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PE 37147

radley A. Roscoe,

Holdup Determination in Horizontal

SPE, Schlumberger-Doll Research

Mght 16S6, %wiefy of Petroleum Engineers, Inc.

is paper was prepared for presen tat ion al the 1996 SPE lntema fional Con ference on

We ll Technology hefd in Calgwy, Canade, l&20 Nmmks 19%.

s pepr was selected for pnssenfat icn by an SPE Program Commiftse fol lowing rev iew of

formation Qnfained in an abstract submitted by the author(s) . Cmtents of the paper , as

esenled. have not been reviewed by the Sockty of Petroleum Engineers and are subject to

rrect ion by the autho fs ). The mater ia l, as presented, does not necessari ly raf lect any

sitionof the .%Mefy ofPetroleum Engineers, i ts ofticers, or members Papers presented at

E meetings are subject to publication rev iew by Edi torial Commit tees of the Socie iy of

t ro leum Engmoers. Permiss ion to wpy is restr ii led to an abstract of not more than 300

ords. Illustrations may not be copied. The abstract should contain conspicuous

hnoMedgment of where and bywhom the paper was presented. Write Librarian, SPE, PO.

x 633636, Richardson, TX 7S0S3.3636, U.S.A., fax 01-214.952-9435.

n horizontal wells, it can be very difficult to interpret

onventional production logging tools due to the fluid

egregation in the borehole. This is a even more of a problem

hen there are more than two phases present in the borehole,

.e., oil, water, and gas, A pulsed-neutron tool measures many

arameters which are differentially sensitive to all three

ossible borehole phases. Therefore, it is possible to combine

e information available from a pulsed-neutron tool to

etermine the 3-phase holdup in horizontal wells.One of the major difficulties in evaluating the response of

tool to 3-phase holdup is obtaining good data under realistic

wnhole conditions, i.e. realistic gas densities. Laboratory

easurements cannot readily be made under these conditions;

erefore, modeling techniques must be used to evaluate and

haracterize tool response. To validate a computer model,

boratory data are needed for benchmarking; therefore, for

is study, over 400 laboratory formation measurements were

erformed using air to simulate gas. These formation

onditions were also modeled using Monte Carlo techniques.

he agreement between measured and modeled data proved to

e good enough that modeling can be used to confidently

redict the tool response with air or realistic gas.

Once the ability to predict tool response under realistic

ownhole conditions exists, it is possible to combine

formation from a pulsed-neutron tool to quantitatively

etermine the holdup of all three phases. This is accomplished

y combining the inelastic near/far ratio with the near and far

arbon/oxygen (C/0) ratio. This approach to the holdup

easurement has been demonstrated using a combination of

boratory data, Monte Carlo modeling, and field data. The

sults of this study have demonstrated that the RMS accuracy

f this measurement is about 6 0/ 0n each of the three phases.

&iltiWells Using a Pulsed-Neutron

Introduction

As horizontal wells have become more prevaient, the ability to

reliably evaluate the production performance of these wells

has become increasingly important. Existing production

logging techniques, such as spinners, that have been

successfidly used in vertical wells cannot always be applied to

horizontal wells with full confidence because of segregatedflow in the borehole. For this reason, new techniques-must be

developed to evaluate oil and water flow rates in horizontal

wells.

To determine the flow rates of the oil and water phases in a

horizontal well, one must either 1) measure the individual oil

and water flow rates directly, or 2) measure the individual oil

and water velocities in addition to their hoidups, (It should be

noted, that for most production logging applications in

horizontal wells, measuring only the holdup or only the

velocitv of the rsroduction fluids is usuallv insufficient to

determ~ne the so&ce of production problems.~ This paper will

address part of the second approach, the measurement of

individual oil, water, and gas holdups. Once determined, these

holdups can be combined with velocity information, obtained

from several possible approaches’”z to obtain oil and water

flow rates.

Background

Pulsed-neutron tools have previously been used to

qualitatively determine the 3-phase holdup in horizontal welis.

This approach uses the borehole sigma and the inelastic

near/far ratio for this determination. The method is considered

qualitative since tool calibration information is not available

~or the ratio measurement or the sigma of the gas.

Recent work reported by Peeters et. al.’ has attempted to

quantify the pulsed-neutron measurement for holdup in

horizontal wells. Their approach utilizes three measurements

from a single pulsed-neutron tool centered in the borehoie:

C/O windows ratio, borehoie sigma, and capture near/far ratio.

The measurements are combined through a iinear response

matrix to produce the desired holdup measurements. The

coefficients for the matrix are determined by regression of

modeled or measured tool responses to known conditions.

A more quantitative approach has been employed with the

RST* Reservoir Saturation TO01.4’5 This tool was primarily

* Mark of Schlumberger

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2 B.A. ROSCM SPE 37147

designed to measure the oil saturation of the formation without

depending on formation water salinity. This was

accomplished by using a carbon/oxygen measurement. A

large part of the problem of converting a C/O measurement

into oil saturation is the effect of the borehole on the

measurement. To properly determine the formation oil

saturation, the borehole composition must be known

reasonably well. In general, a 5 error on the boreholecomposition can cause a 15 S.U. error on the formation oil

saturation. For this reason, the RST tool was designed with

two detectors to compensate for the borehole effect. However,

from previous statements, it is obvious that the RST tool can

also measure the 2-phase borehole holdup in a well.

