SPE
`
`Society of Petroleum Engineers
`
`SPE 21264
`
`Air Drilling and Multiple Hydraulic Fracturing of a 72° Slant Well
`in Devonian Shale
`A.B. Yost II, * U.S. DOE; R Carden, GSM & Assocs.; J.G. Muncey and W.E. Stover,
`Prime Energy; and RJ. Scheper, Gas Research Inst.
`SPE Member
`
`This paper was prepared for presentation at the SPE Eastern Regional Meeting held in Columbus, Ohio, October 31-November 2,1990.
`
`This paper was selected for pre~entation 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 En~lneers, 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 IS presented. Write PubllcallOns Manager, SPE, P.O. Box 833836, Richardson, TX 75083·3836 U.S.A. Telex, 730989 SPEDAL.
`
`ABSTRACT
`
`This paper discusses site selection, air
`directional drilling, stimulations, and
`economic evaluation for a 72" slant well
`drilled in the Devonian shale, Roane County,
`West Virginia. The well was drilled to
`evaluate the concept of using directional
`drilling and multiple hydraulic fracturing
`to improve the gas production from a
`450-foot naturally fractured Devonian shale
`section. The well trajectory remained in
`target for more than 1,500 feet at a pre(cid:173)
`ferred azimuthal direction perpendicular to
`the regional fracture trend and also the
`known production fairway.
`
`The well was conventionally drilled to the
`kickoff point of 2,150 feet. Afterwards,
`positive displacement downhole motors were
`used to build hole angle up to a 72' hole
`angle at the top of the target zone. Once
`the target zone was reached at 67", conven(cid:173)
`tional rotary assemblies were used to rotate
`the well across a l,500-foot target section
`in which inclination varied from 67" to 72"
`of hole angle. After reaching total depth
`of 4,833 feet, the well was logged using
`wireline and drill pipe conveyed methods.
`Then 5 1/2-inch casing was run and cemented
`with 60 rigid centralizers to improve zonal
`isolation.
`
`Four stimulations were designed and com(cid:173)
`pleted. All stimulations will be discussed
`in the paper. Economic analysis was con(cid:173)
`ducted to show expected improvements from
`the application of this technology to frac(cid:173)
`tured Devonian shale. Post-stimulation
`
`References and illustrations at end of
`paper.
`
`well testing has been identifled tor each
`zone to show initial results.
`
`INTRODUCTION
`
`The U.S. Department of Energy, Morgantown
`Energy Technology Center has been inves(cid:173)
`tigating the development of marginal gas
`resources using high-angle and hprizontal
`drilling for more than 20 years. Mobil
`Oil Corporation first tested the concept2
`in the Austin Chalk in the early 1970's.
`DOE also experimented in the r.frly 70's by
`drilling two high-angle wells'
`in the
`Devonian shale but poor well siting and
`mechanical problems plagued early produc(cid:173)
`tion results.
`In 1986, DOE sponsored a
`2,OOO-foot horizontal well in Devonian
`shale5 in which multiple hydraulic frac(cid:173)
`turing experiments were performed. These
`marginal gas resources are contained in
`tight gas formations like Devonian shale,
`tight sands, and coalbeds where non(cid:173)
`damaging drilling fluids like air will be
`used to allow for maximum production from
`an undamaged formation.
`
`The U.S. Department of Energy developed a
`cost-sharing contract with Sterling
`Drilling and Production Company (SD&P) to
`drill, log, test, and stimulate a high(cid:173)
`angle directional well in Roane County,
`West Virginia.
`In addition, SD&P developed
`an R&D agreement with the Gas Research
`Institute to conduct supporting research in
`well siting, well logging, testing, and
`stimulation.
`SD&P had Columbia Gas and
`Pennzoil Company as working interest
`partners in this slant well project.
`
`SITE SELECTION
`
`In order to effectively test this advanced
`gas recovery concept, shown in Figure 1,
`
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`AIR DRILLING AND MULTIPLE HYDRAULIC FRACTURING OF A 72" SLANT WELL IN DEVONIAN SHALE
`
`SPE 21264
`
`careful site selection was conducted to
`ensure the well was placed in an area of
`established production and located in a
`favorable orientation to take advantage of
`the natural fracture permeability
`anisotropy.
