DEA101

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PETE 689 Underbalanced Drilling UBD
Lesson 8 DEA-101 Aerated Fluid Drilling Chapter
4.1
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Definition
Introduction of gas into a drilling fluid for the
purpose of reducing the hydrostatic head.
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Gaseated Mud
Uses

• Control lost circulation.
• Avoid differential pressure sticking.
• Increase drilling rate.
• Reduce or avoid reservoir damage.

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Gaseated Mud
Advantages

• Equivalent Mud Weight reduction down to 4ppg.
• Simple system.
• Not much can go wrong that cant be fixed.

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Gaseated Mud
Hole Problems

• Pressure Surges.
• Velocity Surges.
• These can lead to hole caving and/or reservoir
  damage.

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Gaseated MudEnvironmental Problems

• Large volumes of water or oil.
• Oil emulsifying into mud.
• Residual H2S in cuttings.
• Foaming with oil.
• Room on location.

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Gaseated MudFluid Density Range
FRESH WATER
OIL BASE MUD
OIL
GASEATED MUD
LWSA
FOAM WITH BACK-PRESS
STABLE FOAM
MIST
AIR, GAS
0
1
2
3
4
5
6
7
8
9
FLUID DENSITY (PPG)
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Aerated Systems

• Historical preference - drilling mud.
• Future use - clear fluids and mud.
• Diesel oil.
• Other synthetic or light oils.
• Water.
• Water w/polymers.

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Gaseated SystemsGeneral Pressure andVolume
Requirements

• Maximum pressure
• /- 10,000 kPa (1,500 psi)
• Volume
• 20-45 m3 (700-1,500 scfm)

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Annular Velocity Rules of Thumb for Hole Cleaning

• Conventional AV gt 120 fpm (vertical).
• Conventional AV gt 150 fpm (Horizontal).
• This should be enough to clean the bit and the
  bottom of the hole.

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Planning - Initial Volumes

• Start with 2/3 the amount of mud needed for 120
  ft/min annular velocity.
• This can be increased for horizontal wells or
  other problems.

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Planning - Initial Volumes

• Start with 30 times as much air at STP as mud.
• 30 X (Mud gpm X 7.5) SCF air at STP.

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Gaseated Injection Ratios

• Injection ratio at STP varies.
• 5 gas/1 liquid.
• Less doesnt do much good.
• 40 gas/1 liquid.
• More is very unstable.

40
5
1
1
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Definition

• Ratio - at surface conditions, the Ratio of gas
  to fluid, where fluid is expressed as 1 (e.g.
  20/1)

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Definition

• Quality - The gas in a system. (This is
  ordinarily a foam term).
• This may be expressed as a , a decimal, or as a
  number.

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Gaseated Injection Ratios

• Upper limit of stability is about 10/1 or 90
  Quality at the top of the hole. Beyond that there
  is severe surging.

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Surface Injection Ratio vs. Downhole Foam
Quality

• 8,000 ft Hole
• 9 ppg Mud
• Surface Ratio 101 (Gas Mud)
• Quality 91 at Surface - 3 at the bit

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Foam Quality and Mud Ratio
Surface
Q 91
Air/Mud Ratio 10/1
R 1.4/1
1000 Q 58
2000 Q 18
R 0.23/1
Based on 9 lb/gal Mud
4000 Q 8
R 0.09/1
6000 Q 5
R 0.05/1
8000 Q 3
R 0.03/1
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Aerated Systems

• (-) Gas causes reduction in bottom
  hole pressure.
• ()Friction due to velocity causes addition
  to bottom hole pressure.

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CalculationsGeneral Gas Law
P1 V1 P2 V2 T1 Z1 T2
Z2

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Foam Quality and Mud Ratio
Surface
Q 91
Air/Mud Ratio 10/1
R 1.4/1
1000 Q 58
2000 Q 18
R 0.23/1
Based on 9 lb/gal Mud
4000 Q 8
R 0.09/1
6000 Q 5
R 0.05/1
8000 Q 3
R 0.03/1
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Gaseated Bottom Hole Pressure Check (more or less)
?p P(circ1) - P(circ2) ?p decrease
in BHP P(circ1) mud circulating
pressure. P(circ2) gaseated circulating
pressure.
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Gaseated Bottom Hole Pressure Check (more or less)
?p ?Vmud / (Vann x fg) ?Vmud ? in pits
in bbls Vann Vol ann bbl/ft fg
Fluid gradient psi/ft
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Typical Volume Requirement Chart
(After Phillips Pet. Co.)
Cubic Ft of Air at 14.7 Psia and 60F Per Barrel
of Mud
180
160
20
0
140
120
100
80
60
40
No Dynamic effects Friction Separation
Req'D. Reduction, Ppg
Desired Fluid
Weight, Ppg
5.0
2.0
10
1.0
3.0
4.0
0.5
4
1,000
2,000
9,000
10,000
3,000
4,000
5,000
6,000
7,000
8,000
Drilling Depth in Feet
25
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Aerated Systems
Single Biggest Problem
Pressure Surges
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Aerated System Pressure Surges
Typical Bottomhole Pressure Survey During an
Underbalanced Drilling Operation
Typical Bottomhole Pressure Survey During an
Underbalanced Drilling Operation (N2 Circulation
Prior to Connections)
Hydrostatic Pressure
Connections/Slugs
Hydrostatic Pressure
Reservoir Pressure
Reservoir Pressure
Pressure
Pressure
Time
Time
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Flow Regimes Depend on Velocity Variations
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Threshold Velocities for Maintaining a Mixture
of Gas and Fluid
100
A
Bubble
10
Annular
B
Liquid Velocity ft/sec
1
C
0.1
Slug
Churn
A
B
0.01
0.1
1
10
100
1000

