4.02

PVT Experiments

Introduction P-V-T (Pressure-Volume-Temperature) analyses are performed in a specialized high-pressure laboratory and require well-trained laboratory technologists. Routine PVT analyses generally consist of:

• Constant composition expansion (CCE) • Differential liberation (DL) • Constant volume depletion (CVD) • Multi-stage separation

Each of these tests is described below in detail for both oil and retrograde gas. Constant composition expansion (CCE) Oil The pressure-volume relationship (or constant composition expansion) is studied at reservoir temperature, or at any other specified temperature, starting from a pressure above the initial static reservoir pressure down to a relative volume (Vr) at least equal to two:

r

rr T&intpobubble@Volume

T&pgiven@volumeTotalV = .

HP Pumps

Magnetic Mixer

Side View

Solv

ent

oil

Cathetometer

HP Pumps

Magnetic Mixer

Side View

Solv

ent

oil

Cathetometer

Figure 5.02. 1Schematic of DBR Phase Behavior System

Chapter 4.02 Reservoir Fluid Phase Behavior page 1

PVT Experiments

The reservoir fluid sample is first homogenized in the cylinder by agitation at reservoir temperature and pressure. Then a small portion of the fluid (30 ~ 50 cc) is transferred into the Oilphase-DBR PVT cell at the agitation conditions. Once the fluid has reached thermal equilibrium in the cell at the test temperature, the fluid total volume, liquid/vapor volume are measured at each step. The test is completed until the total volume reaches the cell maximum capacity (~100 cc). The fluid’s bubble point is defined as the pressure at which the first gas bubble evolves from the liquid. The release of the vapor phase usually makes the p-V curve bent upward due to higher compressibility of the vapor phase. A typical CCE data table is presented below:

Constant Composition Expansion for an Oil at 47°C

Pressure Relative Vol Liquid Vol% Liquid Vol% Liquid 106Co Y Function (psia) (Vr=V/Vs) (Vl/Vt) (Vl/Vb) Density (g/cm3) psia-1

6015 0.970 100 0.796 6.09 5515 0.972 100 0.793 6.37 5015 0.976 100 0.791 6.67 4515 0.979 100 0.788 6.97 4015 0.982 100 0.785 7.30 3515 0.986 100 0.783 7.64 3015 0.990 100 0.779 7.98 2530 0.994 100 0.776 8.33 2415 0.995 100 0.775 8.42 2215 0.997 100 0.774 8.57 2015 0.998 100 0.773 8.73

pi 1900 0.999 100 0.772 8.82 pb 1825 1.000 100 100 0.772 8.87

1744 1.012 98.53 99.71 3.88 1636 1.030 96.30 99.23 3.80 1455 1.070 91.98 98.41 3.64 1297 1.116 87.53 97.66 3.52 1174 1.162 83.57 97.12 3.42 1016 1.246 77.26 96.30 3.23 816 1.403 67.97 95.35 3.07 633 1.652 57.06 94.26 2.89

The isothermal compressibility is defined as follows:

pV

V1

C r

ro ∂

∂−=

The Y function value is defined as:


PVT Experiments

)1

VV

(p

ppY

b

b

−

−= .

The Y function, plotted as a function of pressure, is expected to be a straight line for most of the oil fluids and it is used to fine tune the bubble point pressure or to smooth the volume data in two phase region. The following plot shows relative volume as a function of pressure. It can be seen that the fluid volume increases a lot more substantially at pressures below the bubble point than above the bubble point pressure.

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0 1000 2000 3000 4000 5000 6000 7000

Pressure (psia)

Rel

ativ

e Vo

lum

e (V

t/Vt @

Pb)

Retrograde Gas The same PVT cell is used for a retrograde gas CCE test at reservoir temperature or any other specified temperature. The testing procedure is similar in terms of measuring fluid total volume, liquid/vapor volume in two-phase region. But a retrograde gas exhibits a dew point pressure, which is determined by observation of dews or foggy cloud. The dew point pressure is defined as the pressure at which the first liquid drop is formed. The pressure-volume relationship for a retrograde gas typically does not have a break point. In other words, it is a smooth curve for most of the retrograde gas samples. Condensate or liquid volumes in the cell are reported as a percentage of total fluid volume at the current pressure or as a percentage of total fluid volume at the dew point pressure pd. A graph of the retrograde liquid deposit versus declining reservoir pressure is also provided.


