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MODERN DRILLING AND BLASTING TECHNOLOGYAT THE MINNTAC MINE

ByDon Thompson

Senior Engineer, Drill and BlastUS Steel Minntac Mine

Mt. Iron, MN

ABSTRACT

The Minntac Mine drills and blasts approximately 75 million long tons of taconite per year. This requiresdrilling one million feet of 16-inch diameter holes. We are in the process of replacing our old generationrotary blasthole drills (Gardner-Denver 120's and Bucyrus-Erie 61R's) with new generation rotary blastholedrills (Bucyrus-Erie 59R's and P&H 120A's).

This paper compares the primary functions of the two generations of drills, including: main air and it's affectof particle size, penetration rate, and bit life; the differences in performance of the old generation drillchain/hydraulic pulldown systems compared to the new rack and pinion/electric pulldown systems; andmanual controls of the older generation drills compared to Programmable Logic Control (PLC) andAutomation.

Stratalogger Systems have also been installed on the new generation drills, providing pulldown force, rateof penetration, rotary torque, vibration, and compressor data. Examples of actual blast pattern design andbit performance evaluations using this data will be explained. The potential for improved fleet management,on-the-fly pattern design, and real-time drill parameter changes will also be discussed.

INTRODUCTION

The Minntac Mine is a large taconite mining operation in Northern Minnesota. Drilling and blastingrequirements include one million feet of drilling per year producing 75 million long tons of crude ore andwaste rock. Compressive strengths of the taconite range from 30,000 psi to over 90,000 psi. Tricone bitswith tungsten carbide inserts are used in all drilling. Hole diameters have increased from 12 1/4 and 15inches in 1988 to 16 inches exclusively today. Nominal Crude Ore hole spacing has increased from 30 feetby 30 feet for 15-inch bits to 32 feet by 32 feet for 16-inch bits. Hole depths average 43 feet includingsubdrilling. Penetration rates range from less than 10 to 85 feet per driller hour, with an average of 24.5 feetper driller hour. Average bit life ranges from less than 600 feet to over 13,000 feet with an average of 3,500feet.

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Gardner-Denver 120 and Bucyrus-Erie 61R drills are being replaced with Bucyrus 59R and P&H 120ADrills in order to better utilize 16-inch bits. The present drill fleet consists of three old generation Gardner-Denver 120 drills and seven new generation drills, including three P&H 120A drills and four Bucyrus 59Rdrills. With the new generation of drills, significant improvements have been made in the four primary drillsystems which are main air, control, pulldown, and rotary. Bit life and penetration rates for the newgeneration drills are 20 percent greater than the old generation drills, which has led to a substantial decreasein the cost per foot of drilling.

The new generation drills have been equipped with hole logging equipment, data modems, and data radiosfor improved blasting and fleet management.

MAIN AIR SYSTEMS

Sixteen-inch bits were first run at Minntac in 1989. At that time the average tested compressor output was1,538 cfm for the drill fleet. The pipe diameter was increased from 12 1/4 inches for 15-inch bits to 13 3/8inches with the 16-inch bits. This new combination provided an average bailing velocity of 2,822 feet perminute for 16-inch bits. Improved bit life and penetration rates were expected because of larger bearingsand carbides, however, there was no significant change. The primary cause of failure was due to shoulderwear caused by regrinding cuttings. Carbides were still good in most cases. It was strongly felt that moreair would improve penetration and bit life.

