then the total mass of liquid is

where γwhere γ00 = oil gravity (water =1) = oil gravity (water =1) ρρww ＝＝ density of water density of water （（ =62.43 lb=62.43 lbmm/ft/ft33 ））

volumeliquidnM wo

molelbmlbmM

Vn owo 00

volumegasEquivalent

bblsNM

bblsNwhereM

bblsVn

1033.1

52073.105.3505.350

][5.350

61458.54.62

Condensate ReservoirCondensate Reservoir

The dry gas material balance equations can also be The dry gas material balance equations can also be applied to gas condensate reservoir, if the single applied to gas condensate reservoir, if the single phase z-factor is replaced by the ,so-called ,two phase phase z-factor is replaced by the ,so-called ,two phase z-factor. This must be experimentally determined in z-factor. This must be experimentally determined in the laboratory by performing a constant volume the laboratory by performing a constant volume depletion experiment.depletion experiment.

Volume of gas Volume of gas ＝＝ G scf , as charge to a PVT cellG scf , as charge to a PVT cell PP ＝＝ PPii ＝＝ initial pressure initial pressure （（ above dew pointabove dew point ）） TT ＝＝ TTrr ＝＝ reservoir temperaturereservoir temperature

p decrease p decrease by withdraw gas in stages from the cell, and measure gas by withdraw gas in stages from the cell, and measure gas Gp’Gp’

Until the pressure has dropped below the dew pointUntil the pressure has dropped below the dew point

The latter experiment, for determining the single phase z-factor, implicitly The latter experiment, for determining the single phase z-factor, implicitly assumes that a volume of reservoir fluids, below dew point pressure, is assumes that a volume of reservoir fluids, below dew point pressure, is produced in its entirety to the surface.produced in its entirety to the surface.

)35.1('

)46.1('

In the constant volume depletion experiment, however, allowance is made In the constant volume depletion experiment, however, allowance is made for the fact that some of the fluid remains behind in the reservoir as liquid for the fact that some of the fluid remains behind in the reservoir as liquid condensate, this volume being also recorded as a function of pressure condensate, this volume being also recorded as a function of pressure during the experiment. As a result, if a gas condensate sample is analyzed during the experiment. As a result, if a gas condensate sample is analyzed using both experimental techniques, the two phase z-factor determined using both experimental techniques, the two phase z-factor determined during the constant volume depletion will be lower than the single phase z-during the constant volume depletion will be lower than the single phase z-factor.factor.

This is because the retrograde liquid condensate is not included in the This is because the retrograde liquid condensate is not included in the cumulative gas production Gp’ in equationcumulative gas production Gp’ in equation （（ 1.461.46 ）） , which is therefore , which is therefore lower than it would be assuming that all fluids are produced to the surface, lower than it would be assuming that all fluids are produced to the surface, as in the single phase experiment.as in the single phase experiment.

油層工程油層工程蘊藏量評估蘊藏量評估

體積法體積法物質平衡法物質平衡法衰減曲線衰減曲線油層模擬油層模擬

壓力分析壓力分析 (( 隨深度變化，或壓力梯度隨深度變化，或壓力梯度 ), ), 例如例如 , , 求氣水界面。求氣水界面。物質平衡法物質平衡法

井壓測試分析井壓測試分析 (( 暫態暫態 )) （求（求 kk 、、 ss 、、 rere 、、 xfxf 、氣水界面、地層異質性）、氣水界面、地層異質性） Pressure buildupPressure buildup Pressure drawdownPressure drawdown

水驅計算水驅計算 (water drive)(water drive)

chapter 2 chapter 2 volumetric gas reservoir engineering volumetric gas reservoir engineering 1

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