CA1183769A - Device for fracturing a hydrocarbon producing formation - Google Patents
Device for fracturing a hydrocarbon producing formationInfo
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- CA1183769A CA1183769A CA000399949A CA399949A CA1183769A CA 1183769 A CA1183769 A CA 1183769A CA 000399949 A CA000399949 A CA 000399949A CA 399949 A CA399949 A CA 399949A CA 1183769 A CA1183769 A CA 1183769A
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- propellant
- charge
- bore hole
- formation
- oxidizer
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Abstract
DEVICE FOR FRACTURING A HYDROCARBON PRODUCING FORMATION
Abstract of the Disclosure An apparatus for stimulating hydrocarbon well production uses a gas generating propellant placed in the well casing ad-jacent perforations communicating with a formation of known hydro-carbon productivity. The propellant is ignited to generate gas at sufficient pressure and in sufficient volume to fracture the formation to thereby improve its deliverability. The propellant is designed to minimize the production of unburned propellant particles which are prone to plug casing perforations or naturally occuring high permeability streaks and interfere with formation fracturing.
Abstract of the Disclosure An apparatus for stimulating hydrocarbon well production uses a gas generating propellant placed in the well casing ad-jacent perforations communicating with a formation of known hydro-carbon productivity. The propellant is ignited to generate gas at sufficient pressure and in sufficient volume to fracture the formation to thereby improve its deliverability. The propellant is designed to minimize the production of unburned propellant particles which are prone to plug casing perforations or naturally occuring high permeability streaks and interfere with formation fracturing.
Description
7~9 DEVICE FOR FRACTURING A HYDROCARBON PRODUCING FO~MATION
....... _ _ Bac]cground o~ the Invention This invention relates to an apparatus for stimulating hydrocarbon producing wells by fracturing the formation to in-crease its permeability so that hydrocarbon flow from the forma-tion into the well casing is promoted.
It is well known in the art to fracture hydrocarbon producing formations by a variety of techniques. The most commonly used technique is known as hydraulic fracturing in which a liquid loaded 10 with a particulate propping agent is injected into the formation at a pressure adequate to break down the formation and at a volume sufficient to carry substantial quantities of the propping agent into the formation. Although hydraulic ~racturing is widely practiced, it is rather expensive because of the pumping equipment 15 necessary and beca~se of the quantity of fracing liquid and propping agent required.
Another known technique for fracturing a well formation in volves the detonation of an explosive charge in the well bore which fractures the formation by shattering or rubblizing. This technique 20 is somewhat less expensive than hydraulic fracturing ~ut has several significant disadvantages. In i~s oldest form, e~plosive fracturing of a well is accomplished by placing one or more nitroglycerine - charges in the well bore and then detonating them. The first dis-advantage of explosive formation fracturing is that considerable 25 damage is often done to casing in the well and considerable junk is ~, ~
....... _ _ Bac]cground o~ the Invention This invention relates to an apparatus for stimulating hydrocarbon producing wells by fracturing the formation to in-crease its permeability so that hydrocarbon flow from the forma-tion into the well casing is promoted.
It is well known in the art to fracture hydrocarbon producing formations by a variety of techniques. The most commonly used technique is known as hydraulic fracturing in which a liquid loaded 10 with a particulate propping agent is injected into the formation at a pressure adequate to break down the formation and at a volume sufficient to carry substantial quantities of the propping agent into the formation. Although hydraulic ~racturing is widely practiced, it is rather expensive because of the pumping equipment 15 necessary and beca~se of the quantity of fracing liquid and propping agent required.
Another known technique for fracturing a well formation in volves the detonation of an explosive charge in the well bore which fractures the formation by shattering or rubblizing. This technique 20 is somewhat less expensive than hydraulic fracturing ~ut has several significant disadvantages. In i~s oldest form, e~plosive fracturing of a well is accomplished by placing one or more nitroglycerine - charges in the well bore and then detonating them. The first dis-advantage of explosive formation fracturing is that considerable 25 damage is often done to casing in the well and considerable junk is ~, ~
- 2 ~ 37~9 ten left in the hole requiring expensive and time consuming efforts to clean out the well and repair the damage done. Al-though there are more modern explosive fracturing techniques available, these also suffer from this disadvantage.
The second disadvantage of explosive well fracturiny techniques involves the obvious danger in handling, -transporting and detonating such explosives. Personnel of extensive training and experience are required for explosive fracturing techniques and such personnel are not always readily available.
A third type of formation fracturing technique involves the use of a device incorporating a yas generating charge or propellant which is typically lowered into a well and ignited to generate sub-stantial quantities of gas at a pressure sufficient to break down the formation adjacent the perforations. In this type approach, 1~ the desired fracturing is caused by the high pressure combustion products produced by the propellant rather than shock wave fracturing as in the case of explosive techniques. It is this type of ~racturing technique that this invention most nearly relates. Typical dis-20 closures of this type fracturing device are found in U. S. patents3,174,545; 3,264,986; 3,313,234; 4,064,935 and 4,081,031. ~lso of some interest are the disclosures in U. S. patents 3,602,304 and
The second disadvantage of explosive well fracturiny techniques involves the obvious danger in handling, -transporting and detonating such explosives. Personnel of extensive training and experience are required for explosive fracturing techniques and such personnel are not always readily available.
A third type of formation fracturing technique involves the use of a device incorporating a yas generating charge or propellant which is typically lowered into a well and ignited to generate sub-stantial quantities of gas at a pressure sufficient to break down the formation adjacent the perforations. In this type approach, 1~ the desired fracturing is caused by the high pressure combustion products produced by the propellant rather than shock wave fracturing as in the case of explosive techniques. It is this type of ~racturing technique that this invention most nearly relates. Typical dis-20 closures of this type fracturing device are found in U. S. patents3,174,545; 3,264,986; 3,313,234; 4,064,935 and 4,081,031. ~lso of some interest are the disclosures in U. S. patents 3,602,304 and
3,618,521.
One difficulty with gas generating fracturing devices is that 25 there is a tendency for perforations in the well casing to plug during the fracturing process. Assuming, for purposes of illustra-tion, that the fracturing process is proceeding normally, gas ~
generated by the device passes from the interior of the well casing through the perforations communicating with the formation and then 30 into the formation to generate ~ractures therein. Any propellant particles which are free to move independently of the main pro-pellant body will necessarily be picked up by the fairly rapid gas flow through the perforations and moved toward and into the per-forations.
When propellan~ particles break off or move away from the main propellant charge, combustion of a statistically significant number of these particles stops at least momentarily so there is generated a significant number of unburned propellant particles which are free to move toward and into the perfora-tionsO Consequently, plugging 40 of the perforations by unburned propellant particles thrown off by 76~
gas generating Eracturing device~ is a signiEicant problem. What appears to happen is that -the device ignltes causing genera-tion of a substantial quantity of gas which breaks down one or more oP the perforations so that the gas moves rapidly in the direction o~ the broken down perforations. When an unburned propellant par-ticle becomes lodged in the broken down perforation~ gas ~low through this perforation diminishes substantially causing a pressure build up inside the well bore which causes another perforation to break down and begin transmitting gas into the formation through another per-10 foration. Shortly, the second perforation becomes plugged by anotherunburned propellant particle. rrhis process continues until all of the perforations are plu~ged by unburned propellant particles or until the entire quantity of gas generated passes into the formation through unplugged perforations.
