Downloaded from http://onepetro.org/SPEEFDC/proceedings-pdf/97EFDC/All-97EFDC/SPE-38141-MS/1940751/spe-38141-ms.pdf/1 by Elizabeth Fuller on 06 September 2022
`
`SPE 38141
`
`
`
`
`
`Improvements in Perforating Performance in High Compressive Strength Rocks
`P.S. Smith, SPE, BP Exploration, L.A. Behrmann, SPE, and Wenbo Yang, Schumberger Perforating and Testing.
`
`Copyright 1997, Society of Petroleum Engineers, Inc.
`
`This paper was prepared for presentation at the 1997 SPE European Formation Damage
`Conference held in The Hague, The Netherlands, 2-3 June 1997.
`
`This paper was selected for presentation by an SPE Program
`Committee following review of information contained in an abstract
`submitted by the author(s). Contents of the paper, as presented, have
`not been reviewed by the Society of Petroleum Engineers and are
`subject to correction by the author(s). The material, as presented, does
`not necessarily reflect any position of the Society of Petroleum
`Engineers, its officers, or members. Papers presented at SPE
`meetings are subject to publication review by Editorial Committees of
`the Society of Petroleum Engineers. Permission to copy is restricted to
`an abstract of not more than 300 words. Illustrations may not be
`copied. The abstract should contain conspicuous acknowledgment of
`where and by whom the paper was presented. Write Librarian, SPE,
`P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-
`952-9435.
`_____________________________________________
`
`Abstract
`The productivity of wells damaged during drilling is directly
`dependent on the depth of the damage and the performance of
`the perforating guns. If the perforations by-pass the damaged
`zone then the well will have a low mechanical skin.
`Conversely, if the depth of damage is greater than the
`perforation length, the skin factor will be much higher,
`especially when the drilling damage is severe. While we
`normally associate drilling damage with low strength rocks,
`there are reported hard-rock fields with extensive drilling
`damage.
`
`The performance of shaped charges is significantly
`affected by the compressive strength of the rock to be
`perforated; consequently, the ability to bypass drilling damage
`in formations with high rock strength is reduced. Previously
`reported work1,2 has shown a 75% reduction in total target
`penetration, compared to API Section I, in rock with an
`unconfined compressive strength of approximately 25,000 psi.
`
`This paper describes the development and field testing of
`alternative charge designs aimed at improving performance in
`high compressive strength formations. So that the adverse
`effects of drilling damage can be reduced, computer
`simulations and laboratory tests showing the improvements
`achieved are presented. Field testing of the new charges and
`results achieved are shown.
`
`
`Introduction
`The basis for this project was to increase charge penetration
`depth to help optimize the completion efficiencies for the hard
`rock sandstone reservoirs in South America. Due to the
`unique properties of these quartz arenite sandstones, high
`compressive strengths up to 25,000 psi are common, and
`corresponding penetration depths are reduced. Additionally,
`these reservoirs have high permeability and modest porosities,
`resulting in large pore throats.
`
`Formation damage often occurs during the drilling of a
`well. Exposure to a drilling fluid generally results in the
`invasion of the rock matrix by mud filtrate and by mud solids.
`The extent of this invaded zone is dependent on several
`factors, such as the fluid loss characteristics of the mud
`system, the applied overbalance, the pore size distribution of
`the rock matrix and the time taken to drill the zone. The
`invaded zone may range from a few inches to a few feet
`around the well and usually results in a reduction of
`permeability. This permeability reduction, or damage, can
`have a dramatic impact on the potential productivity of the
`well.
`to
`important consideration with respect
`
`The most
`perforation length and well productivity is whether there is
`drilling damage and if the perforation length is sufficient to
`bypass such a damage zone. It is normally expected that
`effective perforating will bypass formation damage around a
`well if this damage is limited to a few inches. In hard rocks,
`the probability of bypassing the damaged zone is reduced due
`to the reduction in observed perforation length. For hard
`rocks with a significant depth of formation damage it is
`unlikely that the perforations will reach beyond the damaged
`zone.
