reservoir fluid sampling & recombination

8/10/2019 Reservoir Fluid Sampling & Recombination

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PETROLEUM SOCIETY OF CIM

PAPER NO. CIM 93-54

THIS IS A PREPRINT SUBJECT TO CORRECTION

RESERVOIR FLUID SAMPLING AND

RECOMBINATION TECHNIQUES

FOR LABORATORY EXPERIMENTS

BY

Jeff Strong,Hycal Energy ResearchLaboratoriesLtd.

F. Brent Thomas,Hycal Energy ResearchLaboratoriesLtd.

D. Brant Bennion,Hycal Energy ResearchLaboratoriesLtd.

PUBLICATION RIGHTS RESERVED

THIS PAPER IS TO BE PRESENTED AT THE CIM 1993 ANNUAL TECHNICAL CONFERENCE IN

CALGARY, MAY 9-12,1993. DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION

MAY BE PRESENTED AT THE TECHNICAL MEETING AND WILL BE CONSIDERED FOR

PUBLICATION IN CIM JOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAM

CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING.

ABSTRA~

investigated.ften times these decisions are based

on properties measured on relatively small fluid

volumes produced from the reservoir at one point in

time. Therefore it is imperative that the fluid samples

used to make these decisions closely match the

characteristic properties of the reservoir fluids at

actual reservoir conditions.

The sampling of oil and gas condensate

reservoirs require that representative fluid samples

be removed by either surface or subsurface

sampling techniques. This paper briefly reviews both

of these techniques and discusses their relative

merits. Several practical examples are provided that

demonstrate the utility of an equation of state model

to verify the quality of separator samples to be useC;

in a recombination. In situations where free gas has

been entrained with the separator samples, the

equation of state model can frequently be used to

synthesize an appropriate gas to be used in a

recombination.

Representative

fluid samples can usually be

obtained from producing reservoirs at either surface

or subsurface locations. Surface samples are

removed at either the separator or at the wellhead,

with the associated gas and liquid subsequently

recombined in proportions to represent the actual

reservoir fluid. Subsurface samples are removed from

within the wellbore at actual reservoir conditions

using bottom hole sampling tools and techniques.

The suitability of the particular sampling technique will

depend on a large nun't>er of factors which may

include economic considerations such as the cost of

sampling and associated loss of production, the type

of surface facilities that are available, the fluid

volumes that will be required and the type of reservoir

and fluid to be sampled.

Introduction

Obtaining representative reservoir fluid samples

has become of increasing importance in the

development and exploitation of oil and gas

condensate reservoirs. This is especially true of

reservoirs where extensive computer simulations are

used o scopeout developmentaltrategies r

where enhanced oil recovery options are

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The sampling technique employed can be of

particular importance in saturated oil or gas

condensate reservoirs where the possibility of

entrainment of disassociated phases decreases the

likelihood of obtaining a truly representative fluid. In

some of these situations, various techniques can be

employed to compensate for entrainment of these

dissociated phases. The focus of this paper is

aimed at determining the techniques used to

recombine the separator samples to represent oil

and gas condensate systems along with many

situations where they have been depleted into the

two-phase region. A brief review of those situations

where bottom hole sampling will more likely provide

a representative sample will also be discussed.

or asphaltenes) or emulsion formation

Removing fluid samples from the wellhead itself is

possible, although the presence of multi-phase flow

will often result in heterogenous samples that will

require modification of the gas phase in order to

obtain representative reservoir fluid. Wellhead

samples are usually taken only In those instances

where chemicals are being added at the surface

separator and there is no other location available for

removing an uncontaminated fluid sample. In rare

instances, If the reservoir is highly undersaturated

with a bubble point pressure that is actually lower

than the wellhead pressure, wellhead samples may

provide fluids that are nearly equivalent to subsurface

samples. However, in general, wellhead samples will

not directly provide representative reservoir fluids

without altering the gas phase to achieve the correct

reservoir fluid.

Sampling Techniques

A thorough review of the equipment and

techniques used to obtain these different types of

fluid samples is outside the scope of this discussion

and individuals who are interested in more extensive

information on sampling procedures should refer to

the cited literature (1,2.3)nd information available

from equipment vendors and service companies that

specialize in sampling. However, a brief review of

the most common sampling techniques will be useful

to establish the fundamental principles that will

discussed later in this paper.