~t first blush, this process of plugging off one perforation and breaking down another may seem advantageous since it sounds like the "balling out" technique used in hydraulic fracturing to assure that all of the perforations receive a fracturing treatment. If this process of plugging perforations with unburned propellant 20 particles were controllable, like balling out a hydraulic frac job is, that would be one matter. Unfortunately, the generation and lodgement of unburnea propellant particles in broken down perfora-tions is not contrallable and has some significant drawbacks.
If, for example, there are a hundred unburned propellant 25 particles thrown off during combustion of the charge and there are ten perforations communicating with the formation to be ~ractured, there is a high degree of probability that all ten perforations will plug off prior to the end o~ gas generation. This necessarily causes a substantial pressure buildup in -the casing which is obviously 30 potentially dange~ous since the pressure may be sufficient to burst the casing or blow out through the lubricator.
In extreme cases, the casing in the well will burst. Bursting normally generates a longitudinal split that extends along one joint of casing, typically 30-40 ~eet~ If this entire interval is pro-35 ductive o~ hydrocarbons, the well may not be Iost. If, on the otherhandl ~he split joint intersects a water bearing formation, the well will no longer produce hydrocarbons in commercial quantities but will instead produce mainly water. The only thing that can be done by way ~83~ E;9 ~, ~f remedy is to a-ttempt to squee~e off ~he split join-t with cemen-t, drill out the cement, reperforate the hydrocarbon ~earirly section and hope that water entry into the casing is s-topped. The successful squeezing of a water bearin~ formation in these circumstances is problematical.
In addition, unless the gas flows rapidly -throuyh the per-forations into the formation, no fractures are generated in the formation and the stimulation attempt is a partial or total failure depending on when plug off occurs. Furthermore, if the perforations 10 are plugged off during stimulation, they remain plugged off when the attempt is made to put the well back on production. If the pipe does not burst, as may happen if plug off occurs at or near the end of combustion, the perforations will be ~ound to be well plugged off so that hydrocarbon entry into the casing is substantially prevented.
15 This may, of course, wholly defeat or counteract any improvement in permeability induced in the formation by the generation of fractures therein. It will be appreciat~d that the perforations cannot be unplugged by normal ~eans, such as swabbing~ since no pressure of suf~icient magnitude can be generated which is opposite in direction 20 to the preSsure created by the gas generatin~ device.
Another di~ficulty with the` prior art fracturing devices of gas generating type is that the devices must be sealed against entry of ~luid or liquid contained in the bore hole. The major reason is that the oxidizexs used in ~as generatin~ $racturing devices are 25 water soluble and water is a verY common liquid present in wells.
When frac~ng a g~s weIl, for example~ the operator will place what is called a water cushion wh;ch is normally brine or KCL water~
Obyiously, if one attempted to frac such a well with a ~as generating type deyice in which a water soluble oxidizer was e~plosed to the 30 action of the bore hole liquid~ the device would not function satis-factorily if it wo~l~ ignite at all since a substantial part of the oxidizer would be dissolyed by the bore hole fluid.
Accordingly, gas ~enerating type deyices have included a me~allic enyelope housin~ the gas ge~erating char~e which envelope acts as a 35 seal against entry of bore hole fluids prior *o ignition of the device.
There is, of courser no problem of exposing the oxidizer to bore hole liquids after ignition for two reasons. First, evolution of gas from the deyice is at a sufficient pressure and volume to preVent bore hole liquids from contacting the propellant. Second, even if some bore ~ ~8~7~
nole llquid were to contact -the propellant, the propellant burns fairly rapid]y so that thexe is not sufficient time Eor a siynificant quantity of the oxidizer to dissolve.
The problem w:ith sealing the propellant charge against contact with bore hole fluids is tw~ fold. First, -there is considerable expense involved compared to a situation where sealing of the charye is not required. Of course, building a tool capable of withstandiny hiyh pressures, as m~y occur in deep gas wells, requires more elaborate and expensive equipment than shallower wells. There is some compromise 10 that must be reached between the desirability of minimizing costs o any particular tool and minimizing the number of tools which must be built and kept in inventory. The second disadvantage of sealiny the propellant charye in a housiny is that considerable eneryy must be spent in burstiny the housiny which could more desirably be used to 15 fracture the formation.
In accordance with this invention, the propellant charye is designed and arranged so that the propellant stays together as a body as long as possible during combustion thereby minimizing the generation of un~urned propellant particles. This i9 accomplished by increasing 20 the mechanical strength of the propellant charge so that it is struc-turally stronger during combustionO Mechanically strenything the propellant body may be accomplished in a variety of ways. One technique is to encapsulate the propellant in a metallic housiny which is designed not to disintegrate during combustion but instead merely to split 25 thereby allowing the passage of combustion products from the device.
Another technique is to strengthen the binder holding the propellant material together in the charge. Another technique for mechanically strengthening the propellant charge is to incorporate therein a rein-forcing material, in much the same way that a fiber glass resin is 30 strengthened by placing glass strands therein or concrete is strengthened by the use of reinforcing steel.
Another fea-ture of this invention is that the propellant body is not sealed inside a housing against the action of bore hole fluids.
Because of the use of a substantially impermeable binder encapsulating 35 the oxidizer particles, the binder provides a sealing function to prevent dissolving of the water soluble oxidizer in the bore hole fluid. This means that the outer surface of the frac tool may comprise the outer surface of the propellant body. In the alternative, the outer surface of the tool may be the propellant body which has 1183~6~
merely been painted or covered with a ve:ry thin metallic layer in order ~o prevent abrasion of the propellant body as it is run in-to the well. It will be appreciate tha-t a me-tal foil wrappiny about the propellant body ls substantially less expensive than a housing which is capable of withstanding many thousands of pounds of pressure and which is fluid -tight. Another advantage is that the energy produced by the burlling propellant is used wholly to frac-ture the formation rather than to burst a housing of significant strength.
It is accordingly an object oE this invention to provide an 10 improved device for Eracturing a hydrocarbon well formation by the gas generating technique~
Another object of this invention is to provide an improved well fracturing device of the gas generating type in which roaketing of the device during combustion is minimized.
A further object of this invention is to provide an improved well fracturing device of the gas generating type which, in use, is substantially less prone to cause plugging of well perforations.
Other objects and advantages of this invention will become more fully apparent as this description proceeds, reference being 20 made to the accompanying drawings and appended claims.
IN THE DRAWINGS:
Figure 1 is a longitudinal cross-sectional view of a typical hydrocarbon producing well demonstrating placement of a fracturing ~evice of the gas generating type adjacent the perforations prior 25 to ignition of the propellant charge therein;
Figure 2 is an enlarged transVerse cross-sectional view of the ~racturing tool o~ Figure 1 taken substantially along line 2-2 thereof as viewed in the direction indicated by the arrows; and Figures 3 and 7 are enlarged tran~yerse cross-sectional views, 30 similar to Figure 2, of a number of different embodiments of this invention;
Figure ~ are cross-section and side elevational views réspectively of another embodiment of this invention; and Figure 6 is a side elevational view of another embodiment of 35 this invention.
- 7 ~ 3~7~9 Referring to Figure 1, there is illustrated a well 10 comprising a bore hole 12 extending into the earth 14 and .intersec-ting a forma-tion 16 which is typically although not universally hydrocarbon beariny.
As will be more fully pointed out hereinafter, it is desirable to improve the productivity of the formation 16 which term is intended to lnclude the ability of the formation 16 to give up fluids contained therein or to accept fluids injected thereinto.
