US2867170A - Explosive device - Google Patents

Description

Jan. 6, 1959 T. A; KIBBY EXPLOSIVE DEVICE Filed June 25, 1954 .2., n.. fau/. 2.". \....v..... I.. JWM uw.; .....F ...M

2,86 7,170 Patented Jan. 6, 1959 tice pta

EXPLOSIV E DEVICE Theodore A. Kibby, San rlfome, Venezuela, assignor to Gulf Oil Corporation, Pittsburgh, Pa., a corporation of Pennsylvania This invention relates to an explosive device which is particularly adapted for use in completing fluid-producing wells in poorly-consolidated producing formations and more particularly to such a device made with explosive cord.

In producing oil, gas, or other fluids from poorly-consolidated formations such as unconsolidated sands, it is desired to avoid the production of sand with the fluids in order to prevent costly shut-down due to clogging of the well bore and to avoid damage to pumps and other well equipment. A number of procedures for sand control have been employed including the use of various types of strainers in the producing formation; for example, wirewrapped slotted pipes, gravel packs, and the like. The prior art means for sand control have not been entirely satisfactory. Abrasion caused by sand particles enlarges the openings in the wire-wrapped slotted pipe and allows the production of substantial quantities of sand which usually accumulates in the well bore, stopping production, and necessitating an expensive cleanout process. The openings in the pipe are also subject to clogging by sand with resulting decrease in production. Gravel packs are also subject to clogging. This is due to the fact that it is difficult to place the gravel in the well without forming voids in some places in the pack. Such voids allow shifting of the gravel during production and consequent movement of sand from the surrounding formation which clogs fluid passages in the pack and stops or reduces production.

The copending patent application of Abraham I. Teplitz, Serial No. 424,862, now abandoned, describes a new .method of completing wells in unconsolidated formations which makes possible the production of fluids from unconsolidated sands without the production of substantial quantities of sand. The new method in general comprises installing in a well drilled in a poorly-consolidated producing formation a tubular casing coaxial with the bore hole, forming a cementitious sheath in the annular space between the casing and the walls of the poorlyconsolidated producing formation, and applying a stress to ,the sheath sufficient to form fractures in the cementitious material. In one modification of the new method the stress is applied by detonating an explosive within the casing opposite the poorly-consolidated formation. The small fractures or cracks formed in the cement allow fluids to lter through the sheath and enter perforations in the well casing while sand is excluded. The fractured sheath retains its general outline and continues to support the surrounding unconsolidated sand formation.

The present invention is directed to an explosive device .which is particularly adapted for use in the method of completing a well described in said Teplitz application. The explosive device of the invention in general com- -prises an elongated, inert core member, a longitudinal strand of explosive cord on the outside of the core member, wrappings of explosive cord attached to the core member, means for lowering the device into a well and means operable from ground level for detonating the Aexplosive cord withinrthe well.

The invention can be described further by referring to the drawing, of which Figure 1 is a diagrammatic view of an explosive device of the invention in position within the producing zone of an oil well; and

Figure 2 is a diagrammatic sectional View of the producing zone of an oil well which has been completed with the use of a device of the invention.

Figure 1 shows, by way of example, a well extending through a consolidated formation 10, an unconsolidated oil sand`11 and into a consolidated formation 12. The well is cased throughout the producing formation with a tubularV casing 14, which is preferably steel casing pipe commonly used in the art. Figure l shows casing 14 as an unperforated pipe which is to be perforated after it is placedin the well, as will be describedl in more detail hereinafter. However, if desired, the pipe can be perforated before it is placed in the well. Conventional shop-perforated or slotted pipes, screen pies and the like can be used instead of the unperforated casing shown in Figure l.

The casing 14 is surrounded by a cementitious sheath 15, formed preferably from a slurry of neat Portland cement. Care is taken to eliminate voids or channels from the cement sheath l5 before it sets, for example, by using scratchers and centralizers in the cementing operation. After the cement has set, the well is ready for .the phase of the completion method in which fractures are formed in the cement sheath with the explosive device of my invention and perforations are formed in Vthe tubular casing if they have not been already formed.

The structure of the well after the cement sheath has been fractured and perforations have been formed in the tubular casing is illustrated diagrammatically in Figure 2. The structure comprises the casing 14 having openings 18 therein and the surrounding cementitious sheath l5 having a network of small fractures or cracks which provide passages for uids from the producing formation 11 into the openings in the casing.

