CA1326207C - Method for selectively treating strata in subterranean formations by co-injection - Google Patents
Method for selectively treating strata in subterranean formations by co-injectionInfo
- Publication number
- CA1326207C CA1326207C CA000613256A CA613256A CA1326207C CA 1326207 C CA1326207 C CA 1326207C CA 000613256 A CA000613256 A CA 000613256A CA 613256 A CA613256 A CA 613256A CA 1326207 C CA1326207 C CA 1326207C
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- Canada
- Prior art keywords
- fluid
- well
- stratum
- rate
- permeable stratum
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K8/00—Compositions for drilling of boreholes or wells; Compositions for treating boreholes or wells, e.g. for completion or for remedial operations
- C09K8/60—Compositions for stimulating production by acting on the underground formation
- C09K8/62—Compositions for forming crevices or fractures
- C09K8/72—Eroding chemicals, e.g. acids
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K8/00—Compositions for drilling of boreholes or wells; Compositions for treating boreholes or wells, e.g. for completion or for remedial operations
- C09K8/60—Compositions for stimulating production by acting on the underground formation
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/14—Obtaining from a multiple-zone well
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
- E21B43/25—Methods for stimulating production
Landscapes
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Mining & Mineral Resources (AREA)
- Geology (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Fluid Mechanics (AREA)
- Physics & Mathematics (AREA)
- Geochemistry & Mineralogy (AREA)
- Environmental & Geological Engineering (AREA)
- Materials Engineering (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Consolidation Of Soil By Introduction Of Solidifying Substances Into Soil (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A method for selectively treating permeable strata in a subterranean formation which is penetrated by a well. In particular, a first fluid is injected into the well to a location which is adjacent to a first permeable stratum at a rate which is greater than the flow rate for the first fluid into the first permeable stratum. Simultaneously, a second fluid is injected into the well to a location which is adjacent to a second permeable stratum at a rate which is less than the flow rate for the second fluid into the second permeable stratum. The second fluid is adapted to treat the second permeable stratum. This method provides an economical technique for selectively treating the first permeable stratum while treatment of the first permeable stratum is avoided.
A method for selectively treating permeable strata in a subterranean formation which is penetrated by a well. In particular, a first fluid is injected into the well to a location which is adjacent to a first permeable stratum at a rate which is greater than the flow rate for the first fluid into the first permeable stratum. Simultaneously, a second fluid is injected into the well to a location which is adjacent to a second permeable stratum at a rate which is less than the flow rate for the second fluid into the second permeable stratum. The second fluid is adapted to treat the second permeable stratum. This method provides an economical technique for selectively treating the first permeable stratum while treatment of the first permeable stratum is avoided.
Description
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
.. . .
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 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.
.
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
.. . .
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 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.
.
Claims (27)
1. A method for selectively treating permeable strata in a subterranean formation which is penetrated by a well which is in fluid communication with said strata, comprising:
injecting a first fluid into said well to a location which is adjacent to a first said permeable stratum at a rate which is greater than the flow rate for said first fluid into said first permeable stratum; and simultaneously injecting a second fluid into said well to a location which is adjacent to a second said permeable stratum at a rate which is less than the flow rate for said second fluid into said second permeable stratum, said second fluid being adapted to treat said second permeable stratum.
injecting a first fluid into said well to a location which is adjacent to a first said permeable stratum at a rate which is greater than the flow rate for said first fluid into said first permeable stratum; and simultaneously injecting a second fluid into said well to a location which is adjacent to a second said permeable stratum at a rate which is less than the flow rate for said second fluid into said second permeable stratum, said second fluid being adapted to treat said second permeable stratum.
2. The method of claim 1, wherein said second fluid is adapted to increase the permeability of said second stratum and said rate at which said second fluid is injected into said well is adjusted as the flow rate for said second fluid into said second stratum increases.
3. The method of claim 2, wherein said second fluid is an acid treatment solution.
4. The method of claim 2, wherein said second fluid is a solvent.
5. The method of claim 1, wherein said second fluid is adapted to reduce the permeability of said second stratum and said rate at which said second fluid is injected into said well is adjusted as the flow rate for said second fluid into said second stratum decreases.
6. The method of claim 5, wherein said second fluid is phenoplast.
7. The method of claim 2, wherein said first fluid is water.
8. The method of claim 1, wherein said first fluid has a known flow rate Q1 into said first permeable stratum and said second fluid has a known flow rate Q2 into said second permeable stratum, and wherein said rate at which said first fluid is injected into said well is approximately equal to Q1 + 0.1 Q2, and said rate at which said second fluid is injected into said well is approximately equal to 0.9 Q2.
9. A method for selective treatment of permeable strata in a subterranean formation which is penetrated by a well which is in fluid communication with said strata, comprising the sequential steps of:
(a) injecting a first fluid into said well simultaneously to a first location which is adjacent to a first said permeable stratum, at a rate which is greater than the flow rate for said first fluid into said first stratum, and a second location which is adjacent to a second said permeable stratum, at a rate which is less than the flow rate for said first fluid into said second stratum; and (b) injecting a second fluid into said well to said location adjacent said second permeable stratum at a rate which is less than the flow rate for said second fluid into said second permeable stratum, wherein said second fluid is adapted to treat said second permeable stratum.
(a) injecting a first fluid into said well simultaneously to a first location which is adjacent to a first said permeable stratum, at a rate which is greater than the flow rate for said first fluid into said first stratum, and a second location which is adjacent to a second said permeable stratum, at a rate which is less than the flow rate for said first fluid into said second stratum; and (b) injecting a second fluid into said well to said location adjacent said second permeable stratum at a rate which is less than the flow rate for said second fluid into said second permeable stratum, wherein said second fluid is adapted to treat said second permeable stratum.
