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SPE-171024-MS
Unique System for Underbalanced Drilling Using Air in the
Marcellus Shale
Chris Maranuk, Ali Rodriguez, Joe Trapasso, and Joshua Watson,
Weatherford
Copyright 2014, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Eastern
Regional Meeting held in Charleston, WV, USA, 2123 October
2014.
This paper was selected for presentation by an SPE program
committee following review of information contained in an abstract
submitted by the author(s). Contentsof the paper have not been
reviewed by the Society of Petroleum Engineers and are subject to
correction by the author(s). The material does not necessarily
reflectany position of the Society of Petroleum Engineers, its
officers, or members. Electronic reproduction, distribution, or
storage of any part of this paper without the writtenconsent of the
Society of Petroleum Engineers is prohibited. Permission to
reproduce in print is restricted to an abstract of not more than
300 words; illustrations maynot be copied. The abstract must
contain conspicuous acknowledgment of SPE copyright.
Abstract
Underbalanced drilling offers significant advantages in terms of
increased rate of penetration (ROP), lessformation damage, reduced
lost circulation material, decreased cost of cuttings disposal, and
increasedproduction. Underbalanced drilling injects gas into a mud
column to lower the overall equivalent mudweight to create a
drilling environment where the pressure in the wellbore is kept
lower than the fluidpressure in the formation being drilled. Air is
the ultimate underbalanced fluid, but diminishes theefficiencies of
mud motors, and prevents the use of mud pulse telemetry MWD tools
due to the lack ofan incompressible fluid. With air drilling, the
only fluid injected into the well is a small amount of oilneeded to
prevent corrosion. Downhole mechanical forces are usually more
violent due to the lack of afluid column for dampening as well as
the higher air volumes going through the bottom hole assembly(BHA)
for cuttings flow. Common drilling technologies to address air
drilling include ElectromagneticTelemetry (EM), mud motors, and
downhole air hammers, but reliability issues are particularly
prevalent,especially for the EM MWD tools and downhole mud
motors.
Air drilling has become popular especially in the Marcellus and
Utica shale reservoirs in the NortheastUnited States because of
higher ROP and less formation damage. As an example, of the 111
rigs drillingin the Marcellus Shale, 27 rigs are drilling
underbalanced and 23 are being drilled with air. A uniquedrilling
system incorporating the use of downhole mud motors, EM MWD, and
air hammers has beenspecifically designed and ruggedized to address
downhole shock and vibration encountered in air drilling.Use of
this system has resulted in significant reduction of non-productive
time (NPT) while drilling withair.
This paper will describe how air drilling is being successfully
utilized in the unconventional reservoirof the Marcellus shale in
the Northeast United States. Drilling fluids and their affect on
various pressureregimes will be discussed. The new drilling system
will be described and drilling parameters highlightingthe
differences between mud and air drilling will be provided.
Modifications to the BHA to increasereliability will be discussed,
and success metrics presented.
IntroductionThe ability to drill wells faster in the Northeast
United States is critical for well profitability. Whileseeking
alternatives to increase ROP and reduce drilling costs, a few
operators implemented batch drillingpractices for pad locations.
This provides an attractive alternative by allowing multiple wells
to share the
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same surface location effectively reducing footprint and
environmental impact. Common well designutilizes streamline well
construction where low costs rigs can drill the top hole sections
and larger, moreexpensive rigs drill the curve and lateral
sections. The typical well plan incorporates surface,
intermediate,curve, and lateral sections that combined may exceed
18,000 feet. The lateral sections are the most criticaland range
between 2,000 and 8,000 feet depending on formation and well
geometry. The goal for closeproximity well design is to minimize
well to well interference and maximize reservoir exposure.
Air drilling provides a significant decrease in hydrostatic
pressure over common mud types resultingin an increased ROP.
Additionally, significantly better hole cleaning can be achieved
resulting from thehigh air velocities used to drill the well.
Finally, mud and cuttings handling costs can be reduced sincethere
are no chemicals to absorb and no cuttings cleaning requirements on
virgin formation. Early testsof this application proved it as a
viable option for the Marcellus and the Utica fields. Initially,
hammerbits were used for air drilling but significant challenges
involving directional control emerged as well plantrajectories
became more advanced. The development of ruggedized mud motors and
MWD tools capableof handling these challenges, and the use of
specialized fluid control systems eventually allowed for
moreconventional BHAs to be successful when used for air
drilling.