To use a device of my invention in completing a well, it is lowered into the well through the cemented casing to a level opposite the oil sand as shown in Figure l. The preferred embodiment of the device, as ilustrated dagrammatically in Figure 1, comprises an elongated inert core member which can be, for example, a length of steel drill pipe. In the preferred embodiment, the core member has a reduced diameter section 21, referred to in the claim as an explosive cord mounting section, which is adapted to receive wrappings of explosive cord. Advantageously the reduced diameter section is fcrmed by turning the cylindrical or tubular core member on a lathe to cut away a section in which the explosive ccrd wrappings are protected from abrasion by the shouders v22 and 23 which have the original diameter of the core.

In the preferred embodiment of Figure 1, the core member is provided with a shallow longitudinal groove 24 in the reduced diameter section 2i. This groove is adapted to receive one or more strands of explo. ive cord which serve as detonating strands for the explosive cord which is wrapped about the core and presses tightly against the detonating strand.

In the preferred embodiment of the explosive device the wrappings of explosive cord are attached to the core member in the following manner. A detonating strand 25 is placed in the groove 24. To the free end of strand 2S is attached a detonating means, preferably an electric blasting cap, not shown in the drawing. The cap may be fired from ground level through an electrical conductor associated with the supporting cable 26 which is the means for lowering the device into a well. Alternatively the cap may be red by means of a go-devil dropped from the ground level or by a time clock in known man` ner. The rest `of the explcsive cord attached. t9 the ser@ member is wrapped as a coil or helix 27 on the core and presses tightly against the longitudinal detonating strand 2,5. The cord forming the helix Q7 can be either a continuation of the strand of cord in groove-24 or a separate length of explosive cord. Y

The explosive device is advantageously provided with means for centering the device withfn a well. ligpre l shows such means in the form of centralizing strips 28 disposed at each end of the device.

An explosive cord which is particularly suitable for use in the device of the invention is known commercially yas Primacord. The cord is l/i inch in diameter and comprises an explosive core of PETN (pentaerythritol tetranitrate) and a water-proofed textile covering. The explosive rate is about 20,000 feet per second. This explosive cord is tough, flexible and substantially impervious to water. lt is relatively insensitive to ordinary shock. friction, fire, or to temperatures met in -oil wells and therefore is safe to handle and resistant to premature explo- 4 sion when used in completing a well. lt is detonated by an electric blasting cap. While Primacord is the preferred explosive cord, other explo1ive cords having the above-described properties can also be used for the explosive device of the invention.

In completing a well the explosive cord opposite the producing formation 1l is detonated as by passing an electric current through a conductor in cable 26 which detonates the blasting cap or other detonator (not shown in the drawing) on the explosive device and which initiates detonation of the explosive cord sections 25 and 27. The amount of explosive cord is predetermined by empirical tests and when this selected amount of cord explodes within the casing 14, the resulting lateral force bulges the casing and applies a limited stress to the surrounding cement sheath 15.

When the limited stress is applied by the explosion to the cement sheath while it is confined between the well casing and the wall of the producing formation, many small fractures form in the portion of the sheath to which the stress is applied. However, the sheath retains its general outline as shown in Figure 2 and continues to support the surrounding sand formation. Consequently, the shifting of sand which results in the clogging of conventional well strainers in oil sands is prevented or greatly reduced. The small fractures in the cement allow fluids to enter the Well from the producing formation but prevent the production of sand of any substantial particle size. The greatest pressure gradient during production from the resulting well structure occurs as the uids pass through the fractures in the cement rather than, as in conventionally lcompleted wells, in the sand formation. Consequently, the forces tending to disturb the sand formation and cause movement of sand are considerably smaller than in conventional well structures.

Following the described fracturing operation, the tubular casing 14 can be perforated if it has not Ialready been perforated. For this purpose a conventional casing perforator, for example, a mechanical perforator of the knife type, can be lowered into the well opposite the producing formation and the fractured cement sheath. The perforator is then actuated to form holes in the casing which admit the uids filtering through the fractures in the cement sheath. The casing perforation can also be performed before the fracturing operation if desired.