10. The method of claim 9, where said second fluid is adapted to treat said second permeable stratum so as to increase the permeability thereof, and wherein said rate at which said second fluid is injected is adjusted as the flow rate for said second fluid into said second permeable stratum increases.
11. The method of claim 10, where said rate at which said second fluid is injected into said well is adjusted by keeping the wellhead pressure at which said second fluid is injected approximately equal to but less than the initial wellhead pressure at which said second fluid was injected.
12. The method of claim 11, wherein said second fluid is an acidizing solution.
13. The method of claim 12, wherein said first fluid is water.
14. The method of claim 13, wherein the step of injecting a second fluid into said well comprises switching over from injection of water to injection of acidizing solution into said well to said location adjacent to said second permeable stratum.
15. The method of claim 9, wherein said first fluid has a known flow rate Q1 into said first permeable stratum and said second fluid has a known flow rate Q2 into said second permeable stratum, and wherein said rate at which said first fluid is injected into said well to said location adjacent said first stratum is equal to approximately Q1 + 0.1 Q2, and said rate at which said second fluid is injected into said well to said location adjacent said second permeable stratum is equal to approximately 0.9 Q2.
16. A method for selectively treating a lower permeable stratum in a subterranean formation having upper and lower permeable strata which are penetrated by a well having a tubing string installed therein which is in fluid communication with said formation at a location adjacent to said upper permeable stratum, comprising:
inserting a conduit into said well through said tubing string so that said conduit is landed with its outlet below said upper permeable stratum;
injecting a first fluid into said well through said tubing string at a rate greater than the flow rate for said first fluid into said upper permeable stratum; and injecting a second fluid into said well through said conduit simultaneously with said injection of said first fluid, at a rate less than the flow rate for said second fluid into said lower permeable stratum, wherein said second fluid is adapted to treat said lower permeable stratum.
inserting a conduit into said well through said tubing string so that said conduit is landed with its outlet below said upper permeable stratum;
injecting a first fluid into said well through said tubing string at a rate greater than the flow rate for said first fluid into said upper permeable stratum; and injecting a second fluid into said well through said conduit simultaneously with said injection of said first fluid, at a rate less than the flow rate for said second fluid into said lower permeable stratum, wherein said second fluid is adapted to treat said lower permeable stratum.
17. The method of claim 16, wherein the step of inserting said conduit into said well comprises inserting said conduit into said well through said tubing string so that said conduit is landed with its outlet adjacent said lower permeable stratum.
18. The method of claim 17, wherein said conduit is a coiled tubing unit.
19. The method of claim 18, wherein said first fluid has a known flow rate Qu into said upper permeable stratum and said second fluid has a known flow rate Q? into said lower permeable stratum, and wherein said rate at which said first fluid is injected is approximately equal to Qu + 0.1 Q?
and said rate at which said second fluid is injected is approximately equal to 0.9 Q?.
and said rate at which said second fluid is injected is approximately equal to 0.9 Q?.
20. The method of claim 19, wherein said second fluid is adapted to treat said lower permeable stratum so as to increase its permeability, and wherein said injection rates are adjusted as said flow rate of said second fluid into said lower permeable stratum increases.
21. The method of claim 20, wherein said second fluid is an acidizing solution.
22. The method of claim 21, wherein said well is a production well and wherein said tubing string is production tubing.
23. The method of claim 18, wherein said second fluid is adapted to treat said lower permeable stratum so as to reduce its permeability.
24. The method of claim 23, wherein said second fluid is phenoplast.
25. The method of claim 24, further comprising the subsequent sequential steps of:
terminating said injection of said acidizing solution into said well;
terminating said injection of said first fluid into said well;
removing said coiled tubing unit from said well; and placing said well back in production.
terminating said injection of said acidizing solution into said well;
terminating said injection of said first fluid into said well;
removing said coiled tubing unit from said well; and placing said well back in production.
26. The method of claim 16, wherein said first fluid is water.
27. A method for selectively treating an upper permeable stratum in a subterranean formation having upper and lower permeable strata which are penetrated by a well having a tubing string installed therein which is in fluid communication with said formation at a location adjacent to said upper permeable stratum, comprising:
inserting a conduit into said well through said tubing string so that said conduit is landed with its outlet below said upper permeable stratum;
injecting a first fluid into said well through said conduit at a rate greater than the flow rate for said first fluid into said lower permeable stratum; and injecting a second fluid into said well through said tubing string simultaneously with said injection of said first fluid, at a rate less than the flow rate for said second fluid into said upper permeable stratum, wherein said second fluid is adapted to treat said upper permeable stratum.
inserting a conduit into said well through said tubing string so that said conduit is landed with its outlet below said upper permeable stratum;
injecting a first fluid into said well through said conduit at a rate greater than the flow rate for said first fluid into said lower permeable stratum; and injecting a second fluid into said well through said tubing string simultaneously with said injection of said first fluid, at a rate less than the flow rate for said second fluid into said upper permeable stratum, wherein said second fluid is adapted to treat said upper permeable stratum.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000613256A CA1326207C (en) | 1989-09-26 | 1989-09-26 | Method for selectively treating strata in subterranean formations by co-injection |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000613256A CA1326207C (en) | 1989-09-26 | 1989-09-26 | Method for selectively treating strata in subterranean formations by co-injection |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1326207C true CA1326207C (en) | 1994-01-18 |
Family
ID=4140692
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000613256A Expired - Lifetime CA1326207C (en) | 1989-09-26 | 1989-09-26 | Method for selectively treating strata in subterranean formations by co-injection |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1326207C (en) |
-
1989
- 1989-09-26 CA CA000613256A patent/CA1326207C/en not_active Expired - Lifetime
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