Subsurface samples

are collected

by lowering a

special sampling tool through the wellhead into the

bottom of the well near the perforations where live

reservoir fluid can be captured and brought back to

the surface. Prior to collecting subsurface samples

the well Is typically conditioned by restricting the

flowrate in order to level out pressure imbalances In

the near wellbore region and then shutting In the well

for a period of time (usually 24 to 72 hours) to allow

fluids to collect and equilibrate in the well bore.

In general, surface samples obtained at the

separator require collection of high stage separator

gas and liquid which must be subsequently

reconDined in a ratio that corresponds to the

relative amounts of gas and liquid produced as the

reservoir fluid travels up through the wellbore and on

through the surface separation facilities. This type of

sample is the most frequently used for several

reasons:

The advantages o subsurfacesampling of fluid

reservoirsare:

-

in situations where nothing is known about the

reservoir fluid, subsurface samples may

provide a good indication of overall fluid

properties such as composition, GOR and

saturation pressure. These results can be

useful in assessing the quality of any

subsequent surface samples.

- the fluid when brought to surface represents

the In situ fluid, and as such, does not need o

be recombined o a target saturationpressure

or target GOR

-

accesso the surface fluid samples is readily

available

- relatively large fluid volumes can be collected

- the cost of

collecting

surface sa~les is

usually much lower than collecting bottom

hole samples

- there is virtually no interruption of production

during the sampling period (although

conditioning of the well prior to sampling may

require some alteration of the oroduction rate).

fluids that have cloud points greater than

surface temperature, those with a propensity to

precipitate solids (such as asphaltenes) with

reduction in pressure or temperature or those

with tendencies to form emulsions will be more

representative from a subsurface sample than

those fluids collected at the surface separator

Surfaceamples will typically yield representative

samples from the well provided that the well is

producing at a stable gas/oil ratio (GOR),

disassociated phases are not entrained in the

produced fluids and there is no solid {such as waxes

-

entrainment f disassociatedhasescan be

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less severe depending on the effectiveness of

the well conditioning.

The recombination of surface separator samples

is achieved by either recombining the gas and fluid to

match the measured separator GOR or to match a

specified saturation pressure at the reservoir

temperature. Ideally, matching the recombination to

one of these characteristics will result in a fluid that

corresponds well in the other, although this is not

always the case. Whether the saturation pressure or

the gas-oil ratio is selected as the fluid characteristic

to be matched will usually be determined by whether

the separator is producing at a stable GOR and

whether an accurate estimate of the saturation

pressure is actually known. In cases where the

reservoir fluid is known to be highly undersaturated,

the target saturation pressure may be significantly

lower than the actual reservoir pressure and therefore

the separator GOR may be a better reservoir fluid

characteristic to attempt to match. For saturated oil

reservoirs where an existing gas cap is known to be

in contact with the oil, the saturation pressure of the

oil will generally be equal to the current reservoir

pressure and therefore, the saturation pressure may

be the better fluid characteristic to match.

While many believe subsurface samples provide

the best opportunity of achieving a representative

reservoir fluid, these samples can be subject to

relatively low capture rates and have a limited

collection volume. Moreover, considerable disparities

have been observed on some oils where multiple

bottom hole samples have been taken.

Consideration of which method to use for

sampling will depend on the aforementioned

variablesand particularlyon the type of well to be

sampled. For the sake of clarity, the issue of

sampling oil wells and condensate wells will bs

treated separately although many of the sams

principleswill apply to both.

Sampling of 011Wells

For undersaturated reservoirs, the recombination

of surface separator samples will usually result in a

representative reservoir fluid provided that the well

is producing at a stabilized gas-oil ratio (GOR). For

undersaturated reservoirs where the producing GOR

is not stable then the possibility exists that the

bottom hole flowing pressure (BHFP) may actuaJly

be lower than the saturation pressure of the fluid. In

this situation, solution gas may be liberated in the

near wellbore area which then must first achieve a

critical gas saturation before it will flow Into the

wellbore and on to the separator. However, once a

steady state equilibrium is established in the near

wellbore region then the producing GOR will usually

stabilize and the surface separator should yield

fluids suitable for recombination.