Although the structure illustrated in Figure l is preferred for the explosive device `of the invention, the device can be modified and still produce advantageous results. For example, instead of being attached as spiral wrappings, the explosive cord can be attached longitudinally to the elongated core `as a number of loops or strands, clipped or otherwise fastened along its length; It should be understood, however, that the structure of the explosive device illustrated in the drawing `is greatly preferred and has important advantages in Yits use in comfil el Dieting a well- For example the Provision Ot a lensitudinal detonating strand under the spiral wrappings of explosive cord has especially proven to have imp rtant advantages. I have found that With this structure the possibility of misres with the device is considerably less than when the longitudinal strand is omitted.

The explosive device of the invention can vary considerably in the length and diameter of the co-re member, depending on such factors as the inside diameter of the casing inthe producing zone and the top to bottom depth of the producing sand formation. A device of suitable size for many operations can be Vmade using as the core member a section of 45/8 inches drill collar tubing of which a 10 foot length is turned on a lathe to form an intermediate reduced diameter section for the Y wrappings of explosive cord. A shallow longitudinal the casing.

-channel is also milled for the detolnating strandf The length of explosive cord attached to the core per llinear foot must be sufficient to cause fraeturing of the cement sheath in the producing formation but insufficient to cause rupturing of the tubular well casing. the amount of cord to be used will thus depend upon the thickness of the casing Wall and the thickness of the cement sheath. As an example, using the 1A inch diameter cord containing PETN, in 51/2 inch J-55 casing surrounded `by a 2 to 3 inch thick sheath of Portland cement, from about 5 to 2O feet of explosive cord per linear foot will fracture the cement without rupturnig Using a diiferent type of explosive cord or a different thickness. of casing or cement sheath, the amount of cord to be used might differ considerably. However, bearing in mind the result desired, i. e., without rupturing the casing to form a fractured cement sheath which retains its general o-utline and supports the oil sand, the amount of explosive cord required can be found by routine experiments in a test ho-le and the information gained used in an actual well completion.

I have Ycarried out particular operations which illustrate the results obtainable using an explosive device of my invention. 1n one such operation, to simulate the structure of a completed well in a producing formation, a cement sheath was formed about a length of slotted liner assembly. This was done by centering a length of 5 inch slotted liner in a length of 8% inch casing. The annular space was lled with cement and after the cement had set the outer casing was removed from a l0 foot section of the assembly. The assembly, with cement exposed over a l0 foot section, was then centered within a boiler flue and the cement-flue annulus was solidly packed with damp sand. The entire assembly was lowered into a test hole and an explosive device of my invention was lowered into the slotted liner. The explosive device had wrapped about it 6 feet of Primacord lper linear foot along a 32 inch length. The Primacord was detonated within the slotted liner, after which the assembly was raised to the surface and examined. It was observed that the cement opposite the 32 inch portion of the explosive device was severely fractured. The cement had both vertical and horizontal fractures, the vertical fractures on the average being about 2 inches apart and the cross-cracking appearing, on the average, about every 6 inches.

If it is found that the fractures formed in the cement by detonating the explosive device of the invention are not large enough to produce fluids as rapidly as desired, the fractures can be enlarged by any of a number of known methods, for example, by treating the well with a chemical agent such as hydrochloric acid.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only such limitations should beimposed as are indicated in the appended claim.

I claim:

An explosive device for fracturing the cement sheath surrounding a well casing which compn'sess an essentially rigid, vertically elongate, metallic core member of substantially circular transverse configuration throughout its length, said core member having intermediate its end portions an explosive .cord mounting section of reduced diameter, each of said end portions being provided with means for eentralizing the core member in a well casing, said explosive cord mounting section having a straight longitudinal groove in its outer surface, a length of explosive cord, said explosive cord extending in one direction along the explosive cord mounting section and in the groove and thence extending along the explosive cord mounting section in the opposite direction wrapped tightly as a helix about both the explosive cord mounting section and the extent of the explosive cord disposed in the groove, the helical extent of the explosive cord pressing tightly against the extent of the explosive cord in the groove to afford a multiplicity of vertically spaced con- References Cited in the tile of this patent UNITED STATES PATENTS 2,362,829 Kinley Nov. 14, 1944 2,414,349 Alexander Jan. 14, 1947 2,609,885 Silverman Sept. 9, 1952 2,650,539 Greene Sept. 1, 1953

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