Note: Descriptions are shown in the official language in which they were submitted.
3262~
METHOD FO~ SELECTIVELY TREATING STRATA IN
SUBTERRANEAN FORMATIONS BY CO-INJECTION
Field of the Invention
This invention relates to a method for selectively
treating vertically disposed permeable subterranean 6trata in
a formation penetrated by a well. More particularly, this
invention relates to a method of selectively acidizing such
subterranean strata by co-iniection of (1) an injection
fluid/treatment fluid through a tubing string installed in the
well, and (2) a treatment fluid/injection fluid through a coiled
tubing unit disposed in the well for treatment purposes.
Background of the Invention
,:
j In the recovery of petroleum oil and gas from
subterranean formations through wells, it is often desirable to
treat the formation in order to improve formation
characteristics, 6uch as production or injection rate6.
As one example, a common treatment technique involves
acidizing the formation by injecting an acid solution via the
well. The acids derive their utility in well stimulation from
their ability to dissolve formation minerals and foreign
-2- 13262~
.,
material, such as drilling mud, that may be introduced into the
; formation during well drilling or workover procedures. The acid
reacts with the oil-bearing formation by etching the rock,
enlarging the pore spaces and passages through which the
reservoir fluids flow. Such acids include hydrochloric acid,
;l
which is often used in connection with limestone formations, and
hydrochloric- hydrofluoric acid, which is often used in
connection with sandstone formations, or organic acids and other
acid mixtures. Typically, the acid i8 held under pressure in
the well for a period of time and then pumped out, after which
the well may be swabbed and put back into production. Chemical
inhibitors may be combined with the acid to prevent corrosion of
piping and other equipment associated with the well. Such
acidizing treatment is frequently advantageous when applied to
the formation in the vicinity of a producing well, or an
injecting well, in order to remove the effect of a formation
permeability reduction near the wellbore by enlarging pore
spaces and dissolving particles plugging these spaces.
i
.! Although acidizing i8 one common treatment which is
; applied to oil-bearing formations, it will be understood by
those skilled in the art that there are a great many well and
reservoir treatment techniques which involve injection of a
fluid into a subterranean formation via a well.
Such subterranean formations, including hydrocarbon
reservoirD, freqtently contain a nttmber of vertically disposed
~3~ 13262~7
layers which include permeable strata interspersed with
impermeable strata. As used in this description and in the
appended claims, the term "strata" refers to dist;nct, usually
parallel beds of rock; an individual bed is a "stratum".
Furthermore, the term "permeable" means a tendency of a material
to permit the flow of a fluid therethrough, while the term
"impermeable" means a tendency of a material to prevent the flow
of a fluid therethrough.
.
Very often, particularly in the workover of wells, it is
; desirable or necessary to perform a selective treatment of a
~; particular layer, which may contain one or more individual
permeable strata, out of several layers in a layered reservoir
which is penetrated by the well. For example, with respect to
the acidizing treatment described above, it may be desirable to
increase the permeability or injectivity of one layer in a
formation without affecting the injectivity of another layer.
~; Conventional techniques for selective treatment of a
single layer in a layered reservoir are typically performed by
packing off the subject layer for treatment, and then injecting
the treatment fluid into the packed off zone. In general, this
. requires the services of a workover rig to pull any tubing
string, such as production or injection tubing, out of the well
before setting the selective acidizing tool. The selective
acidizing tool typically uses packers set in the wellbore
against the well casing to seal off the desired zone for
~4~ 13262~7
selective treatment. The cost involved in obtaining a workover
rig and pulling production tubing from a well in preparation for
such selective treatment of a layer, as well as the C08tS of the
down time of the well during such conventional treatment, are
quite substantial.