~ string of casing 18 extends downwardly into the earth 14 to adjacent the formation 16 and preferably extends somewhat therebelow.
10 The casing string 18 is bonded to the wall of the bore hole 12 by a cement sheath 20. A multiplicity of perforations 22 communicate between the~ interior of the casing string 18 and the formation 16 to allow passage of fluids between the casing string 18 and the formation 16~
Typicall~, the casing 18 is at least partially ~illed with a co~pletion ~luid 24 such as brine, KCl water or the like.
A tool 26 of this in~ention is illustxated as having been run into the'well 10 .in any suitable fashionJ ~s by the use of ~ wire line 28. The tool 26 çomprises, as major componentsJ a main pro-20 pell~nt cha~e.30, ~eans 32 for igniting the charge 30 and means34 for connecting the'wire'line 28 to the tool 26 and for sealing the ig~itin~ means 32 against liquid contamination from the com-ple.tion fluid 2~.
As is e~dent ~om Figures 1 and 2, a peculiar feature of this 25 embodi~ent ~s that it contains no exterior metallic housing for the propellant charge 30 although the char~e'30 may be painted or coated w~th a ~aterial to ~ed`uce o~ minimize abrasion damage to the pro-peIlant charge'as the tool 26 is run into the'casing 18 The ~ain propellant charge 30 is designed to minimize the 30 product~on o~ ~obile unbuxned propellant particl'es durin~ combustion of the'charge'30. In thi~s fashionJ pluggln~ o~ per.~orations during co~bustion of the charge'30 is minimized or eliminated. The first step toward this end is maki~n~ the propell~nt charge 30 a rigid body rather' tha;n a quantity of granules. ~n additional step is increasing 35 the ~echanic~l strength of the'ri~id propeIlant body in advanta~eous directions.
It will be immediately appreciated that a pxopellant charge comprised of a quantity of granules is prone to produce significant - 8 - ~ ~ ~3~6~
quantities of unburned particles. Indeed, all that need occur is for combustion to stop. By providing the propellant charye in a rigid body, the produc-tion of unburned propellant particles is drastically reduced since two steps must occur beore a mobile un-burned propellant particle is capable of plugging a perforation.First, the particle must break off the main charge. Second, com-bustion of the particle must stop. In this fashion, plugging of perforations by the unburned propellant particles is minimized or eliminated. For these reasons, -the propellant charge 30 comprises 10 a rigid body having significant mechanical strength.
One of the factors that bear upon the design o~ a propellant charge is the mechanism whereby the charge is supported during combustion of the propellant. It turns out that if the charge is supported on a wire line and ignited from the interior thereof, the 15 more important mechanical strength facet appears to be tensile strength rather than compressive strength. The reason, of course, is that the propellant charge 30, in the design of Figure 1, is not sub~ected to a compressive load. Instead, any unburned propellant particle that tends to break lose from the main propellant body does 20 so because of a failure in tension adjacent the particle broken off.
The production of mobile unburned propellant particles because of tensile failure is aggreyated by an inherent characteristic of pro-pellant materials which are cast into a rigid body. The characteristic is that these materials are modestly strong in compression but woefully 25 weak in tension.
Accordingly, when designing a wire line supported propellant charge in accordance with this invention, steps are preferably taken to provide a minLmum tensile strength. At a minim~, the tensile strength of the propellant charge should be at least about 200 pounds 30 per square inch. Desirably, the tensile strength is considerably higher and is on the order of 300 pounds per square inch. Preferably, the tensile strength is even higher and is on the order of about 400 pounds per square inch.
There are a wide variety of propellant materials which can be 35 ~ormulated and cast into a rigid body ha~ing the desired strength characteristics. Suitable oxidizers include ammonium nitrater ammonium perchlorate, potassium nitrate, potassium chlorate, potassium per-chlorate, sodium nitrate and other sophisticated oxidizers such as - 9 - 1183~69 lithium perchlorate as will be evident to those skilled in the art.
In order to provide the necessary strength ~or the propellan-t ma-terial, a binder is needed. Although any suitable material may be used a.s a binder so long as it provides the necessary strength to the pro-pellant material, typical binders are long chain polymers such aspolyesters, phenolic, polyethylene, polysulfide, polyvinyl chloride, polyurethane, epoxies and the like. One of the pecularities o~
propellants of the type used in this invention is that the binder nok only acts to agglomerate the oxidizer, but also acts as a fuel which 10 is burned during combustion. The proportion o~ binder and oxidizer should accordingly take into account the requirement for fuel as well as the requirement for strength~ In its broadest aspect, the pro-pellant material of this invention comprises 5-50% by weight binder.
Toward the minimal proportion of binder, the propellant material 15 contains an excess of oxygen and accordingly burns quite clean but does not provide the maximum amount of combustion products. Although the tensile strength of the material declines somewhat, the material remains surprisingly strong. As the proportion o~ binder increases toward a maximum 50%, the propellant material achieves greater strength 20 but begins to suffer from a lack of oxyge:n during combustion so that combustion is quite sooty. For these reasons, the propellant material preferably comprises about 20-40% by weig:ht binder.
Suitable propellant combinations are shown in the following examples:
251. ammonium nitrate - 50% by weight poly~inyl chloride - 50% by wei~ht 2~ ammonium nitrate - 78% by weight polyurethane - 22% by weight 3. ammonium nitrate - 95% by weight 30epoxy - 5% by weight . ammoniu~ pexchlorate - 62% by w2ight polysul~ide - 3g% by wei~ht 5. ammonium perchlorate - 92% by weight polyester - 8% by weight 356. potassium nitrate - 58% by weigh~
phenolic - 42% by weight 7. potassium nitrate - 73% by weight polyester - 27% by weight 8. potassium nitrate - 86~ by weight 40polyurethane - 1~% by weight 1 o - ~ ~83~69 9. potassium chlorate - 52~ by weight polyester - 48% by weight 10. potassium chlorate - 83% by weight polyethylene - 17% by weight 11. potassium perchlorate - 61% by weight polyethylene - 39~ by weight 12. potassium perchlorate - 70% by weight epoxy - 30% by weight 13. potassium perchlorate 88% by weight epoxy - 12~ by weigh-t 14. sodium nitrate - 60% by weight phenolic - 40~ by weight 15. sodium nitrate - 75% by weight polyvinyl chloride - 25~ by weight 5 16. sodium nitrate - 90~ by weight polysulfide - 10% by weight In all of the preceding examples, the oxidizing material and binder are molecularly separate. It is possible that the oxidizing material and binder or fuel material be contained in the same molecule.
20 This is typically the case in thermodynamically unstable molecules in which a reaction may occur to produce products that are more stable.
E~emplary of such compounds is nitrocellulose which is normally con-sidered to be an explosive rather than a propellant. There are, ho~ever, a number of techniques that can be employed to slow down the 25 reaction of nitrocellulose until it loses the characteristics of an explosive material and assumes the characteristics of a gas generating propellant. One of these techniques is to dissolve the nitrocellulose i~ a suitable solvent and then make the dissolved nitrocellulose into a gelled, single grain material which has the characteristics of this 30 invention, i.e. a gas generating propellant.
~ he igniting means 32 may be of any suitable type for initiating combustion of the propellant charge 30 in response to an electrical impulse delivered through the wire line 28. A suitable technique for igniting the propellant charge is shown in United States patents 35 3,174,545; 3,264,986; 3,313,234; 4,064,935 and 4,081,031. In this type igniting system, an elongate blind passage 36 extends into the propellant charge 30 from one end thereof~ The opposite end of the passage 36 is closed in any suitable fashion, as by spacing the bottom wall 38 of the passage 36 from the bottom of the propellant charge 30.