Gas Velocity ft/sec
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Separation of Gas and Fluid

• Large hole - low velocity
• Polymer to increase viscosity.
• Increase velocity as much as reasonable.

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Separation of Gas and Fluid

• Small hole - high velocity.
• Medium depth.
• Best use of gaseated systems.

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Keeping the System Together

• Gas retention in the mixture.
• Viscosity.
• Polymers.
• Gel strength.
• Fluid velocity.
• Small bubbles.
• Jet bit.
• Shut blooie line on connections.

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Gas Fluid Mixing System
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Separating the Gas from the Mud at the Surface

• Spin in the separator.
• Time.
• Shale shaker screen.
• Gentle mixing.
• Low viscosity.
• Low gel strength.
• Deep rather than shallow pits.

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Vertical Separator with Spin
Optional 3 psi Pressure Flapper
Gas
Gas Baffle
Inlet Tangent
Spin Shelf
Float
Level Control
Linkage
To Shaker or Mud Pit
Outlet Valve
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Methods of Controlling Surges
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Aerated System Pressure Surges
Typical Bottomhole Pressure Survey During an
Underbalanced Drilling Operation
Typical Bottomhole Pressure Survey During an
Underbalanced Drilling Operation (N2 Circulation
Prior to Connections)
Hydrostatic Pressure
Connections/Slugs
Hydrostatic Pressure
Reservoir Pressure
Reservoir Pressure
Pressure
Pressure
Time
Time
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Boosting Techniques

• Connection
• Fill DP with gas.
• Turn off gas.
• Fill DP with mud to first string float.
• Turn off liquid.
• Make connection.

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Special Downhole Equipment to Limit Surging

• Jet sub.
• Parasite string.
• Dual casing string.
• Dual drill pipe.
• Constant circulating subs.

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Jet Sub Advantages

• No preplanning or changes.
• Easier to start circulation.
• Lower pressure surges.
• Gas injection pressure is lower.
• Lowest gas use.

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Special Downhole Equipment to Limit Surging

• Jet sub.
• Parasite string.
• Dual casing string.
• Dual drill pipe.
• Constant circulating subs.

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Basic Concept

• Most expansion occurs above 3,000.
• Below 3,000 there is no advantage to increasing
  gas injection above 200 scf/bbl (ratio of 351).

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Advantages of Parasite String

• Avoid heads by circulating during connections and
  trips.
• Easy to get circulation started.
• Simple operations.

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Problems With Parasite String

• Need to drill larger surface hole.
• Slows down casing running and makes it more
  complex.
• Always the possibility of smashing or plugging
  the string when running it.
• Uses more gas than a jet sub.

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Casing Spider Adapter For Parasite String
Slot Cut for 2 1/16 Tubing
3
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Special Downhole Equipment to Limit Surging

• Jet sub.
• Parasite string.
• Dual casing string.
• Dual drill pipe.
• Constant circulating subs.

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Wellhead Setup for Dual Casing String
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Advantages of Dual Casing

• Minimizes the chance of crushing the string.
• Inner string retrieved and used again.
• Can be set very deep.

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Problems with a Dual String

• Requires change in well head.
• Extra string of stream line casing.
• Gas volume storage will unload well.

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Special Downhole Equipment to Limit Surging

• Jet sub.
• Parasite string.
• Dual casing string.
• Dual drill pipe.
• Constant circulating subs.

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Dual Drill Pipe

• It is a mining tool and seldom used in the oil
  field.

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Constant Circulating Subs

• Used in the late 1960s, no longer available.
• Good for connections.
• No advantage on trips.
• Lots of extra subs.

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Constant Circulating Sub
Float Pins
Full opening float
2 Airline
Tool Joint O.D.
Quick connect
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Fluids and Gasses
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Gaseated SystemsGases

• Air
• Nitrogen
• Cryogenic
• Manufactured
• Natural Gas
• Exhaust Gas

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Gaseated SystemsClear Fluids

• Diesel oil.
• Water.
• Saline.
• Other light oils.
• Mineral oils.
• Synthetic oils.

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Gaseated SystemsDrilling Mud

• Conventional gel based mud.
• Low lime mud.
• Xanthan gum mud.
• Thixotropic (holds gas).
• Easily broken gel strength.
• (Several commercial systems available).

