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Use
of
Recording Pressure Gages
in
Drill Stem Tests
R
S.
HRISTIE
ABSTRACT
The author defines briefly the drill-sten1 test em -
when making this test nd discusses at length the inter-
phasizing the inanortanre of obtaining a pressure record
pretation of records obtained.
drill-stem t est may be made to obtain indications of
the potentialities of a suspected oil o r gas sand to test
fo r water shut-off af te r cementing pip? or to determine
the casing point in a known producing horizon. The
drill-stem test is obtained by running on th e bottom of
the drill pipe a valve which can be opened and closed
from the surface. All the formations in th e hole ar e
packed off with the exception of that section of the hole
to be tested. The drill pipe is run into the hole with the
testing-tool valve closed. Aft er t he packer has been set
the valve i s opened; a nd liquid or gas a t the testing point
ente rs the drill pipe. I n all cases the success of the test
depends on the successful operation of the testing tool.
Whether o r not the valve functions properly can be quite
positively determined on the surf ace f or shallower wells
but becomes less positive with increasing depth. It is
important to obtain a s much information a s possible on
each test.
In a test fo r wate r shut-off ther e is little or no open
hole below the casing in the well close to the point of
testing and consequently small hazard. On the other
hand a formation test or casing-point test is usually
made with several hundred feet of open hole with re-
sult ant dang er of sticking the drill pipe by settling cut-
ting s or caving. If i t becomes necessary to run th e test
several times the walls may be weakened and cause
trouble when drilling is resumed.
A kecord of the changes in p ressu re below the testi ng
tool will make more positive the information ordinarily
obtained by such a test and frequently give informa-
tion t ha t could not otherwise be obtained. A recording
pressure gage can be placed into a special container
which can be a part of the anchor of the testing tool.
Fig. shows one type of container.
Purpose of Test
The primar y purpose of a drill-stem te st i s to deter-
mine whether a given formation contains gas oil or
water. The test may give negative results in the face
of other positive indications. This is particularly t ru e
in deep holes a s it i s not always possible to ascertain
whether the packer is holding and the valve function-
ing properly. When the va lve is opened and the fluid
Amerada Petroleum Corp.. Fort Worth Te s.
does not lower appreciably a t the top of th e hole it i s
assumed the test is successful; but this is not always
true because the fluid may drop due to leaky tool joints
Container for Pressure Gage for Use in Drill-Stem Tests.
FIG 1
or because of loss into some formation or may fluctua te
due to gas or ai r pockets-all despite the fac t th at the
packer may have held and the valve opened to give a
test. To eliminate an y doubt a s to the mechanical opera-
tions of the tester a recording pressure ga gs below the
teste r will record differences in pressures occasioned by
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Pressure I l s p m
IDW ooo aow
USE
OF
RECORDING RESSUREAGES
N
DRILL STEMTESTS
Pressure bs
JV
he operation of t he tester.
Fig. 2 shows how the tester
unctioned in a well a t a depth of 5 685 ft. The test was
for a water shut-off. Without th e pressure record the
est would have been int erpreted a s a successful shut-
ff. A subsequent test proved th at wa ter was not shut
off. Fig. 3 shows a tes t taken on a well a t a depth.of
3 463 ft. At the end of about 5 min. the full weight of
the dril l pipe was on the packer. The fluid dropped
Drill-Stem Test of Water Effect of Pressure of Par- Pressure changes in a Drill-
Shut-off at 5 685 Ft.-The tially--plugged Perforations or Stem Test of a Gas Sand.
degree of accuracy t he capacity of th e well to produce
whether i t be gas, oil, or water. In a gas well the open
flow can be determined from the drop in pressure and
the ra te of flow. Fig. 4 is a pressure chart taken in a
well which produced gas a t the rat e of 5 400 000 cu.
ft. per day, with a drop in pressure of 262 Ib., from
a reservoir pressure of 983 lb. The colnputed open-flow
capacity of t he san d is about 20 million cu. ft. of gas
Testing-Tool Valve Failed to Choke in the Testing Tool.