PVT Experiments

The low liquid volume is measured by use of a cone-shaped piston. The liquid occupies the annular space between the glass tube and the piston with an increased liquid column height for better accuracy. The gas deviation factor (z) is calculated by:

RT

pMWz

ρ= ,

where: p = pressure MW = molecular weight (mass) ρ = density R = universal gas constant T = temperature The following table presents an example of CCE test data table for a lean retrograde gas.

Constant Composition Expansion for a Retrograde Gas at 70°C

Pressure % Liquid % Liquid Rel. Vol. Bulk 106Co Z-Factor (psia) (Vl/Vt) (Vl/Vd) (Vt/Vd) Density (g/cc) (1/psia)

(g/cm3)

10015 0 0.792 0.305 29 1.435 9515 0 0.804 0.300 33 1.385 9015 0 0.823 0.293 38 1.343 8515 0 0.837 0.289 44 1.290 8015 0 0.857 0.282 51 1.244 7515 0 0.881 0.274 59 1.198 7015 0 0.908 0.266 67 1.153 6515 0 0.941 0.257 76 1.109 5800 0 0 1.000 0.241 91 1.050 5515 Nil Nil 1.028 0.235 5013 0.06 0.07 1.086 0.222 4730 0.13 0.14 1.125 0.215 4352 0.21 0.25 1.187 0.203 3965 0.29 0.37 1.268 0.190 3454 0.43 0.60 1.412 0.171 2953 0.54 0.88 1.620 0.149 2447 0.57 1.10 1.939 0.125 1942 0.48 1.18 2.459 0.098 1710 0.42 1.17 2.778 0.087

It can be seen from the above table that the dew point was observed at 5800 psia but the first measurable liquid was not available until 5000 psia. This is usually shown on the retrograde liquid deposit (RLD) curve as a tail. The leaner a retrograde gas, the longer the tail.


PVT Experiments

ifferential Liberation (DL)

ifferential liberation is a test considered better describing the separation process, taking place in

Figure 5.02. 2 - Schematic of Diffe

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 60

Pressure (psia)

Liqu

id (%

)

Vl/Vt

Vl/Vd

00

D Dthe reservoir and is also considered to simulate the flowing behavior of hydrocarbon systems above the critical gas saturation. During this test the reservoir fluid is depleted by 6 to 10 steps from saturation pressure to abandonment pressure and the solution gas liberated at each step is continuously removed from contact with its equilibrium oil.

V<V

b 2 V

<Vb

Oil Oil Oil

Gas

3=V 2

b

V=V

1

p <p 2 b p2<pbp =p 1 b

a

Chapter 4.02 Reservoir Fluid Phase B

Gas off
Gas
rential Liberation Test

Oil

Oil

Gas

V<V

b4=

V 3=V 2

V<V

b 5<

V 4

p2<pb p3<p2<pb

ehavior page 5

PVT Experiments

When the pressure of the cell is reduced to a value below pb, the gas cap that is formed is pushed

he liberated gas is analyzed by gas chromatography for molecular composition. The gas gravity

he following parameters are measured or calculated from other sets of data:

Differential Liberation Test – Liquid Properties

out from the cell to a gasometer. The remaining liquid is subsequently depleted down to the next pressure step. Tand viscosity are calculated based on the measured composition. T

Pressure Oil Volume Solution Calculated

Factor Gas GOR Liquid

(Bo) Rs Density

bl/stb f/bb (g/cm ) 3

13138 1.252 682 0.798

12206 1.257 682 0.795

11275 1.261 682 0.792

10344 1.266 682 0.789

9413 1.272 682 0.786

8481 1.278 682 0.782

7550 1.284 682 0.778

6619 1.292 682 0.774

5688 1.300 682 0.769

4756 1.310 682 0.763

1.311 682 0.762

3335 1.227 471 0.790

2688 1.189 375 0.803

2041 1.153 285 0.818

1394 1.118 199 0.832

747 1.084 117 0.848

100 1.043 26 0.865

15 1.024 0 0.872

(psia) (b ) (sc l)

pi 15000 1.245 682 0.803

pb 4629


PVT Experiments

Differential Liberation – Vapor Properties

Pressure Gas FVF Z Calculated Vapor Total FVF

Bg Factor Gas Gravity Bt

Viscosity

(psia) (bbl/mm ft3) (cP) (air=1) (bbl/stb)

