Workover Well Control • Real differences between drilling and workover well control. • In drilling, the uphole shoe is weak zone, while in workovers, the pay is weak zone. • Operations in drilling that might break down the shoe (e.g., shutting in over night with gas migrating up the annulus), would cause no problem in workovers in cased holes. Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
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Workover Well Control
• Real differences between drilling and workover
well control.
• In drilling, the uphole shoe is weak zone, while in
workovers, the pay is weak zone.
• Operations in drilling that might break down the
shoe (e.g., shutting in over night with gas
migrating up the annulus), would cause no
problem in workovers in cased holes.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Massive shallow gas release – West Vanguard
Working Fluids
• Drilling – muds - fluid loss control – allows
operations at high overbalance with
minimum fluid losses.
• Workovers – often clear brines – even small
overbalance causes excessive fluid loss.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Well Control Methods
• Drilling – drillers method
• Workovers – bullheading is common
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Surface Pressure and Equipment
• Allowable surface pressures are higher for
workovers and completions as compared to
drilling operations, at least in relatively new
wells.
• Workovers in older wells may have surface
pressure limitations due to loss of casing
integrity. (Most older wells have lower
formation pressures due to depletion).
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Well Control Considerations
• Well History (mechanical damage, corrosion, abrasion, etc)
• Tubular and wellhead details
• Casing and cementing details
• Annular heating and thermal expansion
• Workover fluid program
• Current mechanical condition
• Directional Survey data
• Expected reservoir and fracture pressures
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Four Well Conditions
• Static Condition – wellbore fluid and tubing are stationary.
• Forward circulating – fluids are pumped down the tubing and up the annulus.
• Reverse circulating – fluids are pumped down the annulus and up the tubing.
• Tripping – tubing is being pulled or run into well.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Static – Hydrostatic Pressure
• Ph = 0.052 * FD * TVD
Ph = hydrostatic pressure, psi
FD = fluid density, ppg
TVD = true vertical depth, ft
0.052 = 7.49 (gal/ft3) / 144 (in2/ft2)
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Static – Hydrostatic Pressure
• Calculate the hydrostatic pressure exerted in a vertical well on the perforations at 5000 ft in a well filled with 10 ppg workover brine.
Ph = 0.052*10*5000 = 2600 psi
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
The Extreme Case – Horizontal
Well • MD or TVD?
Difference in volumes and in pressures
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Bottom Hole Pressure
• BHP = Ph + Ps
– BHP = bottom hole pressure
– Ph = hydrostatic pressure
– Ps = surface pressure
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Ph
P
s
Forward Circulating/Friction
Pressures
• Pump Pressure =
Friction pressure in flow line +
Friction pressure in tubing string +
Friction pressure in downhole tools +
Annular friction pressure +
Annular hydrostatic pressure -
Tubing hydrostatic pressure
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Annula
r F
rict
ion
Tool Fric
Tubin
g F
riction
Forward Circulation
• Circulation Path
• Friction pressures make BHP during circulation higher than at static conditions.
• Annular backpressure is additive to annular friction pressure for BHP
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Annular
friction
Tube
Fric
BHP with a Choke
• During regular circulation,
with 10 ppg brine, there is
200 psi annular friction and
300 psi backpressure is held
with a choke.
• Calculate the BHP.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
BHP?
300
200
BHP with a Choke
• BHP = Ph + Ps + Pfann
Note that this is the back pressure side of the
equation
• BHP = 0.052*(10)(5000)+300+200
• BHP = 3100 psi
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Equivalent Circulating Density
• ECD = Pfann /(0.052 * TVD) + FD
ECD = equivalent circulating density, ppg
Pfann = annular friction (psi)
TVD = true vertical depth (ft)
FD = fluid density, ppg
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Equivalent Circulating Density
• Calculate ECD for circulation with 10
ppg brine and 200 psi annular friction
pressure.
