Note: Descriptions are shown in the official language in which they were submitted.
53~:
FIELD OF THE INVENTION
This invention relates to a method for monitoring
the injectivity of a two phase fluid along a well bore as it
is injected through a well into a subterranean formation.
DESCRIPTION OF THE PRIOR ART
The injection of two phase fluids into subsurface
earth formations has become increasingly wlde spread in the
last few years, particularly with regard to enhanced oil
recovery projects. Unfortunately, this greatly increased use
has not been accompanied by any significant increase in the
knowledge of the behavior of different fluid phases within
the well bore. Detailed information regarding the
permeability of the zone along the well bore to each
component of the two phase fluid is necessary in order to
design an effective injection program. Knowledge of the
injectivity of the gas and liquid phases over an interval of
interest within the well bore provides important information
necessary in many usages, among them are the design of above
ground injection equipment and down hole maintenance projects
such as perforating, fracturing and cementing as well as
process performance analysis.
Two-phase fluids are often injected during the
course of enhanced recovery operations. The injection of
steam containing both water liquid and water vapor during the
course of a steam flooding project is perhaps the most
common. Mixtures of water liquid and air are often used in in
situ combustion processes. Liquid hydrocarbons and carbcn
dioxide are often injected together in the course of a
; miscible flooding project. In a like manner, carbon dioxide
can be mixed with water in certain recovery operations.
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53'~
At the present time there exists no measuring
process which can effectively and accurately describe the
injectivity response for both phases of a two phase fluid as
it is injected over an interval in the well bore. One method
of obtaining an injectivity profile or permeability log for a
one phase fluid in a particular formation traversed by a bore
hole is described in U. S. Patent No. 2,700,734 granted to
Edmund F. Egan and Gerhard Herzog on January 25, 1955. In
this method, two streams of fluid are introduced into a well,
one stream passing through a string of tubing extending
downwardly to a point below th^e formation of interest and the
other stream passing downwardly through the annular space
through the tubing and the casing of the wall of the well.
These streams are introduced or pumped into the well
simultaneously and each stream is carefully metered at the
surface. Fluid pumped down the tubing will, after filing the
exposed portion of the well below the tubing, flow upwardly
around the tubing until it meets the fluid flowing downwardly
through the annular space, thus, forming an interface between
the two streams or bodies of fluid. In order to locate the
interface between the two streams, a small amount of tracer
material, such as a radioactive substance, is added to one of
the streams before it enters the well so that the fluid in
this stream will be radioactive while the other stream will
be nonradioactive. The depth in the well at which the
interface lies may be readily located by lowering the
detector, for example, a radiation detector, into the we]l
and simultaneously and continuously recording the depth of
the detector and the output signal therefrom. The response
of the detector will change abruptly when the detector passes
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from the radioactive fluid into the nonradioactive fluid or
vice versa.
By this method in order to determine the amount of
fluid that is entering into a vertical increment of the
formation of interest, the rates of injection of each of the
two streams are varied but the sum of the rates is maintained
constant. By changing the ratio of the amount of the
radioactive fluid to the amount of nonradioactive fluid
injected into the well the interface will be forced to move
to another depth in the well. The difference in the amount of
either of the fluids injected into the well is the amount of
fluid that is entering the vertical increment of the
formation between two interfaces. It can be seen that, by
making appropriate changes in the ratio of the amount of
radioactive fluid to the amount of nonradioactive fluid
pumped into the well, the interface can be moved in a number
of steps through the well past the formation of interest to
provide an accurate log of the permeability of the formation.
The length of each of the vertical increments between the
successive interfaces depends upon the amount of change of
the rates of the two streams and the permeability of the
increment. After each adjustment or change in the rates of
the two streams and after the interface between the two
fluids has been stabilized, the rate of flow of the two
streams is noted and the radiation detector passed through
the well to determine the depth of the interface.