pb 4629 1.311

3335 895 0.910 0.021 0.652 1.416

2688 1091 0.894 0.019 0.637 1.528

2041 1433 0.892 0.017 0.627 1.729

1394 2129 0.905 0.015 0.623 2.144

747 4102 0.934 0.014 0.636 3.335

100 32114 0.979 0.012 0.827 20.779

15 218967 0.981 0.009 1.588 141.051

Oil Formation Volume Factor:

conditionsstdatoilresidualofVolume

T&pgivenatoilofVolumeB r

o =

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

0 2000 4000 6000 8000 10000 12000 14000 16000

Oil V

olum

e Fa

ctor

Bo

(bbl

/stb

)


PVT Experiments

Solution Gas Oil Ratio:

conditions.stdatoilresidualofVolumeconditions.stdatgasliberatedofVolume

Rs =

0

100

200

300

400

500

600

700

800

0 2000 4000 6000 8000 10000 12000 14000 16000

Solu

tion

Gas O

il Rat

io (s

cf/s

tb)

Liquid Density: Liquid density is computed at a given pressure step, using the mass of gas liberated at that pressure and the mass of oil at the previous step and the volume ratios measured during the DL experiment.

0.74

0.76

0.78

0.8

0.82

0.84

0.86

0.88

0 2000 4000 6000 8000 10000 12000 14000 16000

Pressure (psia)

Liqui

d De

nsity

(g/c

m3)


PVT Experiments

Gas Deviation Factor:

RT

MWpor

VpTz

TpV

Z.std.std

.std.std

ρ==

0.88

0.89

0.9

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

0 500 1000 1500 2000 2500 3000 3500 4000

Gas

Devia

tion

Fact

or (Z

)

Gas Gravity:

)96.28(AirofMW

MWg =γ

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0 500 1000 1500 2000 2500 3000 3500 4000

Gas

Grav

ity (a

ir=1)


PVT Experiments

Gas Viscosity: Gas viscosity is usually calculated by correlations or corresponding states theory.

0

0.005

0.01

0.015

0.02

0.025

0 500 1000 1500 2000 2500 3000 3500 4000

Pressure (psia)

Gas V

iscos

ity (c

P)

Constant Volume Depletion (CVD) Constant volume depletion is a process by which the reservoir fluid is depleted in a PVT cell by usually 8-10 steps from saturation pressure down to a selected abandonment pressure. Abandonment pressure, which is defined as the lowest pressure below which the reservoir is no longer economical, is typically chosen to be in the range of 500 to 1000 psia. The fluid sample, once charged into the PVT cell, is first measured for volume at saturation pressure. For each step, the cell volume is expanded until the pressure is reduced to the specified stage level. After equilibrium is established at this pressure, enough gas is produced from the top of the cell so that the total fluid volume occupies the same volume that it was occupied at the initial step at the saturation pressure. When the gas is produced from the top of the cell, it is analyzed for composition and measured for volume. These data enable the calculation of recovery as a function of pressure. In addition, the retrograde liquid deposit in the cell is measured. It is considered that this process simulates closely the actual depletion that takes place in a gas condensate reservoir, where only gas is produced, the condensate deposits stays in the pores of the formation, the total hydrocarbon pore volume remains constant regardless of pressure change and there is no water influx into the reservoir. The following schematic shows a typical series steps in a constant volume depletion study:


PVT Experiments

Gas

V 2>V d

V 1=V d

Gas Gas

OilOil

p1=pd p2<pd p2<pd

Figure 5.02. 3 - Schematic of a Constant V

Constant Volume Dep

Pressure Total Produced Vapor Properties C

Recovery Z Factor MW Density V

psia (%) (g/gmol) (g/cm3)

5975 0.00 1.067 27.39 0.345

5200 6.34 0.991 27.21 0.321

4500 13.66 0.926 26.72 0.292

3800 23.11 0.878 25.29 0.246

3200 32.96 0.854 24.17 0.204

2700 42.28 0.845 23.47 0.169

2100 54.25 0.848 22.89 0.127

1500 66.94 0.869 22.57 0.088

1000 77.63 0.899 22.55 0.056

Chapter 4.02 Reservoir Fluid Phase Beh

Gas off

Oil

Gas

Oil

Gas

V 3 =V

d

V 4>V d

p2<pd p3<p2<pd

olume Depletion Study

letion

alculated Liquid Properties Two Phase

olume % Density Z Factor

(g/cm3)

