PVT (Laboratory Pvt Measurements)

Petroleum Fluid Properties

Copyright of Core Laboratories (1999 Rev 001) Aberdeen Advanced Technology Centre

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CHAPTER 4.

LABORATORY PVT MEASUREMENTS

4-1 INTRODUCTION

In the previous chapters we have discussed the compositions, phase behaviour andphysical properties of petroleum fluids as well as setting about defining the variousparameters of interest to Engineers. In this chapter the experiments through which theseparameters are determined in the laboratory will be described.

It is assumed that a valid fluid sample is available for the tests, either through i) obtaininga single phase sample in the field, or ii) through recombination of separator gas and liquidin the laboratory. Validation of field samples and recombination are integral parts ofReporting PVT data and are treated in the chapter dealing with PVT Reports, together withQuality Checks applied to the laboratory measurements.

The majority of PVT measurements are derived from a relatively straightforward suite ofexperiments depending upon fluid type. Several of the tests are applied to all types offluid, employing the same principles in different equipment. However, a typicalarrangement of apparatus for volumetric work is shown schematically in Figure 1. Thecell shown in the figure is of the 'long-windowed' type suitable for relative phase volumemeasurements (e.g. in volatile oil studies), but it is common to use other specialiseddesigns such as 'inclined-window' cells and 'blind' cells, (i.e. no windows) for black oils andother studies. Nevertheless the principles of operation remain the same for allconfigurations and specific details and/or limitations will be highlighted as the individualmeasurements are discussed.

4-2 CONSTANT MASS EXPANSION (CME) EXPERIMENT

The Constant Mass or 'Constant Composition Expansion' (CCE) experiment is carried out invirtually all PVT studies irrespective of fluid type. It is used to measure the total fluidvolume and compressibility over a wide range of pressures extending beyond initialreservoir pressure to pressures below the anticipated separator pressures. For black oilsand many volatile oils it is also used to determine the saturation pressure at reservoirconditions. A schematic representation of a CME experiment is shown in Figure 2.

For both oils and gases the (recombined) fluid sample is transferred into the PVT cell andbrought to reservoir temperature and a pressure above the reported initial reservoirpressure. The total volume of the fluid is then measured upon pressure reduction. Thevarious types of Relative Volume (or PV) curves obtained from this experiment are shownin Figure 3. For a black oil or volatile oil, the transition from single to a two-phasesystem is apparent in the plot. For gas condensates the separation of the liquid phase isnot readily apparent from the experimental volumetric data and it must be determinedvisually (see below).



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MERCURYPUMP

THERMOSTAT

CELL

Figure 1: Schematic Representation of PVT Apparatus for Volumetric Measurement

The number of pressure reduction steps and the total pressure range covered is generallydetermined by consultation between the Engineer who commissions the study and thelaboratory personnel. The CME experiment may also be carried out at other temperaturesin addition to the reservoir temperature to help model the producing conditions forsubsequent process calculations.For oil samples, the differential volume expansion below the bubble point pressure issignificantly higher than that above the bubble point pressure. This allows the bubblepoint to be determined from a graphical plot of the total fluid volume as a function ofpressure. The point at which the slope of this point changes is the bubble point of the fluidsample. The change in differential volume expansion is due to the presence of the vapourphase, which has a much greater compressibility than the corresponding liquid phase. For oil samples - where the relative volumes of oil and gas are not usually determined -the CME experiment is often carried out in a "blind" cell to improve the volumetricaccuracy of the experiment.

As mentioned above, for gas condensate fluids the plot of total fluid volume as a functionof pressure does not have a "sharp" change of slope at the saturation pressure unless thesystem has appreciable liquid dropout when crossing the dewpoint pressure. Thisbehaviour can be observed in simple (synthetic) mixtures with 2 to 5 components, but israrely discernible in real systems.