Accordingly, it can be seen that in this manner an
injectivity profile log is made of a formation which clearly
shows the permeability of the various components of the
formation.
l~Z753,2
~ owever, the use of th:is invention is restricted to
the measurement of a single fluid phase. The following
U. S. Patents are similarly restricted and contain various
improvements and refinements based upon the above referenced
patent: U. S. Patent Nos. 2,869,642, issued to McKay and
Egan; 3,010,023, issued to Egan, Widmyer and McKay;
3,100,258, issued to tenBrink and Widmyer; and 3,105,900,
issued to Widmyer. None of these patents however, indicate
that their practice may be extended to a usage which involves
the measurement of the injectivity of both phases of a two
phase injected fluid system.
An accurate description of the different injectiv-
ities of each component in a two phase injected fluid system
is an extremely important piece of information. In most, if
not all cases involving the injection of a two phase fluid
system the injection behavior of one phase is quite different
from that of the other phase. It can be readily appreciated
that, in a case such as an in situ combustion program
comprising injection of both water liquid and air, the
relative amount of each phase being injected at a given point
in the well bore is of crucial importance to the success of
the injection program. Since knowledge of the injectivity
profiles of the two different phases is a critical parameter
in the design of the injection program, such knowledge would
be very useful in a steam injection program, enabling the
practitioner to formulate heat injectivity profiles over the
interval of interest and thereby much more accurately
describe the progress and expected results of the steam
flood.
ll'Z)~S~
SUMMARY OF 1~; INVENTION
This invention discloses a method for making a
permeability log for the injection of a two phase fluid into
a subsurface formation traversed by a bore hole. The method
comprises: first injecting a two phase fluid into the bore
hole above the formation, the two phase fluid containing an
effective amount of one radioactive substance which combines
almost exclusively with the gas phase of the injected two
phase fluid; secondly, simultaneously injecting the same two
phase fluid which contains a radioactive substance which
serves as a tracer for the liquid phase of the fluid into the
well bore below the formation; thirdly, establishing
interfaces between two phases contained in the two fluids;
fourth, determining the depth in the hole of said interfaces
by measuring the radioactivity of the fluids throughout that
portion of the hole being examined; fifth, varying the ratio
of the rates at which the two fluids are injected into the
hole while maintaining the sum of the two rates as nearly
constant as possible so as to cause said interface to move
along the walls of the bore hole to another depth; sixth,
determining the depth of the interfaces produced by the
change of injection rates in the preceding step by the method
of the fourth; and seventh, repeating the fifth and sixth
steps until a series of depth and injection rate measurements
obtained at the various interfaces sufficient to adequately
describe the formation is obtained for both phases of the
injected two phase fluid.
s~
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a vertical sectional elevation through
a well showing the apparatus necessary for making an
injectivity profile for one embodiment of the invention.
Figure 2 is a graph showing the flow rates per foot
of the two phases as a function of the depth in the well.
Figure 3 is a graph showing the total heat injected
as a function of depth in the well.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although use may be made of this invention in the
generation of an injectivity profile for the injection of any
two phase fluid, its practice will now be discussed in detail
as it relates to the making of an injection profile for a
formation undergoing steam injection. Two streams of the
steam fluid are pumped into the well, one stream being
injected through a string of tubing extending downwardly
below the formation and the other being injected downwardly
through the annular space between the tubing and the casing
or the walls of the hole. The streams are injected
simultaneously but separately, and each stream is carefully
metered to provide steam quality and mass flow rate
information. The steam pumped down through the tubing will
emerge at a point below the formation of interest and flow
upwardly around the outside of the tubing until it meets the
steam pumped downwardly around the outside of the tubing in
the annulus. Two interfaces will form in the region in the
annulus where the two streams or bodies of fluid meet. One
interface will form where the gas phase of the fluid injected
into annulus meets the gas phase of the fluid injected into
the tubing; the other interface is formed where the annulus
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liquid phase meets the tubing liquid phase. The gas phase
interface will normally always be located above the liquid
interface due to the density difference between the two
phases.