0.00 1.067

0.40 0.784 0.992

2.03 0.615 0.931

6.48 0.590 0.882

8.69 0.603 0.853

9.38 0.619 0.837

9.37 0.651 0.826

8.90 0.681 0.827

8.29 0.704 0.831

avior page 11

PVT Experiments

Cumulative Recovery:

%100xCellinFluidofMolesInitialTotalGasoducedProfMolesCumulative

Qp =

0

10

20

30

40

50

60

70

80

90

0 1000 2000 3000 4000 5000 6000 7000

Pressure (psia)

Tota

l Rec

over

y (%

)

Retrograde Liquid Deposit:

%100XessurePrintPoDewatFluidofVolume

T&patLiquidofVolume%Vorrld L =

0

1

2

3

4

5

6

7

8

9

10

0 1000 2000 3000 4000 5000 6000 7000

Pressure (psia)

Retro

grad

e Li

quid

Dep

osit

(%)


PVT Experiments

Gas Deviate Factor (Z):

0.8

0.85

0.9

0.95

1

1.05

1.1

0 1000 2000 3000 4000 5000 6000 7000

Pressure (psia)

Gas Z

Fac

tor

Separation Test (ST) A separation test is to simulate the separation process of the reservoir fluid when it is produced to the surface. The data obtained from this test help design the optimal separator conditions for maximal amount of stock tank oil recovery. With the Oilphase-DBR PVT cell, the procedure for performing a separation test is similar to that for a differential liberation test except that the temperature in a ST may vary from stage to stage. A sample of reservoir fluid is charged into the cell and measured for volume at reservoir temperature and saturation pressure. Then the cell pressure and temperature are set to the first specified stage conditions, at which the sample is equilibrated. After the vapor and liquid volume is quantified, the vapor is completely pushed out from the cell for compositional and volumetric measurement. The remaining liquid in the cell is then flashed to the next specified separator conditions and repeat the measurements and analyses, until the last stage typically at atmospheric pressure. The following table presents an example multistage separation test data set. The definition for GOR, oil formation volume factor is the same as that in a DL test. The shrinkage factor is defined as:

StageCurrentatVolumeOilStageeviousPratVolumeOil

Shr = .


PVT Experiments


Four Stage Separation Test

STAGE #1 STAGE #2 STAGE #3 STAGE #4

Pressure-psia 500 250 100 15

Temperature-°F 150 110 90 70

Vap. Den (g/cm3) 0.023 0.012 0.005 0.001

Liq. Den (g/cm3) 0.859 0.872 0.880 0.887

Vap. MW 18.18 18.21 19.46 28.09

Vap. Gravitya 0.627 0.628 0.672 0.969

Vap Heat Val.b 1106 1114 1185 1648

GORc 581 26 20 17

GORd 554 25 19 17 Sep. FVFe 1.049 1.029 1.016 1.004

a) Calculated, at 60°F (air=1) b) Calculated, Dry basis BTU/scf c) scf gas/bbl of oil at STD conditions d) scf gas/bbl of oil at separator conditions e) Liquid volume at sep conditions/liquid volume at STD conditions Residual oil density at standard conditions 0.891 g/cm3 Sep gas gravity (average) Sg = ΣRpjγj/ΣRpj 0.638 Where: Rp: GOR (scf gas/bbl of oil at STD conditions), j: separator stages Sep gas gross heating value (average) Lc =ΣRpj*Lcj/ΣRpj 1124 BTU/scf (dry basis) Where: Rp: GOR (scf gas/bbl of oil at STD conditions), j: separator stages

Shrinkage FVF From Psat & Tres to first Sep conditions 0.814 1.229 From first to second Sep conditions 0.981 1.019 From second to third Sep conditions 0.988 1.012 From third to fourth Sep conditions 0.988 1.012 From fourth to fifth Sep conditions From fifth to sixth Sep conditions

SEPARATOR SUMMARY

aTotal Separator GOR 644 SeparatorSTO Gravity 27.4 bSeparator FVF 1.288

a) scf gas/bbl of STO at STD conditions b) Liquid volume at pb & Ti/STO volume at STD

Documents

4.02