Formation Evaluation Using Wireline Formation Tester Pressure Data - JPT 1978

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Formation

valuation Using Wireline

Formation

Tester Pressure

ata

J. J. Smolen,

SPE-AIME, Schlumberger Well Services

L.

R. Litsey, Chevron U.S.A. Inc.

Introduction

One key to meeting our future energy requirements is

more efficient productionof new and remaining reserves.

To this end, information is needed on conditions down

hole, including accurate down-hole formation pressures.

The Schlumberger Repeat Formation TestetrM (RFT) is

an open-hole wireline device capable of providing such

pressure data with minimal demands for drilling-rig time.

The RFT may be set any number of times during a

single logging run. At each setting depth, a pretest is

made in which small samples offluid are withdrawn from

the formation. During this pretest, the fluid pressure in

the formation adjacent to the wellbore is monitored until

equilibrium formation pressure is reached. These RFT

pressure data are recorded at the surface on both analog

and high-resolution digital scales.

The pretest fluid samples are not saved. However,

after the pretests in a zone

of

interest, another larger fluid

sample can be taken optionally and retained, with the

possibility of retrieving two such fluid samples per trip in

the hole. In this paper, however, interest is directed to the

large number of pressure measurements that can be made

by setting the tool and going through the pretest cycle at

successively different levels.

Recent experience

of

Chevron U.S.A. Inc., in the

Rangely Field

of

Colorado is described to demonstrate

the quality

of

the pressure measurements and the reli

ability of tool operation. Chevron applies the pressure

information to the planning and monitoring

of

a sec-

0149-2136n9/0001-6822 OO.25

© 979 Societyof PetrolelJll Engineers of AIME

ondary-recovery waterflood project. Pressure data, in

conjunction with other data available during the drill

ing

of infill wells, were used to predict which flooded

zones would produce with a high water cut. By eliminat

ing these zones from production and by injecting into

essentially unflooded zones, the effectiveness of the

flood could be enhanced. The pressure measurements

have been used with open-hole and mud-log data to pre

dict the expected water cut. Significant pressure over

balance suppresses hydrocarbon shows on the mud

log. Pressure underbalance exaggerates hydrocarbon

shows. Both lead to erroneous water-cut predictions.

Knowledge of the pressures makes it possible

to

allow for

such errors .

Pressure profiles through the Weber sandstone reser

voir were determined in a number of wells in the Rangely

Field. Reservoir pressures were found to vary greatly and

to be distributed erratically both vertically and horizon

tally. This is attributed to the field s long history of

production and water injection and to the fact that many

of the permeable zones are discontinuous. Plotting these

pressure data on contour maps delineates areas requiring

increased flooding to maintain the effectiveness of the

waterflood program.

During the drawdown phase

of

the pretest, when fluid

is being extracted from the formation, the pressure be

havior is indicative of the minimum local permeability at

that depth. A simple technique is described for computa

tion of permeability from the pressure data, based on a

steady-state spherical flow model. Results are compared

This

paper

describes the Repeat Formation Tester

@

a tool that can make on one open-hole trip

an unlimited number

of

pressure determinations. Down-hole pressure data from the tool are

used to monitor

and

enhance the effectiveness

of

a waterflood in Rangeley Field CO.

atafrom

this tool also are used in a technique to evaluate permeability; results in U.S.

Gulf

Coast wells

are compared with those from sidewall cores.

JANUARY 1979

25



ANTI STICK

PAD

/

b

Fig. 1-Setting section of the Repeat Formation

Tester<fil

(RFT)

in retracted

and

set positions.

FLOW

LINE

EOUALIZING

VALVE

(to

mud

column)

SEAL VALVE

(to lower

sample chamber)

PACKER

;

r - - - ' i ~ F I L T E R PROBE

PRESSURE

GAGE

• CHAMBER I

CHAMBER 2

PRETEST

CHAMBER

SEAL VALVE

(to upper

sample chamber)

Fig. 2-Schematic of RFT sampling system.

26

FORMATION

PRESSURE

Fig. 3-Schematic of RFT analog-pressure recording.

with sidewall-core data gathered primarily from Gulf

Coast wells.