`
`The SD&P operator controlled lease area lies
`within the Allegheny Plateau structural
`province in the Appalachian Basin. The
`study area location and surface expressions
`of local tectonic features are presented in
`Figure 2. The structural style of the area
`is dominated by the intersection of an
`Alleghenian detachment, defined by the north
`to northwest striking Burning Springs and
`Mann Mountain thrust sheets. The well was
`targeted to a tabular fractured zone within
`the Devonian shale; a thick sequence of
`thinly bedded, grey and black shales which
`were deposited in a foreland basin of the
`Allegheny organic belt. The well is located
`on the foreland flank of the Chestnut Ridge
`Anticline which lies on the leading edge of
`the northeast striking foldbelt in Roane
`County, West Virginia.
`
`Computer-contoured structural, isopach, open
`flow, and cumulative production maps of the
`well control in the area were prepared and
`evaluated for high potential fracture trends
`suitable for well siting. The strike within
`the Devonian section is predominately north(cid:173)
`east; dip is on the order of 3' to 4" and is
`predominately northwest. Anomalous thick(cid:173)
`nesses and repeat sections, observed within
`the Devonian section in the study area, have
`been interpreted as small-scale thrust
`faults. Tear faults are interpreted to lie
`within structural low which separate anoma(cid:173)
`lous structural highs and anomalously thick(cid:173)
`ened areas. Locating the well site inside
`favorable cumulative production trends in
`the northeast direction dominated the site
`selection process. The wellbore azimuth was
`designed to cross perpendicular to the
`strong northeast production trends. Linear
`trends of production dominate the geologic
`setting while wells located away from the
`fracture fairways are typically not economi(cid:173)
`cal and have marginal production. Wells
`which are close to or are cut by faults tend
`to exhibit higher initial production rates
`and cumulative production potential. The
`highest production potentials are exhibited
`by wells near the interpreted strike-slip
`faults. These high production fracture
`fairways appear to be several well spacings
`long parallel to strike, but only about one
`to two well spacings wide. Therefore, site
`selection is critical to being in the frac(cid:173)
`tured fairway.
`
`Numerous oriented core natural fractures
`from other Devonian shale wells in the
`Appalachian Basin exhibit near vertical
`angle of dip from splay faults rising out of
`the basement and terminating in the thick
`shale sequence. A conceptual reservoir
`geologic model of the target zone identified
`from geophysical well logs is shown in
`Figure 3.
`
`DRILLING OPERATIONS
`
`The Boggs No. 1240 slant well located in
`Roane County, West Virginia, was to be
`drilled vertically to a kickoff point of
`2,150 feet.
`Inclination was planned at a
`rate of 8.60'/100 feet using a double bend
`motor assembly and a steering tool. An
`inclination of 71" was planned at a mea(cid:173)
`sured depth of 3,239 feet (2,965 feet TVD).
`Inclination was to be built in the slant
`section from 71' to 81" at a rate of 1/2'
`per 100 feet using a rotary build assembly.
`The total measured depth was to be
`5,160 feet (3,426 feet TVD, and 2,499 feet
`departure); however, the exact total depth
`would depend upon where the wellbore went
`outside the target interval. A schematic
`of the slant well profile is shown in
`Figure 5.
`
`The anticipated drilling time for the slant
`hole project was 20 days. Drilling opera(cid:173)
`tions were conducted at the site between
`July 10, 1989, and August 1, 1989. Total
`days on location were 23 compared to the
`anticipated time of 20 days. Figure 4 is a
`plot of depth versus days for the actual
`and planned drilling program.
`
`The vertical portion of the well to the
`kickoff point took 1 day longer than
`anticipated. The build section required
`7 days to drill and was the same as the
`planned time. An additional 2 days were
`needed to drill the slant section because
`of the motor correction made at 3,785 feet.
`The planned versus actual drilling days
`were as close as can reasonably be esti(cid:173)
`mated for a well of this nature.
`
`Drilling progressed as anticipated except
`the motor run at 3,785 feet. The motor was
`run to turn the well to the right. Though
`this was accomplished, the well also
`dropped inclination and ended up dropping
`out of the target interval earlier than
`planned. The wellbore length was 327 feet
`shorter than the planned wellbore
`(5,160 feet planned, 4,833 feet actual).
`
`Vertical Hole To 2,150 Feet
`
`The well was spud on July 10, 1989. A
`17 1/2-inch hole was drilled to 394 feet
`KB. Fresh water was encountered at 75 feet
`KB.