A comprehensive air compressor study was conducted in 1991 to determine if increasing the bailing velocitywould improve 16-inch bit performance. The testing was done on two Gardner-Denver 120 Drills with 133/8 inch diameter pipe. The pulldown force was maintained at 90,000 pounds. Approximately 550 holeswere drilled in soft oxidized waste rock and hard massive crude ore for the test. Bailing velocities werevaried by hooking up a 900 cfm portable compressor to the drills which were equipped with 1,940 cfmcompressors and by varying bit diameters. Compressor output tests were done to verify actual air volumes.Calculated bailing velocities ranged from 3,442 feet per minute with the single compressor and a 16-inchbit to 9,528 feet per minute with double compressors and a 15-inch bit. Every other row was drilled withthe single compressor in order to get an accurate comparison due to changes in the geology. The bit and drillpipe were then changed and the remaining holes were drilled with dual compressors. Average penetrationrate, rotary amperage, and rotary voltage were recorded manually in five-foot intervals. Sizing samples werealso collected in five-foot intervals for approximately 20 holes. Exhibit #1 represents the percent ofmaterial passing a 0.5 inch screen in five-foot intervals throughout the holes. The results of the screeningtests, showed significant particle size increases with increased bailing air velocity. The test concluded thatincreasing the bailing velocity from 3,442 feet per minute to 5,665 feet per minute resulted in increasedpenetration rates from 10 percent for harder taconite to 22 percent for softer taconite. Bit life showed a 22percent improvement in softer drilling with no significant improvement in harder drilling.

As a result of these tests, new drills are now supplied with the largest compressors available. The first P&H120A and Bucyrus 59R drills were delivered with 3,000 and 3,140 cfm compressors respectively. Thesedrills provide between 2,500 and 2,715 cfm at the bit, based on main air tests conducted at Minntac. Thenewest P&H 120A's and Bucyrus 59R drills were delivered with 3,600 cfm compressors. Actual compressortests indicate air volumes between 2,700 and 3,150 cfm for the last four drills delivered. Exhibit #2compares critical flow compressor test results conducted at Minntac to specified compressor outputs. Air

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Compressor plate ratings and actual compressor output values do not match because of altitude, temperatureand mechanical inefficiencies. It is important to test compressors to determine the actual output for thegiven application and location.

Bailing velocities, which are dependent on the bit diameter, pipe diameter, and actual compressor outputhave improved significantly. Bailing velocities for the old generation drills range from 2,100 to 3,700 feetper minute, while new generation drill bailing velocities range from 3,700 to 7,600 feet per minute.

Nelmark’s Formula and Stope’s Law predict the maximum particle diameter ejected from a drill hole at agiven bailing velocity and particle density. Exhibit #3 predicts the maximum particle size based on taconitedensity of 200 pounds per cubic foot. The range of maximum particle sizes for the old generation drills isbetween 1/8 inch diameter and 5/16-inch diameter. The smaller size is based on the least efficientcompressor with maximum pipe wear while the largest size is based on the most efficient compressor withnew pipe. The range for the new generation drills is from 5/16-inch diameter to 3/4 inch diameter.

CONTROL

Propel, rotary, and hoist functions for the old generation drills are provided by DC motors. DC power isprovided by two DC generators driven by an AC induction motor. The output of the generators is controlledby SCR (Silicone Controlled Rectifier) field exciters which are, in turn, controlled by an electronic speedcontrol regulator. Electro-mechanical relays are used for control logic. Old generation drills are equippedwith a deck mounted transmission which is common to propel, hoist, and pulldown. A series of chains andsprockets run from the transmission to the head and propel side frames. Power for pulldown is provided bya hydraulic pump and motor with Dennison controls. Hoist and propel functions are powered by a DCmotor. An auxiliary air compressor provides power for brakes and clutches used for shifting between thepropel, hoist, and hydraulic pulldown functions. Drilling is manually controlled by the operator.

DC power for the new generation drills is provided by static drives. Programmable logic controls (PLC)provide control logic. Propel power is provided by hydraulic motors mounted on the side frames. Powerfor hydraulic functions is provided by pumps driven by the main compressor motor. Head mounted rackand pinion pulldown systems are driven by one DC motor. The transmission, chains, DC generators, andauxiliary air compressor have been eliminated, significantly reducing the number of moving parts on thedrill. This has significantly improved drill availability. Most of the drilling is done in automation. Theautomation parameters are pre-set by mine management.