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Hg

Oil

Gas

34P < P

Hg

Oil

Gas

23P < P

Hg

Oil

Gas

12P < P

Hg

Oil

Gas

sa t1P < P

Hg

ReservoirFluid

satP = P

Hg

ReservoirFluid

P > Psat

Figure 2: Schematic Representation of Constant Mass Expansion Experiment

Pre

ssur

e

Dry Gas

Volume

Pre

ssur

e

Gas Condensate

P (= P )sat d

Volume

Pre

ssur

e

Volatile Oil

P (= P )sat b

Volume

Pre

ssur

e

Black Oil

P (= P )sat b

Volume

Figure 3: Variation in p,V Behaviour for Various Fluids

It is recommended for the measurement of dewpoint pressures that the system be studiedin either a 'long-windowed' (Figure 1) or a 'Sloane-type' Gas Condensate cell where visualobservation of the retrograde liquid volume may be made. The dewpoint is thendetermined by plotting the volume of retrograde liquid Vliq as a function of pressure. Extrapolation as Vliq -> 0 yields the dewpoint pressure for the system.

In the single phase region the relative volume curves are also used to calculate: i) the



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variation of density with pressure, ii) the isothermal compressibility and iii) the thermalexpansivity if the volume of sample is recorded at more than one temperature.

4-3 DEPLETION EXPERIMENTS

4-3-1 DIFFERENTIAL LIBERATION (DLB)

The differential liberation experiment is the classical depletion experiment for reservoiroils. The experiment is carried out at reservoir temperature to simulate the volumetric andcompositional changes in the reservoir during production (depletion).

Initially an 'inclined-window' PVT cell is charged with the reservoir oil sample and the celltaken to reservoir temperature. A series of stepwise pressure reductions is carried outwith the system left after each pressure reduction to separate and for equilibrium to beestablished.

Repeatn

Times

Hg

Reser voirFluid

P > Psat

Hg

Oil

P = Psa t

Hg

Oil

Gas

P < P1 sat

V(Gas, P, 60 F)1

o o

Hg

Oil

1P

Hg

Oil

2P

Gas

2o oV (Gas, P, 60 F)

Hg

Oil

2P

Hg

Oil

or es(P, T )

Hg

Oil

o o(P, 60 F)

Figure 4: Schematic Representation of Differential Separation Experiment

Once equilibrium has been reached, the liberated gas phase is removed from the cell andanalysed (this may be via a simple separator system) and the volume of the liquid phaseand gas phase are determined. The pressure of the liquid phase remaining in the cell isthen reduced again and the procedure repeated until the final pressure (usuallyatmospheric) is reached. The process is shown schematically in Figure 4.

Use of an inclined window allows the outlet of the top valve in the PVT cell to be clearlyseen. This allows the operator to control visually the displacement of the gas phase rightto the oil meniscus. The oil remaining in the cell at Tres, 0 psig is termed 'Residual Oil'. When the Residual Oil has cooled to ambient temperature, its properties (volume at 60 oF,density, composition) are also measured. Note that the volume of Residual Oil necessarilyshrinks due to thermal contraction while cooling. This 'shrinkage' is measured byrecording the volume of oil at both Tres and 60 oF and is an important part of completingthe material balance checks described in the following Chapter. The gas and oilvolumetrics for a DLB experiment are shown in Figure 5.



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Vg1Vg2

Vg3

Vo3 Vo2 Vo1

PsatP1P2P3

V (T )ro res

V (60 F)roo

SampleVolume

Pressure

V @(T , P )

ob

res b

Figure 5: Gas and Oil Volumetrics During Differential Liberation

The quantities determined from DLB experiments are the relative oil and gas volumes, theoil and gas density at stage pressures, the Solution GOR (Rs), and the gas deviation factor(Z) as a function of pressure, and the gas expansion factor.

In modelling reservoir depletion, the DLB experiment makes the assumption that all gasliberated in the reservoir is produced and leaves behind the remaining oil to liberate freshgas. This is not always a valid assumption, particularly for volatile oils, and a ConstantVolume Depletion (CVD) experiment is usually recommended.

4-3-2 CONSTANT VOLUME DEPLETION (CVD) EXPERIMENT

The constant volume depletion (CVD) experiment is generally carried out for volatile oiland gas condensate reservoir fluids. Again, this experiment is a laboratory model ofreservoir fluid depletion during production. It differs from the DLB experiment in that notall the equilibrium gas is removed at each pressure stage. In this respect it models areservoir formation of Constant Volume which is producing rich gas, but retaining theequilibrium liquid phase. This experiment is carried out for volatile oils because of therelatively large volumes of gas liberated under reservoir conditions when the reservoirpressure falls below the bubble point pressure and the high shrinkage of the liquid phasewith pressure reduction. The CVD experiment is shown in Figure 6.



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Condensate

Gas Cond.