Types of Fluid RegimesDrilling fluid and its circulation system
are utilized to clean the borehole, stabilize rock, control
pressures,and enhance drilling rates in all phases of a drilling
program. Drilling fluids allow for sufficient cooling,lubrication,
cuttings removal, and adequate transference of hydraulic energy to
the bit and other downholetools. Though rheology varies,
circulation systems focus on operating under specific pressure
ranges inrelation to formation and reservoir pore pressures. Figure
1 illustrates how various fluid systems areapplied over the range
of formation pore pressure gradients.
Overbalanced DrillingOverbalanced drilling is the state where
the hydrostatic pressure of the fluid column exceeds the
porepressure of the formation. Operating overbalanced is the safest
and most common status of a well in
Figure 1Types of fluids used to address various pore pressure
regimes.
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typical drilling conditions. The hydrostatic pressure exerted by
the mud column functions as the primarywell control device. This
condition is also the most diverse as it may be achieved with a
variety of fluidtypes and operating practices with no
modifications; however, running in an overbalanced condition
doesallow for some detrimental effects. For example, if the
hydrostatic pressure of the drilling fluid exceedsthe formation
fracture gradient, the operator would need to set casing and
continue to drill a smaller holesize. Fluid invasion and formation
damage may result in wellbore instability, leading to losses and
apossible kick scenario, and may also negatively affect
production.
Managed Pressure DrillingManaged pressure drilling is an
adaptive process where the hydrostatic pressure exerted on the
bottom ofthe hole is engineered for balanced differential pressure.
By maintaining drilling fluid pressures that equalthe pore pressure
of the exposed formation during drilling, operators are able to
successfully mitigatesome of the detrimental effects found in an
overbalanced state such as wellbore instability,
lost-circulationzones, over/under pressurized formations, and
shallow flows. These hazards may threaten the operationalviability
and ultimately the economic success of the well. Operations under
these conditions report anincreased ROP, prolonged bit life, and
enhanced drilling efficiency. Additionally, due to pressures
beingconstantly monitored and manipulated, flexibility is afforded
during the drilling process. As formationpore pressure changes from
rock to rock, the managed pressure drilling (MPD) process allows
forsuccessful mitigation and control of bottom hole pressures by
using a combination of tools, techniques,and controls that use
backpressure, fluid rheology, annular fluid level, circulating
friction, and wellgeometry to attain the desired pressure profile.
The methods of MPD consist of controlling bottom holepressure,
maintaining a pressurized mud cap, and utilizing returns flow
control. Successful implementa-tion may allow for flexibility in
the casing design resulting in the elimination of certain strings
leading tosignificant financial savings.
Underbalanced DrillingUnderbalanced drilling is the state where
the hydrostatic pressure exerted by the fluid column is less
thanthe pore pressure of the formation. A simple change of pore
pressure or equivalent fluid density mayunknowingly transition into
underbalanced drilling and represent a kick scenario for an
overbalancedsystem whereas, a constant state of transition exists
in MPD system. However, UBD systems are designedto operate under
these circumstances of constant fluid influx from the formation.
Since the operationalgoal is to maintain fluid density below pore
pressure, several fluid types may be utilized.
Two Phase and FoamWhile many drilling fluid systems are capable
of introducing an underbalanced state, intentional loweringof fluid
density is typically achieved through aeration. Low density
drilling fluids are broken downcoarsely into two phase or aerated
fluids and foams. Aerated fluids are defined by a 46:54 liquid to
gaspercent ratio. When this ratio is exceeded, 4-46:54-96, the
drilling fluid is considered foam. The aerationprocess may utilize
compressed air, natural gas, exhaust gas, or cryogenic or membrane
nitrogen to relievebottom hole pressure. Nitrogen is typically used
due to its low reactivity and lack of combustibility.Additionally,
the introduction of oxygen into an aerated fluid enhances corrosion
potentials and signifi-cantly increases risk for downhole fires.
Aeration may be achieved by an injection unit mixing gas withfluid
pumped down the pipe bore, injecting into the annulus via a
parasite string, or injecting into theannulus via concentric
casing. Aerated drilling fluid is compressible and significantly
attenuates signalfrom pulse based M/LWD tools when gas rates reach
around 10% of total flow. This situation rendersthese tools
incapable of transmitting adequate data to surface. As such, other
telemetry methods arerequired.