40 In the alternative, an imperforate metallic end piece may be cast into 8376~31 the bottom o~ the charcJe 30. ~he passage 36 is Eilled with high veloclty ignition material 42 on top of which is placed an electric bridge wire i.gniter ~4.
The connecting means 34 may be of any suitable design. One technique that has proved quite satisfactory is to embed a loop o:E
wire 46 in the propellant charge 30 as it is being cast. As the propellant solidifies, the wire loop 46 becomes an integral part of the propellant charge 30. The wire loop 46 may then accordingly act as a bail for the device 26. Conse~uently, the wire line 28 is 10 connected to the wire loop 46 in a load transferring relation, as by tying the wire line 28 to the loop 46, by the provision of a hook on the wire line 28 receiving the loop 46 or by physically tying a knot in the wire line 28 with the wire loop 46 inside the knot~ The term-inal end of the wire line 28 is then inserted in the upper end of 15 the passage 36 and sealed, as by the application of a sealant body comprised of wax, quick drying adhesive, silicon sealant or the like.
Operation of the device 26 should now be apparent. The device 26 is lowered into the well 10 on the end of the wire line 28. After the device 26 reaches its desired ignition location adjacent the 20 perforations 22, an electrical impulse is delivered through the wire line 28 to energize the bridge wire igniter 44 which ignites the train o~ ast burning ignition material 42. In response to the ignition material 42 burning, co~bustion of the propellant charge 30 is initiated.
Combustion o~ the charge 30 begins along the surface of the passage 25 36 and commences to burn in ~n outwardly radial direction. Cracks, fissures and the lik~e are generated in the propellant charge 30 to allow escape o~ the hotr high pre~su~e combustion products. Because of the ineXtia of the quantity o~ the completion fluid 24 in the casing 18, the path of least resistance for the combustion products 30 is through the perforations 22 into the formation 16. Because the rate o~ production of combustion gases is rather high, the combustion pxoducts generated by the propel~ant charge 30 pass into the formation 16 at a rate hi~h enough to generate fractures therein.
As mentioned previously, an important part of this~invention is 35 that the propellant charge 30 has sufficient mechanical strength to minimize the production of mobile, unburned propellant particles which are capable of plugging the perforations 22. ~ccordingly, use of the device 26 results in a higher percentage of satisfactorily - 12 _ ~ ~ ~3~S~
stimulated wells than previous types of Eracturing devlces incorporating a gas generating or propellant charge.
~ nother important feature of -the embodiment of Figure 1 is that no metallic housing is employed to protect the propellant charge 30 from the effects of the completion fluid 2~. E~en though the oxidizers used in the propellant charge 30 are water soluable and wa-ter is -the most common constituent of completion fluids, there is no significant deleterious effect on the propellant charge 30. The main reason appears to be that the binder material encapsulates the oxidizer 10 particles thereby minimizing the contact of the oxidizers with the bore hole fluid. It will be appreciated that the only part of the deyice 26 in which any pains are taken to seal are the components of the igniting means 32. It will be seen that this feature greatly simplifies the construction of gas generating type fracturing devices 15 and reduces the cost thereof. It will also be appreciated that the deyice 26 is normally coated with a protective substance, such as paint or the like, in order to minimize abrasion to the propellant charge 30 as the device 26 is run into the casing 18.
Re~erring to Figure 3, there is illustrated another embodiment 20 50 o~ this invention. In the embodiment 50, there is provided a propellant charge 52 and an igniting means in ~hich a blind passage 54 contains a train of rapidly buxning ignition material. The only difference between the devices 26, 50 is that the blind passage 54 is located substanti~lly away from the axis 56 of the device 50 25 whereas the blind passage 56 is substa~tially coaxial with the axis 58 Qf the deyice 26.
Referrin~ to Figures 4 and 5, there is illustrated another embodiment 60 o~ this in~ention comprising a propellant charge 62 including an elonga-te blind passage 64 for recei~ing a quantity of 30 ~ast burning ignition material for igniting the propellant charge 62, a wire line 66 $or supporting the device 60 and igniting the ignition material in the passage 64 and means 68 for connecting the propellant charge 62 to the wire line 66~ As heretofore described, the embodi-ment 60 is identical to the embodiment 26 of Figures 1 and 2. The 35 device 60 differs from the embodiment of Figures 1 and 2 in that the propellant charge 62 is disposed in a metallic cannister or housing 70 comprising a cylindrical wall 72 and a bottom wall 74. The connecting means 68 or ~ire loop extends through suitable openings - 13 - ~ 33769 (not shown) in the wall 72. Preferably, the wire loop is -tied or otherwise connected to the wall 72 so that the wire loop an~l cannis-ter remain connec-ted af-ter the propellant charge 62 is combusted. In order to increase the mechanical strength of the propellant charge 62, 5 the charye 62 is bonded to the interior of the cannister 70. This is conveniently accomplished by pouring a liquid mixture of the desired oxidizer and binder into the cannister. It will be appreciated that several of the binders mentioned previously, e.g. epoxy, have signi-ficant adhesive qualities.
It will be appreciated, of~~course, that the metallic housing 70 does not act to seal the propellant charge 62 and the water soluable oxidizer therein from the action of the bore hole fluid in the well 10.
The metallic cannist:er 70 affords two advantages. First, considerably greater protection is afforded against abrasion o the propellant 15 charge 62 during travel downwardly through the casing string 18.
Second, the cannister 70 provides significant mechanical strength for the propellant charge 62 which either (1) reduces the production of mobile unburned propellant particles or ~2) allows the selection of propellant mixtures which, by themselves, are weaker than would be 20 satisfactory without the reinforcing effect of the cylindrical metallic wall 72.
Referring to Figure 6/ another embodiment 76 of this invention is illustrated comprising an elongate rigid propellant charge 78 bonded to the interior of a metallic housing or cannister 80 having 25 a cylindrical wall 82j a bottom wall 84 and a top wall 86. Secured in any suitable fashion to the top wall 86 is a rope socket 88 into which a wire line 90 extendsO As will be appreciated by those skilled in the art of wire line supported tools, the rope socket 88 connects tp the wire line 90 to provide a load supporting connectiorl for the 30 tool 80~ As in the embodiments of Figures 1-5, the only precautions that need be taken to seal against the entry of bore hole fluids lies in sealing the ignition train. During combustion of the tool 76, the metallic housing 80 splits to allow the exit of combustion products.
Typically, however, the metallic housing 80 remains intact and is 35 remoyed from the well 10 after the fracturing procedure is completed.
Referring to Figure 7, another embodiment 92 of this invention is illustrated. The device 90 is substantially identical to the embodirnent of Figures 1 and 2 except that a multiplicity of elongate fibrous strands 9~ are embedded in the propellant charge 96 during - 14 ~ 83~9 casting thereof. The Eibrous strands 9~ ac-t as a rein~orciny ma-terial to the oxidi~er-blnder mix-ture in much the same manner as steel rods act in cement.
One convenient ~echnique Eor manufacturing devices of this invention should be readil~ apparent. In the case oE -the devices 60, 76 shown in Figures ~-6, the metallic housings 70, 80 act as a mold to receive a well blended liquid mixture of the oxidizer and binder. The blind passage in the propellant charge is formed by a lubricated rod extending into the metallic housings 70, 80. As the 10 propellant sets up, it bonds to the metallic housings 70, 80 but not to the lubricated rod which is withdrawn to leave a receptacle for the ignition material.