A somewhat different technique for acidizing a
formation is described in U.S. Patent 3,730,273 (issued May 1,
1973 to Wilson). Wilson describes a technique in which the
injection fluids are injected into a well completed with two
injection tubes installed therein. Wilson describe6 three
embodiments of his invention, each of which requires a first
injection tube which runs the entire length of the wellbore past
the strata to be treated. This injection tube is perforated at
well elevations adjacent to the respective strata to be treated,
with the number and size of perforations being selected so as to
regulate the volumetric flow rate of fluid introduced into the
wellbore through the injection tube at each elevation. A second
injection tube is also installed in the wellbore, and may be
parallel to or concentric with the first injection tube, or may
discharge into the wellbore above the uppermost oil bearing
strata. Controlled quantities of at least two injection fluids
are injected into the wellbore via the injection tubes, and are
allowed to mix in the wellbore at each of the elevations. A
property promoting additive, such as acid, is incorporated
within at least one of the injection fluids so that the
resulting fluid mixtures at the elevations contain the desired
~ ~5~ 13262~7
concentration of the additive. For example, Wilson describes
one embodiment in which a first fluid travels downward within
the first injection tube, so that controlled quantitie6 of the
fluid enter the wellbore through the perforations located
adjacent to each of the strata. A second fluid i8 injected
through the second injection tube, which discharges into the
borehole above the uppermost strata. As the second fluid then
proceeds downwardly within the borehole, it OEuccessively mixes
with fluid injected into the borehole from the first injection
tube to form mixtures having progressively increasing or
decreasing concentrations of the additive. Perforations in the
wellbore casing allow these mixtures to then enter the
respective strata.
While the Wilson technique describes acidizing of
vertically disposed strata, it should be noted that this
technique involves the mixing of fluids along the wellbore
length 80 as to vary the strength of the acidizing solution, or
the concentrations of other property-promoting additives, at
various elevations in the wellbore, and thus does not appear to
teach or suggest any co-injection technique in which one
selected strata can be acidized, or otherwise treated, while
another strata is not acidized at all. Wilson states that
packers can be installed in the wellbore between the various
subterranean zones to prevent mixing of adjacent fluid mixtures
within the wellbore, however, this is old and well known in the
art, as well as expensive. Furthermore, the technique described
-6- 13262~7
by ~ilson appears to involve at least two permanently installed
injection tubings. The size limitations presented by a wellbore
would present a great difficulty in attempting to install this
plurality of injection tubings.
Accordingly, there exists a need for an economical
method of selectively treating vertically disposed subterranean
permeable strata which are in fluid communication with a well.
More particularly, there is a need for an economical method of
selectively treating one such stratum, or a layer of such
strata, while simultaneously avoiding treatment of another
stratum or layer of strata in the same well. Still further,
there is a need for an economical method of selectively treating
such strata without the necessity of removing production or
other installed tubing from a production well.
Summary of the Present Invention
Briefly, the present invention involves an improved
method for selectively treating subterranean permeable strata
which are penetrated by a well which is in fluid communication
with the strata. Broadly, the invention comprises injecting an
injection fluid into the well to a location which is adjacent a
first permeable strata at a rate which is greater than the flow
rate for that first fluid into the first permeable stratum.
Simultaneously, a treatment fluid is injected into the well to a
location hlch is adjacent to a Gecond per~eable stratt~ at a
-- 13262~7
rate which is less than the flow rate for the treatment fluid
into the second permeable stratum. The treatment fluid is
adapted to treat the second permeable stratum. Hence~ the
treatment fluid will flow into the second stratum and treat it,
but will not flow into the first stratum and cause treatment
thereto. Accordingly, the present invention accomplishes
selective treatment of a stratum by co-injection of fluids.
In a preferred embodiment the first fluid has a known flow
rate Ql into the first permeable stratum and the second fluid has a
known flow rate Q2 into the second permeable stratum. The rate at
which the first fluid is in~ected into the well to the location
ad~acent the first tratum is equal to approximately Ql ~ 0.1 Q2~ and
the rate at which the second fluid is in~ected into the well to the
location ad~acent the second permeable stratum is equal to
approximately 0-9 Q2.