Tool Operation and Pressure Record

While the RFf can retrieve two fluid samples per trip in

the hole, the primary focus here

is

on its multiple-level

pressure-measuring ability. The testing section of the tool

is

illustrated in a photograph Fig. 1). The configuration

and pretest operation have been modified somewhat from

that reported earlier.

1

In Fig. la the tool

is

shown in its

retracted position; when set for a test, the tool is hydrau

lically actuated

to

the position in Fig. lb. Formation

fluids enter the tool through the probe, and the rubber

packer assures that the test is isolated hydraulically from

the well bore fluids. Integral with the probe is a slotted

filter element that is cleaned by the motion of a filter

probe piston during the setting process. Also indicated on

Fig. 1 are antis tick pads that hold the tool off the forma

tion and thereby eliminate any tendency to stick as a

resul t

of

differential pressure.

One of the original purposes of the pretest was to

assure a good retrieved sample by making a preliminary

test for hydraulic seal and sufficient permeability. This is

accomplished by monitoring the pressure with a digital

readout at the surface as small test samples

of

fluid are

withdrawn from the formation. However, the pretest

is

very useful in its own right as a pressure-measuring test.

The

RFf

pretest and sampling system

is

schematically

shown in Fig. 2. After setting the tool, the pretests are

activated automatically and sequentially. The low-flow

rate pretest using Chamber

1

withdraws 10 cm

3

of fluid

from the formation by movement

of

a piston in the pretest

chamber. This is followed immediately by the second

pretest which withdraws another 10 cm

3

at a higher flow

rate using Chamber 2). The rates of withdrawal for

different pretests vary slightly with the tool and down

hole conditions. However, where the fluid

is

produced

from the formation rapidly enough

to

fill the pretest

chambers

as

their volumes increase, the ratio of the flow

rates in the two pretest periods

is

about 1:2.5. The total

time to fill the two chambers is slightly more than 20 s.

Since the pretest withdraws only 20 cm

3

total, the fluid is

essentially all mud filtrate. In some cases, the high-flow

rate pretest may be deactivated to minimize the pressure

drawdown and the likelihood of the tool ingesting debris.

The pressure gauge is located in the flowline down

stream of the filter probe. During a pretest, the pressure

drop in the flowline

is

essentially. negligible and the

pressure indicated by the gauge is that at the formation

face in contact with the probe. A schematic of a typical

pressure profile

is

shown in Fig. 3. The pressure is

initially at hydrostatic mud) condition. When the packer

firs t engages the mud cake, the pressure ma y rise because

of packer or mud compression, followed by a decrease

due to the retraction of the filter-probe piston. When the

piston stops, the pressure builds up due to continued

compression of the packer but suddenly drops again at

the beginning of the pretest. At time t Fig. 3), the

piston in Chamber 1

is

fully withdrawn, and the first

pretest is completed.

It

is followed immediately by the

higher flow rate and, hence, larger pressure drop of the

second pretest. At time

t

2

the piston in the second

chamber is fully withdrawn, and the pressure builds up to

formation pressure.

JOURNAL OF PETROLEUM TECHNOLOGY



TABLE 1-REPEATFORMATION TESTER SPECiFiCATIONS

Pressure rating, psi

20,000

350

6

14

3

4

33

Temperature rating,

of

Minimum hole size, in.

Maximum hole size, in.

Basic make-up length (excluding options),

It

Formation-pressure readings per trip in hole

Sample chamber sizes, gal

Any number

1,2

3

/4

,6,

and 12

Pressure Measurement Specifications

Accuracy· Resolution

(psi)

Repeatability·

( )

No temperature correction

With temperature correction

Special at temperature

calibration

'Based on percent/ulf-scale, 10,OOO-psi gauge.

A typical recording is illustrated in Fig. 4. The left

track of the log

is

the analog pressure recording on a 0

to

10,000 psi (0 to 68.95 MPa) scale. This recording

is

an

excellent means of

evaluating quickly the integrity and

general character of the pretest and the producibility

of

a

formation. However, for any quantitative evaluation, a

four-scale digital recording in the right tracks offers high

resolution and accuracy. For example, hydrostatic pres

sure, indicated before the tool begins setting,

is

4,349 psi

(29.99 MPa). (See numbers beside curves at top

of

log.)