PULLDOWN SYSTEMS

Exhibit #4 represents a series of three weight tests that were performed on a hydraulic/chain pulldownsystem. The tests demonstrate the inconsistencies in pulldown with the old generation drills. There is asignificant range between load cell readings from test to test as compared to the cab reading. The test onAugust 29, 1992, was performed because the drill operator thought the drill was coming off the ground whenthe pulldown was set at 85,000 pounds. The operator was unable set the cab gauge at 90,000 pounds. Thesetest results were quite different from the test done just two weeks earlier. Setting the cab gauge at 85,000pounds did lift the drill off the ground with a load cell reading of 147,000 pounds.

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Exhibit #5 represents a typical pulldown test for a new generation drill. The relationship between thepulldown amps and the load cell readings is linear. Once this relationship is determined for a drill, the headand steel weight settings are changed to match the PLC amps to the PLC pulldown force. Pulldown testsover a period of time for the rack and pinion systems have been consistent. In fact, we have been able toaccurately set pulldown settings for one drill based on tests done on another drill.

ROTARY SYSTEMS

Of the four primary drilling functions, the rotary systems have changed the least. The old generation drillshave dual 105 horse power rotary motors producing a maximum torque of approximately 12,000 footpounds. The new generation drills have dual 135 horse power or a single 235 horse power motor, dependingupon the model. In both cases, the maximum torque is approximately 14,500 foot pounds.

DRILL MONITORING

The new generation drills have been equipped with Stratalogger drill monitoring systems, which are beingused for blast design and bit evaluations. The logging equipment automatically records pre-selected drillingdata in increments of approximately 0.2 feet. Presently, data includes penetration rate, bit loading, bit rpm's,rotary amps, rotary torque, bit air pressure, horizontal and vertical vibration (59R only), and compressortemperature. Drill delays are input by the operator as they occur.

Each Stratalogger unit is connected to a 2,400 baud data modem and a 40 watt data radio. The data istransmitted through a 20 watt repeater to the main drill monitoring server as drilling records are generated.The data is compiled into the data bases at the end of each shift. Binary files are generated on a real-timebasis for equipment fault reporting and real time shift progress monitoring. Supervisor vehicles are equippedwith Mobile Supervisor Terminals or MST’s. Shift information is updated to the MST once each minute.Individual drill shift information sent to the MST includes: feet drilled for present hole; total feet drilled;number of holes drilled; time of last data transmittal; present drill hole number; active delay code; employeecheck number; pulldown force; and rpm’s Employee, hole, bit and pattern numbers can be changed on thedrills remotely from the server or work stations. Delay codes can also be changed remotely. Reports andgraphs can be generated from the server or connected work stations. One of our short term goals is to havea paperless reporting system. We are tracking the average pulldown force for the fleet and the averagedriller log and bit turning times in order to improve fleet performance.

Two penetration rate values are recorded by the Stratalogger System. The E-ROP or effective rate ofpenetration is equivalent to the net rate ( presently recorded by the operator) and represents the total timespent drilling a hole, including hole cleaning, bit retraction times, and minor delays. The E-ROP does notinclude delays such as moving, maintenance, and drill service. The D-ROP or drilling rate of penetrationrepresents raw penetration rate without delays. The D-ROP should be used for bit evaluations and as oneof the blastability indicators.

Exhibit #6 is a typical cross section of the Minntac Mine. The layers are defined by geologicalcharacteristics. Magnetic iron content, after grind silica and grindability, determine the material quality.The Lower Chert layers (LC) from 1 through 4 and the Slate layer contain crude ore. The Upper Chertlayer (UC) is also crude ore, however, only limited quantities have been mined. The Lower Chert 5 layer

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is a waste layer. Oxidized waste pockets are common in the crude layers. The Slate layer also containsInterbedded Chert (IBC). The ability to drill, blast and mill varies throughout the ore body.