`
`Nine joints of 13 3/8 inches, 54.5 Ibs/ft,
`ST&C casing was run and cemented at
`381.75 feet KB (371.85 feet GL) to isolate
`fresh water zones as required by the State
`of West Virginia.
`
`The 12 1/4-inch intermediate hole was
`drilled to 2,010 feet using three bits.
`Water was encountered and the hole was
`soaped after the first bit run at
`1,359 feet.
`
`The intermediate casing was run and set at
`1,988.90 feet KB below the Big Injun. The
`
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`SPE 21264
`
`A. B. YOST II
`
`intermediate casing was 9 5/8 inches,
`36 lbs/ft, J-55, ST&C.
`
`After waiting on cement 12 hours, the
`13 3/8-inch casing was cut off and welded to
`the 9 5/B-inch for support. The BOP's were
`nippled up and the cAsing drilled out with
`an B 3/4-inch bit to the kickoff point of
`2,150 feet.
`
`Angle Build Section
`
`At a measured depth of 2,150 feet, a direc(cid:173)
`tional survey was run prior to kicking tlw
`well off. The survey was 1', N60"E at a
`measured depth of 2,097 feet. The drilling
`assembly was pulled and the 8.6'/100 feet
`motor system was run to start building
`inclination. The motor was oriented using
`wireline steering tool and a side entry sub.
`
`The first motor run was terminated at
`2,362 feet because of the slow penetration
`rate. The penetration rate would vary
`between 30 and 3 feet per hour. The motor
`was drilling less than 3 feet per hour when
`it was pulled.
`Inspection of the bit and
`motor indicated that thRre was no problem
`with either. The slow penetration rates
`were due to changes in lithology from shale
`to the slower drilling siltstones.
`
`The fixed bend motor was designed to build
`inclination at 8.6' /100 feet. The actual
`build rate was 10.5'/100 feet which was more
`than the required build rate. The 2"
`adjustable bent housing motor was run to
`reduce the build rate to the planned rate of
`8.6' /100 feet. This motor actually built at
`a rate of 9.5' /100 feet.
`
`The penetration rate for the adjustable
`motor was comparable to the fixed bend motor
`(9.B feet/hr and 10.6 feet/hr, respec(cid:173)
`tively). Because of the low penetration
`rates and the fact that the inclination was
`ahead of schedule, the tangent section was
`started early. Siltstone stringers were
`causing the low penetration rate. After the
`tangent section, very little siltstone
`stringers would be left and motor penetra(cid:173)
`tion rates were expected to be much higher.
`
`The tangent section was started at
`2,548 feet MD.
`The motor-drilled hole was
`reamed with the same assembly that would be
`used to drill the tangent section in order
`to avoid making an additional trip. After
`the directional hole was reamed, the hole
`was dried out and dusted to 2,B11 feet MD.
`At this point, an estimated build rate of
`9.5'/100 feet would be needed to hit the
`target of 2,965 feet TVD.
`
`The adjustable bend motor was used to drill
`the remainder of the build section. The 2'
`adjustable bent housing motor drilled from
`2,811 feet to 3,165 feet at an average pene(cid:173)
`tration rate of 18.2 ft/hr. Only one
`siltstone stringer was encountered during
`the motor run. The motor built inclination
`at an average rate of 9'/100 feet.
`
`The estimated inclination at the end of the
`build section was 68'. The drilljng plan
`had called fOl stopping the build section
`at 71". However, the build rate experi(cid:173)
`enced with the rotary assembly in the
`tangent section was 1.34'/100 feet. The
`slant hole was supposed to be built at
`0.5"/100 feet. Since the slant hole was at
`a higher inclination in a softer formation,
`it was assumed that the rotary assembly
`would build at a lower rate between 0.5 aud
`1'/100 feet. This rate would be slightly
`higher than the planned rate; therefore,
`the slant section was started at a lower
`inclination to compensate for the antici(cid:173)
`pated higher build rates.
`
`Slant Section
`
`The slant section was drilled from
`3,165 feet to a total depth of 4,833 feet.
`Basically, the rotary assembly that was
`used to drill the tangent section was also
`used to drill the slant section. On the
`first run, the rotary assembly drilled to a
`measured depth of 3,785 feet. The rotary
`assembly had started walking 1.5°/100 feet
`to the left. At thiR rate, the wellbore
`would be too close to the lease line at the
`end of the target interval. Since the
`assembly had walked to the right in tangent
`section, it was assumed that it would walk
`the same in the slant section. Therefore,
`no right-hand lead was set at the end of
`the build section.