Foam is an aggregation of gas separated by liquid that may be
described as stiff, stable, or styrofoam.Foams allow for more
stable underbalanced circulation system than traditional two phase
systems. Theyhave sufficient viscosity properties both downhole and
at surface for successful solids removal. They pose
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no environmental risk because they are acid soluble and are not
affected by evaporates downhole.Additionally, foam systems have a
lower risk of borehole instability due to reduced annular
velocities.
Misting and Dry Air DrillingDry air drilling or dusting is the
utilization of 100% gas as the drilling fluid system. Air rates in
theNortheast United States typically range from 2,000 to up to
6,000 standard cubic feet per minute (scfm).Generally, injection of
5-8 gallons per hour of rock oil or hammer oil is used to keep the
bit, motor ordown-hole hammer lubricated and cool. While air
drilling requires significantly larger flow velocity forcarrying
capacity, it significantly increases ROP and bit life. In addition
to optimal hole cleaning,decreased costs, and maximized ROP,
dusting is able to maintain exceptional shale and clay control.
Airis considered to be the least expensive fluid for operations
since there is no cleanup or disposal for thefluid on surface. It
also drills faster than conventional fluid systems by three to four
times depending ondepth and rock strength (see Fig. 2 for a
comparison of fluid types vs. typical ROP for selected
boreholesizes).
Dusting is ideal for hammer operations but is susceptible to
fluid influx from the formation. Onceinflux has occurred, the fluid
must be switched over to a mist or foam. Liquid influx will result
in mudrings which limit hole cleaning and pose a significant risk
of pack off or stuck pipe. In misting conditions,the liquid to gas
percent ratio exceed 4:96. For operations in the Northeast United
States, liquid injectionrates range from 10 to 50 gallons per
minute (gpm) or higher and typically incorporates surfactants
andcorrosion inhibitors. The additional surfactants prevent the
buildup of the mud rings. Corresponding airinjection rates would
range from 1,000 to 5,000 scfm.
Misting has lower velocity requirements than dusting due to
liquid carrying capacity. ROP typicallyslows 30 to 50 percent in
the transition from air to mist due to increased annular pressure.
Large liquidinflux from the formation would result in rolling back
to a foam or two phase system. In summary, the
Figure 2Typical ROPs of fluid vs. air drilling in 12.25 and 8.75
hole sizes.
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major disadvantages of using air for drilling are its limitation
to handle fluid influx, the reduction of carrycapacity compared to
foam and other normal mud regimes, and the increased flow
velocities required toensure adequate cuttings removal.
Figure 3 demonstrates the trade-off between carry capacity and
flow velocities required for selectiveunderbalanced drilling fluid
types.
Why Use an Air Drilling System?Directional drilling in a dry air
application is widely used in the northeast United States for top
holesections of wells. The benefits previously discussed become
compounded as the batch drilling processbecomes more commonplace.
The reduction in drilling days, drilling fluid costs, and cuttings
handlingexpenses make batch drilling with air economically more
viable.
The simplest application of conventional air drilling is used
for non-directional applications andinvolves nothing more than a
tri-cone or polycrystalline diamond cutter (PDC) bit. Other
straight hole ornon-directional applications use downhole air
hammers or straight housing air motors. These non-directional
assemblies can be run in several different BHA configurations
ranging anywhere from asemi-stabilized, fully stabilized or slick
configuration. The most common method of straight hole airdrilling
is to stack the BHA with additional drill collars to create a heavy
vertical hanging effect that helpsmaintain a vertical well
bore.
Figure 3Types of aerated fluids used for underbalanced drilling.
Carrying capacity increases from left to right. Fluid velocity
requirements increasefrom right to left.
SPE-171024-MS 5
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A motorized air hammer BHA configuration is not as widely used
but is just as effective. Thisconfiguration consists of an air
hammer position at the end of a bent air motor. This BHA is run
with anEM MWD tool and a shock sub. An air by-pass sub is used in
this configuration to divert a certainpercentage of air above the
motor preventing overspinning (see Fig. 4). This added governor
aids inhammer control and increases the longevity of a hammer bit.
While directional control is possible in thisconfiguration care
must be taken when selecting not only the bit to bend distance but
also the bend angleof the motor. Too much angle does inhibit the
flat striking impact of the hammer and can cause damageto the
hammer bit shortening its life. In addition to maintaining a
vertical hole, the heavy BHA mentionedpreviously maintains a
downward impact of an air hammer preventing the BHA from
bouncing.