In another preferred embodiment, the invention comprises
selectively acidizing an upper or lower stratum in a
subterranean hydrocarbon-bearing formation which is penetrated
by a production well having production tubing installed
therein. The production tubing is in fluid communication with
the formation adjacent to an upper permeable stratum. A coiled
tubing unit is inserted into the well through the production
tubing, so that the coiled tubing unit is landed with its outlet
adjacent a lower permeable stratum. Water is injected into the
well simultaneously through both the production tubing and the
coiled tubing unit. If the lower permeable stratum has been
selected for treatment, water is injected through the production
B
-7a- 1326207
tubing at a rate approximately equal to Q + 0.1 QQ, and
through the coiled tubing unit at a rate approximately equal to
0.9 QQ, wherein Q is the flow rate for the water into the
upper permeable stratum and QQ is the flow rate for the
water into the lower permeable stratum. An acid treatment
solution is then cut in and injected into the well through the
coiled tubing unit at a rate approximately equal to 0.9 Q~.
;
B
.. . .
-8- 13262Q7
The rate at which the water is injected into the well through
the production tubing i8 kept constant at QU + .1 QQ.
If, on the other hand, the upper permeable stratum has been
selected for treatment, water is injected through the production
tubing at a rate approximately equal to 0.9 Q, and through
the coiled tubing unit at a rate approximately equal to
QQ + O.1 Qu- An acid treatment solution i6 then injected
into the well through the production tubing at a rate
approximately equal to 0-9 QU.
Brief Description of the Drawing
The FIGURE is an elevation view in section of a well
illustrating the practice of the present invention.
Description of the Preferred Embodiments
The present invention involves a method for selectively
treating permeable strata in a subterranean formation which is
penetrated by a well which is in fluid communication with the
~trata. The details of a preferred embodiment of the present
invention will be described below.
With reference to the FIGURE, a section view of a
formation and well illustrating the practice of the present
invention can be seen. The FIGURE shows a well 10 having a
casing 12 run to the bottom of the wellbore. Well 10 penetrates
-9- 13262~
subterranean formation 14. The well pagses through an upper
permeable stratum 16 and a lower permeable stratum 18. The
casing is shown being bonded to the sides of the borehole by a
cement 20 to secure the casing in place. Permeable stratum 16
is in fluid communication with the interior of casing 12 through
perforations 22, and permeable stratum 18 is in fluid
communication with the interior of casing 12 through
perforations 24.
,~
In a preferred embodiment of the present invention,
permeable strata 16 and 18 represent hydrocarbon-bearing layers
in subterranean formation 14. While the well is in production,
hydrocarbons flow from a strata 16 and 18 through perforations
22 and 24 respectively, and into the interior of casing 12. The
hydrocarbons are transported from the interior of casing 12 to
the surface through production tubing 26. A production packer
28 is installed near the lower end of the production tubing 26
and above upper permeable stratum 16, to achieve a pressure seal
between the production tubing 26 and a casing 12. Production
packer 28, however, is not necessary for the practice of the
present invention, and only represents a preferred embodiment
thereof.
In the preferred embodiment illustrated in the FIGURE,
production tubing 26 terminates adjacent to the upper permeable
strata, as i6 common in typical production wells. Hence, the
tubing string is in fluid communication with the formation at a
-lo- 1 326~7
point which is adjacent to the upper permeable stratum. As used
herein and in the appended claims, the term "tubing strin~"
refers to the entire length of a pipe or tubing which is run
into a well. Production tubing is one example of a tubing
string.
With further reference to the FIGURE, the next step in
the practice of the preferred embodiment of the present
invention is to select either the upper permeable stratum 16, or
the lower permeable 8tratum 18, for selective treatment. As
discussed above, it is frequently desirable to selectively treat
one or more permeable strata in the formation, while avoiding
treatment of other permeable strata. For example, one stratum
or layer of strata may have a lower permeability than another
stratum or layer of strata, and it may be desirable to acidize
the former in order to equalize the permeability of both of the
strata contacted by the well. In the example illustrated in the
FIGURE, lower permeable stratum 18 is, for purposes of
illustration, selected for acidizing treatment to increase its
permeability. As will be further described below, however, the
present invention may be used to selectively treat either upper
or lower layers within a multilayered reservoir.