Hg

Res.Fl uid

P > Pdew

Res.Fluid

Hg

P = Pres

Hg

P < P2 dew

Hg

P 3

Hg

Po

Repeatn

TimesHg

Res.Fl uid

P = P1 dew

dewV = V

} WellstreamGas Cond.

Hg

P 2

dewV = V

Gas Cond.

Hg

P 3

dewV = V

Figure 6: Schematic Representation of a Constant Volume Depletion

Again, the experiment starts with a single-phase reservoir fluid sample of known volumeat reservoir temperature and pressure. As before, the pressure is reduced stepwiseresulting in an expanded volume for the fluid at each stage. After equilibration, sample isdisplaced from the top of the cell to return the fluid volume to the original value. At somepoint during pressure reduction, the fluid passes through the saturation pressure - eithera dewpoint (Pd - gas condensate) or a bubble point (Pb - volatile oil). It is, therefore, part ofthe upper (gas) phase which is displaced from the cell to re-establish constant volume.

The fluid removed at each stage is flashed to near standard conditions and analysed todetermine the composition and C7+ molecular weight. Generally, between 5 and 10pressure reduction steps are completed down to atmospheric. After the final stage, theliquid remaining in the cell is analysed.

The data that is determined from CVD experiments are the volume of gas and liquid atpressure and temperature, the cumulative amount of produced (wet) gas, the gas Z-factorand the composition of the produced wellstream. All of the reported volumetric data isreported relative to the volume of fluid at the dew point, for gas condensate or bubblepoint, for volatile oils. The determination of the correct saturation pressure (and samplevolume) is of paramount importance with small changes in pressure leading to very largeshifts in relative volumes. This can be misleading if the high compressibility of such fluidsis not taken into account in reviewing the volumetrics.

Finally, it should be noted that the compositional changes determined during a laboratoryCVD experiment may be quite different to those for a reservoir under water drive.

4-4 SEPARATOR TESTS

Separator tests (sometimes termed multistage separations) on reservoir fluids are carriedout to simulate potential Production Separator Stages and provide volumetrics and otherinformation on the Stock Tank Oil and liberated gas streams. These days, compositionalinformation obtained from these tests is often used to tune computer models (essentiallyEquations-of-State) to allow determination of optimum separator conditions for productionof the reservoir.

Separator tests are carried out along similar lines to the depletion tests described above,



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except that the temperature is also reduced at each stage and there are only a fewpressure steps (often just one between Pres and Patm). The procedure is set out below.

A known amount of single phase reservoir fluid is charged to a pressure vessel and thepressure and temperature are adjusted to the conditions of the first stage separator. Thesystem is then brought to equilibrium and the liberated gas removed from the cell. Again,the amount of gas is measured (at both separator and standard conditions) as well as itscomposition. The pressure and temperature of the liquid remaining in the pressurevessel is then changed to those of the second stage separator and the above processrepeated. Once the cell is reduced to stock tank conditions, the amount, gravity andcomposition of the remaining liquid (stock tank liquid) can be determined. Any number ofstages can be simulated in this manner in the laboratory. Often several separator testsare requested by engineers to simulate several potential production situations. Themethodology is shown in schematically in Figure 7.

Separator tests are rarely performed on gas condensate systems because there is often thepractical difficulty of obtaining enough reservoir fluid to produce enough sufficient liquidsfor the experiments.

V(Gas, P, 60 F)o o

1 V(Gas, P, 60 F)o o

2

(P, 60 F)o o

Hg

Oil

(P, T )2 2(P, T )1 1

Hg

Oil

Hg

Oil

Gas

Hg

Oil

Hg

Oil

Gas

Figure 7: Schematic of a Separator Test Series

4-5 VISCOSITY MEASUREMENTS

In the petroleum industry there are various techniques available to physically determinethe reservoir fluid viscosity. CoreLab currently uses two methods 1) Rolling BallViscometer (RBV) and 2) Falling Sinker Viscometer (FSV). The RBV is widely used for oilsand liquid phases but is inappropriate and too inaccurate for gases. The FSV has beendeveloped by CoreLab to replace the RB technique and can be used for all types ofreservoir fluids from dry gas systems to heavy bio-degraded oils. The techniques of eachmethod are briefly discussed below.

