FEATURE ARTICLE

A New Approach Toward Elimination of Slug in Shaped Charge Perforating

SOCIETE DE PROSPECTION ELECTRIQUE J. DELACOUR PARIS, FRANCE M. P. lEBOURG

SCHLUMBERGER WELL SURVEYING CORP. Downloaded from http://onepetro.org/jpt/article-pdf/10/03/15/2238755/spe-941-g.pdf by guest on 30 September 2021 W. T. BELL HOUSTON, TEX. MEMBERS AIME

Abstract ing of shaped charge tools, as applied thoroughly explored, both classically to oilwell perforating, has evolved from and experimentally by Pugh, et al."1.4 Since the advent of the perforation simple surface shooting to tests con­ In considering the formation of the flow laboratory, increased attention has ducted in a flow laboratory in which jet and the slug, let us refer to a sim­ been focused on new designs of shaped a study of the charge could be made ple schematic of a conventional, cop­ charges as applied to ai/well perforat­ under simulated down-hole conditions'. per-lined shaped charge (Fig. 1). This ing. Data derived from the flow lab­ In the flow laboratory, pressures and type charge will produce (1) a high oratory, in which charges are subjected temperatures were elevated prior to speed penetrating jet with a velocity to simulated down-hole tests, indicate firing the charge into a target of me­ approximating 8,000 m/sec, and (2) that serious perforation plugging often dium-hard sandstone material. An a relatively slow moving slug or car­ exists due to the elltrance of slugs into elaborate system was devised for firing rot, the velocity of which reaches 800 the perforated hole. the charge under various fluids and m/sec, and which may in turn plug The slug or "carrot" is all intrinsic differential pressures. Provisions were the hole produced by the jet. part of the jet ["ocess and its adverse made for a~tually "flowing" the per­ The charge shown in this classical efJect is inherently characteristic of forated hole under controlled condi­ illustration consists of a cylinder of many commercial-shaped charges tions. It was at this ooint that the term powerful high explosive which is gen­ which otherwise exhibit excellent per­ slug or carrot really emerged into oil­ erally die-formed or molded to a coni­ formance properties. well perforating vernacular. While it cal copper liner and primer explosive. Numerous approaches have been had been noted that a slug of charge Copper is commonly used as the liner tried throughout the industry in an ef­ liner material often entered the per­ material due to its superior combina­ fort to prevent this flow-impeding slug foration, these plugged test targets had tion of plasticity and density charac­ from entering the perforated hole. Yet, drawn little attention. Now in the teristics. Initiation of the explosion is publicly conducted tests indicate that flow laboratory, it was found that this effected at the primer section, after none of these devices are entirely suc­ obstructing slug reduced the resultant which a detonation wave travels from cessful when applied under the clear­ flow through the perforation to as lit­ left to right, sweeping from apex to ance and pressure conditions for which tle as one-half that normally expected! base along the liner. the charges are designed. This paper It was then clear that the slug must be The resulting pressures produced on deals with a new, direct approach either eliminated from the shaped the liner are so large that the copper toward the elimination of the slug or charge entirely or reduced to such pro­ behaves like a perfect fluid. Such pres­ carrot in shaped charf]e perforating. portions that it would be ineffective in sures cause the walls of the liner to Herein is described a newly devel­ plugging the perforated hole. collapse and move inward at high velo­ oped bimetallic liner for shaped cities nearly perpendicular to their sur- charges, which when fired under pres­ Slug Formation in the Jet Process sure leaves no slug in the perforated hole and yet maintains the high per­ MAIN CONICAL To gain an understanding of the CHARGE LINER formance oj the conventional-type slug formation in the jet process, it is PRIMER charge. worthwhile to review basic shaped Introduction charge principles. While it is beyond the scope of this paper to present a During the past few years the test- comprehensive theoretical treatment, the specific phases involved will be Original manuscript received in Society of discussed. Both the mechanisms and Petroleum Engineers office June 6. 1957. Re­ vised manuscript received Feb. 27. 1958. Paper properties of the jet and slug have been presented at 32nd Annual Fall Meeting of So­ ciety of Petroleum Engineers in Dallas, Tex., Fie;. 1 - Schematic of copper-lined Oct. 6-9, 1957. 