The most common and reliable air directional BHA is a bent
housing air motor with a tri-cone bit orPDC bit. This BHA is used
with an EM MWD system above the motor with a shock sub below (see
Fig.5).
Air Drilling SystemAn EM MWD telemetry system offers several
advantages over standard mud pulse telemetry systems andis the
preferred method of transmitting data from a downhole tool to
surface during underbalancedoperations. The key advantage over mud
pulse tools is that it can be used with compressible fluids, suchas
aerated fluids or air. However, the use of air as a drilling fluid
is particularly challenging for any MWDtool and mud motor due to
the extreme levels of axial and cross-axial vibrations generated by
the lack ofa liquid fluid regime.
Figure 4Typical Directional Air Drilling Bottom Hole Assembly
(Option No. 1). This figure shows a typical directional BHA for dry
air drillingwith a mud motor, EM MWD and a hammer bit
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A few operators recognized the potential benefit of drilling
with dry air to increase ROP in the area.But due to congested pads
with a great number of wells, the need for directional measurement
and controlwas recognized. Unfortunately, these early adaptors
experienced a high number of failures when drillingwith MWD and mud
motors due to the harsh air drilling environment.
To mitigate damaging vibration affects, studies were conducted
to identify BHA changes and opera-tional practices to improve
reliability of the air drilling system. A number of common
operationalpractices were identified as exasperating lateral shocks
and vibration. Implemented procedures limited offbottom rotation,
initiated staging compressors, and stopped the practice of drilling
off weight-on-bit(WOB). While differences were seen between BHA
configurations, it became best practice to remove allstring
stabilizers and utilize a inch undergage stabilizer on a bent
housing motor with a bend setting nogreater than 1.5 degrees.
Unfortunately, the impact on service reliability was not as
substantial asanticipated. As such, a new system was required to
withstand these conditions and operate without failuresdownhole.
This upgrade included a modified air drilling motor, a ruggedized
EM-MWD tool, a shock suband in certain instances a fluid by-pass
sub.
The EM MWD tool was ruggedized utilizing more robust electronics
and alternately designed shockabsorbers to mitigate vibration. The
MWD mounting technique was converted to hold the tool in
tensionwhile using redesigned centralizers that allowed other parts
of the tool to move with the vibration instead
Figure 5Typical Directional Air Drilling Bottom Hole Assembly
(Option No. 2). This figure shows a typical directional BHA for dry
air drillingwith a mud motor, EM MWD and a tri-cone bit.
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of trying to eliminate it. The placement of the centralizers
were modeled and engineered to eliminatedrilling harmonics that
could cause damage to the MWD tool. The antenna of the EM tool
wasre-engineered to withstand the high levels of vibration
generated by the air injection. The overall lengthwas increased by
over four times to successfully dampen damaging vibration and
prevent concentrationon critical, small cross-section parts. The
high rate lithium batteries were extensively lab tested lab andthen
modified to qualify them to drill in this environment.
The drilling motor was modified for the air environment by
making modifications that successfullyreduced the necessary
lubrication for the bearing pack. With the changes, the motor only
requires 5 gallonsof oil per hour to successfully extend its
operating life. Additionally, a series of self-lubricating
dynamicsleeves were incorporated to support side loading and
reinforce the bearing pack leading to addedlongevity and better
performance.
A shock sub was designed and utilized as a vibration dampener
within the BHA. The new design is ableto absorb a large quantity of
both axial and lateral vibrations generated by the mud motor. The
toolsuccessfully dampens high frequency vibration that induces
excessive shock to the MWD electronics andsensors.
Figure 6Spider plot of of the case study pad near Washington
County in Western Pennsylvania.
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When air hammers are employed, a by-pass sub is used to minimize
the amount of air pumped throughthe BHA. This by-pass sub helps
protect the hammer bits, which are sensitive to the higher air flow
ratesrequired for successful hole cleaning.
Once the engineering phase of the project was completed, field
trials were initiated. The new systemhas since drilled over 400,000
feet with a success rate of greater than 98%.