4-5-1 THE ROLLING BALL VISCOMETER (RBV)

The RBV has become something of an industry standard method, although it has a



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number of severe limitations in practice. The technique is only suitable for use withliquids and it suffers from a lack of fully developed theory. In this context, it is necessaryto calibrate the instrument with known viscosity standards similar to the fluids to bemeasured. Calibration aside, however, the principle of operation is straightforward:

The RBV consists of a highly polished stainless steel barrel (typically 1/4" (6 mm)diameter) which can be closed at the top by means of a plunger. A steel ball rolls withinthe barrel, its diameter necessarily being slightly smaller than the bore, but dependantupon the viscosity range to be measured. During measurements, the barrel must be filledcompletely with the fluid to be studied. The barrel is inclined at a known angle and theball rolls along it under gravity for a measured distance. The roll time is determined by asuitable timing mechanism. When operating correctly, the roll time interval is a measureof the viscosity. The instrument is shown schematically in Figure 8.

If the clearance between the bore of the barrel and the ball diameter is too small, then theflow of fluid past the ball will be turbulent. Under these conditions the RBV does notmeasure viscosity correctly since the theory (such as it is) assumes laminar flow. Tocontrol the rolling time, measurements can be made at different angles or the stainlesssteel ball and/or the barrel may be replaced with a different diameter. The workingequation is as follows:

oil = (( t x (?ball - ?oil) - a )) x b

Where: t = rolling time in seconds,(?ball - ?oil) = difference in density between ball and oil,a & b = constants of the system which are determined from

calibration with oils of known viscosity.



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l

Detectors

qFigure 8: Principle of rolling ball Viscometer.

Oil viscosities are usually measured at reservoir temperature over a range of pressuresboth above and below the saturation pressure extending down to near atmosphericpressure. Measurements below saturation pressure are made under differentialconditions, ie: matched as closely as possible to the stage pressures used for thedifferential liberation. RBVs are constructed in such a way as to allow a pseudo-differential liberation of gas to be conducted within them, leaving the oil to fill themeasuring chamber. In this way, the viscosity of the oil in the reservoir can be measuredas gas is depleted from it. The change in viscosity with release of gas is usually very large.

Unfortunately, the RBV is susceptible to a number of errors in addition to the limitationsoutlined above. In particular, the ball can bounce and/or slide during its roll timeresulting in scattered and poorly reproducible results. Great care is needed by thelaboratory in order to produce self-consistent, smooth data.



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4-5-2 FALLING SINKER VISCOMETER

The theory of the FSV has a similar principle to the RBV in that it measures the time for asolid body to move through a fluid, but the FSV is based on sound physical principles.Therefore the equations that govern the motion of the body are derived from thefundamental laws of fluid dynamics.

The principle of the FSV is that a self-centring bullet shaped sinker falls under gravityinside a precision bore tube. The sinker initially free falls, eventually accelerating to aterminal velocity and the fall time between two detection coils at this terminal velocity isdetermined. An inductive core in the sinker induces the timing signals. The instrument isshown schematically in Figure 9 and Figure 10. The equation of motion at terminalvelocity yields the viscosity as follows :-

oil = t x (rsinker - rfluid) / A x rsinker

Where:t = fall time in seconds,(rsinker - roil) = difference in density between sinker and fluid,A = constant of the system determined from calibration.

In theory the constant A can be determined from first principles by direct measurement ofthe tube and sinker dimensions. Such a calculation would then only require the fall timeand the buoyancy correction for the sinker. However in practice it is found that accuratemeasurement of the dimensions is very difficult and an error of +/- 1/1000 mm inmeasuring the radius of the tube or sinker will produce an error in A of over 5%.

Therefore the FSV is also calibrated using known viscosity standards to determineconstant A. This value is then checked against the theoretical value for consistency and isnormally found to be within 2-3 %. Unlike the RBV where the barrels interchange forvarious viscosity ranges, here the sinkers, of varying diameters are interchanged. Itshould be noted that as with the RBV, the viscosity measurements are preferably carriedout where calibration data exists for that range. However, as long as measurements are ofthe same order of magnitude or higher than the calibration data the viscosity data shouldbe accurate.