In addition to the obvious 2-phase holdup measurement,

other data (in several forms), already available from the RST

tool, are sensitive to the presence of gas in the borehole.

Quantifying this information and combining it with the

already available C/O measurement provides sufficient

information to quantitatively determine the 3-phase holdup in

the borehole.

To ensure the best quantitative answer, it is advantageous

for the parameters measured by the tool to have approximately

the same depth of investigation. In addition, these parameters

need to be easily quantifiable. For this reason, the following

tool measurements were chosen for this measurement: near

carbordoxygen ratio, far carbonloxygen ratio, and the inelastic

near/far ratio. Since all these measurements are inelastic

measurements, their depth of investigations are about equal

and do not vary with formation and borehole sigma. Other

parameters sensitive to borehole holdup, such as borehole

sigma and capture ratios, can be used, but their varying depth

of investigation can complicate their quantitative use for this

measurement. In addition, their use adds additional

requirements in the form of assumptions about the borehole

salinity.One of the biggest problems in characterizing the tool

response for a 3-phase holdup measurement is the acquisition

of realistic data due to the difficulty and hazards of performinglaboratory measurements with gas under realistic downhole

conditions. Air is usually used to simulate the effects of gas

for these measurements; however, real downhole gas has

appreciable density and elemental constituents that cannot be

ignored. Therefore, tool characterization depends heavily on

tool response modeling. For this reason, this study uses a

multistep process to develop the quantitative analysis required.

These steps include:

1. Performing benchmarked laboratory measurements

simulating 3-phase holdup conditions in horizontal wells

using air to simulate gas.

2. Modeling the tool response using Monte Carlo techniques

under the conditions measured in the laboratory to

benchmark the model,

3. Develop interpretation procedures that quantitatively

predict tool response using air to simulate gas.

4. Model the tool response to 3-phase holdup using realistic

downhole gas characteristics.

5. Apply interpretation procedure to modeled data.

6. Apply interpretation procedure to acquired field data.

Finally, the issue of running the tool centralized or

eccentralized in the borehole must be addressed. In a

horizontal well, it can be difficult to centralize the tool in the

borehole. An argument can be made that it is better to

eccenter the tool and really be sure of its location in the

borehole. However, eccentering the tool can introduce some

problems such as a nonlinearity of the borehole tool response.

Therefore, this study has also tried to quantify the effects of

tool position in the borehole.

Laboratory Measurements

Measurements to characterize the RST-A response under

conditions simulating a horizontal well with 3-phase holdup

were performed at the Schlumberger Environmental Effects

Calibration Facility (EECF) in Houston, Texas. Since these

formations are usually used to simulate vertical conditions,

borehole liners were fabricated to divide the borehole fluid

into three equal volumes. This simulated the effect of

segregated fluids in the borehole. Different liners were

designed for the centered and eccentered measurements to

allow for the fluid displacement by the tool and the

requirement of a rathole for the tool (Fig. 1).

Dividing the borehole into three equal volumes allows 10

possible borehole fluid combinations that simulate holdup in a

horizontal well. These are shown in Table 1. Since it is not

possible to have a realistic downhole gas present in the

borehole for these laboratory measurements, air was used to

simulate gas.

A series of measurements was performed using various

formations at the EECF. The formations were selected to

provide variations in lithology, porosity, borehole diameter,

and formation oil saturation (see Table 2). Most of these

formations w run with the tool both eccentered and

centered in the borehole. All of the measurements used a 7-

inch, 23-lb/ft casing. Fresh water was used in the formation

and borehole for all measurements. Some additionalmeasurements were performed with 100 kppm brine in the

borehole. Over 400 laboratory measurements were performed

for this study.

Monte Carlo Model of RST-A Response

Since laboratory measurements with realistic gas conditions

are not possible in the laboratory formations, characterization

of the tool response depends heavily on tool response

modeling. For this reason, a large part of the effort in this

study has been to develop and test Monte Carlo models to

predict tool response. To this end, a RST-A model for the

inelastic measurement has been developed using the Monte

Carlo modeling code MCBEND,’ which was developed by

AEA Technology in Winfrith, UK. This model requires about18 hours to mn on a SUN Spare 20 for each set of formation

and borehole conditions. The outputs from the model are

tallied such that the following tool responses are available:

near and far carbon and oxygen yields, near and far

carbon/oxygen ratio, near and far carbon/oxygen window

ratio, and near/far inelastic countrate ratio.

To benchmark the RST-A model (as applied to 3-phase

holdup problems), the laboratory measurements described

above were modeled with MCBEND. The results from the

modeling were then compared with the results obtained with

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E 37947 Three-Phase Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source 3

e measurements as shown in Fig. 2. These figures show that

e modeling can be used to predict tool response. Each figure

s a line fit to the data that can be used as a calibration to

nvert from modeled results to measured results. Both

ntered and eccentered data are shown on the plot.

orehole Gas Holdup

e inelastic near/far countrate ratio can be used to detect and

antify gas in the borehole. This is demonstrated in Fig. 3

r centered and eccentered tool positions with measured data

which air was used to simulate the presence of gas. In this

ure, the lines on the plot show the data trends for various

actions of air in the borehole.