As a preliminary matter in the practice of the present
invention, it is necessary to identify the respective flow
rates, ie. the rate at which a particular fluid will flow into a
material, fcr the strata ~hich have heen selected Eor treatment
132~2~7
and the strata which have been selected for non-treatment. This
may be done by any number of techniques well known to those
skilled in the art, and may preferably be done by means of
injection profile logging, most preferably by use of a flow
meter. As used herein and in the appended claims, the variable
"Q" is assigned to represent the flow rate for a particular
stratum with respect to a particular fluid.
In preparation for the next step in the practice of the
present invention, the well 10 is taken off production. A
conduit is then inserted downwardly into well 10 through
production tubing 26. The conduit in the preferred embodiment
illustrated in the FIGURE is a coiled tubing unit 30. Coiled
tubing units, which are known to those skilled in the art,
represent an economical and convenient form of conduit for use
in the practice of the present invention. However, any suitable
conduit, such as another tubing string concentric with or
parallel to the existing tubing, may also be used. In the
preferred embodiment illustrated, coiled tubing unit 30 is
inserted into well 10 through production tubing 26 until it has
landed with its outlet 31 adjacent to lower permeable stratum
18. This arrangement represents a preferred embodiment of the
invention, however, the invention may be practiced so long as
coiled tubing unit 30 is landed in well 10 so that outlet 31 is
located below the lowermost of the upper permeable strata which
have been selected for treatment/nontreatment relative to the
lower strata.
-12- 13262~
In the next step in the practice of the preferred
embodiment of the present invention which is illustrated in the
FIGURE, an injection fluid, preferably water, is injected into
well 10 to locations adjacent permeable strata 16 and 18.
Preferably, the water is injected into the well simultaneously
through production tubing 26 and coiled tubing unit 30. The
water is injected into the well to a location which is adjacent
to the layer which has been selected to remain untreated at a
rate which is greater than the flow rate for the water into that
layer. The water is injected into the well to a location which
is adjacent to the layer which has been selected for treatment
at a rate which is less than the flow rate for the water into
that layer.
In the illustrated embodiment, the preferred initial
injection fluid is water. However, the injection fluid can
comprise any liquid which is readily pumpable, relatively inert
to the formation substructure, and which is abundant and
relatively inexpensive. Exemplary injection fluids include
water, brine, aqueous carbon dioxide, hydrocarbon oils,
liquified petroleum gas, etc. The most preferred injection
fluids include water, brine, and petroleum oil.
,
As described above, the injection rate for the water to
the location adjacent to the strata selected for non-treatment
should be greater than the flow rate for the water into those
strata, and the injection rate for the water to the location
-13-
13262~7
adjacent to the strata selected for treatment should be less
than the flow rate for the water into those strata. Most
preferably, the injection rate of the water to the strata
selected for non-treatment i8 equal to about the flow rate for
the water into those strata, plus 10% of the flow rate for the
water into the strata which have been selected for treatment,
while the injection rate for the water to the location adjacent
to the strata which have been selected for treatment is equal to
about 90% of the flow rate for the water into those strata. In
the preferred embodiment illustrated, the upper strata have been
selected for non-treatment, and the lower strata have been
selected for treatment. The flow rate for the water into the
upper stratum may be represented by the variable ~Q ~~, while
the flow rate for the water into the lower stratum may be
represented by the variable ~QQ~. Accordingly, in the
embodiment illustrated, the most preferable injection rate for
the water injected into the well to a location adjacent to the
upper stratum may be approximately equal to Q + 0.1 QQ,
while the injection rate for the water injected into the well to
a location adjacent to the lower stratum may be approximately
equal to 0.9 QQ.
~ uring the initial injection of the water, the water
exits production tubing 26 in the directions shown by arrows 34
and enters upper permeable stratum 16 through perforations 22.
