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tasks like defining the sampling method to be used, the sample volume required and the type of PVT/EOR experiments neededto complete the project may be discussed and dealt with accordingly. Figure 1 shows the typical work flow that needs to befollowed.

Sampling TechniquesThe properties of the reservoir fluid and the overall quality of the study depend upon the quality of the samples

collected from the field. Hence the objective of the fluid sampling should be to collect samples that are representative of the

down-hole conditions. This means that the samples are to be collected when the well is flowing in single phase conditions i.e.when original saturation pressure is less than the reservoir pressure. If observed bottom hole flowing pressures (BHFPs) areless than the reservoir pressure, then the sample is considered to have been collected in single phase conditions if the originalsaturation pressure is less than the BHFP.

Depending on the reservoir and fluid type sampling can either take place at sub-surface, wellhead or surface. Each ofthese sampling techniques is shown schematically in Figure 2. For undersaturated oil reservoirs it may be an idea to carry outsub-surface sampling using single phase sampler. In this method the sampler is lowered into the well using wire linetechnology and the samples are captured at the desired depth. The sub-surface sampling is good for black oil, volatile oil anddry gas fluid types. However sub-surface sampling may not result in a representative sample if the reservoir is depleted or thefluid type is a gas condensate.

For gas condensate fluids it is advisable to carry out surface sampling. In this method of sampling the reservoir fluid

is made to flow through a separator, which is stable at particular temperature (separator temperature) and pressure (separatorpressure). A separator oil and separator gas are sampled at the same time. This pair of separator samples is then recombined to

the producing GOR to give a representative reservoir fluid composition. This method of sampling is applicable for black oils,volatile oils, gas condensates, wet gases and dry gases.

Wellhead sampling is good for any fluid that is flowing in single phase at the wellhead conditions. Under theseconditions the wellhead sampling is a very reliable and cost effective method of sampling.

Sample ValidationOnce the samples are collected in the field, it is important to carry out the validation of the samples to check whether

they really represent the reservoir fluid. The quality of the PVT study and of a possible EOS model developed for the reservoirfluid depends on the quality of the samples. Hence it is very important to carry out the validation of the samples beforeperforming the reservoir fluid study. The validation process depends on the type of samples collected, i.e. whether they arebottom hole, wellhead or separator samples. In this section we present an appropriate validation procedure.

Prior to any sample removal, the sample chamber (received by the laboratory from the field) should be heated to

approximately 200 F to redissolve any wax that may have precipitated during sampling or shipping. Thereafter the validation

procedure varies depending on the sample collection type.

Bottom Hole/Wellhead Samples Validation Procedure

When the sample arrives at the PVT laboratory to carry out the PVT study the following procedure is to be followed:

Record the opening pressure of the sampler at ambient temperature.

Determine the chamber content from the top and bottom of the chamber by removing sample approximately 5 mlfrom each side. Take out approx 10 ml of sample (if oil) and determine its density. The density of the fluid iscompared with the density of the reservoir fluid (if known). A too high density may signal the fluid is mixed withwater.

Carry out gas chromatography (finger printing) analysis. Compare it with a fingerprint analysis of the drillingfluid. A resemblance may indicate that the sample is contaminated.

Determine the saturation pressure of the fluid at reservoir temperature.

Separator Samples Validation ProcedureThe separator gas and separator liquid samples should be taken at the same time and be in equilibrium when sampling

is performed. To have the separator gas and liquid samples in equilibrium it is important that

Minimum choke size is used so as to ensure stable flow.

Gas and liquid flow rates are accurately measured.

The retention time in the separator is adequate for the gas and liquid to reach equilibrium.

Separator Gas Samples Validation ProcedureThe separator gas should ideally be at its dew point at separator temperature. To make sure there is not a significant

liquid carry over the gas cylinder is heated to a temperature at least 20 F above the separator temperature. It the cylinder

contains any liquid at these conditions, it suggests either liquid carry over, that the sampling temperature was incorrect or the

cylinder was contaminated. Such sample should be discarded. Otherwise the opening pressure of the cylinder and composition

of the separator gas are to be measured. Flash carried out using a PVT simulator can be used to check consistency betweenseparator pressure and opening pressure.