The Hole Summary is a series of data and graphs representing a logged hole. Exhibit #7 is the HoleSummary for hole #004 203, and represents drilling for competent, massive, and hard taconite in the lowerchert 1 and 2 layers. The top of the page shows all pertinent hole data, while the lower portion shows aseries of graphs. In this case, rpm's, rotary torque, rate of penetration (D-ROP) and specific energy areplotted against hole depth. The specific energy is the correlation between mechanical work done by the bitto the energy required to crush a unit volume of rock expressed in ft-lb/ft-in2. The driller set the collaringmode for about two feet and the drill had no trouble maintaining the pre-set pulldown force of just over100,000 pounds after collaring. The D-ROP of 45.3 feet per hour was relatively low. Rotary torque of 5,429foot-pounds was also low which is an indication that the carbides were unable to penetrate the formation.The average specific energy for this hole is 17,645 ft-lb/ft-in2,, which is high. The average specific energyfor Minntac is 11,095 ft-lb/ft-in2.

BLAST DESIGN

Drill monitoring has the potential to decrease the probability of fly rock and noise, while improvingfragmentation by optimizing powder placement. Flyrock control will become even more critical as the mineexpands to the south and west.

Presently, the net penetration rate is used as a primary factor in determining rock hardness and blastability,which can be misleading. For instance, in areas where the rock is fractured, drill rates can be relatively lowwhich is because of drilling conditions such as caving, not rock hardness. Additionally, the net penetrationrate normally decreases with bit age. It is not unusual for the net penetration rate for a bit to decrease by 50percent or greater through it's life. However, occasionally the penetration rate increases with bit life becauseof steel wear around the carbides.

Exhibit #7 is typical of competent Lower Chert 1 and Lower Chert 2 drilling. Historically, these layershave been difficult to blast, especially in the top half of the pattern. This hole was over drilled by 12 feetfor head maintenance and was later back filled. Thirty-six feet of the hole were loaded with 4,236 poundsof emulsion blend. The top 17 feet were stemmed. The ANFO equivalent powder factor was 0.82 poundsper ton. Because of the shallow collaring depth and the high specific energy, stemming could have safelybeen reduced to 14 feet without increasing the 3,000 foot blast clearing radius. This would have provideda ANFO equivalent powder factor of 0.89 pounds per ton.

Exhibit #8, the Hole Summary for hole #428 415, represents drilling in the main sump area. The drill wasunable to hold a constant pulldown force for the top 38 feet of the hole because of pre-fractured rock. TheD-ROP was 86.9 feet per hour. The average specific energy for this hole was 9,259 ft-lb/ft-in2. This holewas drilled to 65 feet, however, 22 feet of the hole caved in. Twenty-two feet of this hole were loaded with2,589 pounds of emulsion blend. The top 17 feet were stemmed. The ANFO equivalent powder factor was0.80 pounds per ton. The entire powder column was in the fractured area. The peak air blast from thispattern was excessively high. The stemming could have been increased by 5 feet or more. The ANFOequivalent powder factor would have been 0.65 pounds per ton with 22 feet of stemming. As a footnote,

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there was a change in drilling in this hole at about 38 feet which coincides with the Lower Cherty 1/Quartzitecontact based on core samples.

Exhibit #9, the Hole Summary for hole #006 433, represents a 65 foot hole that was drilled in inter-beddedchert and slate in the East Pit. The drill was able to maintain a constant pulldown force after about 5 feetof drilling, which is normal. The D-ROP was 86.9 feet per hour. The specific energy was 7,489 ft-lb/ft-in2.Although this number is low, the drilling was competent. Changes in lithology can be seen in this HoleSummary when compared to drill core data. The top 25 feet of the hole contained an after grind silicacontent of 3 percent and a magnetic iron content of 30 percent, which is lean crude ore. The materialchanged to waste rock from 25 to 37 feet. The magnetic iron content dropped to 7 percent and the aftergrind silica increased to 5 percent. The material was again lean crude ore from 37 feet to 52 feet with amagnetic content of 28 percent and an after grind silica of 5 percent. The material changed to slate at 52 feetand contained 16 percent magnetic iron and 7 percent after grind silica, which is marginal crude ore. This hole was also over-drilled for maintenance purposes. Twelve feet of this hole were back-filled. Thirty-six feet of the hole were loaded with 4,236 pounds of emulsion blend. The top 17 feet were stemmed. Thisprovided a ANFO equivalent powder factor of 0.86 pounds per ton which is probably adequate for this rock.However, because of the higher specific energy in the top 25 feet, a loading scheme providing more energyin this region would most likely reduce chunks in this layer.