The water injected through coiled tubing unit 30 exits in the
directions shown by arrows 38 and enters lower permeable
-14- 1326~7
stratum 18 through perforations 24. During this injection the
flow rate of the water into the formation may fluctuate as the
water fills the fissures and pore spaces in the strata adjacent
to the wellbore. Preferably, the injection of the water as
described above is continued until the flow rates into the
formation stabilize. Once these flow rates have stabilized, the
next step in the practice of the present invention may
preferably be commenced.
With further reference to the FI~URE, the next step in
the practice of the present invention will be described. Once
i! the flow rate of the water into lower permeable stratum 18 has
stabilized, selective treatment of lower permeable stratum 18
' may preferably commence. The injection of the water into the
well through coiled tubing unit 30 may be switched over to the
treatment fluid, or a suitable treatment chemical may be mixed
with the water as it is injected through the coiled tubing unit
if the treatment chemical is suitably soluble in or miscible
with the water.
.i
The treatment fluid may be any fluid which, when
injected into the well, may treat the permeable strata in a
desired fashion. In the preferred embodiments illustrated, the
treatment fluid is an acid, such as hydrochloric, hydrofluoric,
citric, formic, acidic, etc., and combinations thereof. Other
exemplary treatment fluids may include other permeability
enhonclng agene~, ~uch t. solve~t~; plugging ngentt, tuch ~s
..
-15- ~32~ Q7
phenoplast (phenolic-aldehyde) gel, polymers with or without
cross-linking agents, talc, calcium carbonate, sand, etc., scale
inhibitors, such as phosphorus containing inhibitors like
alkylphosphonic acids; wax inhibitors such as calcium petroleum
sulfonate and aromatic fractions containing alkylenapthylenes;
viscosity increasing agents, such as micro-emulsions, fatty acid
soaps, and water-soluble polymers; surface tension reduction
agents, such as alcohols, and surfactants, and a wide range of
other treatment chemicals and solutions known to those skilled
in the art.
Phenoplast gel i6 a preferred plugging agent. The
gelling solution is prepared as is known in the art by adding
any commercially available mixture of a phenolic resin and an
aldehyde to water to which caustic has been added. The caustic
acts as the catalyst for the polymerization reaction which forms
a stiff, i.e. highly viscous, impermeable gel. Rate of
polymerization is controlled, as is known in the art, so that
the solution will not stiffen in the wellbore. The phenolic
component may be one or re of any phenolic compounds such as
phenol, resorcinol, catachol, and the like, as well as selected
oxidized phenolic compounds such as 1, 4-benzoquinone and
natural or modified tannins. The aldehyde may be either a
monoaldehyde, such as formaldehyde and acetaldehyde, or a
dialdehyde, such as glyoxal. Formaldehyde is the most preferred
aldehyde. The aldehyde may also be generated in-situ by an
aldehyde precuKor, such A~ parafDrm~ldehyde. The phe=olic
-16- 132~2~7
compound to aldehyde ratio may be any ratio suitable to form a
stiff gel upon polymerization.
In the preferred embodiment illustrated, the acid
treatment solution is cut in and injected into well 10 through
coiled tubing unit 30 in the direction shown by arrow 36. The
acid treatment solution discharges from the discharge end of
coiled tubing unit 30 in the directions indicated by arrows 38,
adjacent to lower permeable stratum 18, proximate which coiled
tubing unit 30 has been landed. The acid treatment solution
fills the interior of well 10 in region 42, while the water
which continues to be injected through production tubing 26
continues to occupy region 40. Hence, the treatment fluid and
injection fluid are co-injected. The acid treatment solution
flows from region 42 into lower permeable stratum 18 via
perforations 24, thereby penetrating and treating lower
permeable stratum 18.
It will be understood by those skilled in the art that
the treatment fluid may have a different flow rate into the
selected stratum than did the earlier injected water. In the
practice of the present mvention, the flow rate for the
treatment fluid into the selected stratum is determined, and the
injection rate of the treatment fluid through coiled tubing unit
30 will preferably be selected to be approximately equal to 90%
of the flow rate for the treatment fluid into that stratum. The
practice of the present invention is most preferably conducted
-17- 13262~7
using a treatment fluid which has an initial flow rate into the
selected permeable stratum which is similar to that of the flow
rate for water into that stratum.
