Special Well Control Applications

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Lesson 23

• Special Well Control Applications
• Underbalanced Drilling (UBD)

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Special Well Control Applications

• Underbalanced Drilling
• Well Control in Unconventional Hole Programs
• Casing and Cementing Operations

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Special Well Control Applications

• Homework 12
• On the Web
• Due Dec 02, 2002

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Underbalanced Drilling (UBD)

• In drilling UBD wells the ECD is
  intentionally kept below the formation pore
  pressure.

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Reasons for UBD

• Minimize formation damage
• Faster penetration rates
• Longer bit life and fewer trips
• Eliminating one or more casing strings

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Reasons for UBD

• Reduced risk of lost circulation
• Reduced risk of differential sticking
• Lower mud costs
• Earlier oil sales

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Drilling Fluids used in Underbalanced Drilling

• Air and Natural Gas Drilling
• Mist and Foam Drilling
• Underbalanced Drilling w/Mud

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Air and Natural Gas Drilling
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BOP System for Air and Natural Gas Drilling
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Rotating Head
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Air and Natural Gas Drilling
Casing gauge is used to predict BHP
Compressor at surface Drillpipe Float Gas in
drillstring All make DP gauge unreliable for BHP
Pore Pressure
Casing Head Pressure
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Example 7.1
Excessive gas rates from a sandstone at 7,200 ft
threaten a blowout on an air-drilled hole in the
Arcoma Basin. The well is shut-in and, after
wellbore temperatures reach equilibrium, the
casing pressure gauge reads 1,250 psig.
Estimate the kill-mud density requirement.
What is the maximum pressure at the 1,500-ft
shoe depth before and during the kill procedure?
Assume the average temperature in the annulus
is 160 oF and use a 0.70 specific gravity gas in
the calculations (ignore the effect of the air
mixed in with the hydrocarbons).
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Example 7.1 - Solution
Use Eq. 7.1 and iterate by first assuming the
compressibility factor is 1.00
Pore pressure, pp 1,472 psia
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Example 7.1 - Solution - contd
Take the pseudoreduced properties on wellbore
averages to determine the average z factor ppr
(1,264 1,472) / (2 666) 2.05 Tpr 620
/ 389 1.59 z 0.855
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Example 7.1 - Solution - contd
Substitute this z-factor value into Eq. 7.1 and,
after one more iteration, obtain pp 1,512
psia. The density equivalent at 7,200 ft is
rkwm (1,512 - 14) / (0.052 7,200) rkwm
4.0 lbm/gal The result is invalid if a
substantial liquid column is above the kick zone,
but it should be apparent that a low-solids mud
will be satisfactory.
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Example 7.1 - Solution - contd
The maximum shoe pressure will be the shut-in
pressure if the casing side is held constant
until the string is filled with mud. The
preceding calculations are repeated for the
1,500-ft depth
Shoe pressure, pshoe 1,305 psia
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Example 7.1 - Solution - contd
Also,
ppr (1,264 1,305) / (2 666) 1.93 Tpr
620 / 389 1.59 (same as before) z
0.860 Ultimately we obtain pshoe 1,298
psig which gives a pressure gradient of 0.865
psi/ft. The fracture integrity is obviously
higher or control would have been lost when the
preventer was closed.
( 1,298 / 1,500 0.865 )
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Mist and Foam Drilling

• Mist drilling may be used when small water
  flows would cause mud rings with air or natural
  gas drilling.
• Water is injected downstream of the
  compressors until the air is nearly saturated
  with water vapor.

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Mist and Foam Drilling

• Foam drilling can tolerate still more water
  than mist.
• Foams are generated by shearing water and gas
  together with a foaming surfactant and
  bentonite or polymers added for better hole
  cleaning.