1References given at end of paper. shaped charge ready for detonation. SPE 941-0 MARCH, 1958 15 faces. The collapsing copper retains DETONATION charge containing an ordinary copper practically a conical shape as shown WAVE cone liner. This charge, which is rep­ in Fig. 2. Point P moves to the right ,------.. resentative of those used in commer­ I along the axis with the collapsing cone I JET cial application, possessed the follow­ describing the indicated angle, 2f3. iJ-l ing specifications: ( 1) angle at the During this process, the inner part I [ apex = 2 0: = 60°, (2) diameter at L,_.J of the cone is "squeezed out" from the base = 32.6 mm, and (3) mean thick­ [ inner apex of the liner and forms the L ______.;. ness liner = 0.65 mm. jet, which travels to the right. To the Substituting maximum and mini­ left behind the moving apex (P) is SLUG mum values of the angle f3 as observed found a section of collapsed liner con­ Fig. 2 - Schematic of copper-lined on radiographs, Eqs. 1 and 2 were ap­ shaped charge after detonation show­ taining the slug metal from the outer ine; collapse of liner. plied. It was thus determined that the part of the cone. In other words, the slug should weigh between 5.0 and 7.5 metal of the liner divides into two \ Y gm. Actual slugs recovered in shooting parts with the dividing surface between through water weighed 6 gm, which these two parts being a cone lying represents a reasonable check on the somewhere between the inner and theoretical values. outer surface of the original liner. Observations on many test shots in The manner in which the metal of the laboratory on hollow carrier the collapsing liner divides (between charges of various types bring out the the jet and the slug) can be precisely following further facts. Downloaded from http://onepetro.org/jpt/article-pdf/10/03/15/2238755/spe-941-g.pdf by guest on 30 September 2021 determined. Let us assume that the 1. Slugs produced from successive liner (Fig. 3) which is in the process shots with the same type of copper­ of collapsing under the detonation lined charge may have a wide, random wave is considered from the point of variation in final dimension, but hold view of a moving observer s"tationed to approximately the same weight. The Fig. 3-Formation of slue; and .iet from at the moving junction, P. The liner paint of view of observer stationed at variation in dimensions depends on will thus appear to flow toward P with moving junct'on P. many different factors which vary in a velocity, V, and after passing the the individual charge characteristics. junction, the jet and slug will appear ZINC OUTER 2. Plugging action does occur when to recede with the same velocity. CONE the slug enters the perforated hole. Therefore, in accordance with the 3. Slugs produced by the solid cop­ principle of conservation of momen­ per liner are found to plug from a tum, we may equate the horizontal smalI percentage to 100 per cent of the components of momentum before, to perforations, depending on the type those after passing the junction P in of charges and the down-hole shooting the moving coordinate system of Fig. 3, conditions. m V cosf3 = m., V - mj V , (1) 4. Large differential pressures in where m represents the total mass per these cases generally do not dislodge unit length of collapsing liner, m; that the slug. part of m going into the jet, and m., COPPER INNER CONE that going into the slug.' Then, ac­ Methods Applied in Eliminating the cording to the principle of conserva­ Slug from Perforations tion of mass, we have Fill:. 4-Bimetallic cone liner. m = m; + m.. , (2) Throughout the industry, numerous and can therefore obtain attempts have been made to eliminate mj = m/2(1 - cosf3) = m sin 'f3/2 the slug. Various approaches have m., = m/2(1 + cosf3) = m cos 'f3/2 been tried. Among these, an effort has (3) been made to stop or "catch" the slug. Mechanical devices such as spe­ Now, from Eqs. 1, 2, and 3, the size AI of the slug is a function of the design cial porthole covers and washers have and weight of the liner. The slug been employed. Charges have been weight resulting from a given liner can modified to throw the main perform­ therefore be readily predicted if the ance factors out of balance in an ef­ various parameters are known. Thicker fort to produce a hole large enough so liners will naturally produce larger and that the carrot would not remain in heavier slugs. Thinner liners, con­ the perforation. Others have been fitted versely, produce smaller slugs. Unfor­ with asymmetrical cone liners in an ef­ tunately, however, experience shows fort to force the carrot to one side of the path of the jet and prevent its en­ that reduction of liner thickness is ac­ BI companied by reduction in perform­ trance into the hole. Various other ance. modifications to liners have been made, such as grooving and truncation Experimental Verification of of the cone. A novel approach con­ Preceding Theory sisted of the "disintegration" of the As a check on the theoretical con­ Fig. 5-(A) Slug-jet formation with slug through use of a baffle-type cepts, radiographic or X-ray studies copper cone liner shaped charge. (B) charge. Jet process with bimetallic cone liner. (as shown in Fig. 6) were conducted Most of the slug was eliminated due While some of these approaches during the detonation process on a to volatilizing of zinc outer liner. seem somewhat indirect, the problem