Furthermore, if, as in the preferred embodiment shown,
the injected treatment fluid is acid, it will be understood by
those skilled in the art that the flow rate of the treatment
fluid into the selected stratum may increase as the treatment
fluid reacts with the formation material, as wa6 described
above. Accordingly, as the treatment fluid is injected, it may
be necessary to adjust the injection rate for the treatment
fluid to compensate for the increased flow rate into the treated
stratum. The amount of rate increase is determined by keeping
the injection pressure of the coiled tubing unit close to but
less than the initial injection pressure. Conversely, if the
injected treatment fluid i6 a permeability reducing agent, such
as phenoplast, it may be necessary to adjust the injection rate
for the treatment fluid to compensate for the decreased flow
rate into the treated stratum.
It will be observed that, by maintaining the injection
rates of the water and the treatment fluid in the
above-described relationship to their respective flow rates into
the stratum which have been selected for non-treatment and
treatment, selective treatment of the latter îs accomplished
without affecting the former. In the embodiment illustrated in
the FIGURE, so long as the water is injected into well 10 to a
-18- ~2~2~7
location adjacent to upper permeable stratum 16 at a rate which
exceeds the flow rate of the water into upper permeable stratum
16, while the treatment fluid is injected into well 10 to a
location adjacent to lower permeable stratum 18 at a rate which
i6 less than the flow rate of the treatment fluid into lower
permeable stratum 18, the treatment fluid cannot flow upwardly
through the wellbore towards perforations 22. Rather, inasmuch
as the water is being injected into well 10 at a rate in excess
of the flow rate for the water into permeable stratum 16, while
the acid solution i8 being injected into well 10 at a rate which
is less than its flow rate into its respective stratum, the
water will tend to flow downwardly through well 10 in the
direction of perforations 24, displacing the treatment fluid in
the wellbore in the course of its advance. Accordingly, while
some water may flow into lower permeable stratum 18, no
treatment fluid will flow into upper permeable 16.
In the embodiment illustrated in the FIGURE, selective
treatment of the lower permeable stratum has been accomplished,
while treatment of the upper permeable stratum has been
avoided. The reverse procedure can be used to treat the upper
permeable stratum, while avoiding treatment of the lower
permeable stratum. For example, the treatment fluid can be
injected into well 10 through production tubing 26 to a location
adjacent to upper permeable stratum 16 at a rate which i8
preferably approximately equal to 90% of the flow rate for the
treatment fluid into the upper permeable stratum, while water
-19- ~32~2~7
may be injected into well 10 through coiled tubing unit 30 to a
location adjacent to lower permeable stratum 18 at a rate which
is preferably approximately equal to the flow rate for the water
into stratum 18, plus 10~ of the flow rate of the treatment
fluid into permeable stratum 16. Accordingly, for the reasons
described above, this embodiment permits upper permeable
stratum 16 to be treated, while lower permeable stratum 18
remain~ untreated.
It will also be recognized by those skilled in the art
that, following the injection of the treatment fluid as
described above, a pusher fluid or overflush, such as water, may
subsequently be injected to the location adjacent to the stratum
selected for treatment, so as to drive the treatment fluid into
the formation.
Once the desired treatment of the selected stratum has been
completed as described above, the well may be placed back into
production. With reference to the FIGURE, the injection of the
water, treatment fluid, and pusher fluid (if any) through
production tubing 26 and coiled tubing unit 30 is terminated,
preferably commencing with termination of injection of the
treatment fluid/pusher fluid. Following the termination of
injection of the fluids, coiled tubing 30 can be withdrawn
upwardly through production tubing 26 and out of well 10. It
may also be desirable to remove the water, treatment fluid, and
pusher fluid from well 10, using any of several techniques well
` -20- 13262~7
known to those skilled in the art, particularly if the treatment
fluid is an acidizing solution which may be harmful to pumps and
other production equipment if it produced Erom the well along
with hydrocarbons. Following this~ the well may be placed back
in production and the recovery of hydrocarbons from formation 14
resumed.