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Underbalanced Drilling with Mud

• Air can be injected into the mud stream to
  lighten the mud column.
• One way is to inject at the standpipe

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Used to determine volume of air to inject
rA 9 lb/gal rD 5 lb/gal rA - rD 4
Desired MW 5 lb/gal
( not always very accurate )
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Example 7.2
A well has been drilled to 9,500 ft with aerated
mud and the decision is made to shut-in and pump
a kill. Air rate at the standpipe was 1,500 scfm
and an 8.7 lbm/gal mud was being pumped at 250
gal/min. Estimate the kill-mud density if the
SIDPP is 800 psig. Assume the average
temperature in the drillstring is 150 oF and use
the compressibility factors given in Fig. 7.7.
KWM ??
What is the BHP?
Fig. 7.7 - Compressibility chart for Air
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Example 7.2 - Solution
We chose to solve this problem by numerically
integrating the mixture densities and pressures
with depth. Eq. 2.20 gives the air density
underneath the drillpipe gauge
Fig. 7.7 - Compressibility chart for Air
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150 oF
814 psig
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Example 7.2 - Solution - contd
pVZnRT
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Example 7.2 - Solution - contd
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UBD with Weighted Mud
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UBD with Weighted Mud
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Tripping in UBD
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Well Control in Unconventional Hole Situations

• Horizontal and ERD Wells
• Slim-Hole Applications
• Coil-Tubing Operations

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Horizontal and ERD Wells

• For Horizontal and ERD Wells the window for
  acceptable mud weights narrows.
• The high angle reduces fracture pressure,
  lowering the maximum mud weight allowed
• Hole collapse increases the minimum mud
  weight.
• Mud weight may limit the length of the lateral

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Fig. 7.12
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Heel
Terminus
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Additional Pressure Concerns

• Cuttings beds require high annular velocity
• Surge and Swab pressures higher

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Less overbalance during trip
Swab Pressure
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Additional Pressure Concerns

• ERD wells are more prone to kicks and lost
  circulation
• SICP lower for ERD (for given kick size)
• Gas migration less of a problem

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Gas trapped in washouts reduces migration
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Vertical height remains constant in lateral
section
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DrillPipe Pressure During Well Controlwhile kill
mud is filling drillstring
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Slim-Hole Applications

• High annular friction during circulation
• Small pit gains yield long vertical height of
  kick fluid resulting in high SICP

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Coil-Tubing Operations

• Continuous, non-jointed pipe
• which is stored on a reel
• and transported to a wellsite
• to perform a specific operation

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4. Lift tubing
5. Close blind rams
3. Close shear rams
1. Close slip rams
2. Close pipe rams
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CT growth with time
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Casing and Cementing Operations

• Running the Casing
• Cementing the Casing
• The Annular Flow Problem
• Liner Top Tests

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Running the Casing

• Operator should replace upper DP rams with
  casing rams.
• SIP could result in large upward forces on the
  large diameter casing.
• Large diameter casing results in high surge
  and swab pressures.

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Comparison of surge/swab pressures for casing vs.
DP
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Cementing the Casing

• pbh pch ph Dpf - Dpss Dpa
• pbh BHP
• pch choke backpressure
• ph HSP
• Dpf circulating friction pressure
• Dpss surge or swab pressures
• Dpa pressure resulting from fluid
  acceleration

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Cementing Consideration

• Spacer density and volume
• High viscosities
• U-tubing of cement slurries
• Freefall of cement
• Flash setting of cement

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Effect of cement flash setting
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Cement Channeling
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The Annular Flow Problem

• The transition period between development of
  gel strength and setting sometimes allows
  flow
• High gel strength of cement can support the
  HSP of mud column above and allow flow of gas
  into the cement
• Gas may then percolate upward

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Gas percolation may be possible
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Liner Top Tests

• Getting good cement job on liner can be
  difficult.
• Inadequate liner isolation can cause well
  control problems
• Liner top needs to be tested

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Liner Top Tests

• Casing cleaned out to liner top.
• Pressure applied to liner top to test for leak
• Differential pressure test should be
  conducted by decreasing the HSP above the
  liner top.
• If liner leaks during differential test, a
  liner-top-isolation, LTI packer may need to
  be installed