The effect of porosity on the inelastic near/far ratio is small

comparison with its response to air in the borehole. The

ta points along each air fraction line lie high or low around

e line depending on the “total hydrogen index” of the fluids

the borehole. For example, the 660/0air line has a lower

ar/far ratio for oil in the borehole than it does for water in

borehole.

As far as sensitivity to 100 air in the borehole, the

elastic near/far ratio changes by about the same amounthether the tool is centered or eccentered. However, for air

ldups ranging from O to qs~., the centered tool has

bstantially more sensitivity to the air because of its

oximity to the air region. For similar reasons, the

centered tool has more sensitivity than the centered tool

hen the air holdup is between 66 and 100 .

Modeling was used to predict the RST-A inelastic near/far

tio under the same measurement conditions as those in Fig.

This was used to benchmark the modeling, and a

rrelation was developed so that modeled results could be

nsformed into equivalent measured results (see Fig. 2).

is means that the calibrated modeled response gives the

me results as the measurements.

After the benchmarking of data of Fig. 3 via Fig. 2, thermations were remodeled substituting realistic downhole gas

to the model. The gas was modeled as 0.3 g/ml methane

H4). The modeled inelastic near/far ratio under these

nditions is shown in Fig. 4 for the tool centered and

centered, For this “realistic” gas, the dynamic range of the

near/far ratio is reduced by about 30 to AOO/o relative

the air data for both tool positions. Like the air data, the gas

ldup lines drawn on the figure show the same general trend

that the centered tool has its maximum sensitivity between

qq~. holdup while the eccentered tool has its maximum

nsitivity between 67 and 100°/0gas.

o-Phase (Oil/VVater) Holdup From C/O

s in the gas phase measurement discussed above, thesitioning of the tool in a segregated borehole impacts the

earity and sensitivity of the holdup measurement. This

ction will address this issue with respect to the 2-phase,

l/water, situation with a near/far C/O interpretation

For this section, the interpretation approach utilized will be

e standard RST approach’ in which carbon and oxygen

emental yields can be expressed as sums of the contributions

e to the matrix, pore space, and borehole as:

Cn = N, +N2cDS0 +N3Y0 ..... ........................................(l)

0 n =N4+N,0 l -So + N, l-Yo ........................... 2

where the N’s are the near detector sensitivity factors of

carbon and oxygen to the matrix, pore space, and borehole.

(Similar equations can be written for the far detector where the

far detector sensitivity factors are designated by F’s.) To

determine the sensitivity factors, four laboratorymeasurements (or calculations) are performed where only the

formation or borehole fluids are changed i.e. formation

porosity, lithology, borehole size, and casing size are fixed).

These measurements are usually referred to as the endpoint

measurements since they correspond to the endpoints of the

tool response. From these data, the near and far sensitivity

factors are determined using the four eIemental yields from the

measurements with the above equations. This results in four

equations and three unknowns for both the carbon and oxygen

expressions, allowing the determination of the three sensitivity

factors for each elemental yield. To characterize the complete

tool response, this process is repeated varying porosity,

lithology, etc.

Once the tool response is known from the above

calibration, for every depth, the sensitivity factors are

calculated based on known borehole size, lithology, porosity,

etc. and the near and far C/O ratios are measured. Combining

these data results in the following two equations:

c N, +N2cPS0 +N3Y0

‘n ‘~= N, +N,4 1 -SO +N, I- YO ““”””””””-’’’””” 3

R,=:=F 1 + F IOS O + F 3 Y0

F4+F ,cD 1 -SO + F , l-YO................

r

,....(4)

which have two unknowns. The solution to these equations

yields the borehole holdup and the volume of formation oil.The above analysis assumes that the borehole fluid is a

homogeneous mixture of oil and water as is the case with

vertical wells. However, in horizontal wells, the segregated

borehole fluid can introduce nonlinearities into the analysis.

The purpose of the following analysis is to characterize the

effect of this nonlinearity on the determination of holdup in a

horizontal well. The data used for this analysis will include

eight points per formation porosity, i.e., oil and water in the

formation with four different oil/water combinations in the

borehole depicting several holdups with segregated flow (see

Table 3). The data points shown will be from modeling (the

measured data show the same result.)

For this analysis, the coefficients for the interpretation

model are calculated from the four endpoint measurements ofthe formation being considered as described above. These

coefficients are then applied to the other four data points of the

data set to look at the linearity effects with segregated

borehole fluid. (This approach solves for both the oil

saturation and holdup simultaneously). The results of this

analysis can be seen in Fig. 5 for a centered and eccentered

tool .

The top plot of Fig. 5 shows the reconstructed oil holdup

for data modeled in a 7-inch casing (8.5- and 10-inch

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4 B.A. ROSIXX3 SPE 37147

borehole). The reconstructed holdup is extremely nonlinear

if the tool is eccentered. In this case, it appears that the tool

does not see the top third of the borehole due to the

nonlinearity. If the tool is centered, then the tool response is

fairly linear.

Since the effect being observed is a nonlinearity caused by

the tool’s limited depth of investigation, the effect should be

reduced in a smaller casing size. The bottom plot of Fig. 5shows modeled results for a 5.5-inch casing in a 8.5-inch

borehole. As can be seen, the effect is reduced, but still a

significant issue.