Manufacture of the devices 26, 50, 92 is quite similar except that a reusable mold is employed rather than the metallic housings 15 70, 80. The mold as well as the rod are lubricated to allow separation of the propellant charge from the molds.
Gne of the peculiar aspects of this invention is that the pro-peIlant materials do not burn very well, if at allr unless the pressure in the well where the device is located is at least about 20 200 pounds. This is normally no problem since the well 10 is typi-cally at least partially filled with the completion fluid 24.
It will accordingly be eyident that there is provided a device for fracturing ~ormations of the ~as generating type which minimizes the production of unburned propellant particles and which does not 25 req~ire a metallic housing to seal the propellant against entry of bore hole fluid.
Although the preferred embodiments of this invention have been disclosed in substantial detail~ it is understood that this invention is defined in the appended claims and that numerous changes in the 30 details o~ construction and selection o~ materials may be resorted to without departing ~rom the spirit and scope of the invention as hereinafter claimed.
One difficulty with gas generating fracturing devices is that 25 there is a tendency for perforations in the well casing to plug during the fracturing process. Assuming, for purposes of illustra-tion, that the fracturing process is proceeding normally, gas ~
generated by the device passes from the interior of the well casing through the perforations communicating with the formation and then 30 into the formation to generate ~ractures therein. Any propellant particles which are free to move independently of the main pro-pellant body will necessarily be picked up by the fairly rapid gas flow through the perforations and moved toward and into the per-forations.
When propellan~ particles break off or move away from the main propellant charge, combustion of a statistically significant number of these particles stops at least momentarily so there is generated a significant number of unburned propellant particles which are free to move toward and into the perfora-tionsO Consequently, plugging 40 of the perforations by unburned propellant particles thrown off by 76~
gas generating Eracturing device~ is a signiEicant problem. What appears to happen is that -the device ignltes causing genera-tion of a substantial quantity of gas which breaks down one or more oP the perforations so that the gas moves rapidly in the direction o~ the broken down perforations. When an unburned propellant par-ticle becomes lodged in the broken down perforation~ gas ~low through this perforation diminishes substantially causing a pressure build up inside the well bore which causes another perforation to break down and begin transmitting gas into the formation through another per-10 foration. Shortly, the second perforation becomes plugged by anotherunburned propellant particle. rrhis process continues until all of the perforations are plu~ged by unburned propellant particles or until the entire quantity of gas generated passes into the formation through unplugged perforations.
~t first blush, this process of plugging off one perforation and breaking down another may seem advantageous since it sounds like the "balling out" technique used in hydraulic fracturing to assure that all of the perforations receive a fracturing treatment. If this process of plugging perforations with unburned propellant 20 particles were controllable, like balling out a hydraulic frac job is, that would be one matter. Unfortunately, the generation and lodgement of unburnea propellant particles in broken down perfora-tions is not contrallable and has some significant drawbacks.
If, for example, there are a hundred unburned propellant 25 particles thrown off during combustion of the charge and there are ten perforations communicating with the formation to be ~ractured, there is a high degree of probability that all ten perforations will plug off prior to the end o~ gas generation. This necessarily causes a substantial pressure buildup in -the casing which is obviously 30 potentially dange~ous since the pressure may be sufficient to burst the casing or blow out through the lubricator.
In extreme cases, the casing in the well will burst. Bursting normally generates a longitudinal split that extends along one joint of casing, typically 30-40 ~eet~ If this entire interval is pro-35 ductive o~ hydrocarbons, the well may not be Iost. If, on the otherhandl ~he split joint intersects a water bearing formation, the well will no longer produce hydrocarbons in commercial quantities but will instead produce mainly water. The only thing that can be done by way ~83~ E;9 ~, ~f remedy is to a-ttempt to squee~e off ~he split join-t with cemen-t, drill out the cement, reperforate the hydrocarbon ~earirly section and hope that water entry into the casing is s-topped. The successful squeezing of a water bearin~ formation in these circumstances is problematical.
In addition, unless the gas flows rapidly -throuyh the per-forations into the formation, no fractures are generated in the formation and the stimulation attempt is a partial or total failure depending on when plug off occurs. Furthermore, if the perforations 10 are plugged off during stimulation, they remain plugged off when the attempt is made to put the well back on production. If the pipe does not burst, as may happen if plug off occurs at or near the end of combustion, the perforations will be ~ound to be well plugged off so that hydrocarbon entry into the casing is substantially prevented.
15 This may, of course, wholly defeat or counteract any improvement in permeability induced in the formation by the generation of fractures therein. It will be appreciat~d that the perforations cannot be unplugged by normal ~eans, such as swabbing~ since no pressure of suf~icient magnitude can be generated which is opposite in direction 20 to the preSsure created by the gas generatin~ device.
Another di~ficulty with the` prior art fracturing devices of gas generating type is that the devices must be sealed against entry of ~luid or liquid contained in the bore hole. The major reason is that the oxidizexs used in ~as generatin~ $racturing devices are 25 water soluble and water is a verY common liquid present in wells.
When frac~ng a g~s weIl, for example~ the operator will place what is called a water cushion wh;ch is normally brine or KCL water~
Obyiously, if one attempted to frac such a well with a ~as generating type deyice in which a water soluble oxidizer was e~plosed to the 30 action of the bore hole liquid~ the device would not function satis-factorily if it wo~l~ ignite at all since a substantial part of the oxidizer would be dissolyed by the bore hole fluid.
Accordingly, gas ~enerating type deyices have included a me~allic enyelope housin~ the gas ge~erating char~e which envelope acts as a 35 seal against entry of bore hole fluids prior *o ignition of the device.
There is, of courser no problem of exposing the oxidizer to bore hole liquids after ignition for two reasons. First, evolution of gas from the deyice is at a sufficient pressure and volume to preVent bore hole liquids from contacting the propellant. Second, even if some bore ~ ~8~7~
nole llquid were to contact -the propellant, the propellant burns fairly rapid]y so that thexe is not sufficient time Eor a siynificant quantity of the oxidizer to dissolve.
The problem w:ith sealing the propellant charge against contact with bore hole fluids is tw~ fold. First, -there is considerable expense involved compared to a situation where sealing of the charye is not required. Of course, building a tool capable of withstandiny hiyh pressures, as m~y occur in deep gas wells, requires more elaborate and expensive equipment than shallower wells. There is some compromise 10 that must be reached between the desirability of minimizing costs o any particular tool and minimizing the number of tools which must be built and kept in inventory. The second disadvantage of sealiny the propellant charye in a housiny is that considerable eneryy must be spent in burstiny the housiny which could more desirably be used to 15 fracture the formation.
In accordance with this invention, the propellant charye is designed and arranged so that the propellant stays together as a body as long as possible during combustion thereby minimizing the generation of un~urned propellant particles. This i9 accomplished by increasing 20 the mechanical strength of the propellant charge so that it is struc-turally stronger during combustionO Mechanically strenything the propellant body may be accomplished in a variety of ways. One technique is to encapsulate the propellant in a metallic housiny which is designed not to disintegrate during combustion but instead merely to split 25 thereby allowing the passage of combustion products from the device.
Another technique is to strengthen the binder holding the propellant material together in the charge. Another technique for mechanically strengthening the propellant charge is to incorporate therein a rein-forcing material, in much the same way that a fiber glass resin is 30 strengthened by placing glass strands therein or concrete is strengthened by the use of reinforcing steel.