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Separator Liquid Samples Validation Procedure

For a good separator sample the separator liquid is at its bubble point at separator temperature. A partial ConstantComposition Expansion test is performed at separator temperature to check the bubble point pressure of the sample. Thecomposition of the separator oil is measured and a Karl Fischer (ASTM D6869) test is performed to measure the concentrationof entrained water.

Sample Blending

Once the samples are validated the selected samples, which will come from the same reservoir, depth and location,are comingled into one large cylinder. This is to avoid any compositional variation that might arise had the samples been usedseparately.

Compositional AnalysesBefore carrying out a compositional analysis a bottom hole sample is flashed to atmospheric pressure. To ensure a

good separation it can be convenient to flash it to a temperature higher than the ambient temperature (say 120 F). The gas and

the liquid from this flash are analyzed separately using gas chromatography (GC) to obtain the composition in weight% andare recombined on a weight basis to produce a C36+ weight% reservoir fluid composition. The mole% data is calculated using

either the default molecular weights and densities from Katz and Firoozabadi (1978) data or the molecular weights anddensities of the True Boiling Point (TBP) distillation cuts.

For separator samples, the gas is injected directly into the gas chromatograph and the composition measured. Theseparator liquid is flashed at the same conditions and analyzed in the same way as a bottom hole sample. The composition of

the separator oil is then calculated using the compositions of the liquid and gas and recombining them mathematically usingthe measured GOR (Pedersen and Christensen, 2006).

Injection GasThe injection gas will often be either a CO2, N2, a lean hydrocarbon gas or a rich hydrocarbon gas, which could be the

gas obtained from the field. A gas obtained from the field is readily available whereas CO 2, N2 or a light hydrocarbon gas

mixture has to be prepared in the laboratory. In either case, the gas must be available in sufficient quantity to complete theentire project.

Carbon Number DistillationIt is recommended to carry out the Carbon Number Distillation also known as the True Boiling Point Distillation

(ASTM D2892) on a sample of stock tank oil to provide measured molecular weights and densities for (at least) the individualcarbon number fractions from C11 to C19 and for the C20 plus fraction. A carbon number fraction of a high density is rich in

aromatics while a carbon number fraction of a low density is rich in paraffins. An aromatic fraction should have different EOSmodel parameters assigned than a paraffinic one. Measured densities therefore help building a robust EOS model. The TBPdistillation procedure is schematically illustrated in Figure 3.

EOR PVT DataProcedures for measuring routine PVT data like Constant Mass Expansion, Differential Liberation, Constant Volume

Depletion, Separator Tests and Viscosity Tests are well established and that type of experimental PVT data will be sufficientto develop an EOS model for a reservoir fluid to be produced by natural depletion.

For reservoir fluids undergoing gas injection specialized EOR PVT experiments are required in addition to the routinePVT experiment. For Black Oil type of reservoir fluids the relevant type of laboratory experiments are solubility swellingstudies, equilibrium contact (also known as equiphase) studies, multi contact (either forward or backward) and minimummiscibility pressure studies. If the reservoir fluid is a gas condensate. a gas revaporization study may be carried out in addition

to the routine PVT studies. The EOR studies are performed to define the phase behavior and fluid composition in a reservoirwith gas injection.

Solubility Swelling

A solubility swelling test is performed on oil mixtures (Figure 4) in a long window cell. A known volume of reservoirfluid is charged into the cell at working pressure and reservoir temperature. A constant composition expansion (CCE) isperformed to determine the saturation pressure, relative volume, liquid density and liquid shrinkage data. The CCE is carriedout in a number of steps in the single and two phase regions. In the two phase region the liquid shrinks due to the release oflighter components in the gas phase region. This is measured in a number of steps and is important to determine the fluid type.The liquid will shrink more if the fluid type is a volatile oil than if the fluid is black oil.

Once the experiment is complete for the first gas-oil mixture, the sample is brought back to the working pressure(typically 1,000 psig above the reservoir pressure) and a predetermined volume of injection gas is added to the fluid and thesample is got back to the working pressure. The total sample volume is measured and the increased (swelled) volume isdetermined. The newly created mixture is subject to a new CCE experiment and the saturation pressure and liquid shrinkage

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pressure the fluid is in two phases and is allowed to stabilize for 24 hours. The volume of the gas and liquid equilibriumphases formed and the total volume of the sample are measured. The equilibrium gas phase composition, volume and thedensity are determined. As in the equiphase experiment the gas phase viscosity is determined from correlations. The remainingequilibrium gas is displaced from the PVT cell to a second PVT cell and is stored at working pressure and reservoirtemperature. The total volume of gas is now the total volume pumped out from the cell. The liquid volume is equal to the total

volume minus the gas volume. A partial CCE is performed on the liquid phase to confirm the saturation pressure used for themulti contact phase separation. The density, viscosity and the composition of the liquid are also determined.