`
`The effect of perforation length on well productivity has
`been reported by McDowell and Muskat3, Harris4 and Klotz et
`al.5 These studies showed that the well productivity could
`only be maximized if the perforations penetrated beyond the
`damaged zone. Even when only a few perforations just pass
`the damaged zone the observed impact on productivity is
`significant. Computer models, such as the method of Karakas
`and Tariq,6 can be used to predict the impact of perforating
`parameters and drilling damage on the expected mechanical
`(Darcy) skin factor for a well.
`
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`P.S. Smith, L.A. Behrmann and W. Yang
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`SPE 38141
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`Downloaded from http://onepetro.org/SPEEFDC/proceedings-pdf/97EFDC/All-97EFDC/SPE-38141-MS/1940751/spe-38141-ms.pdf/1 by Elizabeth Fuller on 06 September 2022
`
`frame, Behrmann and Halleck16 showed that penetration in
`concrete, in addition to rock, was degraded by compressive
`strength.
` An additional target effect exhibited by rocks, as
`opposed to concrete, was illustrated by Aseltine17. This is
`called an “active target” effect and is a result of a rock
`"reaction" which destroys portions of the perforator jet prior
`to its penetration into the rock. This effect was discovered
`during WW II and its many variants are used today to protect
`modern armored vehicles from shaped charge and projectile
`impact.
`
`Rock Lithology.
`is another recently
`lithology
` Rock
`recognized variable affecting rock penetration. Perforating
`through layers of “weak” and “hard” rock reduces the
`penetration depth when compared to a homogeneous rock of
`the same compressive strength2 . Data from the work of Ref. 2
`was expanded and reported in Ref. 18. Additional analysis
`was performed on this data and is shown in Table 1.
`Calculations of
`the
`theoretical penetration depth were
`performed and compared with the experimentally observed
`values. CT-Scans of Targets BPC-3, -5’, and -8 showed dense
`(“hard”) layers of rock near the middle and end of the
`perforations. The
`ratio of
`experimental-to-theoretical
`penetrations averaged 0.69 for these three tests, whereas for
`the homogeneous outcrop Carbon Tan cores, CT-1, and -2
`tests, this ratio was 0.96. Figs. 1 and 2 show CT-Scans of
`Tests BPC-5' and BPC-8 demonstrating the effect of hard rock
`layers on the penetration process. Note the reduction in hole
`diameter as the perforation passed through the hard layer, Fig.
`1. Figure 2 shows that the penetration stopped and was
`diverted along a less dense (weaker) path when a hard layer
`was encountered. Although not seen in the gray scale CT-
`scan of Fig. 2, there is liner debris just after the open portion
`of the perforation.
`
`Recent indications are that sand-grain size may also be an
`important determinate of rock lithology. Experiments using
`concrete targets19 show that the size of the aggregate sand can
`affect penetration by 10% or more: Penetrations were deeper
`using targets made with finer-grain sands than with coarser-
`grain sands. The effect seems to relate to grain size alone and
`not to the compressive strength of the target, since the deeper-
`shooting targets had the same or slightly higher compressive
`strength.
`
`Thus we see that rock lithology can severly impact the
`penetration process even with targets of equal strength.
`
`Charge Development
`For many wells, the relationship between perforation length
`and well productivity requires that improvements are made in
`either perforation damage skin and/or perforation length.
`Penetration flow tests in high compressive strength reservoir
`rock18 showed that perforation damage skin for the most part
`was zero or negative. Due to the extensive depth of drilling
`damage, increasing effective penetration depth was the focus
`of this reported work.
`
`Reservoir in-flow simulators showed that the combined
`
`reduction in penetration depth and the large depth of
`formation damage in the quartz arenite sandstone resulted in
`the perforations not extending beyond the drilling damage,
`resulting in low productivity. To regain productivity, a three
`step program was initiated: 1) reformulate the drilling fluid
`solids to reduce the depth of drilling damage; 2) increase
`charge penetration into hard rock, and 3) double the shot
`density. The later step could be immediately applied and has
`been effective as predicted by the in-flow simulators and
`shown in a recent paper.7 Work on the design of drilling fluids
`to minimize damage in these sandstones has also been
`performed8.