Exhibit #10, the Hole Summary for hole #045A 1209, represents a soft waste rock hole. The D-ROP was182.8 feet per hour. Rotary torque averaged 3,906 foot-pounds. The reason for the adjustments in thepulldown force were most likely caused from the penetration rate exceeding the pre-set limit which keepsthe bit from being jammed into the formation. The average specific energy of 3,506 ft-lb/ft-in2, is quite low.This hole was loaded with 2,236 pounds of emulsion blend and 19 feet of stemming, providing a ANFOequivalent powder factor of 0.55 pounds per ton, which is adequate. Based on core samples, there was alayer of crude slate from approximately 30 feet to 45 feet and inter-bedded chert below that. These breakscan be seen in the Hole Summary. Exhibit #11 is a set of contours representing drilling characteristics for Pattern # 96096. The above Hole#045A 1209 was part of this pattern. This pattern was 1,750 feet from the Main Gate and 550 feet fromCounty Road #102, causing concern for flyrock. The D-ROP averaged 137 feet per hour. The highlightedportion of the D-ROP contour shows relatively low and consistent penetration rates, which might normallyindicate harder drilling than the shaded portion of the pattern. The ridge in the shaded area on the right sideof the D-ROP graph shows an area that appears to be softer than the remainder of the pattern, however thehighlighted area of the pulldown force shows that the drill could not maintain a constant pulldown force.The shaded area to the right of the highlighted area shows an increase in the pulldown force that correspondsto the ridge of higher penetration rates. The Rotary Torque Contour shows the affects of the varyingpulldown forces. Specific energy averaged 3,019 ft-lb/ft-in2. There were significant adjustments in thepulldown force in the softest portions of the pattern that held the penetration rate relatively constant. Thisarea proved to be soft and loose which was cause for concern. Stemming adjustments of two feet were madefor the soft drilling and four feet for the very soft drilling, based on specific energy rather than thepenetration rates.

The Stratalogger information for pattern #96096 was also used to point out to the drill manufacturer, thatthere were problems with their drill automation logic. Because of the automation problems, performance

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for the bit used in the soft zone might have been unfairly evaluated by the present method. Drillperformance on this pattern demonstrates the need to re-think how we evaluate drill bit performance.

DRILL BIT PERFORMANCE EVALUATIONS

Exhibit #12 compares the net drilling rates as recorded by the operator for “bit #1” and “bit #2”. Bit #1,was an older bit design. Historically, this model was the best performing bit model at Minntac. Bit #2 wasa new design which was subject to a time study by the manufacturer. The 58th hole drilled for bit #1 wasreported to have been drilled at 700 feet per net hour compared to an E-ROP of about 26 feet per drillinghour (recorded by the Stratalogger). This was a manual reporting error that was made by either the drilleror the mine clerk. Other factors that cause discrepancies between the Net and E-ROP rates include drillerperformance and drilling conditions. Initial results based on operator reports for bit #2 were impressive.The average net penetration rate was 51 percent greater than the penetration rate for bit #1.