Sampling of Gas Condensate Wells

Similarly, gas condensate systems may also

exhibit extreme sensitivity with respect to pressure

and temperature conditions and it is difficult to get

representative samples from the surface separator

unless one has access to a large separator, where

large fluid volumes have been averaged, and where

stable flow rates are observed. In the case of

subsurface samples, these are also sometimes

subject to temperature and pressure sensitivities and

their results need to be quantified and scrutinized

closely. Consequently, for gas condensate systems

there is no guarantee that a bottom hole sample will

be superior to surface samples and therefore all

results need to be closely examined. Indeed, for gas

condensate systems the most important factor may

be when the sample was taken once production had

been initiated. Simulation studies conducted by

McCain and Alexander3) on gas condensate wells

have suggested that gas samples should be obtained

early during the first 30 days of production in order to

obtain a representative reservoir sample. After that

point the gradual buildup of a condensate ring around

the wellbore will prevent the collection of

representative samples regardless of the amount of

well conditioning that is performed.

Sampling saturated oil reservoirs provide a

special challenge since the production of any gas

cap or previously liberated solution gas will usually

result in a non-representative recombination.

Reducing the flow rate of a well and observing if

there is a corresponding drop in the measured

separator GOR may reveal if gas coning or gas

liberation effects are being observed in the well.

Figure 1 shows the standard relationship between

GOR and flow rate which one might expect for such

a well. For the best opportunity to obtain

representative samples, the surface separator

should be operated at a condition below the

threshold flowrate that will induce entrainment of

disassociated gas phase. If operated above, then

the likelihood of having excess gas (over and above

the solution GOR) along with a leaner gas phase is

much increased.

Equation of State Modelling

Equation of State modelling can be particularly

useful o evaluate he quality of the surface samples

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and provide a method of recombiningphases in

order o predictoverall phasebehaviourat reservoir

conditions. In instances where the entrainmentof

disassociated hases s suspected, he Equationof

State can be a valuable tool to determine f the

separator gas collected is representativeof the

evolved reservoir solution gas. This can be

illustratedwith the followingexample.

EOS we can detennine whether the separator

samples can be recombined to represent the present

in situ reservoir liquid. The results of this comparison

are provided in Table 2.

Table 2 . Comperi80n of ~.tor ~

.- - -

EOS

G.s

Measured

Gas

omponent

Example 1. A saturated oil reservoir had an original

pressure of 15,168 kPag (2200 psi) at 65C (14eoF).

Since that time the reservoir has been depleted to a

current reservoir pressure of 11,032 kPag (1600

psig). In order to perform laboratory tests on the

field It was desired to recombine separator oil and

gas samples to represent the present in situ liquid

phase. An original compositional analysis was

available from a bottom hole sample taken in 1960

and co~ositional analyses of the current separator

gas and liquid

were

also available.These are

summarized in Table 1. Figure 2 shows a general

schematic of a depleted reservoir as used in this

exa~le.

N.

co.

HaS

c,

c.

c.

I-c.

n.c.

.,c.

n.c.

~

0.0 1

0.0271

0.1231

0.6808

0.0840

0.0277

0 0025

O.~

0.0013

0.0011

0.0013

0.0056

0 0264

0.1295

0.6790

0.0812

0.0351

0 0034

0.0084

0.0016

0.0013

0.0008

This comparison shows that the methane content

of the EOS-generated separator gas was slightly

higher than that of the sampled gas. This suggested

that there was no gas cap gas entrained in the

separator gas since the EOS predicted a gas cap gas

containing approximately 76% methane which would

have contributed to a higher methane content in the

separator gas.

A subsequent recombination of the separator gas

and liquid samples according to the measured

separator GOR provided a recombined oil sample

with a saturation pressure that was within 100 kPa of

the present reservoir pressure. The compositional

analysis of the final recombined oil is provided in

Table 3 which shows relatively good agreement with

the composition predicted by the EOS. Therefore, by

using the EOS, the quality of the recombination has

been evaluated and the confidence level is improved.

As previously mentioned, the presence of a gas

cap in a saturated oil reservoir can frequently result

in the entrainment of the disassociated gas phase

into the separator fluids thereby making a direct

recombination of fluids invalid. However, based upon

preliminary EOS analysis a suitable separator gas

can be synthesized in the laboratory in order tc

obtain a representative recombined oil sample.

In order to determine whether the separator gas

and liquid could be used to recombine a

representative sample of the in situ liquid an

equation of state was used simulate the reservoir

fluid. Based on the original compositional analysis of

the bottom hole sample an EOS model was tuned to

fit the original bubblepoint pressure of 15 168 kPag

at 65C. Once the EOS had been tuned to match

the original reservoir conditions, the oil was then

partially depleted to current reservoir conditions and

then flashed to the present separator conditions.