`
`Perforating also produces debris in the created tunnel,
`consisting of crushed rock and the remains of the shaped
`charge liner. Liner debris is typically seen near the end of the
`tunnel and, if sufficient, may reduce productivity (and
`injectivity) compared to a clean perforation tunnel. A
`laboratory study of hard-rock perforating showed that, in the
`majority of tests, debris remained in the perforation tunnels,
`despite perforating with high underbalances. This reduced the
`effective length of the perforations, sometimes by as much as
`50%2.
`
`Perforator Penetration Mechanics
`Shaped charge perforators have been used in the oilfield for
`over 50 years. However, a detail understanding of the
`penetration physics is insufficient to predict penetration
`performance from first principles without the use of empirical
`data. In this section, we review our understanding of the effect
`of various reservoir parameters on penetration depth.
`
`Concrete Targets. Oilfield perforators are qualified for
`penetration and casing hole diameter in surface concrete
`targets as specified in API RP 43 5th Ed.9 Furthermore, most
`service companies use small concrete targets for quality
`control tests. As a result, over time, perforators become
`optimized for concrete. Thus, the question of optimum
`performance in harder rock is raised. Will a perforator
`optimized for penetration in concrete (or a weak rock) also
`perform optimally in a much harder rock? The short answer is
`no, and that is part of the focus of this paper, as well as the
`effects of the reservoir properties on formation penetration
`and the development of a deep penetrating charge optimized
`for hard rock.
`
`Strength, Stress and Active Target Effects. Thompson10
`published the first paper showing the effects of rock
`compressive strength on perforator performance. Saucier and
`Lands11 published the first paper demonstrating that the
`effective stress (average rock stress minus pore pressure)
`severely degrades penetration in rock. Continuing work on the
`stress effect by Halleck, et al12-15, 1 showed that smaller
`charges were affected more than large charges and that weak
`rocks were affected more than strong rocks (concrete
`penetration is not stress dependent). During this same time
`
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`SPE 38141
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`Improvements in Perforating Performance in High Compressive Strength Rocks
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`for different explosive equation-of-states with active target
`considerations, whereas Model Three does not account for the
`"active" target effect.
`
`Interpretation of Fig. 8 shows that Theoretical simulation
`One matched the early experimental jet velocity/penetration
`up to a penetration depth of about 6-in.. The deviation in the
`TOA between experiment and theory after 6-in. suggests
`either a stronger active target effect than used in the simulator
`or higher
`jet velocities
`than
`shown experimentally.
`Theoretical simulation Two suggests an early jet velocity
`lower than shown experimentally, but a closer match of the
`tail portion of the jet velocity. Both simulations used the same
`active target parameters and the static UCS in the penetration
`model. The curve labeled Theoretical Three used the
`AutodynTM simulation of curve One, but ignored the active
`target effect in the penetration simulator. To obtain the same
`theoretical penetration, the rock strength was increased from
`1.2 Kbar
`to 7 Kbar, which
`is unrealistic and again
`demonstrates
`the need
`to duplicate
`the experimental
`penetration TOA.
`
`Performance Results. A production baseline 34 g. charge
`was chosen for hard rock optimization before optimizing. This
`charge had an overall penetration in the Nugget sandstone of
`12.6-in., which included a 3/8-in. steel face plate. The first
`optimized design was restricted to a liner geometry change
`only, the liner material and case geometry were those of the
`original production charge. The average penetration for this
`first optimized charge was increased to 14-in. as measured
`from 30 QC shots during an 8000 unit production run. The
`14" was short of our goal of 16-in., thus, the only restrictions
`on the next design was to maintain the outer case geometry
`and liner material. This second design reduced the explosive
`charge from 34 g. to 30 g. and reached an average penetration
`of 15.9-in. from 14 QC shots in a 3000 unit production run.
`
`Field Results
`To date, a limited number of field trials have been performed.
`The hard-rock charges have been used on both oil producers
`and gas injectors, but quantitative data regarding their
`performance have not been obtained. Qualitative data in one
`field suggests an improvement in performance has been
`achieved: a gas injector perforated at 4 shots per foot is
`outperforming the majority of gas injectors in the field which
`were shot at a higher shot density (up to 12 shots per foot).