Figure #13 is a comparison of the D-ROP data collected from the Stratalogger, which represents the rawpenetration ability of the drill bit. The results showed a much less impressive 20 percent performanceincrease when comparing all holes drilled. Closer review showed the first 15 holes for bit #1 were drilledon another pattern that was much harder. This probably caused some premature carbide damage and bearingwear. Because of this, another comparison was made with holes 15 through 98 for each bit (shaded areasof Exhibits #12 and #13). The results from this comparison showed an insignificant difference of only 2percent. Examining the data showed significant deviations between the net rates as recorded by the drillers and theE-ROP recorded by the Stratalogger. The deviations between the net rate and E-ROP for bit #2 were muchless then for bit #1. This is most likely because the drillers knew that bit #2 bit was being closely monitored.Bit #1 was drilled during the month of January 1996, which was one of the coldest months ever recorded,while bit #2 was run in June of 1996.

Table #1 represents performance results for four drill bits. The shaded areas represent values that can onlybe collected from the Stratalogger System. The unshaded values are based on driller reports.

The bit performance ranking based on the Net Rate is: C, B, D, A compared to B, C, A, D for the E-ROP,and D, C, B, A for the D-ROP. The Net Rate and E-ROP indicate the drilling problems that occur duringdrilling, such as caving, compressor kick-outs, rotary stalls, and hole clean out. Again, D-ROP is the rawbit penetration ability.

The average pulldown force of 63,874 pounds for bit “D” indicates that the drill was unable to maintainmaximum pulldown because of exceeded parameter limits caused by soft and broken material. On the otherhand, the average pulldown force of 98,200 pounds for bit “A” is typical of drilling in hard competent rockwith shallow collaring depths. This example suggests that present cost per foot calculations, (Drilling Costper Foot = Bit Cost / Total Feet + Gross Drilling Rate / Hourly Rig Cost), are often times not representativeof the ability of the drill bit. A task for the future is to find a more realistic method of evaluating drill bitperformance. Indexes such as the rock quality or rock hardness will one day be worked into the Cost perFoot evaluations, which will not be possible without the use of automated drill monitoring.

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FORMATION DRILLING CHARACTERISTICS

The ability of the drill monitoring system to record changes in drilling parameters is the basis for materialrecognition. Approximately 2,500 Stratalogger records in 0.2 foot increments were compared to diamonddrill data in the West Pit. Data was filtered to include pulldown forces of 97,500 pounds or greater. Resultsshowed that drilling characteristics could be identified for various strata. Table #2 and Exhibit #14 representthe results. The graphs are statistical distributions of D-ROP, rotary torque, and specific energy for eachmaterial type. The large, lightly shaded distribution curve is the total distribution for all material types. Thesmaller, darker shaded distribution curve is for the specific material identified to the left. Most of thespecific energy distributions are somewhat normal.

The distributions show that there is not a correlation between drillability and silica or magnetic iron content.Instead, the drillability for each material is based on the macro structure and degree of oxidation. Forexample, LC1 , LC2 which are marginal crude, and LC5 which is waste, are very hard although themagnetic and silica contents are low. The LC3 layer is also hard although the magnetic content is high.Crude slate drilling is similar to the drilling of oxidized waste LC3 and LC4 layers.

This type of information is valuable in terms of drilling and blasting optimization. For example, based onpresent experience, bit manufacturers' recommendations, and gut feelings, drilling at 80 rpm's with 100,000pounds of pulldown force will provide optimum drilling results. This is probably not true. One day, withthe help of tools such as automated data collection and transmission we will be able to change drillingparameters as the drilling characteristics change.

CONCLUSION

Significant technical advancements have been made with the Minntac drill fleet, however, we have justbegun to scratch the surface. New drill features combined with hole logging technology, are major steppingstones for further technical advancements such as material recognition, on-the-fly drill parameter adjustment,on-the-fly blast design, custom hole loading, and GPS.

Real time data transmission is also important for maximizing drill productivity and improving maintenanceresponse times. Accurate delay data collection and analysis will be used for parts failure projections andmore meaningful maintenance service schedules.

Drill and blast technology is changing rapidly. Continued advancements at Minntac are necessary if we wantto step up to the plate and play the twenty-first century technology game.