Open.
FIG.
FIG.
3
bout 30 ft. During the total time of 33 min. there was
o change in the fluid. level until the valve was closed..
From th e irreg ular ity of the ch art an d from indications
n the surface, it is reasonable to believe the bottom
hoke wa s part ially plugged. With a small opening and
h differential pres sure when t he valve is opened,
uch a result may be obtained.
An analysis of the pressures recorded shows whether
testing tool functioned properly, and indicates other
ulportan t factors. I t is possible to obtain with some
FIG. 4
per day. The sta tic reservoir pressure was ,obtained
by not unseating the packer for 2 min. after the bot-
tom valve had been closed, which was sufficient time
fo r a p ressure ,reading. A st atic reservoir pressure ob-
tained a t this s tag e of completion will be
Inore nearly
the original formation pressure than any subsequent
pressure.
Oil-Well Application
A similar method can also be applied to an oil well,
bu t sufficient tes ts have not a s yet been obtained to de-
term ine dependability. Fig.
5
presents data obtained
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on a drill-stem te st taken a t a depth of 4,418 ft. With
th e tes ter valve open 20 min., 614x1. dril l pipe filled 900
ft . with oil and 180 ft. with drilling mud. On this basis
the calculated flow was 102 bbl. per hour, with a bot-
tom-hole pressure of approximately 300 lb. A produc-
tion test after completion obtained approsinlately 60
bbl. an hour, wi th bottom-hole pres sure of 1,150 lb.
and a low gas-oil ratio. Fig . 6 shows the results of a
drill-stem test th at produced water. The test was taken
5m
Pressure l s q
m
lorn 1600 2000
may show a section of mud th at is gas-cut and much
lighter than the remainde r of the column. Fu rth er test-
ing may prove this to be caused by a gas- or oil-bearing
formation which has been passed up.
Even though no
other formation is obtained, a pressure gradient gives a
graphic p ictu re of the condition of th e mud in the hole.
From such a picture it may be found advisable to con-
dition the mud before further testing or drilling ahead.
I n Fig. th e measured hydrostatic head was 2,915 Ib.
Pressure Changes in a Drill- Drill-Stem Test of a Water Drill-Sten1 Test of a Water
Stem Test of an Oil Sand.
Sand at 3 270 Ft.-Calculated
Sand at 5 704 Ft. and Check for
Plow
60
Bbl. Per Hour.
Mud Condition.
FIG.
5
FIG.
6
FIG. 7
a t a depth of 3,270 ft . The decrease in pres sure between
the making up of the last stand of drill pipe and the
setting of the packer may have been caused by the
formation tak ing fluid. Fr om the volume of fluid in
the drill pipe, the estimated flow was approximately
60 bbl. per hour. The bottom-hole pressu re while flow-
in g at this ra te w as 1,424 lb., which is sufficient pres-
sure for an artesian well.
The hydrostat ic head calculated from the weight of t he
mud was 2,930 lb., which proved the entire colunln of
mud was in good condition. Occasionally mud is heav ier
than necessary, bu t tests of formation pressure ar e more
likely to prove th at for safety the mud should be
heavier.
Well Completion
Pressure Gradient
While running th e gage, a i~ res su re radient of the
drilling fluid is also obtained. A study of this gradient
Many wells a re completed by landiiig pipe on bottom,
then perfora ting the pipe a t those points where indica-
tions of oil were encountered while drilling. drill-
sten1 test of t he perfora tion may frequently he quicker
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and more economical than bailing. The pressure record
of such a test shown in Fig.
8
will aid in interp retation
of th e test. When th e bottoln valve was opened the
pres sure dropped to allnost 0 and only small gas blows
by heads occurred during the hour the valve was open.
I t was known from a previous drill-stem test th at th e
potential of t he s and wa s much gre ater tha n was ob-
tained by the lat er test. Several additional perforations
and test s were required before sufficient information w as
gained to permit p rope r completion of t he well.