`Although reservoir quality could also be the cause of the
`higher performance, the estimation from logs is that this well
`is similar to the other gas injectors in the field. It is therefore
`likely that it has a lower than average skin factor. A similar
`result has been observed on an oil producer, which is on
`production at a shot density of 4 shots per foot, compared to a
`typical shot density of 8-12 shots per foot for other wells in
`the field.
`It is hoped that quantitative data of the performance of
`hard rock charges will be obtained in the near future. In
`addition, field trials of the latest charge design will commence
`
`
`QC Target. The first problem
`the
`in
`to be solved
`development of a “hard rock” charge was to select a QC
`target. A comprehensive perforator testing program by
`Exxon20 clearly showed that the best perforators for concrete
`or steel were not necessarily the best for rocks (Figs. 3 and 4).
`However, other work1 suggested that steel might be a good
`QC target for hard rock.
`
`Due to the very high cost of hard natural rock targets,
`initial experiments were performed to evaluate steel versus
`hard rock QC targets. Figure 5 shows penetration time-of-
`arrival (TOA) data for steel and Nugget sandstone (UCS =
`16,000 - 18,000 psi). Note that both the penetration “path” and
`final penetration values are different implying that steel will
`not be a good surrogate target. Fig. 6 further confirms this
`showing that penetration in rock may be more influenced by
`the explosive loading force than for penetration in steel.
`
`As a result, the QC targets were Nugget sandstone 2.75
`in. by 2.75 in. by 16-in. to 18-in. long backed up with a QC
`concrete target. These targets were cemented into a 6-in.
`sonotube and then placed into a steel “C-clamp” to simulate
`an infinite thick target, Fig. 7. If the target was not placed in a
`steel clamp,
`total penetration would have
`increased
`approximately 20% to 40%. The Nugget was vacuum
`saturated with brine and maintained in a saturated condition
`until shot. Penetration was perpendicular to the Nugget
`bedding plane. Inconsistent and deeper penetrations were
`obtained when charges were shot parallel to the bedding
`plane.
`
`Charge Optimization. A combination of computational,
`analytical and instrumented tests are used to understand, first,
`the physics of jet formation and, second, the jet/target
`interaction. The AutodynTM finite difference code is used to
`calculate the jet velocity and mass versus time/position21.
`Penetration TOA tests are conducted to obtain jet quality and
`dynamic target properties. These test data with the AutodynTM
`results are then used in an analytical penetration code.
`Finally, X-rays of the jet versus target penetration are used to
`help determine the "active" target effect which is then used to
`update the penetration model. Design iterations are then
`performed to obtain an optimum design.
`
`Experimental - Theoretical Design Results. X-rays of the
`residual jet versus target thickness were also used to obtain the
`jet velocity and penetration time versus target penetration.
`These data were then compared with theoretical calculations
`using both AutodynTM and penetration simulations. Two
`simulations were used with different explosive equation-of-
`states to represent design/production uncertainities.
`
`Figure 8 shows the comparison between experiments and
`
`theory for the penetration-time-of-arrival for the final design.
`(The final experimental penetration was 15.9"; its time-of-
`arrival is not shown since it is usually not obtainable.) Three
`theoretical models are also shown. Models One and Two are
`
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`P.S. Smith, L.A. Behrmann and W. Yang
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`SPE 38141
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`
`12. Halleck, P.M., Saucier, R.J., Behrmann, L.A., and
`Ahrens, T.J.: “Reduction of Jet Perforator Penetration in
`Rock Under Stress,” paper SPE 18245 presented at the
`1988 SPE Annual Technical Conference and Exhibition,
`Houston, TX, Oct. 2-5.
`13. Halleck, P.M., “The Effects of Stress and Pore Pressure
`on Penetration of Jet Perforators in Berea Sandstone,”
`Final report, API Project 86-36, 1987.
`
`14. Halleck, P.M.,: “Further Effects of Stress on Penetration
`Flow Performance of Jet Perforations,” Final Report of
`API Project 86-36, 1988.
`15. Halleck, P.M. and Behrmann, L.A.,: “Penetration of
`Shaped Charges in Stressed Rock,” in Rock Mechanics
`Contributions and Challenges, Balkema, Rotterdam
`(1990); presented at the 31st U.S. Rock Mechanics
`Symposium.