`
`The 2" adjustable bent housing motor was
`used to turn the wellbore back to the right
`with the tool face set 70· to the right of
`high side. After drilling only 70 feet (to
`3,B55 feet), the steering tool indicated
`that the motor was not only turning the
`wellbore to the right but it was also drop(cid:173)
`ping inclination rapidly. The motor was
`pulled to prevent the wellbore from
`dropping too much inclination.
`
`Initially, the key in the mule shoe sub was
`thought to have been knocked out with the
`steering tool. When trying to seat the
`steering tool, it continually stopped at
`the top of the drill collars (change in
`internal diameter). To overcome the prob(cid:173)
`lem, the steering tool was run at a higher
`velocity than normal. Later inspection of
`the mule shoe sub in the daylight showed
`that there was no damage to the key. The
`tool was also seated properly because the
`weight indicator showed that the latch in
`assembly pins sheared when the tool was
`pulled from the hole.
`
`The exact reason the motor assembly dropped
`inclination was later found to be related
`to loose set screws in the adjustable bent
`housing causing misalignment of the motor
`bend.
`
`Directional surveys showed the motor had
`dropped 7° and turned 6' to the right in
`the 70 feet that it had drilled. Because
`of problems associated with hole cleaning
`
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`4
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`AIR DRILLING AND MULTIPLE HYDRAULIC FRACTURING OF A 72° SLANT WELL IN DEVONIAN SHALE
`
`SPE 21264
`
`using air mist through the motors and bud(cid:173)
`getary constraints, additional motor runs to
`bring the inclination back up were aborted.
`
`The motor-drilled hole was reamed with a
`rotating bottomhole assembly. One (1) day
`was spent circulating to clean the hole and
`drying out the hole before rotary drilling
`continued to TD of 4,833 feet.
`
`The lower inclination experienced after the
`motor run caused the wellbore to drop out of
`the target zone earlier than had been
`planned. The wellbore dropped below a TVD
`of 3,400 feet at approximately 4,400 feet
`MD. Drilling continued until no more shows
`were encountered. The last mud log show was
`at 4,674 feet MD. Total measured depth was
`4,833 feet at a TVD of 3,571 feet. The plan
`view and vertical view can be seen in
`Figures 6 and 7, respectively.
`
`After logging, 113 joints of 5 1/2-inch,
`17 Ibs, N-80, LT&C casing was run and
`cemented at 4,787 feet KB. A total of
`60 rigid centralizers (manufactured by
`Gemeco) were run with a spacing of one
`centralizer per joint. The top centralizer
`was at 2,316 feet.
`h latch down collar was
`placed in the top of the first joint.
`
`MOTOR PERFORMANCE, BOTTOMHOLE ASSEMBLIES,
`AND HOLE CLEANING
`
`The first motor to be run was a fixed double
`bend motor with two stabilizers. The motor
`drilled from 2,150 feet to 2,363 feet
`(212 feet) in 20 hours and was pulled
`because of low penetration rate. The aver(cid:173)
`age penetration rate was 10.6 ft/hr with a
`low of 3 ft/hr and a high of 20 ft/hr.
`Originally, it was thought that there was a
`problem with the motor or the bit, but the
`low penetration rate was caused by siltstone
`stringers in the formation.
`
`For a period of time, the fixed bend motor
`was run at 1,600 scfm with 25 barrels per
`hour (BPH) of mist. The rig's mist pump
`was not capable of pumping less than 25 BPH
`at the required pressure. The standpipe
`pressure, while pumping 25 BPH, was approxi(cid:173)
`mately 800 psi. A replacement pump was
`rigged up and the mist rate dropped to
`10 BPH. The standpipe pressure fell to
`approximately 600 psi with the same air
`volume. The air flow rate was measured
`continuously to ensure proper motor operat(cid:173)
`ing conditions and to maintain adequate hole
`cleaning.
`
`The equivalent flow rate through the motor
`while drilling was 340 gallons per minute.
`There were no problems running the motor and
`no repairs were necessary. The motor was
`functioning properly when pulled.
`
`The averaged dogleg severity generated by
`the fixed bend motor was 10.5°/100 feet.