16 JOURNAL OF PETROLEUM TECHNOLOGY BUORE 8!fORE Finally, a new type of liner was de­ OElUllUION DETDNATKlH veloped which fulfills the foregoing .. ~ specifications. This liner, appropriately termed "bimetallic", consists of zinc ... . and copper cones arranged as shown IAj • in Fig. 4. Zinc represents the outer 6 4 MtRl).SEtS. MtRO·S£CS element of the cone and copper the inner. This liner produces only a very small slug which does not enter the perforated hole under down-hole con­ 11.5 10.5 ditions of pressure and temperature. M~RO-Sf£S MICRO·SEeS. IBI The slug produced is so small that plugging of the perforation could not be effected even if it entered the hole.

~ICRO16·S£CS i:., [1,...... -:::::....--...... MICR(l.15S££S . The Bimetallic Liner lei According to the principles of the jet process, the outer portion of the lA' (8] COPPER' lINER BIMETAUJ: UHER solid copper liner, adjacent to the ex­ Fig, 7-Flash radiographs of shaped charge slugs, (A) Full size copper liner Fio· 6-Flash detonation radiographs plosive, forms the slug. During the slug. (8) Interlllediate slug resulting Downloaded from http://onepetro.org/jpt/article-pdf/10/03/15/2238755/spe-941-g.pdf by guest on 30 September 2021 o(;'opper and hillletallic liner shaped slug formation, the metal is subjected frolll reduced copper and increased charges. to very high temperatures approach­ zinc. (C) Slllaller billletallic slug re­ ing, but not exceeding, the melting sulting frolll further increase in ,'atio has certainly been approached in true point of copper.' of zinc to copper. earnestness. This is attested by the It was reasoned that, if this outer fact that many of the aforementioned section of liner were replaced with in reducing the slug weight, through volatilization of a part of the liner ma­ methods have actually been developed some metal of low melting point, it terial, is evident in Fig. 7. This radio­ into production models. should be volatilized under the high graph shows relative sizes of slugs Yet, numerous publicly and pri­ temperatures and pressures attending produced as the proportion of zinc to vately conducted tests have amply the process-thus, eliminating most of copper is progressively increased. demonstrated, to those who have fer­ the slug' (Fig. 5B). At the same time, reted out the available data, that none if the metal chosen possessed proper of these devices have been success­ density and plasticity, performance of Production Considerations ful in eliminating the slug while main­ the jet should not be impaired. In ac­ While the fabrication of a perfect taining high performance specifica­ tual practice this was found to be the "no slug" bimetallic liner is theo­ tions. Where one charge might appear case. Where copper is maintained as retically possible without reduction in promising on surface tests, serious the inner liner and zinc is used as the performance this did not prove eco­ slug problems result under the condi­ outer portion, both of these conditions nomically feasible in practice. Such a tions of varying gun clearance and are satisfied. The jet produced has the liner would require a perfect balance elevated pressure and temperature ap­ same approximate penetrating power between zinc and copper beyond nor­ plied in the flow lab. Questions of in­ as the jet from a plain copper liner, mal production tolerances. terpretation of test results may thus yet the outer zinc portion rapidly vola­ The use of an excessive thickness arise in evaluating the merits of any tilizes during the course of jet forma­ of zinc in combination with a thin particular method or device until those tion, producing no slug or a greatly copper element (over-all liner thick­ tests are standardized and related to reduced one. ness remaining constant) eliminates the field conditions in the flow lab. In the development of the bimetallic slug but results, unfortunately, in starv­ liner, considerable testing was done ing the jet of copper material, with Basic Approach to the Problem using different metals such as lead, subsequent reduction in performance. zinc, aluminum, and their alloys. While On the other hand, if the zinc por­ Early in the project, it was con­ several of these functioned equally tion of the outer liner is reduced in cluded by the authors that the slug is well, zinc was chosen for reasons of thickness by degrees and the copper an inherent and inseparable part of production. correspondingly increased, a small slug the jet process in charges with a solid begins to form which increases in size copper liner. It was decided that the The firing process of shaped charges, and weight with pursuance of this best approach to its elimination lay with both the solid copper liner and trend. The thickness of the respective in going directly to the root of the new bimetallic type, are illustrated in metals may therefore be adjusted to problem and developing a new liner­ the series of radiographic pictures of give the proper balance between maxi­ a liner which, without interfering with Fig. 6 (A and B). These X-ray photo­ mum performance and negligible slug the power of the charge, would either graphs show the development of the welgnt-"negligible slug weight" being produce no slug at all or one so small explosions at intervals of a few micro­ defined as that weight incapable of en­ it would be incapable of plugging the seconds after detonation. The collapse action of the liners, the slug, and jet tering and plugging the perforated perforation. Many tests were con­ formation, are all apparent. In the hole. ducted using various approaches, but solid copper series, the slug formation This approach was taken for the in all cases either the slug remained is clearly defined, while in the bimetal­ mass production of the liner. The point or the performance of the charge was lic series, the volatilizing zinc portion is of "maximum performance and negligi­ so reduced that the test devices could seen in the increasing cloudiness ble slug weight" was determined ex­ not be considered as efficient perfora­ around the collapsing liner. The effec­ perimentally and verified through pro­ tors, tiveness of the bimetallic construction duction flow tests in the laboratory.