Table 4 summarizes the results of this 2-phase holdup

analysis using C/O ratios, This table includes RMS accuracy

estimates and 18-second precision for this measurement.

From these data, it is obvious that a centered tool always gives

better accuracy and precision than an eccentered tool.

However, if the tool must be run eccentered, accuracy can be

improved by making corrections based on calibrations of tool

response as shown in the plots of Fig. 5. The effect of this

approach is reduced sensitivity to low oil holdups.

Another interesting result of this analysis is that for

measurements using the RST-A tool, where the borehole fluid

is unknown, a centralized tool will give better precision on

formation oil saturation than an eccentered tool (see Table 5).

This is because centralizing the tool makes the near and far

carbonloxygen response more orthogonal with respect to

borehole compensation. For some conditions, this can mean

reducing station times by a factor of 4. For measurements in

which the borehole fluid is known, station time requirements

show no improvement.

3-Phase Holdup Determination

To determine the holdup of all three phases using the near and

far C/O ratios and the inelastic near/far ratio, a two-step

interpretation is currently used (these could be combined intoa single step). The interpretation is based on the assumption

that the three borehole holdups sum to unity and that

formation gas is zero, i.e., YO+YW+Y =1 and SO+SW=l.

khe first step is to obtain the gas oldup from the inelastic

near/far (N/F) ratio. This is obtained from calibration data

similar in form to those shown in Figs. 3 or 4 and can be

expressed as:

Yg = f y~ ........................................................................ 5

Once the gas holdup is determined, the water and oil holdups

can be determined using a modified version of the normal

RST C/O interpretation, The modifications made are to allow

for the inclusion of gas holdup into the formulation as shown

below:

(6)~ = N1 +N20S0 +N3(Y0 +Y,~yo)

“ N, +N,O l-SO +N, l-YO -Y, ““”’””””””””””””””””

F , +F ,OS O +F 3 Y0 + Y, p~m

R,= F4 +F ,@ l_so +Ff l_yo _y, . . .. . . . . . . .- 7

where ps and PO are the downhole gas and oil densities. In

these equations, the numerator and denominator have been

modified from the original RST interpretation (Eqs. 3 and 4)

to allow for the contribution of gas in the borehole. In the

denominator, this results in a reduction of the amount of

oxygen present in the borehole. In the numerator, it is

assumed that gas will increase the carbon response by a factor

approximated by the relative densities of gas and oil.

As with the normal C/O processing, the sensitivity factorsare obtained from laboratory calibrations which use the

borehole size, casing size and weight, lithology, and porosity

as inputs. Once the C/O ratios are measured and the gas

holdup is calculated horn Eq. 5, Eqs. 6 and 7 reduce to two

equations and two unknowns that can then be solved for the

holdup and oil saturation.

In this formulation of the problem, it is assumed that the

tool is equally sensitive to all sections of the borehole. As has

been shown, this is a reasonable assumption if the tool is

centered; however, if the tool is eccentered, this formulation

will introduce some bias. Under many conditions, this bias

can be reduced by providing a correction based on data similar

to those in Fig. 5.

3-Phase Holdup Results - Centered Tool

The interpretation procedure outlined above has been applied

to the measured and modeled 3-phase data obtained in this

study to evaluate the expected performance of this

measurement. This includes the measured data (with air) and

the modeled data (with air and 0.3 g/ml gas).

An example of the measurement parameters for a 16-p.u.

oil-saturated limestone formation with an 8.5-inch borehole

and 7-inch, 23-lb/ft casing is shown in Fig. 6 in a three-

dimensional plot. In this plot, the comers of the plots

represent the conditions when only a single phase is present in

the borehole, i.e., YW=1 is all water, YO=1 is all oil, and Ys=l

is all gas (0.3 g/ml in this example). From one comer toanother, the fraction of each phase changes linearly along that

line. The point in the center of the plot has water, oil, and gas

present in equal fractions. The ‘z’ axis of the plot shows the

magnitude of the various parameters being displayed.

The data from Fig. 6 are obtained from modeling since a

realistic gas was used (0.3 g/ml). As can be observed, the

modeled data are dominated by the physics of the

measurement, while the statistics of the Monte Carlo

calculations are small by comparison. This makes identifying

the tool response trends fairly straightforward. Also note that

the inelastic near/far ratio response is quite orthogonal from

the other responses, giving a clean gas signal. The near and

far C/O ratios show a similar response; however, they are

orthogonal enough to differentiate the borehole and formationsignals.

When the holdup data of this study are analyzed through

the above interpretation model, one can compare the predicted

holdups to the known holdups to give an estimate of the

accuracy of the approach. An example of these errors in

reconstruction is shown in Fig. 7 for the input data of Fig. 6.

These data show that the reconstruction is fairly good. The

overall accuracy of this approach can be estimated by taking

RMS errors of the data. This was calculated using data for

several formations with different borehole size, porosity, and

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PE 37147 Three-Phase Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source 5

aturation. This resulted in RMS errors of 6.3, 6.1, and 4.8°/0

or the gas, oil, and water holdups, respectively.