`The system was designed to build at 8.6°/
`100 feet. Because of the higher than
`
`anticipated build rate, the adjustable bent
`housing motor was run to reduce build rate.
`
`The adjustable bent housing motor had the
`bend in the motor set at 2°. The motor
`drilled from 2,362 feet to 2,548 feet
`(186 feet) in 19 hours and was pulled
`because of low penetration rate. The aver(cid:173)
`age penetration rate was 9.8 ft/hr with a
`low of 2 ft/hr and a high of 20 ft/hr.
`Again, the low penetration rate was due to
`formation effects.
`
`The motor was run with a little over
`1,600 scfm and 10 BPH of mist. The equiva(cid:173)
`lent flow rate through the motor was calcu(cid:173)
`lated to be 346 gpm. No problems were
`experienced in running the motor and the
`motor was still functioning properly when
`it was pulled.
`
`The adjustable bent housing motor created
`an average dogleg severity of 9.5°/
`100 feet. A longer tangent section was
`planned to compensate for the higher build
`rate.
`
`The third motor run was made with the same
`adjustable bent housing motor with the
`bend set at 2°. The motor drilled from
`2,811 feet to 3,165 feet (354 feet) in
`19.5 hours. The average penetration rate
`was 18.2 ft/hr. There was only one short
`section that drilled at less than 3 ft/hr.
`The rest of the motor run drilled at
`approximately 20 ft/hr.
`
`The motor was pulled because the build
`section had been completed. There were nc
`problems running the motor and the motor
`was functioning properly when pulled.
`
`The fourth motor run was made to make a
`direction change in the slant section of
`the hole. Again, the same adjustable bent
`housing motor was run with the bend set at
`2 0
`The motor drilled from 3,785 feet to
`3,855 feet (70 feet) in 4 hours. The aver(cid:173)
`age penetration rate was 17.5 ft/hr. The
`motor was pulled because the hole had
`unexpectedly started dropping inclination.
`
`•
`
`The flow rate through the motor was
`1,400 scfm and 15 BPH of mist. The mist
`rate had been increased to aid in hole
`cleaning. The standpipe pressure was
`470 psi and the equivalent flow rate
`through the motor was 305 gpm.
`
`No problems were encountered in running
`both the fixed bend and the adjustable bent
`housing motors. The motors started shortly
`after the air was turned into the drill
`pipe even after connections. The motors
`did stall several times but it was due to
`excess weight being placed on the motor. A
`maximum weight of 12,000 Ibs was run on the
`motors while drilling the harder siltstones
`at less than 4 feet per hour. The maximum
`weight that could be run on the motors
`while drilling shale at 20 ft/hr was
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`SPE 21264
`
`A. B. YOST II
`
`5
`
`approximately 6,000 Ibs before the motor
`would stall.
`
`Neither the adjustable bent housing motor
`nor the fixed bend motor showed any exces(cid:173)
`sive wear when torn down and inspected. The
`fixed bend motor drilled for 20 hours and
`the adjustable bent housing motor drilled
`for a total of 42.5 hours in three runs.
`The adjustable bent housing motor also had
`an additional 11.25 hours circulating off
`bottom to clean the hole. Most of that time
`(8.75 hours) was at the end of the last run
`in the slant section. Total hours (circu(cid:173)
`lating and drilling) on the adjustable bent
`housing motor was 53.75 hours.
`
`One directional, rotary assembly was used to
`drill the Boggs No. 1240. The assembly
`consisted of a bit, float sub, six-point
`reamer, and two nonmagnetic drill collars.
`The float sub was run below the six point on
`BHA Nos. 4 and 9, and above the six point on
`BHA Nos. 6 and 7. Other than that, the
`bottomhole assemblies were the same.
`
`The tangent section was drilled with the
`bottomhole assembly that was planned to be
`used in the slant section. The intention
`was to collect data that could be used to
`fine tune the assembly for the slant section
`and to compensate for bit walk.
`
`The directional rotary assembly drilled the
`tangent section from 2,548 to 2,811 feet
`(263 feet). The average penetration rate
`was 45.7 ft/hr with 25,000 Ibs of bit weight
`and 80 rpm. The assembly built 1.34°/
`100 feet and walked 0.30°/100 feet to the
`right as calculated from the surveys between
`2,553 and 2,799 feet.