Early Case StudyAn operator was drilling a series of wells near
Washington County in southwestern Pennsylvania. Thetarget reservoir
was the Marcellus Shale. Because this is a populated area, drilling
surface locations haveto be constructed as small as possible. In
order to make drilling econmonical, as many wells as possiblewere
needed to drill on a single pad. The case study pad was designed to
drill up to nine complex threedimensional wells (see Fig. 6).
A typical well plan includes drilling from the surface to about
800 feet and setting 13-3/8 casing. Fromsurface casing, a hammer
bit without directional measurements is used to drill to about
4,500 feet. Thedirectional air drilling system is used to drill
from about 4,500 feet to approximately 7,000 feet. Finally,a rotary
steerable tool is used to drill the curve and lateral. Wells
typically reach total depth at about14,500 feet (see Fig. 7).
Figure 7Section and plan views of Well 6 drilled with dry
air.
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The first five wells were drilled with a low angle nudge up to
about 30 degrees. The significance ofwell 6 is that this well used
the directional air drilling system to drill a complex three
directional well from4,600 feet to about 7,200 feet with an
inclination of over 62 degrees (see Fig. 8). Almost 70% of the
curvewas drilled using the directional air drilling system. This
would not have been possible in an airenvironment without the use
of this system. Previously, the customer would use an expensive RSS
sytemto drill this intermediate section. Once they picked up the
RSS, they had to change to a fluid based drillingsystem which
significantly increased mud cost and reduced ROP.
Well 6 was drilled with air at a rate of 3,800 scfm while
injecting 22 gpm water. The 2,565 foot sectionwas drilled in 34.33
hours. The profile of this well kicked off from zero degrees at
around 4,600 feet(measured depth), built a 30 degree tangent at a
60 degree azimuth, and then turned to a 340 degreeazimuth while
building the curve to 62 degrees. Through this section the MWD tool
was able to remainin its lowest power transmission setting,
maximizing tool battery life and the potential time it can
remaindown hole. Achieving this amount of build while drilling with
air cut the time spent drilling the curve toless than 12 hours. The
average drilling time to build a curve section using conventional
methods in theMarcellus takes between 21 and 26 hours.
Drilling plans include the use of the directional air drilling
system for wells 7 through 9 (at the timethis paper was published,
wells 7 through 9 were not drilled).
The objective of the drilling program was to reduce pad drilling
costs using the directional air drillingsystem. As stated earlier,
other benefits included the reduced rig, mud, and cuttings disposal
cost, betterhole cleaning and increased ROP.
Conclusion
1. A new directional air drilling system has been designed that
allows operators to reliably drill wellsusing air as a drilling
fluid.
2. The new system has two basic configurations: one that uses an
air hammer and the other uses anair bent housing motor and a
standard bit.
3. EM MWD is critical for obtaining directional and formation
evaluation data in the air drillingenvironment.
4. Air drilling significantly improves hole cleaning due to the
high volumes of air needed to removecuttings.
5. Use of the directional air drilling system decreases
hydrostatic column to a minimum which resultsin an increase in ROP
over standard fluid based drilling systems.
Figure 8Footage drilled vs. inclination for the 6 wells drilled
to date on the case study pad.
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6. Introduction of this technology has allowed the drilling
program to be more economical due toincreased ROP, reduction in mud
cost, less cuttings disposal, and the use of economical rigs.
AcknowledgementsThe authors wish to thank the Weatherford
management team for allowing us to publish and present thispaper.
We wish to thank the rig crew and service specialists who ran the
direction air drilling system andcollected field data for
analysis.
Nomenclature
BHA Bottom Hole AssemblyEM ElectromagneticEM MWD Electromagnetic
MWDgpm Gallons per MinuteMWD Measurement-While-DrillingMPD Managed
Pressure DrillingM/LWD Measurement or Logging-While-DrillingNPT
Nonproductive TimePDC Polycrystalline Diamond CutterROP Rate of
PenetrationRSS Rotary Steerable Systemscfm Standard Cubic Feet per
MinuteTD Total DepthUBD Underbalanced DrillingWOB Weight-on-Bit
SPE-171024-MS 11
Unique System for Underbalanced Drilling Using Air in the
Marcellus ShaleIntroductionTypes of Fluid RegimesOverbalanced
DrillingManaged Pressure DrillingUnderbalanced DrillingTwo Phase
and FoamMisting and Dry Air Drilling
Why Use an Air Drilling System?Air Drilling SystemEarly Case
StudyConclusion
Acknowledgements