While the holdup calculations are performed, it is possible

o estimate the precision of the holdup measurement by

ropagating the measurement errors through the analysis.

his was done assuming 18 seconds of data accumulation for

he measurement, which is equivalent to logging at 500 ft/hr

sing a 5-level depth average. For these conditions, the I-a

recision were estimated to be 1, 15, and 15 for gas, oil,

nd water holdup, respectively. These precision will vary

epending on the formation porosity, borehole size, and casing

For an unambiguous interpretation, it is recommended that

oldup measurements in horizontal wells be performed in

onjunction with velocity measurements. Since the velocity

easurements are stationary measurements, a holdup

easurement could be performed at the same time as the

elocity measurement, giving more than adequate precision to

measurement.

entralization Error Effects on 3-Phase Holdup

s stated previously, one of the advantages of eccentralizinge tool is that its position is well known, while attempting to

entralize the tool can introduce tool positioning uncertainty.

o evaluate the effect of errors in tool centralization, a series

f Monte Carlo calculations were performed in which tool

osition was varied from fully centralized to fully

ccentralized. These calculations were performed for

ormations with an 8.5-inch borehole in both 16 and 33 p.u.

andstone formations. The calculations were performed with

o different casing sizes: a 7-inch, 23-lb/ft and a 5.5-inch,

lb/ft casing.

The data were analyzed to show what would happen to the

alculated borehole holdups if the tool was assumed to be

entralized, even when it was not. This was accomplished by

alculating the near and far detector sensitivity coefficientsased on data with the tool centralized. In addition, the tool

as response (Eq. 5) was based solely on the centralized tool

ata. These coefficients were then usedwith the modeled tool

ata for all tool positions to predict the holdups one would

lculate with this centralization error.

Figure 8 shows the RMS error on the borehole holdups as

function of centralization error for the two casing sizes

odeled. The RMS error calculation includes data at both

orosities and with both water and oil in the formation. In the

gure, the RMS error at zero centralization error is a

ackground error level due to the nonlinearity of the response

nd statistics of the modeling. For both casing sizes, it

ppears that there is no significant increase in the RMS error

p to about 0.5 inches. Based on this analysis, as long as the

ol is within 0.5 inches of being centered, there is negligible

fect on holdup accuracy.

eld Example

in 1993 as the first well in the offshore section

f this reservoir (18 p.u. sandstone), the well has been on

ntinuous production with interruptions only for workovers.

he horizontal section was drilled with a 8.5-inch bit and

ompleted with a 5,5-inch, 17-lb/ft casing and cement. The

well inclination across the reservoir section varies from 75 to

88.5 degrees. It penetrates the original OWC, enabling

movement of the OWC to be monitored. The objectives of the

production logging program were to 1) determine the flowing

production profile, 2) determine the source of the water

production, and 3) determine any movement of the OWC.

Both velocity measurements and holdup measurements

were performed with the RST tool in this well. Unfortunately

for this study, the well did not have any gas in the borehole (as

verified by the inelastic data). However, it is an excellent

example of 2-phase oil/water production in a horizontal well

and is still a good example of what can be accomplished in a

horizontal well.

The RST tool was run eccentered in the borehole for this

well. As mentioned earlier, if not corrected, this results in

some bias in the final holdup estimates. For this example, the

biases were removed by a calibration similar to that shown in

Fig. 5 (derived from modeling). The RST holdup

interpretation was based solely on modeling results.

The logging results are shown in Fig. 9. The top graph in

the figure shows the comparison of PVL* Phase Velocity Log

and WFL* Water Flow Log water velocities. The agreementbetween the two techniques is excellent. The next graph down

shows the oil velocity measured with the PVL. The following

graph shows a comparison of water holdup measurements

from the RST-A tool and the LIFT” Local Impedance

Flowmeter Tool. (The LIFT tool measures the water holdup

in a wellbore by scanning across the wellbore with six

separate impedance probes which individually measure the

local water holdup.g In both conventional and horizontal

wells, the data from these six probes can be processed to give

a global water holdup measurement.) The agreement between

the LIFT and RST-A holdups is excellent except where known

problems occurred with the LIFT measurement. (Note that

the uncalibrated LIFT points were due to operational problems

and the over-ranged point was past the maximum LIFTsensitivity). The bottom graph in the figure shows the oil and

water flow rates calculated from the velocities and holdup.

These data indicate that most of the water production is from

below 900 ft while the oil production is fairly uniform over the

interval.

This log shows an example of when the holdup answer, by

itself, in a horizontal well, does not completely diagnose the

production problem and why additional velocity information is

usually required for an accurate interpretation. Looking at

only the holdup data from this well, a rather uniformly

decreasing water holdup going up the well is observed. This,

in itself, does not readily identify that most of the water

production is coming from the lower part of the well.

Summary and Conclusions

The ability of the RST-A tool to measure 3-phase holdup in

horizontal wells has been quantified in terms of accuracy and

precision of the measurement. This has been accomplished by

merging experimental (using air to simulate gas) with modeled

data (using 0.3 g/ml CH4 for gas) into a database with over

400 borehole/formation conditions. This approach has

allowed for the benchmarking of the Monte Carlo model (for

the inelastic measurement) and predictions of the tool

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6 B.A. ROSCOS SPE 37747

response with gas under realistic downhole conditions.

The inelastic neadfar ratio was shown to be an excellent

stand-alone indicator of gas holdup. This measurement is

more accurate with the tool centralized in the borehole than

when it is eccentralized due to nonlinearities caused by tool

positioning.