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MountingFrame

End Plug

Shr inkageChamber

IsolationValve

PressureVessel

Homogeniser

End Plug

CleaningPlug

RotatingCradle

SampleValve

ViscometerAssembly

ElectricalLeadthrough

Sinker Plug

Figure 9 : Schematic of Falling Sinker Viscometer

DETECTOR COILS

TITANIUM SINKER

INDUCTIVE CORE

PRECISION BORETITANIUM TUBE

RESERVOIR FLUID

D

L

d

l

In freefall, at terminal velocity,Equations of Motion yield:

Figure 10: Principles of Falling Sinker Viscometer



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4-5-3 CAPILLARY TUBE VISCOMETER

The Capillary Tube Viscometer is employed in many laboratories to determine theviscosities of effluent fluids from core floods and slim tube studies as well as throughdirect application in PVT experiments by connection to a cell (Figure 11). Measurement ofthe viscosity is determined by measuring the pressure drop ? P across a capillary atconstant fluid flowrate by applying Poiseuille's equation:

DP = 32. .v.l

d 2m

where l and d are length and internal diameters of the capillary respectively and v is theaverage fluid velocity. Fluid velocity is normally derived from a volumetric flowrate on ahigh pressure pump in which case the equation becomes:

DP = 128.Q.l.

.d 4m

p

where Q is the volumetric flowrate. It is immediately apparent that the diameter of thecapillary is critical to the measurement and high pressure capillary viscometers areusually calibrated to avoid severe measurement errors in d. Nevertheless, long capillariesrarely if ever have a uniform internal diameter and the differential pressure ? P is easilydominated by 'end effects' due to burring in cutting the tubing. Capillary viscometry is amore difficult laboratory experiment than it at first appears and results from it should bereviewed carefully where important calculations (eg: mobility ratio) are involved.

CONSTANTFLOWRATE

PUMP

PRESSU REGAUGE

THERMOSTAT

CELL

BPR

DPT

CAP.COIL

FS

Figure 11: Operation of a Capillary Tube Viscometer

4-6 SPECIAL MEASUREMENT REQUIREMENTS

4-6-1 PVT CELL DESIGN

Gas condensate reservoir fluids generally require special attention. For these fluids we aretrying to measure small liquid phase volumes in equilibrium with a very large volume ofvapour phase. However, these small volumes of liquid are important because they mayimpact substantially on the economics of a reservoir development.

A schematic of a PVT cell optimised for gas condensate fluids is shown in Figure 12 and



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Figure 13. The hour glass shape maximises the height of condensate liquid in the"capillary" section of the cell. This cell design is sometimes referred to as a 'Sloane' cell. These cells are typically vary in internal volume from 1 to 4 litres, which will provide agood surface area to volume ratio and thus reduce any surface hold-up effect, as willgood design.

SAMPLE

MERCURY

B

A

Mercury

V1

P0

P2

P6

P3 P4

C1

C2

C3

C4

VideoCamera

P 7 P8

Computer-controlledMercury Pumps

C5

StirringMotor

LightSource

G as

Condensate

ACB Cell

Figure 12: High pressure Gas Condensate Cell

4-6-2 EQUILIBRATION TIMES

In 1988 Eyton presented a paper talking about the practical limitations of obtaining PVTdata for Gas Condensate systems. He reported that the amount of liquid seen in the celldepends upon the times the cell is left to stabilise, which was assumed to be due todrainage from liquid hold-up on the internal surfaces. He also stated that the laboratoryconsidered that most of the liquid had drained in the first 2.5 hours. In the paper Eytonpresented a plot of the liquid saturation curve for a CME (CCE) experiment with the liquidvolume as a function of time at each pressure step included.



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However, other workers have found that by leaving the cell overnight after waiting over twohours for cell stabilisation that the volume of liquid in the morning has increasedsubstantially. Many research workers in this area believe that the time required to reachequilibrium at each pressure step may be of the order of 24 hours or more. However, thereare no literature reports of liquid saturation curves as a function of time. At the presenttme, this question of liquid losses within cells has not been resolved satisfactorily.

S

N S

N

Upper Chamber

Lower Chamber

Piston

Stirrer Assembly

Suppor t Stand

Viewing Windows

Sampling Por tand Valve

Sliding Seal

End ClosureAssembly

Hg Inlet Port

Hg Inlet Port

High PressureSeal

Figure 13 : Gas Condensate (Sloane) Cell for Operation to 392 oF (200 oC) and17,500 psi (1200 bar)

PVT (Laboratory Pvt Measurements)

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