Near the bottom

ofthe

log, the pressure ultimately builds

up smoothly to a shut-in formation pressure of 3,850 psi

(26.55 MPa). The pressure during the first pretest is

drawn down to about 1,850 psi (12.76 MPa), while it is

drawn down

to

about 100 psi (0.69 MPa) during the

second pretest.

RFT specifications are summarized in Table 1.

ANALOG PRESSURE

DIGITAL

PRESSURE RECORDING,

psi

RECORDING, psi

0

10000

1

1000

T e ~ N o

,I 100

1 ) J )

0

0

I

r ~

1

~

HYDROSTATIC PRESSURE-

t

40901 1300

i O ~ ·

~ ~ ~

TOOL SETTING

6

Sec

I ~

•

I

1.

-=

r = ~

It

I · 0

'-

I

p ~

r il

'

J

I

t

EiS1

-, .

I

fCD

lJ

1=:::=

::;t::

F-

t

1 .12

I

:..r-.

'

':=

~

-1=

i

h

i

I

1'-

I

i

i

rl...

I

~

i

R

i

SHUT-IN PRESSURE

FIitOM PRETEST

i

l

-

Fig. Typica l RFT pretest pressure record, showing both

analog and digital pressure scales. Recording

is

made in camera

at surface.

JANUARY 1979

( )

0.98

0.29

1.0

1.0

0.05

0.05

0.18

1.0

0.05

Pressure Measurements in Rangely Field

Rangely Field (operated by Chevron) is an anticlinal

closure located in northwestern Colorado on an arch

between the Piceance and Uintah basins (Fig. 5). The

principal production

is

from the Permo-Penn Weber for

mation, which is composed of 600 to 900 ft(183

to

274

m)

of

interbedded sands, silts, and shales. This formation

interfingers across the structure

to

the south and southeast

into the arkosic Maroon formation. This reservoir com

prises a series

of

irre gular porous zones within the thicker

sand units. The porous sands are separated by imperme

able intervals

of

silt, shale, and tight, well-cemented

sand. In general, porosity within the reservoir averages

about 15 . Permeability ranges up to several hundred

millidarcies but averages between 5 and 50 md.

2

Well

to-well correlation of the producible sands

is

difficult

• PROOUCING WELLS

INJECTION WELLS

STUOY RE

Fig.

SB--Structure

of Rangely Field as mapped on top of

Weber formation.

27



T BLE 2 RESULTS OF PRESSURE TESTS REPE TED T OR NE R THE

S ME DEPTHS N R NGLEY FIELD

Well

Number

4

8

Depth

5,767

5,768

6,055

6,055

6,063

6,064

6,165

6,165

6,166

6,135

6,135.5

6,276

6,276

Porosity, q

( )

17.0

18.0

15.5

15.5

16.0

16.0

15.5

15.5

14.0

15.5

15.0

17.0

17.0

9 6,520 14.5

6,520 14.5

6

6,551 14.5

6,551 14.5

8

~ 7 8

15

5,785 15

6,100 16.0

6,100 16.0

6,276 8.0

6,276 8.0

21

5,980 13.5

5,980 13.5

22 5,967.5 4

5,968 4

23 5,995 13

5,997 13

24 6,104.5 12

6,105 12

because of lateral heterogeneity. This is partly because

porous zones may not necessarily correspond to deposi

tional units. The field

is

now under waterflood.

The RFf has been used in 25 recent infill wells to

obtain pressures in the Weber sandstone reservoir. No

fluid samples were taken. The tool was set in zones

having porosities, as determined from the density log,

from

10

to 22 . In wells tested between July 1976 and

June 1977, 643 pressure measurements were attempted in

390 different porous intervals. Of these attempts, there

were 21 seal failures and 274 dry tests. Seven

of

the

21

seal failures occurred on the first two logging runs. The

dry

tests are attributed to setting the tool in tight streaks in

the formation, although it is possible that some dry tests

may have been caused by either plugging of the probe or

formation damage from drilling operations. Pressures

were obtained in 317 of the porous intervals, or about

80 of those tested. While the tool was stuck in one well

(apparently as a result of key seating of the cable), stick

ing

of

he tool i tself has not been a problem.