Small amounts of one radioactive substance are then
added to the steam being pumped down the annulus, the
radioactive substance being of such a nature as to cause it
to combine almost exclusively with the gaseous phase in the
steam. The fluid injected into the tubing does not contain
this tracer. In order to locate the interface between the
radioactive annulus gas phase and the nonradioactive tubing
gas phase, a radioactivity detector is passed through the
tubing, its depth being recorded continuously. From the
record of the output of the detector, the depth of the
interface can be ascertained since the response of the
detector will change more or less suddenly while the detector
passes from the radioactive gas into the nonradioactive gas
or vice versa.
In a preferred embodiment, the radioactive matter
added to the fluid injected into the annulus is not
introduced continuously but in small slugs. The radio-
activity detector is positioned within the tubing slightly
above the expected position of the interface. Once the
response from the detector indicates that the slug of
radioactive material in the annulus has passed by the
detector, the detector is then lowered further into the
casing to detect and record the exact position of the
interface. This embodiment has the advantage of requiring
smaller amounts of radioactive material.
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Similarly, small amounts of another radioactive
substance are added to the fluid injected into the tubing.
This radioactive substance acts as a tracer for the liquid
phase of the fluid and, as such, should combine almost
exclusively with the liquid phase of the fluid. In this
case, the liquid tracer matter must be added in small
discrete slugs rather than continuously because the radiation
detecting tool positioned in the tubing would otherwise be
constantly immersed in radioactive fluid in the tubing and be
therefor unable to effectively detect the desired fluid phase
interfaces at the wellbore.
It is preferable that the small slugs of both
radioactive tracers be added more or less simultaneously to
their respective fluid streams. The radioactivity detecting
tool is preferably positioned near the expected depths of the
interfaces. The tool response is monitored in order to
detect the passage of one or both of the radioactive slugs
past the position of the tool in the tubing. Actual
measurement of the depth of the two interfaces by traversing
the tool through the tubing is preferably begun only after
both of the smaller radioactive slugs have at least either
reached the interface or begun to enter the formation. The
tool response as the detector is raised up the tubing past
the interfaces should appear as follows: a low level
slightly above background radiation levels indicative of the
prior passage of the liquid tracer tapering up to a
relatively high level immediately below the liquid interface;
then an abrupt drop to near background radiation level in the
interval above the liquid interface but below the gas
interface; then another abrupt rise in radiation level at the
112~5j3~
gas interface followed by a tapering down to near background
level as the tool is raised farther above the gas interface.
Both interfaces are then markecl by this abrupt change in
radiation level, the liquid interface being lower in the well
than the gas interface.
The rates of injection of the two fluid streams can
be varied by means of pumps, chokes, or other means at the
surface. The rates are adjusted that so at at all times the
sum of the rates remains constant. One preferred method for
controlling the rates is to utilize a single fluid source and
divide its output between the tubing and the annulus.
Another preferred control method is to utilize two separ~te
fluid sources. By increasing the ratio of the amount of the
fluid injected into the annulus to the amount of fluid
injected into the tubing the interface will be forced
downwardly through the well past the exposed walls of the
formation or zone to be examined. As the rates of injection
of the two fluid streams are varied by increments, the
interface will move downwardly by steps. The vertical length
of such steps will depend upon the permeability of the
formation to the fluid. After each change in the rates of
injection for the two different fluid streams, the radiation
detector is passed through the well and a record is made of
the depth of the two interfaces after each such change. In
this manner, an injectivity profile for the two phases of the
steam is made of the formation of interest. This record will
clearly show variations in the permeability of all the
sections or portions of the formation in relation to the
injection of the gas and liquid phases thereinto.