The 2-phase holdup (oil/water) measurement was

evaluated using the near and far carbon/oxygen ratios for theeffects of tool positioning. This analysis was the normal dual

detector C/O analysis but used to evaluate the nonlinearities in

the measurement due to segregated flow. These results

indicated that the tool response could be nonlinear if the tool

was run eccentered with segregated flow, but that it could also

be corrected over a large part of the dynamic range with

proper calibration. It was also shown that running the tool

centralized in segregated flow does not exhibit this nonlinear

behavior.

A filly integrated 3-phase holdup analysis was evaluated

using the inelastic near/far countrate ratio and the near and far

carbou/oxygen ratios. This analysis was performed using a

RST analysis modified to take into account the gas

contribution. The analysis used both measured and modeled

data in combination for a centralized tool. For this analysis,

the estimated accuracies are approximately 6 p.u. for each of

the three phases, The estimated precision (18 seconds of

data) for this measurement are approximately 1, 15, and 15°4

for the gas, water, and oil holdups, respectively.

A field example indicated the accuracy and precision of the

3-phase holdup measurement are in line with the predictions

from the modeling. Comparison of the holdups obtained with

the RST tool compared quite favorably with holdup

measurements performed by another totally independent

technique, establishing the validity of both measurements. In

addition, this field test example demonstrated that modeling

results could be used to interpret log data accurately.Finally, this field example reaffirmed the notion that

velocity or holdup measurements by themselves will not

always give a correct interpretation of production problems;

but that holdup and velocity measurements together can.

Acknowledgments

The author would like to thank British Petroleum for

permission to publish their data and Steve Bamforth for his

assistance in obtaining this permission. The author would also

like to thank Norman Winkelmann of Schlumberger Houston

Product Center for his assistance in acquiring the 3-phase

holdup measurements under laboratory conditions.

Nomenclature@= Porosity

BOPD = Barrels Oil Per Day

BPD = Barrels Per Day

BWPD = Barrels Water Per Day

C“ = Near Carbon Yield

Cr = Far Carbon Yield

C/O = Ratio of Carbon to Oxygen

F,= Far Detector Sensitivity Factors

N,= Near Detector Sensitivity Factors

N/F = Inelastic Near to Far Ratio

on=of=

owc ‘

R“ =

R~=

RMs =

so=

Sw=

Y.=

Yg =

Y.=

Y.=

Near Oxygen Yield

Far Oxygen Yield

Oil Water Contact

Near C/O Value

Far C/O Value

Root-Mean-Square

Formation Oil Saturation

Formation Water SaturationAir Holdup

Gas Holdup

Oil Holdup

Water Holdup

S1 Metric Conversion Factors

BPD X 1.589873 E-O] = m’/d

fiX 3.048 E-01 =m

R/h x 8.466667 E-05 = lllk

ft./rein x 5.08 E-03 = 111/S

in x 2.54 E+OO = cm

lb/ft X 1.488164 E+OO= kg/m

References

1.

2.

3.

4.

5.

6.

7.

8.

McKeon, D.C., Scott, H.D., and Patton, G.L.: “Interpretation of

Oxygen Activation Logs for Detecting Water Flow in

Production and Injection Wells,” Trans. SPWLA 32nd Annuaf

Logging Symposium Midland, Texas, (June 16-19, 1991), paper

BB.

Roscoe, B.A., Lenn, C., Jones, T.G. J., and Whittaker, C.:

“Measurement of the Oil and Water Flow Rates in a HorizontalWell using Chemical Markers and a Pulsed-Neutron Tool,”

Paper SPE 36563 presented at the 1996 SPE Annual Technical

Conference and Exhibition, Denver, Colorado, (October 6-9).

Peeters, M., Oliver, D., and Wright, G.: “Pulsed Neutron Tools

Applied to Three-Phase Production-Logging in HorizontalWells,” Trans. SPWLA 35th Annual Logging Symposium, Tulsa,

Oklahoma, (June 19-22, 1994) Paper L.

Scott, H.D., Stoner, C., Roscoe, B.A., Plasek, R. E., and

Adolph: , R.A. : “A New Compensated Through-Tubing

Carbon/Oxygen Tool For Use In Flowing Wells,” Trans.

SPWLA 32nd Annual Logging Symposium Midland, Texas,

(June 16-19, 1991), Paper MM.

Stoner, C., Scott, H.D., Plasek, R.E., Lucas, A.J., and Adolph,

R.A.: “Field Tests of a Slim Carbon/Oxygen Tool for Reservoir

Saturation Monitoring,” Paper SPE 25375 presented at the SPE

Asia Pacific Oil & Gas Conference & Exhibition, Singapore,

(Feb. 8-10, 1993).

MCBEND: A Monte Cario Program For General Radiation

Transport Soiuiions Users Guide AEA Technology, Wirrfrith,

U.K., 1995.

Roscoe, B. A., Stoner, C., Adolph, R.E., Boutemy, Y.,

Cheeseborough, J., Hall, J., McKeon, D., Pittmsn, D., Seeman,

B., and Thomas, S.: “A New Through-Tubing Oil-Saturation

Measurement System,” Paper SPE 21413 presented at the

Middle East Oil Show & Conference, Bahrain, (Nov. 16-19,

1991).