Duplication of the pressure measurements has been

excellent. Pressure tests have been repeated at or near the

same depth in 15 different porous zones. These results are

summarized in Table 2. In two-thirds of the tests, the

pressures repeat to within less than 5 psi (0.035 MPa). In

the remaining tests, the highly discrepant readings appear

to be caused by failure

of

the pressure to fully build up to

that of the formation when the pretest ended. This may be

caused by the tool probe being positioned in a low

permeability streak, which results in an abnormally long

buildup period.

28

Pressure

(psi)

1,619

1,618

2,625

2,625

2,627

2,629

2,535

2,533

2,535

2,909

2,908

2,809

2,810

3,236

3,147

1,436

1,437

2,876

2,864

3,018

3,020

3,060

2,810

2,913

2,909

2,035

2,037

3,010

2,953

892

870

Buildup

Time

(minutes)

0.5

3.5

2.0

2.5

2.0

1.0

2.0

2.0

1.5

3.0

3.5

1.0

1.0

3.9

8.3

0.9

0.8

1.0

1.3

1.3

3.1

3.0

11.4

2.0

1.8

1.8

1.9

0.9

4.6

4.6

4.0

Pressure

Difference

(psi)

o

2

2

89

12

2

250

4

2

57

22

Pressures have been obtained with pressure bombs run

after swab testing in 29 of the zones where RFf pressures

were measured.

RFf

pressures averaged about 75 psi

(0.52 MPa) higher than those measured with the pres

sure bomb after swabbing the well and allowing it to fill

up and stabilize, but pressure differences

as

high as 460

psi (3.172 MPa) were seen. The pressure differences

were less than 200 psi (1.379 MPa) in 22 cases and less

than 100 psi (0.69 MPa) in 14 cases. The values of stabi

lized pressures obtained after swabbing are subject to

some error because the buildup times generally were

less than 15 hours, and considerable extrapolation was

required in many cases.

stimation of Water

ut

Rangely Field has been under waterflood since 1958 and

has many wells that produce with a high water cut.

n

this

field, the RFf is run immediately following wireline

logging. The density log is used to select the porous test

intervals and the gamma ray curve

is

used to delineate the

cleaner zones in the porous interval. A porosity cutoff of

10

is

used in the Weber reservoir

to

define producible

reservoirs. Generally, pressure measurments have been

attempted in 15 to 20 zones in each well. Fig. 6 shows a

typical pressure profile and open-hole logs. For conve

nience, the Weber reservoir traditionally has been sub

divided into at least five zones, designated A through E

in Fig. 6.

In recent development drilling, a method was needed

to estimate oil and water cuts in the various porous sands

without testing to eliminate high-water-cut sands from

JOURNAL

OF

PETROLEUM TECHNOLOGY



the productive intervals. One method that has proven

useful is to combine RFf pressure data with information

provided by monitoring the drilling mud.

The mud log has been used extensively

to

detect and

evaluate potentially productive reservoirs in exploratory

wells by monitoring the drilling mud for hydrocarbons.

In some areas it also has been used for evaluating de

velopment wells. Gas-detection equipment monitors the

drilling fluid

to

detect the presence of hydrocarbon and to

measure the relative amount. The amount of hydrocar

bon entering the drilling mud while drilling through a

hydrocarbon-bearing reservoir

is

affected by several

factors. These are primarily formation pressure, mud

weight, saturation, porosity, and penetration rate. Poros

ity information is available from a porosity log, and

penetration rate and mud weight are recorded during

drilling operations, but until the advent of the RFf

formation pressures were not readily available.

Formation pressures obtained with the

RFf

are used to

determine the pressure overbalance (amount the hydro

static pressure in the mud column at time of drilling

exceeds the formation pressure). Mud-log gas shows will

be strongly suppressed in zones where the pressure over

balance is high, whereas they will be exaggerated in

zones where the overbalance is low or negative. The

pressure ovetbalance therefore is

used

to

qualify the

interpretation

of

mud-log shows when estimating oil and

water cuts. To date, these pressure overbalances have

been used in a qualitative sense only.

COMPENSATED

FORMATION DENSITY

LOG

P R E S S ~ E PROFILE

REPEAT FORMATION TESTER

GAMMA RAY

BULK DENSITY

RESERVOIR

PRESSURE

2.0

psi)

2000 3000

A

<==--I: j

1

f----- J-----t;r ---- ~

C

~ . 1 1 - - - - - - - ~ - - - - - - - - - - - - 18T

~

Fig. 6-Density log and pressure profile showing typical Weber

reservoir and Reservoir Zones A through E

JANUARY 1979

Fig. 7 shows a typical portion of the reservoir in which

five pressure tests were conducted in porous intervals.