MARCH, 1958 17 TABLE 1 - TYPICAL COMPARATIVE RESULTS - Bi· METALLIC VS SOLID COPPER LINERS (AS USED IN IDENTICAL CHARGES) Carrot Probe Flow Weight Depth Penetration Index Location (gm) 4·in. Bimetallic liner Charge 7.38 8.75 1.13 Gun 1.6 7.50 8.75 1.23 Gun 1.8 8.30 9.38 1.18 Gun 1.4 (Al (8) (el 7.75 8.50 1.20 Gun 2.1 6.00 9.00 1.08 Gun 1.4 Fig. 8-Typical slugs or carrots recov­ Avg.7.39 8.88 1.16 1.6 ered from 4-in. bimetallic and coppei' 4-in. Solid Copper liner Chorne liner charge tests. (A) Bimetallic tner, 3.25 8.87 0.62 Perlo. 6.4 1.25 9.00 0.48 Pe.lo. 6.3 (B) conventional copper liner, and 3.13 8.75 0.67 Perb. 6.6 (C) balle.type parabolic liner. 3.00 8.20 0.70 Perlo. 6.1 2.30 8.80 060 Perlo. 5.5 Results Avg.2.59 872 0.61 62 These flow lab tests showed that a numerous attempts have been made in slug weight of 1.6 gm, or 25 per cent Fig. 9. - Typical ex­ amples of cro 's -sec­ connection with its elimination, very of the conventional slug weight (Fig. tioned flow lab Berea little success has been experienced. 8), does not interfere with the perfora­ sandstone target. (A) A new solution is now offered in the tion and allows competitive perform­ Target ~ hot with 4-in. form of the bimetallic liner in which ance to be maintained. b imetalLit' liner charge Downloaded from http://onepetro.org/jpt/article-pdf/10/03/15/2238755/spe-941-g.pdf by guest on 30 September 2021 - l'esults in singles the formation of a large slug is elimi­ A further safeguard against possible perfOl·alion. (B) Tat'gel shot with 4 -in. nated in the jet process. reduction of efficiency by plugging of COI)per liner charge-avel'uge position Laboratory tests of this new-type bi­ the perforation was established when of slug. metal liner have shown that it greatly it was found that this small slug pro­ enhances the performance and effi­ duced by the bimetallic charge liner was found that while the small slug ciency of the individual perforation. does not enter the perforated hole did not enter the perforation under when fired under down-hole condi­ pressure, it did enter the perforation Acknowledgments tions. Tests conducted in the flow lab­ on surface tests. This is judged to be oratory at 1,500 psi and 180 0 F indi­ of minor importance since very few The authors wish to express their cate that the very small bimetallic liner wells are completed without hydro­ appreciation to the personnel of the slug remains within the shaped charge static pressure. Furthermore, under various Schlumberger organizations carrier. these conditions the small slug travels and laboratories who participated in The tests further prove that the to the very end of the perforation, this work-in particular to H. C. Fa­ slug-free perforation results in greatly yielding consistently high probe depths