Figure 3 provides a schematic description of this

EOS procedure. By co~ring the composition of

the actual separator gas to that generated by the

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appropriateGOR. In order to reduce the saturation

pressure to the target reservoir pressure, the GOR

had to be decreased by almost 20% thereby altering

the transport properties and composition of the oil.

However, when a second recombination was

performed using a blended synthetic separator gas

based on the composition predicted by the EOS, the

GOR obtained was 82.5 m3/m3 with a saturation

pressure within 100 kPa of the current reservoir

pressure. A comparison of the recombined oils is

provided n Table 5.

Example 2. This second example shows a situation

where the separator oil and gas as sampled cannot

be used in a direct recombination due to the

entrainment of gas cap gas. As in the first example,

an initial reservoir fluid composition was available

from very early in the productive life of the reservoir

which was used to input into the EOS model. The

model was tuned to give an oil with a bubblepoint

pressure of about 15 860 kPa (2300 psig) at 75C

(16~F). The schematic of Figure 4 shows the EOS

process used for this situation. However, unlike the

first example a comparison of the sampled separator

gas and the EOS-generated separator gas showed

considerable differences. Table 4 shows the

comparison of these two gas compositions.

Example

3. The next example considers a situation

where separator oil is not available and the

recombination must be performed with dead stock

tank oil. This can occur when an emulsion has

formed in the separator and the oil must first be

degassed and then centrifuged in order to remove

the water. However in this example the oil actually

came from an overseas well which was accidentally

degassed during transport. A compositional analysis

of the reservoir fluid was available along with the

saturation pressure and single stage flash GOR

which were used to tune the equation of state model.

Tuning of the EOS was performed by adjusting the

temperature and pressure of the EOS flash until the

composition of the liquid from the EOS closely

matched the actual analysis of the present separator

liquid. Once the model had been tuned, the

necessary gas was synthesized in the laboratory and

then recombined with the remaining dead oil. Table

6 provides the properties of the resulting recombined

oil sample compared with the original properties of

the bottom hole sample.

Table 4

.

Comparisonof Separator Gases

EOS

omponent

N.

co.

0.0059

0.0512

H.s

0.0064

Ct

c.

c.

c.

~

0.6387

0.1362

0.0907

0.0554

0.0155

0.7309

O.~

0.0641

0.0392

0.0080

c.

There

is a significantamountof excess methane n

the sampled gas which is most likely the result of

gas cap gas entrainment into the separator. Using

the actual separator fluids in a recombination

resulted in a fluid with a saturation pressure much

higher than the maximum reservoir pressure for the

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from a free liquid leg. The producing GOR was in

the range of 1500 m3/m3and the liquid had a gravity

of 43 API. An equation of state model was

developed to detennine if the liquid could have

possibly resulted from a gas-condensate system.

This was perfonned by using the equation of state to

model the gas and liquid phases produced through

the separator and then recombining these two phases

with the EOS in varying amounts to detennine the

GOR that would be required to achieve a single

phase at reservoir conditions. The results indicated

that a GOR in excess of 100000 m3/m3 would be

required at 63C in order to achieve a saturation

pressure of 15 MPa. Since this GOR is far in excess

of what was observed in the field, this would appear

to suggest that the production from this well was a

combination of gas and entrained free liquid rather

than resulting solely from a gas condensate system.

The sampling of gas condensate wells has

already been discussed in terms of the relative

merits of subsurface versus surface sampling. One

particular problem that is sometimes encountered is

determining whether a sample collected at surface

represents a gas condensate, a free liquid leg in the

reservoir or a combination of both. The distinction

between oils and condensates is usually evident

from a compositional standpoint and as a general

rule it has been suggested by Moses(4)hat reservoir

fluids which are less than 12.5 mole% heptanes plus

are usually in the gas phase in the reservoir. This

can also be confirmed with an equation of state

model performed on separator samples taken early

in the life of the reservoir. The key question to

answer is if the separator gas at the reservoir

temperature and pressure can vaporize the liquid

corresponding to the measured separator GOR. If

not, then the GOR needs to be increased until the

liquid s vaporized. If the saturation

pressure

of that

fluid Is within the realistic limits of the reservoir then

the produced liquid may be derived from the vapour

phase in situ. However, if the corresponding

saturation pressure is much higher than the

maximum pressure of the reservoir then there is

good evidence that a free liquid leg exists in situ.