The pressure overbalance is indicated by a bar at the

setting depth. The drilling penetration rates in Zones I

and IV were about equal. In Zone IV, the mud log shows

a strong 112-gas-unit anomaly with a pressure ovetbal

ance of 32 psi (2.21 MPa). Swab tests proved this zone

to be 100 oil productive. In Zone I, the mud log is more

pessimistic, with a maximum reading of only 63 gas

units, leading to an interpretationof significant expected

water cut. Overbalance pressure, however,

is

520 psi

(3.59 MPa), significantly larger than for Zone IV. Swab

tests showed this zone also to be 100 oil productive,

suggesting that pressure overbalance may have sup

pressed the mud-log reading. While Zone V was not

swab tested, the extreme pressure overbalance

of

2,073

psi (14.29 MPa) indicates a highly suppressed mud-log

show, and consequently, even in view of the indications

on the mud log in Fig. 7, the zone might be expected to

yield a low water cut.

Analysis of Pressure ProfIles

and

Maps

Besides estimating oil and water cuts, pressures from the

RFf used with wireline logs can provide information

about reservoir continuity and the effectiveness of the

waterflood program. When such pressures are plotted

as

profiles for well-to-well comparison (Fig. 8), one strik

ing feature that emerges is the great variation in pres

sures, both laterally and vertically. However, with

PRESSURE

OVERBALANCE

1000 2000

PSI

Fig. Typical mud-log gas curve with pressure overbalance

excess of hole pressure at time of drilling over formation

pressure) and open-hole logs.

29



WELL

II

WELL 2 WELL 4 WELL 8 WELL 5 WELL 4 WELL 24 WELL 2

WELL I

psi)

2000

2000 0 1000

2000

3000 2000 2000

2000

2000 2000

2000

O ~ - L - t T ~ O = = P ~ W ~ E ~ B ~ E ~ R = r ~ - - ~ ~ ~ - L - - - - L - ~ - L - - ~ - - r - ~ - - ~ - t ~ L - - - ~ ~ ~ - - - - ~ i - ~ - - ~ - - r - ~

100

200

300

500

600

ft.

®

®

Fig.

8-Cross-section

(along line of section shown in Fig. 9) illustrating lateral and vertical pressure variations.

further study

of

the cross-sections and with average pres

sure data displayed and contoured on maps, coherent

trends appear.

Some anomalous pressures are exemplified

by the tests

in the lowermost sand

of

Zone B in Wells

11

and

12

in

Fig. 8. While these sands occur at the same stratigraphic

position, their pressures differ by about 1,000 psi (6.9

MPa) in these adjacent wells. They also differ greatly in

pressure from the sands above and below this zone. These

sands apparently are isolated from the main reservoir and

from each other.

Maps of average pressure in the various reservoir

zones portray reasonable patterns and trends of pressure

variation, in spite of the radical local pressure changes

noted above. A pressure-contour map of Zone D (Fig. 9)

illustrates trends established in the area of study. Similar

trends of low- and high-pressure areas persist through all

reservoir zones, although significant deviations from the

trends do occur. The pressures and variations

in

pressure

(as demonstrated by profIles, cross-sections, and maps)

•

•

.

,.

•

.

•

.

LINE OF SE TION

•

FOR FIGURE 8

•

•

•

•

•

•

•

•

•

2.

•

•

?

•

.

•

can be used as one measure

of

the effectiveness of the

Rangely Waterflood Project. The pressure maps clearly

document the presence

of

low-pressure areas that need

additional water injection to restore the reservoir pressure

to a value near the original pressure

of

2,750 psi (18.96

MPa). About 50

of

the study area has pressures greater

than 2,500 psi (17.24 MPa) and is being flooded effec

tively, but about 20 of the area is greatly underpres

sureq at less than 2,000 psi (13.79 MPa).