gan. Special acknowledgment is due improved flow characteristics. Fig. 9 and no plugging action. the Laboratoire de Recherches Tech­ illustrates a typical example of the bi­ nique of St. Louis, France, for their metallic liner flow target as compared Commercial Significance assistance. to that of the plugged conventional The first production bimetallic liner copper type. While physical properties References was introduced to the field in Sept., of the two holes are almost identical, 1. Allen, T. 0., and Worzel, H. c.: "Pro· 1955. Improved versions were intro­ the bimetallic perforation resulted in ductivity Method of Evaluating Gun duced during the middle of 1956. Perforating," Paper 906·1·H, API approximately 90 per cent more flow To date, some 360,000 bimetal (1956) 9. than that of the conventional type. charges, termed "No-Plug" charges, 2. Birkhoff, G., MacDougall, D. P., Pugh, Production flow indices were 1.18 have been fired in the field. While the E. M., and Taylor, G.: "Explosives With and 0.62, respectively. Repeated tests, Lined Cavities," Jour. Appl. Phys. greater part of these were of the 4-in. of which those shown in Table 1 are (1948) 19, 563. carrier size, five different size charges typical, gave similar results. 3. Pugh, E. M., Eichelberger, R. J., and have been redesigned and fitted with Rostoker, N.: "Theory of Jet Forma­ Tests were also performed in a ma­ this type of liner. tion by Charges with Lined Conical Cav­ jor oil company's flow laboratory for ities," Jour. Appl. Phys. (1952) 23,532. verification that perforations made Conclusions 4. Eichelberger, R. J., and Pugh, E. M.: with the bimetallic liner charge under "Experimental Verification of the Theory down-hole pressures are slug-free. The fact that a real problem in of Jet Formation by Charges with Lined Conical Cavities," Jour. Appl. Phys. These tests confirmed the results ob­ shaped charge perforating exists due (1952) 23, 537. tained in the Schlumberger laboratory. to slug or carrot plugging of the per­ 5. Proc., Third World Pet. Congress (1955) In the course of laboratory tests, it forated hole cannot be denied. While Sec. II, 215. ***

J. DE LACOUR, a drilling and production engineering graduate, joined Schlumberger in 1948 as project engineer and is now section head for explosive and shaped charge research in Paris. M. LEBOURG is assistant manager of engineering in Houston. He holds mechanical and electrical engineering degrees from Paris schools. Lebourg joined Schlumberger in 1934. W. T. BELL heads the perforating design section of Schlumberger's Engineering Dept. He joined the department in J 952 after four years in South J. Delaconr M. P. Lebourg w. T. Bell America and has held his present position for two years.

18 JOUR"'AL OF PETROLEUM TECHNOLOGY