• ECD = 200 / (0.052*5000) + 10
• ECD = 10.77 ppg
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Reverse Circulating
• BHP = Ph + Ps + Pftbg
BHP = bottom hole pressure, psi
Ph = hydrostatic pressure
Ps = surface pressure
Pftbg = tubular friction (psi)
The bottom hole pressure is highly influenced by the tubing friction – annular friction is not felt except as a loss of surface pressure or hydrostatic – only important in close annular clearances.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Reverse Circulating
• Calculate the BHP at 5000 ft while
reverse circulating with 10 ppg brine
and holding 300 psi back pressure on
the tubing choke. Friction in the
tubing is 500 psi. Friction in the
annulus is 0.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Reverse Circulating
• BHP = Ph + Ps + Pftbg
• BHP = 0.052(10)(5000) + 300 + 500
• BHP = 3400 psi
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
ECD Reverse Circulating
• ECD = Pftbg/(0.052* TVD) +FD
Pftbg = friction pressure in the tubing, psi
TVD = true vertical depth, ft
FD = fluid density, ppg
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Swabbing
Swab occurs in upward
tool/tube movement.
Any large BHA can swab.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
BHPs<BHPi
Red Flags for Swabs
1. Fast Pipe Movement
2. Viscous Fluids and High Gel Strengths
3. Tool OD > 80% Pipe ID
1. Packers
2. Liners
3. Perf guns
4. Scrapers
5. Pumps
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Possible Indicators of a Kick
1. Cutting of fluid
2. Change in chloride content of water
3. Incorrect fill-ups
4. Decrease in pump press while circulating – decrease in hydrostatic pressure from kick.
5. Increase in flow line temperature
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Handling a Workover Kick
• Minimize the influx
– Early recognition and quick shut in
– Estimate of type of kick from pressure
difference
– Problems with hole geometry
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Trapped Pressures and Fluids
• Completions well work may encounter trapped fluids or pressures
• Trap areas:
– Plugs, packers, SSSV, surface valves – need equalization path
– Tools – below large tools with viscous fluids in wellbore
– Debris plugs – fill, paraffin, scale, corrosion
– Well Design – large annuli (minimize), dual strings, cross-overs, deviation, washouts, cross-flows
– Annular thermal expansion Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Special Cases of Control
• Trapped Gas
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Gas in the annulus above the end of
tubing, is stored energy. High stored
energy after fracturing or bullheading
will pressure up after shut down. It
may bleed off rapidly by displacing
fluid into the formation.
Mechanical Failures
• Equipment malfunction or failure is
involved in 20% of blowouts.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Effect of Casing Size
• 10 ppg fluid, 20 bbl gas kick, 2-7/8” tubing
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Effect of Kick Type and Density
• 10 ppg fluid,
20 bbl kick,
7” Casing,
2-7/8” tbg
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
As Gas Migrates Upward…..
• Gas migration is the upward movement of a gas
bubble – occurs without circulation – fluid
density difference is the driver.
• Calculate the increase in BH pressure and
surface pressure when a gas kick migrates 500 ft
in a 5000’ deep well filled with 10 lb/gal brine.
Initial SICP is 400 psi and initial reservoir
pressure is 3000 psi.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Gas Migration Problem
BHP = bubble press + hydrostatic below bubble
BHP = 3000 + (0.052)*(10)*(5000’-4500’)
BHP = 3260 psi
4500 ft
5000’
3000 psi
400 psi
3260 psi
660 psi
Ps = bubble press +
Phyd to bubble = 3000-
(0.052(10)(4500) = 660
psi
Gas Expansion
• Expansion – simple approach – Boyle’s Law
• P1V1 = P2V2
Calculate the volume of gas downstream of the
choke when a 1 bbl kick at 3000 psig (3014.7
psia) is bled off.
V2 = P1V1/P2 = (3014.7 psi)(1 bbl) / (14.7 psi)
V2 = 205 bbl of gas (standard conditions) vented.
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Gas Expansion
• One barrel of gas, vented to atmospheric pressure,
expands to:
– From 500 psi => 35 bbl
– From 1000 psi => 69 bbl
– From 1500 psi => 103 bbl
– From 2000 psi => 137 bbl
– From 3000 psi => 205 bbl
– From 5000 psi => 341 bbl
Large kick volumes under high pressures will take very long
time to vent!
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
Calculation – 10 bbl gas kick
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text
• 10 bbl gas kick in casing, perforations at
5000 ft. 7”, 23 lb/ft casing, 2-7/8” tubing, 9
ppg brine. SITP = 260 psi. Gas gradient =
0.1 psi/ft, Calc BHP and SICP. Annulus
capacity = 0.0313 bbls/ft.
Calculation – 10 bbl gas kick
Workover Well Control and Blow Out Prevention Guide – BP-Chevron Alliance Text