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Referring to the drawing, a well or bore hole 10 is
shown as traversing several subsurface formations including
the formation 12 for which it is desired to make a steam
injection profile. The upper portion of the well is shown as
being provided with a casing 14 having a closed casing head
at 16. A string of tubing 18 passes through the casing head
16 and downwardly through the well to a point below the
formation 12. At the surface a pump 20 is connected to the
casing head through a meter or meters 22 and is adapted to
pump a stream of fluid 24 downwardly into the well through
the space between the casing 14 and the tubing 18. A small
amount of one radioactive substance which combines almost
exclusively with only one of the two phases contained within
the fluid is added to the fluid 24 by means not shown,
preferably before the fluid is taken into the pump 20.
Another pump Z6 is shown as connected to another meter or
meters 28 to the upper end of the tubing and is adapted to
pump fluid 30 downwardly through the tubing. This fluid
contains a tracer that combines almost exclusively with the
other phase. This fluid passes out of the bottom end of the
tubing and upwardly around the tubing until it meets the
other radioactive fluid 24. The gas phase from the annulus
fluid 30 meets the gas phase from the tubing fluid 24 at the
upper interface 32. The liquid phase from the annulus fluid
30 meets the liquid phase from the tubing 24 at the lower
interface 33. 2. It will be seen that, if the pumps 20 and
26 are adjusted to change their rates of pumping while the
total amount of steam pumped remains constant, the interfaces
32 and 33 will be caused to move up or down in the hole
depending upon the two pumping rates.
--10--
11 ~ 7~Z
Shown as suspended within the tubing 18 is a
radioactivity logging instrument 34 containing a detector of
gamma rays the output of which is conducted upwardly through
the suspended cable 36. This cable passes over a suitable
cable measuring device 38 which continuously indicates the
depth of the instrument 34 in the hole and then to a suitable
amplifier 40 and a recorder 42. As the instrument 34 is
traversed through tubing it will respond to the radioactivity
in the well, thereby sensing the location of the two
interfaces 32 and 33.
A record of the output of the detector is made
continuously by the recorder and is correlated with the depth
of the detector in the hole as measured by the device 38.
Thus, by passing the detector 34 through the hole and
correlating the points in the record from the radioactivity
recorder 42 at which the detector passes the two interfaces
32 and 33 with the depths in the hole at which those points
are registered, accurate measurements are made of the depths
of the interfaces 32 and 33.
Selection of the particular radioactive substances
to be used as tracers for each phase will of course depend on
the particular two phase system to be examined. In a steam
two phase system one effective radioactive tracer for the
liquid phase is tritiated water comprising an H20 molecule
with one of the hydrogen atoms being replaced by a tritium
atom. This is a particularly effective tracer material for
the liquid water phase inasmuch as the tritiated water
molecule is slightly denser than a normal H20 molecule and,
as a consequence, will tend to stay in the liquid phase
rather than freely changing between the liquid and gas states
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as would a normal H2O molecule. However the effectiveness of
tritium as a tracer is limited in wells where the tubing in
the interval of interest is made from steel. This limitation
could be circumvented by the use of non-ferrous tubing in the
interval of interest. Other preferred liquid phase tracers
include radioactive sodium iodide and sodium irridium
chloride. Suitable radioactive tracers for the gaseous phase
in the steam include tritiated hydrogen gas, radioactive
ethyl iodide, radioactive methyl iodide, Krypton 85 or any
number of other radioactive gaseous isotopes which have
relatively short half lives on the order of ten days or less.
Application of other radioactive isotopes to different two
phase fluid systems such as liquid water and air or liquid
hydrocarbon and CO2 is well within the expertise of one
skilled in the art.
Reference is now made to the following Example to
more particularly describe the practice of this invention in
a steam injection well.
E X A M P L E
The surveyed well has casing in the hole to a depth
of 1300 feet which is perforated in the zone of interest from
1200 to 1240 feet with the tubing string extending below the
zone of interest. The appropriate pumping and metering
equipment has been installed at or near the surface location
of the well. Sodium iodide has been selected as the
radioactive substance for the tracer for the liquid water
phase and ethyl iodide gas has been selected as the
radioactive tracer for the gaseous phase of the steam. The
steam will be injected at 500 pounds per square inch (psig)
gauge pressure at 70 percent quality (70 percent water vapor,
30 percent water liquid by mass).