Another fea-ture of this invention is that the propellant body is not sealed inside a housing against the action of bore hole fluids.
Because of the use of a substantially impermeable binder encapsulating 35 the oxidizer particles, the binder provides a sealing function to prevent dissolving of the water soluble oxidizer in the bore hole fluid. This means that the outer surface of the frac tool may comprise the outer surface of the propellant body. In the alternative, the outer surface of the tool may be the propellant body which has 1183~6~
merely been painted or covered with a ve:ry thin metallic layer in order ~o prevent abrasion of the propellant body as it is run in-to the well. It will be appreciate tha-t a me-tal foil wrappiny about the propellant body ls substantially less expensive than a housing which is capable of withstanding many thousands of pounds of pressure and which is fluid -tight. Another advantage is that the energy produced by the burlling propellant is used wholly to frac-ture the formation rather than to burst a housing of significant strength.
It is accordingly an object oE this invention to provide an 10 improved device for Eracturing a hydrocarbon well formation by the gas generating technique~
Another object of this invention is to provide an improved well fracturing device of the gas generating type in which roaketing of the device during combustion is minimized.
A further object of this invention is to provide an improved well fracturing device of the gas generating type which, in use, is substantially less prone to cause plugging of well perforations.
Other objects and advantages of this invention will become more fully apparent as this description proceeds, reference being 20 made to the accompanying drawings and appended claims.
IN THE DRAWINGS:
Figure 1 is a longitudinal cross-sectional view of a typical hydrocarbon producing well demonstrating placement of a fracturing ~evice of the gas generating type adjacent the perforations prior 25 to ignition of the propellant charge therein;
Figure 2 is an enlarged transVerse cross-sectional view of the ~racturing tool o~ Figure 1 taken substantially along line 2-2 thereof as viewed in the direction indicated by the arrows; and Figures 3 and 7 are enlarged tran~yerse cross-sectional views, 30 similar to Figure 2, of a number of different embodiments of this invention;
Figure ~ are cross-section and side elevational views réspectively of another embodiment of this invention; and Figure 6 is a side elevational view of another embodiment of 35 this invention.
- 7 ~ 3~7~9 Referring to Figure 1, there is illustrated a well 10 comprising a bore hole 12 extending into the earth 14 and .intersec-ting a forma-tion 16 which is typically although not universally hydrocarbon beariny.
As will be more fully pointed out hereinafter, it is desirable to improve the productivity of the formation 16 which term is intended to lnclude the ability of the formation 16 to give up fluids contained therein or to accept fluids injected thereinto.
~ string of casing 18 extends downwardly into the earth 14 to adjacent the formation 16 and preferably extends somewhat therebelow.
10 The casing string 18 is bonded to the wall of the bore hole 12 by a cement sheath 20. A multiplicity of perforations 22 communicate between the~ interior of the casing string 18 and the formation 16 to allow passage of fluids between the casing string 18 and the formation 16~
Typicall~, the casing 18 is at least partially ~illed with a co~pletion ~luid 24 such as brine, KCl water or the like.
A tool 26 of this in~ention is illustxated as having been run into the'well 10 .in any suitable fashionJ ~s by the use of ~ wire line 28. The tool 26 çomprises, as major componentsJ a main pro-20 pell~nt cha~e.30, ~eans 32 for igniting the charge 30 and means34 for connecting the'wire'line 28 to the tool 26 and for sealing the ig~itin~ means 32 against liquid contamination from the com-ple.tion fluid 2~.
As is e~dent ~om Figures 1 and 2, a peculiar feature of this 25 embodi~ent ~s that it contains no exterior metallic housing for the propellant charge 30 although the char~e'30 may be painted or coated w~th a ~aterial to ~ed`uce o~ minimize abrasion damage to the pro-peIlant charge'as the tool 26 is run into the'casing 18 The ~ain propellant charge 30 is designed to minimize the 30 product~on o~ ~obile unbuxned propellant particl'es durin~ combustion of the'charge'30. In thi~s fashionJ pluggln~ o~ per.~orations during co~bustion of the charge'30 is minimized or eliminated. The first step toward this end is maki~n~ the propell~nt charge 30 a rigid body rather' tha;n a quantity of granules. ~n additional step is increasing 35 the ~echanic~l strength of the'ri~id propeIlant body in advanta~eous directions.
It will be immediately appreciated that a pxopellant charge comprised of a quantity of granules is prone to produce significant - 8 - ~ ~ ~3~6~
quantities of unburned particles. Indeed, all that need occur is for combustion to stop. By providing the propellant charye in a rigid body, the produc-tion of unburned propellant particles is drastically reduced since two steps must occur beore a mobile un-burned propellant particle is capable of plugging a perforation.First, the particle must break off the main charge. Second, com-bustion of the particle must stop. In this fashion, plugging of perforations by the unburned propellant particles is minimized or eliminated. For these reasons, -the propellant charge 30 comprises 10 a rigid body having significant mechanical strength.
One of the factors that bear upon the design o~ a propellant charge is the mechanism whereby the charge is supported during combustion of the propellant. It turns out that if the charge is supported on a wire line and ignited from the interior thereof, the 15 more important mechanical strength facet appears to be tensile strength rather than compressive strength. The reason, of course, is that the propellant charge 30, in the design of Figure 1, is not sub~ected to a compressive load. Instead, any unburned propellant particle that tends to break lose from the main propellant body does 20 so because of a failure in tension adjacent the particle broken off.
The production of mobile unburned propellant particles because of tensile failure is aggreyated by an inherent characteristic of pro-pellant materials which are cast into a rigid body. The characteristic is that these materials are modestly strong in compression but woefully 25 weak in tension.
Accordingly, when designing a wire line supported propellant charge in accordance with this invention, steps are preferably taken to provide a minLmum tensile strength. At a minim~, the tensile strength of the propellant charge should be at least about 200 pounds 30 per square inch. Desirably, the tensile strength is considerably higher and is on the order of 300 pounds per square inch. Preferably, the tensile strength is even higher and is on the order of about 400 pounds per square inch.
There are a wide variety of propellant materials which can be 35 ~ormulated and cast into a rigid body ha~ing the desired strength characteristics. Suitable oxidizers include ammonium nitrater ammonium perchlorate, potassium nitrate, potassium chlorate, potassium per-chlorate, sodium nitrate and other sophisticated oxidizers such as - 9 - 1183~69 lithium perchlorate as will be evident to those skilled in the art.
In order to provide the necessary strength ~or the propellan-t ma-terial, a binder is needed. Although any suitable material may be used a.s a binder so long as it provides the necessary strength to the pro-pellant material, typical binders are long chain polymers such aspolyesters, phenolic, polyethylene, polysulfide, polyvinyl chloride, polyurethane, epoxies and the like. One of the pecularities o~
propellants of the type used in this invention is that the binder nok only acts to agglomerate the oxidizer, but also acts as a fuel which 10 is burned during combustion. The proportion o~ binder and oxidizer should accordingly take into account the requirement for fuel as well as the requirement for strength~ In its broadest aspect, the pro-pellant material of this invention comprises 5-50% by weight binder.
Toward the minimal proportion of binder, the propellant material 15 contains an excess of oxygen and accordingly burns quite clean but does not provide the maximum amount of combustion products. Although the tensile strength of the material declines somewhat, the material remains surprisingly strong. As the proportion o~ binder increases toward a maximum 50%, the propellant material achieves greater strength 20 but begins to suffer from a lack of oxyge:n during combustion so that combustion is quite sooty. For these reasons, the propellant material preferably comprises about 20-40% by weig:ht binder.