These pressure measurements have complemented log

analysis when interpreting reservoir continuity in this

complex reservoir. In some areas, where log correlations

are difficult and sands appear to be discontinuous, pres

sure uniformity suggests that the reservoirs are, in fact,

continuous or connected. In contrast, in other areas

where porous sands can be correlated more easily, large

pressure variations suggest reservoir discontinuity, or at

least greatly reduced lateral permeability. Uniformity of

pressures vertically across zone boundaries suggests that

vertical reservoir continuity also exists in many areas. In

.

•

•

•

.

,.

- -------.-

•

I

•

•

•

•

•

•

?

•

20

@15

236

•

0 :

•

.

•

854

,.

0

0

•

.

•

0

@13/

29

•

,

377

/..

•

?

.

Fig.

9-Contour

map showing average pressures

in

Zone 0 of Weber reservoir. Pressures corrected to datum of -900ft - 274m).

30




summary, log correlation alone is not enough to predict

reservoir continuity with any degree

of

accuracy in this

type of depositional environment, where stratigraphic

changes occur in distances less than the well spacing.

Permeability Evaluation

The pressure differentials during the pretests (indicated

by

t::.Pl

and

t::.P2

in Fig. 3) are sensitive to the formation

permeability. Generally, the greater the pressure de

crease required to maintain the pretest flow rate, the

lower the permeability. For very low permeabilities, the

chambers are drawn to near-vacuum conditions since the

formation is not capable of producing at the required rate

and flow rate

is

reduced. I f the formation

is

isotropic and

the flow is spherical in character, the analog pressure

record may be used for a quick indication

of

permeability

(Fig. 10). (Compare the difference values between draw

down and final formation pressures.)

Quantitative evaluation of permeability is based on

steady-state spherical type flow into the probe. The per-

u

«

til

o

5

>

x

--

1

I

1

J

10.1

a::

::>

( /)

( /)

10.1

a::

a..

I

1

..

-10.1

I-a::

«::>

til /)

oa::

> 0..

,x

I

I

i

\

-

\

\

..

1

. I

I

ABOUT

I O O ~ d

a

I

I

L

I I I I

ABOUT IOmd

b

-

III

ABOUT I md

C

I

1I I I I I I I I I l l l i l

ABOUT.I md

d

I

I

I

T I G H T I

e

[

. 1

-

Fig. 10-Permeability estimates from pressure records

assuming isotropic formations.

JANUARY 1979

meability is given b

y

3

k

=

Fqf-t . . . . . . . . . . . . . . . . . . . . . . . . .

(la)

21T1 t::.p

which, in more convenient units, becomes

k = 3,300 qf t

t::.p

or, in metric units,

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

(lb)

k = 22.75 qf t

t::.p

where the quantities in Eqs. 1a and 1b, and the units in

Eq. 1b, are

k = permeability, md

q = flow rate, cm

3

/s

r = probe radius = 0.21 in. (5.33 mm)

f-t = fluid viscosity (usually filtrate), cp

(mPa s)

t::.p

= drawdown from formation pressure, psi

(MPa)

F

= flow-shape factor =

1.00 for hemispherical flow,

0.75

for

'quasispherical borehole

corrected flow for 8-in. (20.3-cm)

wellbore,

0.50 for spherical flow.

The flow regimes associated with different values

of

the

flow-shape factor, F are summarized in Fig. 11. Eq. 1b

incorporates the value

F

= 0.75, based on computer

simulations. This approximates flow conditions for the

tool set in an 8-in. (20.3-cm) borehole (lowermost flow

F =

1 0

HEMISPHERICAL

FLOW

F

=.75

\

/\

F =.5

SPHERICAL

FLOW

BOREHOLE

CORRECTED

FLOW

Fig.

11-Aow

regimes and associated flow shape factors F.

31



,:

. .

+

+

100

ct {'

t

It'

·

+

....

.,) '

.

r

t

;

k ,·i

j

10

..

r

q

l , , / ~ ; i

i '

I

ii

l,,)'/

1 ,, /'

. Cor. Data Point

verOQe

t

Cor. Data

0.11 ,)'

0 1

1.0

10

100 1000

PRETEST

DRAWDOWN

PERMEABILITY md.)

Fig. 12-Comparison of permeability estimates based on

average of nearby sidewall cores with those computed from

high-flow-rate pretests.

regime in Fig. 11).