`16. Behrmann, L.A. and Halleck, P.M.,: “Effect of Concrete
`and Berea Strengths on Perforator Performance and
`Resulting Impact on the New API RP-43,” SPE paper
`18242 presented at the 1988 SPE Annual Technical
`Conference and Exibition, Houston, TX, Oct. 1988.
`17. Aseltine, C., “Flash X-ray Analysis of the Interaction of
`Perforators with Different Target Materials,” SPE paper
`14322 presented at the 1985 SPE Annual Technical
`Conference and Exhibition, Oct. 1985.
`18. Behrmann, L.A. and McDonald, B., “Underbalance or
`Extreme Overbalance,” paper SPE 31083 presented at the
`SPE International Symposium on Formation Damage
`Control, Lafayette, LA., 14-15 Feb. 1995.
`19. Brooks, J. E., “Effect of Sand-Grain Size on API Section
`I testing,” Schlumberger Perforating and Testing Center,
`Jan. 1997.
`20. Gidley, J.L., et al: “Results From Phase I of the Exxon
`Perforator Evaluation,” Exxon report, March 1984;
`Gidley, J.L., et al: “Results From the Phase I Retest of the
`Exxon Perforator Evaluation, Exxon report, August 1985;
`Ott, H.E.: “The Evaluation of the Potential Degradation
`of Perforation Charges as a Result of Exposure to
`Elevated temperature,” Battelle Report, Oct. 29, 1985;
`Gidley, J.L., et al: “Exxon-Battelle Perforator Evaluation,
`Phase III, Berea Sandstone Targets,” Exxon report, Aug.
`1985.
`21. Autodyn-2dTM, “An Interactive Non-Linear Dynamic
`analysis software,” from Century Dynamics, Danville,
`Ca.
`
`
`Acknowledgments
`The authors would like to thank BP Exploration and
`Schlumberger for permission to publish this paper.
`
`
`shortly. These charges should provide a further improvement
`in performance.
`
`Conclusions
`1. Perforating penetration in high compressive rocks can be
`increased by optimizing the perforator geometric design.
`2. The penetration physics of oil field perforators into rock
`is only partially understood and semi-empirical analysis is
`still required.
`3. A substitute QC target for hard rock has not been found.
`4. Qualitative field data confirms an increase in well
`productivity/injectivity.
`
`
`
`References
`1. Halleck, P.M., Wesson, D.S., Snider, P.M., and
`Navaretta, M.: “Prediction of In-Situ Shaped Charge
`Penetration Using Acoustic and Density Logs,” SPE
`paper 22808 presented at the 1991 SPE Annual Technical
`Conference and Exhibition, Oct. 1991.
`2. Blosser, W.R.:
`“An Assessment of Perforating
`Performance for High Compressive Strength Non-
`Homogeneous Sandstones,” paper SPE 30082 presented
`at The European Formation Damage Conference, The
`Hague, 15-16 May 1995.
`3. McDowell, J. M. and Muskat, M.: "The Effect on Well
`Productivity of Formation Penetration Beyond Perforated
`Casing," Trans. AIME (1950) 189, 309-312.
`4. Harris, M. H.: "The Effect of Perforating on Well
`Productivity," JPT (Apr. 1966) 518-528.
`5. Klotz, J. A., Krueger, R. F. and Pye, D. S.: "Effect of
`Perforation Damage on Well Productivity," JPT (Nov.
`1974) 1303 (SPE 4792).
`6. Karakas, M., and Tariq, S.: "Semianalytical Productivity
`Models for Perforated Completions," paper SPE 18247
`presented at 1988 SPE Annual Technical Conference and
`Exhibition, Oct. 1988.
`7. Brooks, J.E.,: “A Simple Method for Estimating Well
`Productivity,” SPE paper 38148 presented at the 1997
`SPE European Formation Damage Conference,
`the
`Hague, The Netherlands, 2-3 June 1997.
