PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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The Majority of Problems in
Drilling are Somehow Related
to the Mud Program
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Under-Balanced Drilling
Underbalanced Drilling is a drilling technique
where the hydrostatic pressure of the drilling
fluid is less than the formation pressure.
Formation fluids flow into the wellbore during
underbalanced drilling.
Under-Balanced Drilling is also referred to as
Managed-Pressure Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
• Overbalanced condition:
𝑃𝑓 < 𝑃hyd
• At balanced condition:
𝑃𝑓 = 𝑃hyd
• Underbalanced condition:
𝑃𝑓 > 𝑃hyd
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
UBD Techniques
•
•
•
•
Light-weight Drilling Fluids
Gas Injection down the drillpipe
Annular Gas Injection via parasite string
Foam or aerated drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Light-weight Drilling Fluid Technique
• Light-weight drilling fluids such as water (freshwater or
saltwater), crude oil, diesel are the most common techniques
to achieve UBD.
• In normally pressured areas with 8.65 ppg pressure
gradients, it may not be possible to reduce the hydrostatic
pressure to be less than formation pressure.
• Using MW less than formation pressure is a very common
UBD technique.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Gas Injection down the Drill Pipe
This technique requires compressors and Underbalanced
Drilling (UBD) is achieved by injecting gas into the mud
discharge line and aerate the mud. Air and nitrogen can be
used to aerate the mud and reduce its density.
Introducing air or oxygen into a hydrocarbon environment
can cause fire and corrosion issues.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Annular Gas Injection
The annular drilling fluid can be aerated by running a
parasite string or spaghetti string into the annulus.
This procedure is more expensive than gas injection into the
Drill Pipe due to increased rig time to handle more pipe. It
does however maintain constant pressure on the bottom of
the hole.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Foam Drilling
Foam Drilling is a common technique for UBD. Nitrogen is
the gas used to create the foam.
These foams can be very stable and safe to use.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Advantages
• Increased penetration rate
• Elimination of Formation Damage due to mud filtrate
invasion.
• Reduced risk of differential sticking of pipe
• Minimized and/or eliminated lost circulation while drilling
• Improved formation evaluation
• Increased well productivity
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Advantages of Underbalanced Drilling
Increased Penetration Rate
Rate of penetration increases with reduced MW.
Rock removal is easier and faster with less hydrostatic
pressure at the bottom of the hole. The formation pressure is
greater than the hydrostatic pressure of the drilling fluid with
aerated drilling fluid.
In an underbalanced condition, the formation pressure actually
assists in rock removal from underneath the bit.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Advantages of Underbalanced Drilling
Elimination of Formation Damage due to
mud filtrate invasion
With the formation pressure greater than the wellbore pressure
of the drilling fluid, mud filtrate cannot flow into formation.
Instead, formation fluids flow into the wellbore.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Advantages of Underbalanced Drilling
Reduce the risk of Differential Sticking
Differential Sticking is caused by wellbore pressure that is
greater than formation pressure “pushing” the drill string
against the side of a permeable formation in the wellbore.
This cannot occur if the formation pressure is greater than the
pressure in the wellbore.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Advantages of Underbalanced Drilling
Minimize and / or Eliminate Lost Circulation
Issues While Drilling
Weak formations can break-down when the hydrostatic
pressure of drilling fluids exceed the fracture pressure of the
rock. This results in the drilling fluids flowing into that
formation instead of circulating back to surface with the
cuttings.
Aerated Drilling Fluids can be used to drill through Lost
Circulation formations.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Advantages of Underbalanced Drilling
Increased Well Productivity
This is related to Elimination of Formation Damage in that
some formations are “water sensitive” due to clays, minerals,
etc. Water from FWBM filtrate invasion can cause these
shales and minerals to swell which will cause a reduction in
porosity and permeability in the near-wellbore region.
Underbalanced drilling can prevent this from occurring by
preventing the water filtrate from contacting the formation.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Disadvantages
Safety – the well is essentially in a blow-out condition while
drilling underbalanced.
Wellbore instability problems – sloughing, caving, etc
Water influx issues – mud contamination
Downhole fires (if air is used to aerate the drilling fluid)
Directional / horizontal drilling equipment – more expensive
Economic factors - same
Environmental Issues regarding the disposal of oil and/or
saltwater contaminated mud.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Disadvantage of Underbalanced Drilling
Wellbore Instability Issues
With the formation pressure greater than the wellbore
pressure, Unconsolidated Formations can slough into the
wellbore and prevent further penetration as well as stuck pipe.
The solution to drilling with underbalanced fluid through
unconsolidated formations could require setting additional
casing strings to seal-off the sloughing formation.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Disadvantage of Underbalanced Drilling
Water Influx
Permeable water-bearing formations are able to flow into the
lower-pressure wellbore. If the rate of water influx is great
enough, air drilling or aerated drilling will be “drowned-out”
and conventional drilling fluids will be necessary to continue
drilling operations.
If UBD is being achieved by using lightweight drilling fluids
or reduced mud weight, the water influx most likely will be
salt water. This causes salt contamination and disposal issues
with the drilling mud.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Disadvantage of Underbalanced Drilling
Downhole Fires
Introducing oxygen into a hydrocarbon environment can result
in unpleasant situations including corrosion and downhole
fires.