Halford, F.R., MacKay, S., Bamett, S., and Petler, J. S.: “A

Production Logging Measurement of Distributed Local Phase

Holdup,” Paper SPE 35556 presented at the European

Production Operations Conference and Exhibition, Stavanger,

Norway, (April 16-17, 1996).

900

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PE 37147 Three-Phase Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source 7

ablal: Borehole fluldconfigurations used in3-phase holdup

asurement to simulate a horizontal well.

Yw Y. YA Bottom Middle ToP Table4: Accuracy and precision (18 second) of the RST.Aoll

% % % Section Section SectIon holdup meaaurementj Yo, for the tool eccentered and centered In

1 100 0 0 Water Water Water

the borehole with sevaml casings and aegregatad borehole fluids.

2 66 33 0 Water WaterThis analysis solves for +SO and Y. at the same time.

3 33 66 0 Water

4 66 0 33 Water

: ~ ~

Water

5 33 0 66 Water

6 33 33 33 Water

7 0 100 0 oil oil oil

8 0 66 33 Oil oil Air

9 0 33 66 Oil Air Air

10 0 0 100 Air Air Air

ble 2: Formations used for >phaae holdup chamcterkatlon. Table 5: Accuracy and preclalon (18 eecond) of the RST-A

Tool Borehole Formationvolume of formation oil measurement, $S., for the tool eccentered

Poeition Diameter Llth. (:) Fluidand centered In the borehola with several casings andaegmgatedborehole flulda. This analyala solves for $430 and Y. at the came

Eccent. & Cent. 8.5 Lime 17 Water & Oil time.

Eccent. & Cent. 8.5 Lime 43

“ ~

Water & Oil

Eccent. & Cent. 10 Lime o

Eccent. & Cent. 10 Lime 47 Water &Od

Eccent. 10 Sand 33 Water & Oil

able 3: Fluid combhratlons used with the data for 2-phaae

oldup calculatlona from C/O. The holdup combinations used

mulated segregated borehole holdup In a horizontal well. Thosembinations used for calculation of interpretation coefficientse labeled as “andnolnte’.

r—–

I I s. I Y. I Y. I Endpoint I,

1 0.00 0.00 1.00 yes

2 0.00 0.33 0.67

3 0.00 0.67 0.33

4 0.00 1.00 0.00 yes

5 1.00 0.00 1.00 yes

6 1.00 0.33 0.67

7 1.00 0.67 0.33

8 1.00 1.00 0.00 yes

eig. 1: Tool positioning In borehole with different Ilners allowing

for cantered and eccedered 3-phaae holdup measurements.

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8 B.A. ROSC06 SPE 37147

; 0.8 —mw

o- 0.4 -0

L~ (),2

z

0.0

0.0 0.2 04 0.6 0.8 1.0 1.2

Near C/O (Modeled with MCBEND)

l,o~ I I I I J

’ nslar C/O Ratio .......-..~..-.......-..6..~.00--.*...-

0.0 0.2 0.4 0.6 0.8 1.0

Far C/O (Modeled with MCBEND)

o.=

z --?.0au~:

:s 1.8 –awl

.........7 .. ..... .................... ..............

o=.= w

~1.4 –

alc

0.8 1.0 1.2 1.4 1,6 1.8 2.0 2.2

Inelastic Near/Far Ratio (Modeled)

s:2~’.’..:.::1..::lz

‘-k-~-‘ 2s:.*.:*

2.0 .. . ..;.................i ................j...*. .. .....

y Y . O.(Mz . --”1

~ 1.2k------*-”----;-~m---

*I*

* v=lnn d‘a ““-- .0

l.glo’r, I I I I

0 10 20 30 40 50

porosity (%)

2.4

; 2.2 4

2 ..,..,........................................ . .. ,

:

%1.8

:

1.6 — G..; ...... : ~ ..........t?. ....._..... . .. ..........

0 +~ ‘j Yi=0,67i - ~.-

Z=1,4 — ...... . . . .......... . ................... ......... .

g Y = 1.00: ... ....... ... ... ....... .

1,2 — ................ . .. .......... . ... . .

+

}Illlil lllillj lilllli lllilllli‘“%0 o 10 30 40 50

Poroa& (“A)

Fig. 3: RST-A Inelestic nedfer countrate mtio es e funotion ofporosity measurad In IC logging mode. The tool was run

eccentered and centered Irr a 7-inch, 23-lb/ft casing in both 8.5-and 1 inchorehoies. Air was used to simuiste gaa for thasemeaauramenta. Tha iinee ara drawn to show tha trenda In the

data for various air hoidups In the borehoie.

Fig. 2: Near C/O ratio, far C/O ratio, and Inelastic nearlfar MO

from maasuramenta versus the MCBEND modelad reaulta. The

line showe a Ilnear flt to both the eccentered and centerad tooi

data.

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E 37147 Three-Phaae Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source 9

2.2- , , r I I I

Cantered Tool [j;

.0 - Ineleatlc NIF Ratio .......................{...................................

0.3 glml ges ;.

1.8 - ... .i . .. ,.. . . . .. .. .. .. .. .. . . . . . ... .;Yg=o.od “;’””””””””””’’’’”””s”x”””””

x~ =m

=-. .. ...... ..;...,.., .. .. .....:.