`8. Smith, P.S., Browne, S.V., Heinz, T. J. and Wise, W. V.:
`"Drilling Fluid Design to Prevent Formation Damage in
`High Permeability Quartz Arenite Sandstones", SPE
`paper 36430 presented at the 1996 SPE Annual Technical
`Conference and Exhibition, October 1996.
`9. Recommended Practices
`for Evaluation of Well
`Perforators, (RP 43), fifth edition (Jan. 1991), API,
`Publications and Distribution Section, 1220 L. Street,
`NW, Washington, D.C. 20005, Order No. 811-08600
`10. Thompson, G.D..: “Effects of Formation Compressive
`Strength on Perforator Performance,” Drilling Prod.
`Practice (1962) 191-197.
`11. Saucier, R.J., and Lands, J.F.: “A Laboratory Study of
`Perforations In Stressed Formation Rocks,” paper SPE
`6758, J. Pet. Ttech. (Sept., 1978), 1347.
`
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`SPE 38141
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`Improvements in Perforating Performance in High Compressive Strength Rocks
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`5
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`Downloaded from http://onepetro.org/SPEEFDC/proceedings-pdf/97EFDC/All-97EFDC/SPE-38141-MS/1940751/spe-38141-ms.pdf/1 by Elizabeth Fuller on 06 September 2022
`
` Figure 1 Test 5’ From SPE30082
` The light colored regions are high density rock
`
`
`
`
`
`
`
`
`Figure 3 Concrete versus Berea slab penetration data
`
`Figure 4 Concrete versus steel penetration
`
`S te e l
`N u g g e t
`
`3 0 0
`
`2 5 0
`
`2 0 0
`
`1 5 0
`
`1 0 0
`
`5 0
`
`0
`
`Time, microsec.
`
`0
`
`1 0
`5
`P e n e tra tio n , in c h e s
` Figure 5 Penetration time-of-arrival
`
`1 5
`
` Figure 2 Test 8 from SPE 30082
` The light colored regions are high density rock
`
`
`
`
`
`
`
`
`
`
`
`
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`P.S. Smith, L.A. Behrmann and W. Yang
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`SPE 38141
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`Downloaded from http://onepetro.org/SPEEFDC/proceedings-pdf/97EFDC/All-97EFDC/SPE-38141-MS/1940751/spe-38141-ms.pdf/1 by Elizabeth Fuller on 06 September 2022
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`Experimental
`
`Theoretical One
`
`Theoretical Two
`
`Theoretical Three
`
`600
`
`500
`
`400
`
`300
`
`200
`
`100
`
`0
`
`Time, microsec
`
`0
`
`4
`
`8
`Penetration, inches
` Figure 8 Penetration time-of-arrival
`
`12
`
`16
`
`Steel
`Nugget
`
`14
`12
`10
`
`02468
`
`Penetration, inches
`
`0
`
`1
`Normalized Loading Force
`
`1
`
`Figure 6 Penetration versus loading force
`
`
`
`
`
`
`
`
`
`Figure 7 Hard rock target
`
`Table 1 Test Data in Hard Rock, From SPE31083
`3000 psi underbalance, 5500 effective stress
`
`
`Test
`
`BPC-1
`BPC-3
`BPC-5
`BPC-5
`BPC-8
`CT-1
`CT-2
`
`
`
`Permeability
`to kerosene
`(md)
`
`Charge
`weight (gm)
`
`0.096
`17.75
`7.66
`9.17
`4.72
`10.4
`10.4
`
`23
`14
`3.5
`6.5
`6.5
`3.5
`3.5
`
`
`
`Unconfined
`compressive
`strength
`(Kpsi)
`25.1
`13.58
`16.06
`16.06
`20.88
`8.8
`8.8
`
`Single shot
`skin
`
`Experimental
`penetration
`(inches)
`
`Theoretical
`penetration
`(inches)
`
`-0.75
`0.34
`-2.1
`-2.0
`170
`-0.23
`-1.0
`
`
`
`6.0
`5.13
`1.9
`3.0
`2.6
`3.0
`3.15
`
`7.0
`7.4
`2.37
`4.08
`4.0
`3.2
`3.2
`
`Ratio of
`exp’t.-to-
`theoretical
`penetration
`0.86
`0.69
`0.80
`0.74
`0.65
`0.94
`0.98
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