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REFERENCES

Fichna B. A., 1991, "Bailing Velocities for Rotary Drilling in Open Pit Mines", MinTech 91, SterlingPublications International Limited, London, England

"Blasthole Drill Air Test Procedure Recommended Drill Air Practice and Blasthole Drill Weight TestProcedure" Baker Hughes Mining Tools, Inc., Grand Prairie, TX.

D. Campbell, M. Scoble and J. Peck "Identification of Indurated Siltstone Layers in Advance of Oil SandsMining" McGill University, Montreal, Quebec, Canada.

Stratalogger Plus Software User Manual "Thunderbird Mining Systems, Redmond, WA.

EQUATIONS

Nelmark’s Equation: V = (54,600 DC 0.6) / (D + 62.4)Where; V = Bailing Velocity (ft/min)

C = Cuttings Diameter (ft)D = Rock Density (lb/ft3)

Stope’s Law: Vt = [2G Dp (Pden - Fden)] / (1.12 Fden)Where; Vt = Bailing Velocity (ft/min)

G = 32 ft/sec2

Dp = Particle Diameter (ft)Pden = Density of Particle (lb/ft3)Fden = Density of Fluid (lb/ft3)

Specific Energy: SE = (2pi NT) / [R / 60pi(0.5D)2] + W/[pi (0.5D)2] Where; SE = Specific Energy (ft-lb/in2-ft)

N = RPM’sR = Drilling Rate of Penetration or D-ROP (ft/hr)T = Rotary Torque (ft-lb)W = Bit Load (lb)

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Table #1 Drill Bit PerformanceTotal Rates Rotary Pulldown Air

Bit Drill Feet Net E-ROP D-ROP Torque Force rpm's Pressure

"A" 41 2,345 27 27 65 5,923 98,200 76 38

"B" 41 3,836 40 37 83 6,007 92,933 75 41

"C" 42 1,717 59 28 91 3,512 92,213 80 44

"D" 42 1,782 33 18 146 3,283 63,874 69 47

Table #1 Drilling Characteristics vs. FormationPenetration Rate Crude Waste

Feet per Net Hour All LC 1&2 LC 3 LC 4 Slate LC 3&4 LC 5 Slate

Mean 152 85 73 158 198 245 95 251

Median 111 68 60 149 157 185 81 217

Mode 68 64 55 162 111 149 72 166

Minimum 17 21 43 43 17 51 38 60

Maximum 914 378 434 506 914 897 642 897

Range 897 357 391 463 897 846 604 837

Rotary Torque Crude Waste

Foot - Pounds All LC 1&2 LC 3 LC 4 Slate LC 3&4 LC 5 Slate

Mean 7,888 6,839 5,642 7,869 8,804 8,462 6,965 9,899

Median 7,568 6,588 5,176 7,765 8,549 8,431 6,823 9,804

Mode 6,823 5,647 4,706 8,314 7,529 6,510 6,510 10,117

Minimum 4,000 4,627 4,000 5,176 5,255 5,569 5,020 6,823

Maximum 14,745 12,549 11,137 11,686 14,588 13,333 11,686 14,745

Range 10,745 7,921 7,137 6,510 9,333 7,765 6,667 7,921

Specific Energy Crude WasteFoot Pounds per Foot - Inch^2 All LC 1&2 LC 3 LC 4 Slate LC 3&4 LC 5 Slate

Table #1 Mean 11,447 15,654 13,971 10,040 8,610 8,564 13,196 7,492

Median 10,664 15,225 13,597 8,640 8,147 6,549 12,962 7,257

Mode 13,080 12,381 10,995 18,077 5,680 na 13,080 7,518

Minimum 1,456 3,102 2,295 2,756 1,456 1,988 2,136 1,866

Maximum 69,660 69,660 28,221 29,852 55,997 26,656 31,840 25,901 Range 68,204 66,558 25,926 27,095 54,542 24,669 29,704 24,035

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