== : Y, = 0.33 {-’-j’’’=- -- -

~

1,0 — .......... .......... .......... .............. ...

0.8I I I I I

-lo 0 10 30 40

Poros~t~ (Y.)50

2.2

,I

,I

, I ,I

, 1’i

L

...-. - .. . .. . .. . .. . . .. . .. . .. . .. . ...+Y.- o.oo.:....~ ...........j...i.....-.J

~ :-:

*: Y= 0,67::=

1.6 — --’,.’.’.” : - .-- ’’’’..’.; ’-.- -- .’.~..’.: :’ 0. ............~

z

1,4 — .................. ... ... . . . . ... ... ... .. ...... .... ......... . ... .... ...* yY9=l. oc E

1,2 — .......... .......... ... ....... . ... . .............

Eccentered Tool\

1.0 –Inelastic NIF Retio .,: ......................................... , .,., :

0.3 g/ml Gas ;\

na I I I 1 I“

-lo 0 10 30 40 50

Poros~/y (%)

. 4: RST-A Inelaatic nearlfar countrate ratios aa a function ofrosity modeled in IC logging mode. The tooi was modeiedentered and centered in a 7-inch, 23-iblft casing in both 8.5-

d Ill-inch boraholes. Gaa was modeiad with a dansity of 0.3

i. Tha Iinss are drawn to show the trands in tha data foroua eir holdups in the borehoia.

120Tb

100R Open Points - Eccantbrsd 100I ““”””””””””””:’’””””””””””””

z

x

.= 60 --- - .;.. ... ... ... .-..-i.. .. .. .. .. .. ... ...........,.

0

zz 20 —- - -..-+.............

~

Modeied Data:

:a

-20 0 20 40 60 60 100 12C

Acutai Oii Hoidup (%)

g100 .. . ... ... ... .. .. .. . . . . ... ... .. .

na

x

E 60

0

...... .... . . .... . . .. . .

z

z 20 —’.’++

~

000

:1,

-2020 0 20 40 60 80 100 120

Actual Oli Hoidup (%)

Fig. 5: Reconstructed hoidup from modelad dsta in which thainterpretation coefficianta ware calculated from the endpoint

measursmente in 7- and 5.L&inchcasing. Thasa dsta include onlyoii and water in tha borahola and formation. Tha interpratstion

solved for both Y. and $SO simultansousiy. The line is drawn toindicata perfect reconstruction.

903

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10 B.A. ROSCOf? SPE 37747

NearC/O Ratio ,,+-----

FarCIORatio ,/T-.

Inelastk N/F Ratio,+ -----

Fig. 6: Modeled response of the RST-A tool to changes In

segregated borehole f luld compoaltion for a 16-P.u. Ilmeatoneformation with a 8.5-inch borehole and a 7-inch, 23-lb/ft casing.The date were modeled ueing gaa with e density of 0.3 g/ml.

Fig. 7: Error in the reconstruction of borahole holdups using the

modeled data of the RST-A tool. The formation used for thisanaiysia waa a 16-p.u. iimestone formation with a 8.5-inchborehole and a 7-inch, 23-lb/ft casing. The data were modeled

using gas with a density of 0.3 g/ml.

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SPE 37147 Three-Phase Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source 11

0 0 0.5 1.0 1.5 2,0 2.5

Centralization Error (inches)

50I“’I’r’ l’’’ l’’’ l’’’ l’r’ l)’ ‘l’.-

8.5-inch Borehole ~

g40 :.... 5.5-inch, 17-lb/ft Casing ~:;

st

~n~;30 —=y .............................. ..... .. ....... ...........

w

a? ~ Y.

u ~Y320 n ., ,. ..... .

z

(n~lo ---;-” , (

......... . .. ... .. ... .. .. .. .. . .. ... ... . .. .. .. .. . . ...

1b

oI I , 1,,11 I I 11 l,-

0.0 0,2 0.4 0.6 0.8 1,0 1.2 1.4 1.6

Centralization Error (inchee)

Fig. 8: Error on the 3-phase holdup estimate for atool that isassumed to becantralized when It Isreally off-center. The RMS

error calculation Includes date for both 16 and 33p.u. sandstoneformations with both water and oil Intheformatlon. In addition,the data include boreholea with 3.phase segregated fluids. Forboth casing sizes, i t appeere that there Is no signif icant IncreaaeIrr the RMS error up to about 0.5 Inches. Baeed on this analysla,

as long ss the tool is within 0.5 inches of being centered, thenthere is negligible effect on the holdup accuracy.

600

0

I I I I d

100 I I 1 4

.::”” ::t:..$..::””:”+”++~

{’:[le~ ‘

~’o~l’000 ..:.7.:............. .......’., ...-.U...:.....:..+..~ oil

I I I 1 3 3

600 700 600 900 1000 tloo

Relative Depth (ft)

Fig. 9: Example of production logging meaaurementa performedin a horizontal well drilled with an 8.5-inch bit and completad witha 5.54nch, 17-lb/f t cemented casing. The meaaurernante includePVL and WFL water velocity maasurementa, PVL oil veloclty

measurements, and RST and Ll~ holdup measurements. TheIinas on the plot connecting the data hava been added to halp

indicate the trenda.

905

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spe_37147