Suitable propellant combinations are shown in the following examples:
251. ammonium nitrate - 50% by weight poly~inyl chloride - 50% by wei~ht 2~ ammonium nitrate - 78% by weight polyurethane - 22% by weight 3. ammonium nitrate - 95% by weight 30epoxy - 5% by weight . ammoniu~ pexchlorate - 62% by w2ight polysul~ide - 3g% by wei~ht 5. ammonium perchlorate - 92% by weight polyester - 8% by weight 356. potassium nitrate - 58% by weigh~
phenolic - 42% by weight 7. potassium nitrate - 73% by weight polyester - 27% by weight 8. potassium nitrate - 86~ by weight 40polyurethane - 1~% by weight 1 o - ~ ~83~69 9. potassium chlorate - 52~ by weight polyester - 48% by weight 10. potassium chlorate - 83% by weight polyethylene - 17% by weight 11. potassium perchlorate - 61% by weight polyethylene - 39~ by weight 12. potassium perchlorate - 70% by weight epoxy - 30% by weight 13. potassium perchlorate 88% by weight epoxy - 12~ by weigh-t 14. sodium nitrate - 60% by weight phenolic - 40~ by weight 15. sodium nitrate - 75% by weight polyvinyl chloride - 25~ by weight 5 16. sodium nitrate - 90~ by weight polysulfide - 10% by weight In all of the preceding examples, the oxidizing material and binder are molecularly separate. It is possible that the oxidizing material and binder or fuel material be contained in the same molecule.
20 This is typically the case in thermodynamically unstable molecules in which a reaction may occur to produce products that are more stable.
E~emplary of such compounds is nitrocellulose which is normally con-sidered to be an explosive rather than a propellant. There are, ho~ever, a number of techniques that can be employed to slow down the 25 reaction of nitrocellulose until it loses the characteristics of an explosive material and assumes the characteristics of a gas generating propellant. One of these techniques is to dissolve the nitrocellulose i~ a suitable solvent and then make the dissolved nitrocellulose into a gelled, single grain material which has the characteristics of this 30 invention, i.e. a gas generating propellant.
~ he igniting means 32 may be of any suitable type for initiating combustion of the propellant charge 30 in response to an electrical impulse delivered through the wire line 28. A suitable technique for igniting the propellant charge is shown in United States patents 35 3,174,545; 3,264,986; 3,313,234; 4,064,935 and 4,081,031. In this type igniting system, an elongate blind passage 36 extends into the propellant charge 30 from one end thereof~ The opposite end of the passage 36 is closed in any suitable fashion, as by spacing the bottom wall 38 of the passage 36 from the bottom of the propellant charge 30.
40 In the alternative, an imperforate metallic end piece may be cast into 8376~31 the bottom o~ the charcJe 30. ~he passage 36 is Eilled with high veloclty ignition material 42 on top of which is placed an electric bridge wire i.gniter ~4.
The connecting means 34 may be of any suitable design. One technique that has proved quite satisfactory is to embed a loop o:E
wire 46 in the propellant charge 30 as it is being cast. As the propellant solidifies, the wire loop 46 becomes an integral part of the propellant charge 30. The wire loop 46 may then accordingly act as a bail for the device 26. Conse~uently, the wire line 28 is 10 connected to the wire loop 46 in a load transferring relation, as by tying the wire line 28 to the loop 46, by the provision of a hook on the wire line 28 receiving the loop 46 or by physically tying a knot in the wire line 28 with the wire loop 46 inside the knot~ The term-inal end of the wire line 28 is then inserted in the upper end of 15 the passage 36 and sealed, as by the application of a sealant body comprised of wax, quick drying adhesive, silicon sealant or the like.
Operation of the device 26 should now be apparent. The device 26 is lowered into the well 10 on the end of the wire line 28. After the device 26 reaches its desired ignition location adjacent the 20 perforations 22, an electrical impulse is delivered through the wire line 28 to energize the bridge wire igniter 44 which ignites the train o~ ast burning ignition material 42. In response to the ignition material 42 burning, co~bustion of the propellant charge 30 is initiated.
Combustion o~ the charge 30 begins along the surface of the passage 25 36 and commences to burn in ~n outwardly radial direction. Cracks, fissures and the lik~e are generated in the propellant charge 30 to allow escape o~ the hotr high pre~su~e combustion products. Because of the ineXtia of the quantity o~ the completion fluid 24 in the casing 18, the path of least resistance for the combustion products 30 is through the perforations 22 into the formation 16. Because the rate o~ production of combustion gases is rather high, the combustion pxoducts generated by the propel~ant charge 30 pass into the formation 16 at a rate hi~h enough to generate fractures therein.
As mentioned previously, an important part of this~invention is 35 that the propellant charge 30 has sufficient mechanical strength to minimize the production of mobile, unburned propellant particles which are capable of plugging the perforations 22. ~ccordingly, use of the device 26 results in a higher percentage of satisfactorily - 12 _ ~ ~ ~3~S~
stimulated wells than previous types of Eracturing devlces incorporating a gas generating or propellant charge.
~ nother important feature of -the embodiment of Figure 1 is that no metallic housing is employed to protect the propellant charge 30 from the effects of the completion fluid 2~. E~en though the oxidizers used in the propellant charge 30 are water soluable and wa-ter is -the most common constituent of completion fluids, there is no significant deleterious effect on the propellant charge 30. The main reason appears to be that the binder material encapsulates the oxidizer 10 particles thereby minimizing the contact of the oxidizers with the bore hole fluid. It will be appreciated that the only part of the deyice 26 in which any pains are taken to seal are the components of the igniting means 32. It will be seen that this feature greatly simplifies the construction of gas generating type fracturing devices 15 and reduces the cost thereof. It will also be appreciated that the deyice 26 is normally coated with a protective substance, such as paint or the like, in order to minimize abrasion to the propellant charge 30 as the device 26 is run into the casing 18.
Re~erring to Figure 3, there is illustrated another embodiment 20 50 o~ this invention. In the embodiment 50, there is provided a propellant charge 52 and an igniting means in ~hich a blind passage 54 contains a train of rapidly buxning ignition material. The only difference between the devices 26, 50 is that the blind passage 54 is located substanti~lly away from the axis 56 of the device 50 25 whereas the blind passage 56 is substa~tially coaxial with the axis 58 Qf the deyice 26.
Referrin~ to Figures 4 and 5, there is illustrated another embodiment 60 o~ this in~ention comprising a propellant charge 62 including an elonga-te blind passage 64 for recei~ing a quantity of 30 ~ast burning ignition material for igniting the propellant charge 62, a wire line 66 $or supporting the device 60 and igniting the ignition material in the passage 64 and means 68 for connecting the propellant charge 62 to the wire line 66~ As heretofore described, the embodi-ment 60 is identical to the embodiment 26 of Figures 1 and 2. The 35 device 60 differs from the embodiment of Figures 1 and 2 in that the propellant charge 62 is disposed in a metallic cannister or housing 70 comprising a cylindrical wall 72 and a bottom wall 74. The connecting means 68 or ~ire loop extends through suitable openings - 13 - ~ 33769 (not shown) in the wall 72. Preferably, the wire loop is -tied or otherwise connected to the wall 72 so that the wire loop an~l cannis-ter remain connec-ted af-ter the propellant charge 62 is combusted. In order to increase the mechanical strength of the propellant charge 62, 5 the charye 62 is bonded to the interior of the cannister 70. This is conveniently accomplished by pouring a liquid mixture of the desired oxidizer and binder into the cannister. It will be appreciated that several of the binders mentioned previously, e.g. epoxy, have signi-ficant adhesive qualities.