The next contact (usually referred to as the first contact) is between the recovered original equilibrium gas from theprevious stage and original reservoir fluid. A known amount of reservoir fluid is charged to the second cell where theequilibrium gas is stored and brought to single phase at reservoir temperature and working pressure. The mixture is brought tothe selected multi contact test pressure. At this pressure the fluid is in two phases and is allowed to stabilize for 24 hours. The

volumes, compositions and densities of each phase and the viscosity of the liquid phase are measured as described previously.The above procedure is continued with up to 4 contacts (dependent on sufficient volume of equilibrium gas) between theequilibrium gas recovered from the previous stage and the original reservoir fluid. For a backward (reverse) multi contactstudy, a portion of reservoir fluid is charged into the PVT cell and stabilized at reservoir temperature and working pressure. Ameasured portion of injection gas is added to the cell. The quantity of injection gas is expressed as mole fraction of injectiongas per total moles in place at the start. The cell content is mixed thoroughly and stabilized in a single phase at workingpressure. The mixture is brought to the selected multi contact test pressure. At this pressure the fluid is in two phases and is

allowed to stabilize for 24 hours. The volume of the gas and liquid equilibrium phases formed and the total volume of thesample are measured. The equilibrium gas phase compositions, volumes and densities are determined. The remaining

equilibrium gas is displaced from the PVT cell. The total volume of gas is now the total volume pumped out from the cell. Theliquid volume equals the total volume minus the gas volume. A partial CCE is performed on the liquid phase to confirm thesaturation pressure used for the multi contact phase separation. The density, viscosity and the composition of the liquid aredetermined.

The next contact (usually referred to as the first contact) is between the equilibrium liquid phase and the injection gas.

The above procedure is usually followed for 4 steps or until the equilibrium liquid phase is insufficient to continue the test.The forward multi contact experiment basically follows the injected gas during the first few contacts with the fresh

oil, thereby simulating (in a very simplistic manner) conditions near the injection gas front. A forward multi contact test isnormally performed when the process is thought to be a predominantly vaporizing drive. A lean gas is mostly preferred for theforward multi contact test.

A backward contact experiment measures phase changes taking place with the oil near the injection well during the

first few contacts with fresh injection gas. When a rich gas (e.g. a separator gas) is used as the injection gas, a backward multicontact test may be preferable.

Gas Revaporization

A gas revaporization study is designed for gas condensates undergoing gas injection. A portion of reservoir fluid ischarged into the gas condensate PVT cell and stabilized at reservoir temperature and working pressure. A constant

composition expansion test is performed on this reservoir fluid to determine the dew point pressure and the volume at the dewpoint. The test is continued until the fluid is expanded to the gas revaporization pressure. A known portion of gas is pumpedoff from the cell till the volume returns to the volume at dew point pressure. The composition of the pumped off gas phase ismeasured.

A known volume of injection gas is charged to the cell and the content in the cell is then mixed to get a new mixtureat the test conditions. At these conditions the sample is allowed to stabilize to have the gas and liquid phases in equilibrium.

Once equilibrium is attained the total sample volume and the volumes of the gas and liquid phases are determined. The excessvolume of the gas phase is pumped off to return the cell volume to the constant volume at the dew point pressure. Thecomposition of the pumped off phase is measured. The test is repeated for (typically) 4 more steps and the change in fluidproperties, composition and liquid volume as a function of gas revaporization process is determined. The experiment issketched in Figure 9.

ConclusionGas injection studies are performed to evaluate the interaction of the injection gas with the reservoir fluid at reservoir

temperature. The reservoir fluid can bean oil or a gas condensate. The data obtained from such studies gives an idea about theincrease in production by injecting gas. The three types of injection gas processes commonly covered in EOR PVTexperiments are solubility swelling, miscibility and multi contact processes.