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Corrosion
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Always Remember Corrosion cannot be stopped.
It can only be controlled
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Corrosion Control pH

• Keep pH above 9
• Steel becomes passive above pH 11

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Corrosion Control

• Dont use air.
• Oxygen corrosion is the most common type of
  corrosion.
• There are other types of corrosion and scaling.

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Corrosion Inhibitors
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Organo-Phosphate Esters
ANIONIC

• Foamer compatible.
• Good solubility.
• Limited scale inhibition.
• Relatively high temperature.
• Biodegradable.
• Excellent oxygen control.
• 500-2,000 ppm concentration.

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Chromates
ANIONIC

• Extremely soluble in water.
• Foamer compatible.
• Relatively high temperature.
• 200-1,200 ppm concentration.
• Low solids system.
• pH gt 8
• Heavy metal carcinogen.

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Nitrates
ANIONIC

• Foamer compatible.
• 60-70 chromate effectiveness.
• Require high pH.
• Require high concentration.

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Phosphates
ANIONIC

• Primary scale inhibitor.
• Foamer compatible.
• Raise pH to 8-12.
• Not sufficient by themselves.
• Produce calcium phosphate. scale in high calcium
  fluid.

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Amines
Cationic

• Effective In Mud Systems.
• Work Well In Gas/Air.
• Form Protective Film.
• Film Easily Penetrated by Monatomic Oxygen.
• Foamer Incompatible.

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Equipment for Gaseated Systems
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Rotating Head
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Drill String Floats
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Separators
Typical Closed System UBD Operation

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Simple Aerated Mud Separator
Overflow Pipe
De-Aerator
Mud Flow Line
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Vertical Separator with Baffles
Gas to Flare
Gas
Gas Baffle
14 Feet
Gas vent
Inlet flow
Mud Baffle
From Choke Manifold
Mud
From Choke Manifold (2,3 or 4 inlets)
Mud to Pit
Solids or Cleanout
8 Fee t
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Vertical Separator with Spin
Optional 3 psi Pressure Flapper
Gas
Gas Baffle
Inlet Tangent
Spin Shelf
Float
Level Control
Linkage
To Shaker or Mud Pit
Outlet Valve
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SWACO Super Mud Gas Separator
Gas Out
83 Mmcf/d
From Well
U Tube Level Control
Fluid Out 50,000 bpd
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Closed Separator
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Schematic of Closed Separator
Well Effluents In
Gas Out
Adjustable Partition Plate
Gas
Solids Transfer Pump
Oil
Mud
Solids
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Typical Horizontal Separator

• 9 x 50 (3m x 15m).
• 50 psi (345 kpa) wp.
• 5 mmcf/d and 500-600 bbls.
• 141,000 m3/d and 87m3 liquid.
• Orifice meter run.

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Vertical Separator
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Typical Vertical Separator

• 4 x 15 (1.3m x 3m).
• 500 psi (3450 kPa).
• 25 MMcf/d and 10,000 bbls/d
• (700,000 m3 gas/d and 1,590 m3/d liquid).
• Orifice Meter, Level Control, Dump Valves.

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Surface Equipment Closed System
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Underbalance Drilling Spread
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Gas Separator System
Gas Separation System
Ensures all fluid is degassed prior to return to
the system.
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Open Skimmer
Oil Trap
Oil Overflow
Drilling Fluid/Water
Solids
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Sample Catcher for Closed System(Alpine, 1996)
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Gaseated Drilling Techniques
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Techniques
Flowline returns do not represent downhole
conditions.
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Initiating Gaseated DrillingUnloading the Hole

• Start Pumping gas/mud system.
• When pump pressure gets too high for the air
  compressor, stop pumping air.
• Pump mud until the pressure goes back down.

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Drilling

• There will be alternate heads of gas and mud.
• Heading cycle may be as long as 5 minutes.

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Control Heading

• Increase viscosity.
• Increase mud volume(velocity)
• Decrease gas.
• Add 2 TO 3 atm of backpressure 30 to 50 psi.
• Decrease bit jet size.

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Connections

• Gas and fluid will separate.
• Fill the drill pipe with gas before connection.
• Then fill to first string float with mud to keep
  connection dry.

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Pressure on the Annulus

• Shut in the well on connections to keep the gas
  under pressure.
• If the well is strongly flowing, the choke may
  have to be left slightly open.

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Gaseated System Making a Trip

• For lost circulation (well not flowing) no
  problem with trip.
• For flowing well
• Displace clean clear fluid over reservoir and
  kill with a floating mud cap.
• Use a deployment valve.

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Gaseated System

• Stripping is a reasonable and safe technique
• Planning on snubbing out the pipe light stands is
  a bad planning practice because of the time
  expense.

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After a Trip

• Go to bottom and unload hole with alternate mud
  and gas.
• Working back to bottom while gassing us each zone
  will not be successful unless you keep the well
  shut and bleed a little pressure.

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End
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