This technique should be only employed early in the

life of a suspected condensate well since the loss of

liquids in the near wellbore region will distort the

phase behaviour of the recombined fluids and give

erroneous results.

When separator samples are recombined to

represent very lean gas condensate systems with

GOR's in excess of 5000 m3/m3, the resulting

mixtures are very difficult to use in the laboratory tc

measure volumetric properties due to the small

volume of liquid. Constant volume depletion (CVD)

tests require that exact dewpoint pressures and

phase volumes be determined experimentally using

relatively small volumes of overall sample. The

resulting experimental error associated with these

measurements can result in inaccurate estimates of

the two phase formation volume factors. An

alternative methodology that can be e~loyed when

dealing with reconmined gas condensate systems is

to recombine the samples to a GOR that is low

enough to be able to accurately measure the phase

behaviour of the mixture. Once this has been

accomplished then an EOS model can be tuned with

the experimental data and then subsequently used to

predict phase behaviour at the actual field GOR.

Recommendations

The collection of representative samples from a

reservoir can be accomplished through the use of

surface separator samples or subsurface samples but

results should be scrutinized carefully to ensure that

the final reservoir fluid Is consistent with the

properties of the reservoir. Equation of state models

can be employed to great advantage to assist in the

evaluation and synthesis of separator samples to be

used in recombinations. As with all computer

simulations, the quality of the input data should be

evaluated before it is used to model prospective

recombination fluids.

Example 4. An example of this previous situation is

where a gas well at 15 MPa and 63C was

producing significant quantities of liquid at the

surface separator although it was not known whether

the liquidswere he resultof a gas condensate r

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Conclusions

References

1. The selection of an appropriate sampling

technique will generally be made based on

factorssuch as overall samplecosts, he surface

facilities hat are available, the fluid volumes hat

are required and the type of reservoir to be

sampled. Regardless f the methodemployed o

collect the sample, the resulting reservoir luid

should be scrutinizedcarefully o ensure hat It

accurately epresents he in situ fluid before t is

used n any laboratorystudy.

1. Reudelhuber,F.O.,Sampling Procedures or Oil

Reservoirs; Journal of Petroleum Technoloav.

Dec. 1957.00. 15-18.

2. API RP 44. API Recommended Practice

for

SamDlina Petroleum Reservoir Fluids; 1st addition,

Jan. 1966

3. McCain, W.D. and Alexander, A.A., SalT1>lingGas

Condensate Wells; SocietY of Petroleum

Enaineers Reservoir Enaineerina. Aua. 1992 . DD.

358-362.,

. An equationof state model can be used to

evaluate he qualityof surfacesamplesespecially

those where the entrainment of disassociated

phases s suspected.

4. Moses, P L., EngineeringApplicationsof Phase

Behaviourof Crude Oil and CondensateSystems;

Journal of PetroleumTechnoloay.July 1986. DD.

715-723.

. In situations where separator gas phases have

been contaminated or are not available, an

equation of state model can be used to

synthesize he appropriateseparator gas to be

used in the reco~ination. The accuracyof the

recombinationwill depend on the quality of the

original luid compositionused n the model.

4. For suspectedgas condensate ystems,an EOS

model can be used o determine f the produced

liquid is a condensate resulting from the

productionof gas or a free liquid resulting rom

entrainmentwith the produced gas. The EOS

model should only be based on samples

producedearly in the life of the well since later

samples may be nonrepresentative.

Acknowledgments

The authors would like to acknowledge the

supportof Hycal EnergyResearchLaboratories td.

in providing esearch acilitiesand sampledata used

to prepare his presentation.

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FIGURE 1

GAS ENTRAINMENT AS A FUNCTION OF FLOWRATE

/

,/

~

~

~

Well Flowrate

FIGURE 2

GENERAL SCHEMATIC OF DEPLETED RESERVOIR

Separator Gas

Separator Oil

Gas Cap

Saturated

Oil System

Depleted Oil


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FIGURE 3

EXAMPLE 1 - EOS SCHEMATIC

FIGURE 4

EXAMPLE 2 - EOS SCHEMATIC

reservoir fluid sampling & recombination

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