The usual method of t estin g a cement job i s by bail-
ing th e hole and allowing i t to stan d to determine if fluid
is entering the hole. Freq uen tly a drill-stem test can be
substituted . This method will prove whether a shut-off
valve
v lve
opened
closed
Drill-Stenn Test after Perforating Pipe in a Gas Horizon
at
4 005
Ft.
FIG.
8
is obtained and often a t less expense. The pressure
gage will show the amount a nd ra te -a t which the wate r
is corning into the hole if th e shut-off job is unsuccessful.
Fig. 9 is a pressu re record of a n unsuccessful wate r
shut-off. The hole contained wate r a s a drilling fluid.
The drill pipe was run into the hole empty; and upon
opening the tes ter valve approxima tely 30 min. was
required to fill the drill stem up to the top and flow a
small amount of water.
Fig.
10
is a pressure chart on a drill-stein test of a
successful shut-off. When pulled out the drill pipe con-
tained nothing bu t 5 ft . of mud in the bottoni.
CONCLUSION
drill-stem test is usually a satisfactory method of
testing a formation to determine the advisability of
setting pipe fo r a production test. I t is much more sat-
isfactory when t l~e
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estimated, the reservoir p ressure measured, an d the con-
dition of the drilling fluid determined. Additional ap-
plications are determining the success of perforation
jobs, wate r shut-off jobs, a nd i n testing fo r casing seats.
Just as all improvements in the drilling and produc-
ing of oil wells become st an da rd practice, so also will
the use of recording pressure gages in drill-stem tests
hecorne an essential pa rt of th e test.
Chairman
J
T. Hayw ard (Barnsda ll Oil Company)
We have heard a very interesting paper, and on some-
thin g th at is absolutely new. I understand th at a num-
ber of tests shown here were only taken during the
las t month. We have all stood around the top of the
hole when drill-stem tests were made, and watched
the mud. Aft er a longer or shorte r time, depending
on conditions, it begins to fall; and, when the tester
was pulled out, we had to guess whether there was
anyt hing ther e or not. Usually, 50 per cen t of the
guesses have been wrong, I suppose. Now we have a
method t ha t enables u s t o overcome th at difficulty.
G
E.
Cannon (Hum ble Oil and Refining Company)
The author states that 2 min. was long enough time to
obtain the forma tion pressure. We used similar methods
in making tests, and found that the maximum pressure
was not obtained until the end of 15 min. Of course,
our plan vari es fr om the, one you use.
Another peculiar t hi ng was th at, in some of these
tests, just before the packer was set, a record was ob-
tained of the hydrostatic pressure, and aft er unseating
the packer, there was a reduction in the hydrostatic
pressure by as much a s to 10 per cent, and there was
no loss of fluid.
I
thought that was quite interesting.
Mr. Christie: Th at is a n unusual condition. I t might
be well to ru n a recording gag e in the pipe and outside
of it. Pe rha ps some of th e loss is due to leaky tool
joints, and going into th e tool pipe.
Mr. Cannon: No, th e hole wa s full of mud. We set
the packer; and, aft er unseating the packer, it was full.
There was no loss there.
Mr. Christie: I have no explanation for that.
A Voice: The aut hor cites a case where the calcu-
lated flow was 102 bbl. pe r hour with a bottom-hole pres-
sure of approximately 300 lb., and another instance
where the production t est w as 60 bbl. a n hour, with
bottom-hole press ure of 1,150 Ib. I s t ha t indicative of
the accuracy of the test?
Mr. Christie: As a m att er of fact, the pressure
obtained on the drill-stem test fell almost esactly on
the production curve aft er completion. We had a pres-
sure here of 1,150 Ib., with 60 bbl. a n ho ur on the pro -
duction test; and on the drill-stem test we had 102 bbl.,
with a pres sure of 300 lb. Of course, th e first test, unless
taken for a considerable length of time, is not a t ru e
test, because i t takes some time for th e sand to clean up.
We have subsequently taken a second test in that well,
and i t shows the well to have dropped off some. I t
didn t follow the fir st curve.
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