The method of the present invention will be more
readily understood by reference to the following example.
Example 1
This example illustrates the technique of the present
invention in a selective acidizing process.
A phenoplast gel treatment was carried out in order to
modify injection profiles on an injection well in an oil
producing formation. The well penetrated a formation having a
permeable top zone, a nonpermeable middle zone, and a permeable
bottom zone. The gel treatment was successful in modifying the
injection profile of the injected fluids. However, the total
injection rate dropped to about 437m3/day at 12.06 MPa
wellhead pressures after the gel treatment. This low injection
rate was insufficient to maintain the desired reservoir
voidage. In order to maintain reservoir voidage, it was
desirable to increase the total injection rate by selectively
acidizing the bottom zone, which was located at a depth of
approxi~Ately Z550 mKB, while not ~ffecting the injectivity of
-21- ~3~62~7
the top zone. Accordingly, the technique of selective acidizing
by co-injection in accordance with the present invention was
applied.
The well included an installed 88.9mm tubing string. A
coiled tubing unit was run into the hole down the 88.9mm tubing
string at a rate of 30 meters per minute. The coiled tubing
unit was landed with its discharge end at a depth of 2526 mKB.
A feed rate of .09 m /min. of water at 8 MPa wellhead pressure
was established down the 88.9mm tubing string, using produced
water from the formation. Simultaneously, produced water was
pumped down the coiled tubing unit at a rate of .13m3/min at
11 MPa wellhead pressure. A 15% hydrochloric acid solution was
then cut in to the injection down the coiled tubing unit, which
continued at .13m3/min. at 11 MPa wellhead pressure. Pumping
of the water down the 88.9mm tubing string continued at
. O9m3/min .
A total acid volume of 5.0m3 was injected at four
levels adjacent to the treated zone as follows: .5m was
injected at 2526.0 - 2527.0 mKB, l.Om3 was injected at 2529.0
2531.0 mKB, 2.25m was injected at 2539.0 - 2543.5 mKB, and
1.25m was injected at 2566.0 - 2568.5 mKB.
Following the acid injection, the coiled tubing unit
was overflushed with 2.0m of produced water, and the 88.9mm
tubing string was overflushed with 3.0m3 of produced water.
-22- 13262~7
Pumping was continued down the 88.9mm tubing string and the
coiled tubing unit until the coiled tubing unit was withdrawn
inside the 88.9mm tubing string. The coiled tubing unit was
subsequently pulled out of the hole at a speed of 30m/min.
Following completion of the above operation, the well
was placed back on injection. The total down time of the well
was 16 hours and 45 minutes.
The foregoing treatment was successful in increasing
the total injection rate of the well from 437m3/day at 12.06
MPa, to 1203m /day at a lower wellhead pressure of 9.6 MPa,
without affecting the upper zone significantly. The following
Table 1 s } rizes the results of the foregoing treatment.
Table l
Comparison of injection rates before and after
selectively acidizing by co-injection
Injection Before After
(m3/d) (m3/d)
SS top zone ~133 54
S4 middle zone 0 0
S3 bottom zone 304 1149
Total rate 437 1203
Wellhead pressure
(MPa) 12.06 9.60
-2~- 13262~7
The total cost of the foregoing treatment job was
approximately $15,900. This compares favorably with the cost of
$80,000 to conduct a comparable acidizing job at another well in
the same formation using a conventional selective acidizing
tool. Accordingly, it will be observed that the present
invention represents a much more economical approach to
selective treatment of permeable strata within a reservoir.
In addition to selective acidizing treatments, the
present invention is also particularly well suited to selective
phenoplast treatments, which reduce formation permeability.
Inasmuch as the present invention is subject to many
variations, modifications, and changes in detail, it is intended
that all subject matter disclosed above or shown in the
accompanying drawing be interpreted as illustrative and not in a
limiting sense. Such modifications and variations are included
within the scope of this invention as defined by the following
claims.
.