A solubility swelling study is carried out to get to know the effect on saturation pressure and reservoir fluid volume of

adding the injection gas to the reservoir fluid. As miscibility between injection gas and reservoir oil is developed through acritical point, one of the most important results of a swelling study is the amount of injection gas required to produce a critical

fluid. Hence the swelling study should be designed in such a way that it shows all aspects of fluid behavior. If the fluid isblack oil, the swelling study should, as illustrated in Figure 10,include injection gas concentrations for which the fluid is avolatile oil, a near critical fluid, and finally a gas condensate. One can use PVT simulation data to design the swelling study.

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A slim tube (miscibility) study usually consists of 4 to 6 runs. The design of a miscibility study is to be carried out insuch a way that equal numbers of runs are measured in the miscible and immiscible zones from which the minimummiscibility pressure can be interpolated. A miscible run can, for design purposes, be assumed one which yields a recovery ofmore than 90% at 1.2 pore volume of gas injected. PVT simulations can give an approximate minimum miscibility pressureand help in the design of the experiment to ensure that the number of miscible and immiscible runs is equal. It is recommended

to carry out the maximum and the minimum pressure runs first. The data measured can then help re-design the study todepict the real situation. A guideline for the minimum pressure is 100 psig above the saturation pressure.

An EOS model is only suited for gas injection reservoir simulation studies if it accurately simulates the propertiesinside the phase envelope and near the critical phase area. The design of an Equilibrium Contact study should include at leasttwo points of the critical composition at the critical temperature and pressures lower than the critical pressure and one point forthe volatile oil phase. This will also help to define what happens near the injector and producer zone as gas comes in contact

with the reservoir fluid and moves through the reservoir.The multi contact studies are interesting as they provide information on what happens after multiple contacts between

gas and oil. For an oil mixture the multi contact experiment may give experimental evidence of the first few stages towardsdeveloping a miscible drive. With black oil samples, it is appropriate to start a forward multi contact study at the oil zone butnear the critical zone area. Since miscibility develops through a critical point it is advantageous that the next gas additionbrings the fluid into the near critical zone. After this the next two additions are performed in the gas condensate region. Forthe backward multi contact study the design should be made such that the original oil is vaporized to the maximum (about 80%

of the initial oil in place).With gas condensate samples; the design is made such that the injection gas vaporizes as much heavier end present in

the reservoir as possible as the heavier end otherwise would have been unrecoverable.The importance of carrying out high quality EOR PVT experiments cannot be over-stated as operators have a very

high investment in miscible gas injection projects. Fluid phase behavior plays a key role in such processes and goodexperimental data leading to a strong EOS model can result in a successful case study.

Nomenclature

CCE Constant composition expansionCND Carbon number distillationECM Equilibrium contact mixEOR Enhanced oil recovery

EOS Equation of stateGC Gas chromatography

GOR Gas/oil ratioMMP Minimum miscibility pressureTBP True boiling point

References

Katz, D.L. and Firoozabadi, A., Predicting phase behavior of condensate/crude-oil systems using methane interaction coefficients, J.

Petroleum Technol. 20, 1978, pp. 1649-1655.

Lee, A., Gonzalez, M., Eakin, B., The Viscosity of Natural Gases, SPE Paper 1340, Journal of Petroleum Technology, 18,1966, pp. 997-1000.

Pedersen, K.S. and Christensen, P.L., Phase behavior of petroleum reservoir fluids, Taylor & Francis, Boca Raton, 2006.

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Figure 1Workflow for EOR PVT experiments

Bottom Hole Sampling Separator Sampling Wellhead Sampling

Figure 2Sampling methods

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C7C

19asdistillation

cuts

C20+asresidue

C C molw ts

Figure 3Schematic illustration of Carbon Number Distillation (CND), which is also called True Boiling Point (TBP) distillation.

Figure 4Swelling experiment in a Px diagram

O il O il O ilO il

G a s G a sG a s

M o l % G as

P s a t

Figure 5Slim tube apparatus

Gas

HgHg

OilGas

Slim tubeBack pressure

valve

Separator

Oil

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Pressure

% Recovery

90-

Figure 6Slim tube recovery 3 miscible and 3 immiscible runs.

Figure 7Equiphase mixing ratios and pressures

O il

O i l O i l O i l

G a s

G a s

G a s G a s

F o r w a r d c o n ta c t

R e v e r s e c o n ta c t

Figure 8Forward and backward (reverse) multi contact studies

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H O2Oil

H O2

H O2H O2H O2

Gas Gas Gas Gas Gas

P =P P