`
`The planned build rate in the slant section
`was 0.5°/100 feet.
`It was assumed that the
`assembly would build slower at the higher
`inclinations in the slant section but not as
`slow as 0.5°/100 feet. Therefore, the float
`sub was also moved to the top of the six
`point to reduce the building tendency.
`
`The directional program used indicated the
`assembly would still build inclination at
`approximately 0.70°/100 feet. To compensate
`for the higher build rate, the build section
`was stopped at 68° instead of the planned
`71" .
`
`Bottomhole assembly No. 6 twisted off while
`reaming the motor drilled hole. Bottomhole
`assembly No. 7 (with the float sub above the
`six point) drilled from 3,165 to 3,785 feet
`(620 feet). The average penetration rate
`was 33.5 ft/hr with 20,000 Ibs bit weight
`and 70 rpm. The assembly was pulled because
`it was walking to the left and the well
`would have ended up too close to the lease
`line. A motor was run to turn the well back
`to the right.
`
`The rotary assembly built inclination at a
`rate of 0.85° /100 feet and walked 1.00" /
`100 feet to the left. The build rate was
`
`close to the predicted value; however, the
`bit walked in the opposite direction of
`that predicted in the tangent section.
`
`Bottomhole assembly No. 9 was run after the
`motor run. The motor had unexpectedly
`dropped inclination so the float sub was
`placed below the six point to increase
`build tendency. The assembly drilled from
`3,855 to 4,833 feet (978 feet). The aver(cid:173)
`age penetration rate was 29.0 ft/hr with
`20,000 lbs bit weigQt and 60 to 70 rpm.
`
`The assembly built inclination at a rate of
`0.45°/100 feet and walked 0.89°/100 feet to
`the left. Apparently, the float sub below
`the bit actually slowed the build rate.
`
`Hole cleaning was the primary problem in
`drilling the Boggs No. 1240.
`It was
`expected that cleaning would be a problem
`when misting with the motors at high incli(cid:173)
`nations. When dry air is used, cleaning is
`generally not a problem. To alleviate hole
`cleaning problems, the well was drilled
`with dry air as much as possible during the
`tangent section and after the second build
`curve.
`
`Initially, stiff foam was pumped at a rate
`of 10 BPH along with 1,768 scfm of air
`and no hole cleaning problems were noted
`until the end of the third motor run at
`3,165 feet. There was no problem while
`drilling, but there were problems pulling
`the pipe out of the hole during a trip. A
`total of 4.75 hours were spent circulating
`and pulling pipe until it came free. The
`inclination at the end of the build section
`was 67". The pipe came free above an
`inclination of 45".
`
`The next motor run was made at 3,785 feet
`and the inclination was 72°. Again, there
`were problem pulling the motor out of the
`hole after drilling only 70 feet. The hole
`became tight after pulling only two stand
`of pipe (124 feet). A total of 7.5 hours
`were spent circulating to clean the hole
`and pulling pipe while circulating.
`
`The air rate for this motor run was
`1,400 scfm and the foam rate was 15 BPH.
`The foam rate had been increased to aid in
`hole cleaning.
`
`After the motor was pulled, the rotary
`assembly was run back in the hole. This
`time it was difficult to dry the well out.
`A total of 24 hours were spent trying to
`clean the well with foam and then dry it
`out. The daily reports dated July 27,
`1979, and July 28, 1989, detail the
`efforts.
`Increasing the air rate to
`2,300 scfm finally dried the air well out
`and drilling continued to TD.
`
`The 2,300 scfm air rate was still a minimum
`required to clean the slant section of the
`well. Though no problems were encountered
`while drilling, a downhole TV camera showed
`
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`6
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`AIR DRILLING AND MULTIPLE HYDRAULIC FRACTURING OF A 72 0 SLANT WELL IN DEVONIAN SHALE
`
`SPE 21264
`
`there was still a significant amount of dust
`in the well.
`
`All indications are that the stiff foam
`accomplished very little over using a simple
`mist. The obvious solution to the hole
`cleaning problem with mist is to use higher
`air volumes. Unfortunately, higher air
`volumes will be detrimental to the life of
`downhole motors unless R partial bypass can
`be devised.