Air is not a wise choice of gas to use when using gas injection
techniques to achieve UBD.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Disadvantage of Underbalanced Drilling
Directional and Horizontal Drilling
Air is compressible therefore MWD tools that rely on mud
pulse telemetry cannot be used.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Disadvantage of Underbalanced Drilling
Economic Factors
Directional and Horizontal UnderBalanced Drilling is usually
more expensive than vertical UnderBalance Drilling.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling
Safety Issues
Underbalanced drilling is working on a live well that is
constantly in a blow-out condition. Well control must be
maintained.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Good Candidates for UBD
• Pressure depleted formations
• Areas prone to differential pressure sticking
• Hard rock (dense, low permeability, low
porosity).
• “Crooked-hole” country and steeply dipping
formations
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Good Candidates for UBD
•
•
•
•
•
Fractured formations
Vuggy formations
High permeability formations
Highly variable formations
Once the optimum candidate has been identified, the
appropriate technique must be selected, based on
much of the same data required to pick the candidate
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Density of Various Drilling Fluids
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling Techniques
• Air Drilling Gases
Dry air
Nitrogen
Carbondioxide
Natural gas
• Aerated Fluids
Air-liquid mixtures (gasified liquids)
Mist
• Foams
• Emulsions
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling Fluids
6.7 ppg
5 ppg
8.33 ppg
1.7 ppg
10.0 ppg
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling Flow Patterns
LIQUID
AERATED
LIQUID
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
FOAM
MIST
GAS
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Underbalanced Drilling Equipment
• Rotating head
• Closed-pressurized surface system
• Separator
• Compressor
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Rotating Head
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Rotating Head
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling Equipment
Rotating Head
• A Rotating Head is used to permit drilling under pressure.
The Rotating Head has a rotating, steel reinforced rubber
element that seals around the drill pipe or kelly. A diverter
below the seal causes the returning drilling fluid to be
routed to either the mud return line or to the choke
manifold.
• The sealing elements rotate with the Drill Pipe or Kelly to
minimize wear
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling Equipment
Rotating Head
Iron Horse Tools, LLC
Iron Horse – 5
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Underbalanced Drilling Equipment
•
•
•
•
Closed-pressurized surface system
Separator
Compressor
MWD tools
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Air Drilling
• Air/Gas drilling (also called “dust drilling”) is a technique
used in areas where the formations are “dry” i.e., there is no
influx of water or liquid hydrocarbons.
• Also used to drill conductor hole / surface hole in areas with
thick, hard limestone caprock at the surface.
• This medium requires significant compressed gas volumes
to clean the well with average velocities of over 3,000 ft per
minute
• Air Drilling uses percussion type bits that pulverize the rock
and the recovery of samples for analysis is difficult, at best.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Air Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Air Drilling Applications
• The most common applications for Air Drilling are:
1. In hard, dry rock drilling where less than 15 ft/hr
ROP’s are obtained using conventional drilling
techniques.
2. Crooked hole problems.
3. Lost Circulation areas.
4. Water sensitive formations.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Air Drilling Disadvantages
• Disadvantages to Air Drilling include:
1. Air Drilling can only be conducted in relatively
“dry” rock. Water influx from formations can
“drown” circulation.
2. Downhole and surface fires due to adding oxygen
to hydrocarbon and creating an explosive mixture.
3. Sloughing due to the absence of hydrostatic
pressure to maintain wellbore stability.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Air Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Air Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Mist Drilling
• Mist is created by the addition of 6 to 30 bbl/hr of fluid to
the gas stream. The added fluids are typically water,
surfactant and inhibitors.
• The mist cleans, cools and lubricates the bit.
• Cuttings are transported to the surface as a mist or more
normally in a modified two phase flow.
• Mist Drilling has slower ROP than Air Drilling but higher
than conventional drilling.
• If excess water influx is encountered, it will be necessary to
change to either Foam, Aerated or Conventional Drilling
Techniques.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Foam Drilling
• Water, surfactants and gas are combined to create a stiff foam that is
used for the drilling fluid. The Cuttings-Carrying Capacity is up to 7
times greater than Air Drilling.
• Foam is the most versatile of the gas-generated systems
• Effective operating range from 0.2 to 0.6 s.g.
• Mixture of gas phase and foaming solution
• Foam flow varies with depth in the hole
• An adjustable effective BHP is possible.
• The disadvantage to Foam Drilling is that the returning foam requires
time and sometimes chemical breakers to settle. Large reserve pits
are required to contain the foam while it “breaks”.
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Foam Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Foam Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Foam Drilling
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Aerated Fluids
• Aerated Drilling Fluids are created by gasifying the Primary
Drilling Fluid. This is accomplished by adding air or nitrogen into
the drilling fluid.
• Methods
Standpipe injection at surface
Jet Sub above the bit
Parasite String
• Most drilling fluids can be aerated however nitrogen must be used
when aerating OBM. Corrosion inhibitors must be added when air
is the gas being used.
• Aerated Drilling Fluids were initially designed as a technique to
lighten mud to reduce lost circulation
• As an UBD fluid, it is easiest to control in small diameter holes
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Aerated Fluids
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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Aerated Fluids
PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018
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