It will be appreciated, of~~course, that the metallic housing 70 does not act to seal the propellant charge 62 and the water soluable oxidizer therein from the action of the bore hole fluid in the well 10.
The metallic cannist:er 70 affords two advantages. First, considerably greater protection is afforded against abrasion o the propellant 15 charge 62 during travel downwardly through the casing string 18.
Second, the cannister 70 provides significant mechanical strength for the propellant charge 62 which either (1) reduces the production of mobile unburned propellant particles or ~2) allows the selection of propellant mixtures which, by themselves, are weaker than would be 20 satisfactory without the reinforcing effect of the cylindrical metallic wall 72.
Referring to Figure 6/ another embodiment 76 of this invention is illustrated comprising an elongate rigid propellant charge 78 bonded to the interior of a metallic housing or cannister 80 having 25 a cylindrical wall 82j a bottom wall 84 and a top wall 86. Secured in any suitable fashion to the top wall 86 is a rope socket 88 into which a wire line 90 extendsO As will be appreciated by those skilled in the art of wire line supported tools, the rope socket 88 connects tp the wire line 90 to provide a load supporting connectiorl for the 30 tool 80~ As in the embodiments of Figures 1-5, the only precautions that need be taken to seal against the entry of bore hole fluids lies in sealing the ignition train. During combustion of the tool 76, the metallic housing 80 splits to allow the exit of combustion products.
Typically, however, the metallic housing 80 remains intact and is 35 remoyed from the well 10 after the fracturing procedure is completed.
Referring to Figure 7, another embodiment 92 of this invention is illustrated. The device 90 is substantially identical to the embodirnent of Figures 1 and 2 except that a multiplicity of elongate fibrous strands 9~ are embedded in the propellant charge 96 during - 14 ~ 83~9 casting thereof. The Eibrous strands 9~ ac-t as a rein~orciny ma-terial to the oxidi~er-blnder mix-ture in much the same manner as steel rods act in cement.
One convenient ~echnique Eor manufacturing devices of this invention should be readil~ apparent. In the case oE -the devices 60, 76 shown in Figures ~-6, the metallic housings 70, 80 act as a mold to receive a well blended liquid mixture of the oxidizer and binder. The blind passage in the propellant charge is formed by a lubricated rod extending into the metallic housings 70, 80. As the 10 propellant sets up, it bonds to the metallic housings 70, 80 but not to the lubricated rod which is withdrawn to leave a receptacle for the ignition material.
Manufacture of the devices 26, 50, 92 is quite similar except that a reusable mold is employed rather than the metallic housings 15 70, 80. The mold as well as the rod are lubricated to allow separation of the propellant charge from the molds.
Gne of the peculiar aspects of this invention is that the pro-peIlant materials do not burn very well, if at allr unless the pressure in the well where the device is located is at least about 20 200 pounds. This is normally no problem since the well 10 is typi-cally at least partially filled with the completion fluid 24.
It will accordingly be eyident that there is provided a device for fracturing ~ormations of the ~as generating type which minimizes the production of unburned propellant particles and which does not 25 req~ire a metallic housing to seal the propellant against entry of bore hole fluid.
Although the preferred embodiments of this invention have been disclosed in substantial detail~ it is understood that this invention is defined in the appended claims and that numerous changes in the 30 details o~ construction and selection o~ materials may be resorted to without departing ~rom the spirit and scope of the invention as hereinafter claimed.
Claims (17)
1. Apparatus for stimulating productivity of a subterranean formation comprising a charge of propellant material for generating a large quantity of gaseous combustion products at an elevated pressure and means for igniting the propellant charge inside a well bore adjacent the formation, the charge comprising an elongate rigid body for reducing the production of mobile unburned propellant particles during combustion of the charge.
2. The apparatus of claim 1 wherein the rigid bodied propellant charge comprises an oxidizer and a binder agglomerating the oxidizer into the rigid body.
3. The apparatus of claim 1 wherein the rigid body exhibits a tensile strength, prior to combustion, on the order of at least 300 pounds/square inch.
4. The apparatus of claim 3 wherein the rigid body exhibits a tensile strength, prior to combustion, on the order of at least 400 pounds/square inch.
5. The apparatus of claim 4 further comprising means affixed to the rigid body for receiving a wire line.
6. The apparatus of claim 5 wherein the rigid body is free of an encapsulating metal container.
7. The apparatus of claim 6 wherein the rigid body is covered with a coating.
8. The apparatus of claim 7 wherein the coating is paint.
9. The apparatus of claim 2 wherein the charge further comprises elongate fibers interspersed in the binder oxidizer material.
10. The apparatus of claim 1 further comprising a housing en-closing the charge.
11. The apparatus of claim 9 wherein the charge is bonded to the housing.
12. The apparatus of claim 10 wherein the rigid body has a tensile strength, when not bonded to the housing, on the order of at least 250 pounds/square inch.
13. Apparatus for stimulating productivity of a subterranean formation through a fluid filled borehole, comprising a body of propellant including a mixture of water soluble oxidizer and a substantially impermeable combustible binder for generating a large quantity of gaseous combustion products at an elevated pressure; and means, sealed against the entry of bore hole fluid, for igniting the propellant body;
the propellant body not being sealed against contact with the bore hole fluid.
the propellant body not being sealed against contact with the bore hole fluid.
14. The apparatus of claim 13 further comprising means for lowering the propellant body into the bore hole.
15. The apparatus of claim 14 wherein the lowering means includes a connection rigid with the propellant body and a wire line secured to the connection.
16. Apparatus for stimulating productivity of a subterranean formation through a bore hole at least partially filled with a liquid, comprising a self supporting body of propellant comprising a mixture of water soluble oxidizer particles and a substantially impermeable combustible binder encapsulating and agglomerating the oxidizer particles into the body;
means for lowering the propellant body into the bore hole;
and means, sealed against the entry of bore hole liquid, for igniting the propellant body;
the impermeable binder comprising means for sealing the oxidizer against action of the bore hole liquid.
means for lowering the propellant body into the bore hole;
and means, sealed against the entry of bore hole liquid, for igniting the propellant body;
the impermeable binder comprising means for sealing the oxidizer against action of the bore hole liquid.
17. A method of stimulating productivity of a subterranean formation through a cased bore hole having perforations communicating between the formation and the bore hole, comprising lowering into the bore hole, to a location adjacent the formation, a rigid bodied propellant charge;
igniting the charge;
combusting the charge and maintaining the propellant charge as a unitary body during combustion to reduce plugging of the per-forations by unburned propellant particles.
igniting the charge;
combusting the charge and maintaining the propellant charge as a unitary body during combustion to reduce plugging of the per-forations by unburned propellant particles.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US26866781A | 1981-06-01 | 1981-06-01 | |
| US06/268/667 | 1981-06-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1183769A true CA1183769A (en) | 1985-03-12 |
Family
ID=23023973
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000399949A Expired CA1183769A (en) | 1981-06-01 | 1982-03-31 | Device for fracturing a hydrocarbon producing formation |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1183769A (en) |
-
1982
- 1982-03-31 CA CA000399949A patent/CA1183769A/en not_active Expired
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