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The injectivity profile survey is commenced and it
is quickly ascertained that a total fluid injection rate of
1000 barrels per day, with 0 barrels per day into the annulus
and 1000 barrels per day of st:eam into the tubing will
produce a stable gas phase interface at the top of the
interval of interest at 1200 feet. From this point it is
decided to increase the injection rate into the annulus by
200 barrel per day increments while decreasing the injection
rate into the tubing by a corresponding amount. After each
such adjustment, the gamma ray tool is utilized to make a
measurement of the depth of the new interfaces. The results
of the injectivity profile survey for the gas phase and the
liquid phase of steam are reported in Tables I and II. Note
that the total rate of gas phase injected is 700 barrels per
day while that of the liquid phase injected is 300 barrels
per day (1000 bpd of 70 percent quality steam = 300 bpd liquid
phase and 700 bpd vapor phase). A graphical representation
of the gas phase mass flow rate per foot as a function of the
depth in the well is shown as the dashed curve in Figure 2
while the liquid phase mass flow rate per foot is represented
by the solid curve.
The results of the survey are reported in Table I
and show that the majority of the water vapor has entered the
top 15 feet of the interval while the majority of the water
has entered into the lowest 15 feet of the interval.
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11~7s~z
TABLE I
DEPTH TO DEPTH TO
INJECTION RATE INJECTION RATE GAS PHASE LIQUID PHASE
INTO ANNULUS (BPD) INTO TUBING (BPD) INTERFACE (FT) INTERFACE (FT)
0 1000 1200 1215
(700 gas-300 liq.)
200 800 1205 1225
(140 gas-60 liq.)(560 gas-240 liq.)
400 600 1210 1230
10(280 gas-120 liq.)(420 gas-180 liq.)
600 400 1215 1233
(420 gas-180 liq.)(280 gas-120 liq.)
800 200 1225 1235
(560 gas-240 liq.)(140 gas-60 liq.)
151000 0 1235 1240
(700 gas-300 liq.)
Since both the steam entering the annulus and the
steam entering the tubing contain the same proportions of
liquid and gaseous phases and further since the pressure and
guality of the steam is known, it is also possible, using
readily available steam tables, to calculate a total injected
heat profile over the entire interval from the injectivity
data produced by the practice of this invention. A heat
profile over the interval of interest has been calculated
from the preceding data, and the results are given in Table
II. The rate of heat injection is graphed as a function of
the depth of the well in Figure 3. Such information would be
exceedingly useful to one studying the effects of the steam
injection program.
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TABLE II
Depth Flowrate Into Interval Steam 1 Injected 2 Injected Heat
Interval (BPD) Quality Heat Per Foot
(ft.) _ Vapor Phase Liquid Phase (%) (mmBtu/day) (mmBtu hr-ft)
1200-1215420 0 100 % 177 .492
1215-1225140 60 70.0% 68.5 .285
1225-123070 60 53.8% 39.0 .325
1230-123342 60 42.2% 27.2 .378
1233-123528 30 48.3% 16.6 .345
1235-1240 0 90 0 % 14.3 .119
1 Where steam quality = Vapor Phase Mass Flowrate/Foot
Liquid Phase Mass Flowrate/Foot + Vapor
Phase Mass Flowrate/Foot
2 Where the enthalphy of the steam at 500 psig is hf = 452.94
Btu/lb & hf = 751.66 Btu/lb.
The above example has been presented for the purpose of
illustration and should not be considered as limitative.
Obviously, many other modifications and variations of the
invention as hereinbefore set forth are possible and may be made
without departing from the spirit and scope thereof, and only
such limitations should be imposed as indicated in the following
claims.