`
`Hole cleaning is a problem because of the
`additional time required (and, therefore,
`cost) and the.possibility of sticking the
`drill string. To prevent the latter( addi(cid:173)
`tional float valves were placed in the drill
`string. A hammer float was placed on top of
`the bit with rotary and downhole motor
`assemblies. A second float valve was placed
`in the float sub. With motor runs ( a third
`float was placed above the side entry sub to
`keep the drill string exhausting through the
`motor during connections.
`
`STIMULATION
`
`Four sAparate hydraulic fracturing treat(cid:173)
`ments were conducted in the Bogg No. 1240
`slant well. A pre-frac nitrogen breakdown
`was conducted on the bottom zone to deter(cid:173)
`mine pre-frac reservoir properties. Stimu(cid:173)
`lation intervals ranged from 6 feet to
`570 feet along the slant section. All
`intervals were treated with straight nitro(cid:173)
`gen treatments which were the fracturing
`methods used on vertical wells in the field.
`Nitrogen volumes varied from 1(170 to
`1,800 million standard cubic feet (MSCF).
`Typical nitrogen injection rates were
`relatively uniform between 70(000 and
`82(000 standard cubic feet per minute (SCFM)
`among zones. Each zone was broken down with
`500 gallons of 7 1/2 percent acid ahead of
`the nitrogen treatments. A retrievable
`bridge plug was used to isolate individual
`stages of fracturing. Perforating was
`performed using wireline conveyed hollow
`carrier guns which were conveyed with a
`specially designated set of rollers to
`enhance the ability to move the perforating
`guns opposite the interval of interest. A
`summary of stimulation treatments is pro(cid:173)
`vided in Table 1.
`
`Production from each stimulation was mea(cid:173)
`sured after the nitrogen contamination was
`less than 10 percent. An orifice runway and
`back pressure regulator were used to simu(cid:173)
`late line pressures of 40 to 45 psig and
`flow rates were monitored against line pres(cid:173)
`sure for several weeks in order to establish
`the stabilized flow rates for each zone.
`Stabilized flow rates ranged from 30 to
`80 thousand cubic feet per day (MCFD) with a
`total flow rate of 195 MCFD against 45 psig
`for the entire well.
`
`ECONOMICS
`
`Production histories were evaluated for
`36 wells surrounding the slant well. The
`
`average open flow for vertical wells was
`450 MCFD compared to 1,400 MCFD for the
`slant well which represents an improvement
`ratio of 3:1.
`Initial open flow data for
`vertical wells in the field is shown in
`Figure 8. Vertical well flowed an average
`of 64 MCFD for the first year while the
`slant well was projected at 195 MCFD which
`represents a 3:0 improvement ratio.
`
`A review of the ultimate gas recoveries for
`vertical wells in the area indicates recov(cid:173)
`erable gas ranging from 20 to 350 million
`cubic feet (MMCF) as shown in Figure 9.
`Average vertical wells only produce
`100 MMCF of ultimate recovery. Using cur(cid:173)
`rent cost of drilling a vertical well at
`$180,000 and a slant well at $500(000 point
`requires ultimate recoveries to be 150 MMCF
`and 500 MMCF, respectively, as shown in
`Figure 10. Therefore, the three-fold
`increase in production will simply offset
`the 2.7 fold increase in cost and until the
`cost of slant well directional drilling
`technology using air can be reduced to less
`than twice the cost of vertical wells(
`improved economic advantage cannot be
`realized.
`
`CONCLUSIONS
`
`1. Site selection remains a critical
`factor in establishing the benefits of
`slant well drilling.
`
`2. This project demonstrated for the first
`time that positive displacement down(cid:173)
`hole motors can be operated with air
`for more than 50 hours without failure.
`
`3. Downhole motors are extremely sensitive
`to formation hardness (sand versus
`shale) using air to operate them.
`Rates of penetration in sandstones
`using motors were up to an order of
`magnitude slower than shales. Tech(cid:173)
`niques to improve drilling rate of
`penetration will be required for hard
`sandstone/limestone formations.
`
`4. Slant well drilling can improve produc(cid:173)
`tion three-fold, but the cost of the
`technology must be reduced to offer
`economic advantage.
`
`5. Hole cleaning is a major problem when
`using mist while operating downhole
`motors on air.
`
`ACKNOWLEDGEMENTS
`
`The authors wish to thank Frank J. Schuh of
`Drilling Technology, Inc., for his support
`and advice on improving drilling efficiency
`during this slant well project.
`
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`SPE 21264
`
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`
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`
`7
`
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`
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