As an illustration, this equation may be applied to the

recording in Fig. 4. Formation pressure, as indicated by

the digital log, is 3,850 psi (26.55 MPa). The pressure

during the first pretest

is

drawn down to about 1,850 psi

(12.76 MPa) , and ~ =

3,850-1,850

= 2,000 psi

(13.79 MPa). Assuming the pretest withdraws

m

3

at a

constant rate, and since the time for the first pretes t is 16

s,

the flow rate, q, equals 10 cm

3

/16 s = 0.625 cm

3

/s.

Assuming a viscosity of 0.5 cp (0.5 mPa

s),

which is

typical for mud filtrate down hole, the permeability indi

cated is

=

3 300

x

(0.625) (0.5) 0 52

d

, 2,000

=

m

which, in the metric units, is

= 22.75

x

(0.625)(0.5) = 0.52 md.

13.79

In a similar manner, the permeability indicated by the

second pretest is about 0.54 md.

This technique has been applied to Texas Gulf Coast

area data, where sidewall-core information was available

for comparison. Because of the high variability in down

hole permeability, a comparison was made between an

average

of

permeability from sidewall cores within 2 ft

(60.96 cm)

of

the setting depth and permeability based on

the high-flow-rate pretest. This comparison

is

plotted in

Fig. 12. In nearly all cases, the permeability from the

pretest is less than or about equal to that indicated by the

average core. This is consistent with what is expected on

32

the following bases .

1. Sidewall-core permeabilities have been reported

as

tending to be too high for permeabilities less than about

20

md

and too low for greater permeabilities in Gulf

Coast sands.

4

2. Pretest permeability may be too low becauseof skin

damage.

3.

f

the permeability

is

actually layered and aniso

tropic, larger pressure drawdowns would be required to

maintain flow rate than if permeability were isotropic .

This would result

in

a lower permeability value,

as

calcu

lated on the basis of a spherical model.

Therefore, it appears that permeability measurements

using this technique are credible and realistic.

Summary

The Repeat Formation Tester is an open-hole wireline

tool capable of providing accurate and repeatable down

hole measurements of formation pressures. Such pres

sure information has been applied by Chevron to monitor

the effectiveness of its waterflood program in the

Rangely Field of Colorado. Down-hole pressures, in

conjunction with mud logs and other open-hole logs, can

improve estimates of predicted water and oil cuts. Con

tour maps of pressure for each zone delineate areas

of high and low pressures and, hence, provide insight

into the geographic dis tribution

of

the waterflood

effectiveness.

Experience, primarily in Gulf Coast sands, tends to

support permeability estimates based on the drawdown

phase of the pressure measurement.

cknowledgments

We thank Chevron U.S.A. Inc. for permission touse and

publish pressure data acquired in Rangely Field, CO,

during 1976 and 1977. We also thank R. E. Hobart of

Chevron, who

is

responsible for developing much of the

pressure-mud log applications in Rangely Field, and

T.

H.

Zimmerman

of

Schlumberger for his assistance

with the RFT permeability evaluations.

References

I. Schultz, A. L. Bell, W.

T.,

and Urbanosky,

H.

J.: Advancements

in Uncased-Hole Wireline-Formation-Tester Techni ques, 1 Pet

Tech.

(Nov. 1975) 1331-1336.

2. Larson, T. C.: Geological Considerations of the Weber Sand

Reservoir, Rangely Field, Colorado, paper SPE 5023 presented at

the

SPE-AIME 49th Annual Fall Meeting, Houston, Oct. 6-9, 1974.

3. Moran, J. H. and Finklea, E. E.: Theoretical Analysis

of

Pressure

Phenomena Associated with the Wireline Formation Tester,

1 Pet. Tech. (Aug. 1962) 899-908.

4. Reudelhuber, F. O. and Furen, J. E.: Interpretation and Applica

tion of Sidewall Core Analysis Data,

Trans.

Gulf Coast Assn.

of

Geological Societies (1957) VII. PT

Orginal manuscript race iIIed in Society of Petroleum Engineers office Aug. 26, 1977.

Paper accepted for publication June 9, 1978. Revised manuscript received Sept. 11,

1978. Paper SPE 6822) first presented at the SPE-A IME 52nd Annual Fall Technical

Conference

and

Exhib.ion, held in Denver, Oct. 9-12, 1977.


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Formation Evaluation Using Wireline Formation Tester Pressure Data - JPT 1978

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