Surge and Swab
This section will cover:
• Understanding Surge & Swab
– Magnitude of ECD fluctuations
– Causes
– Effects
• Options to reduce Surge & Swab
– Planning stage, Implementation stage
– Special equipment
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Surge and Swab
What is Surge and Swab? (With Pumps OFF)
Surge: Swab:
Results from Results from upward
downward pipe pipe movement
movement
Dynamic Pr Dynamic Pr
ECD = ESD + ECD = ESD -
g x TVD g x TVD
Dynamic Pr
Static Pr
Static Pr
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Dynamic Pr
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Surge and Swab
Why is Surge & Swab a particular concern for ERD?
• ERD wells have much higher surge & swab ECD fluctuations
– For same reasons ECDs are higher (see next plot)
• ERD wells require more swab margin for normal tripping
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Surge and Swab
1 @ 10,000’ MD/ 4,000’ TVD
2 Same 0.3 ppg over-balance = 62 psi
5
Consider two identical scenarios (but one vertical, other ERD)
6
• The same Swab margin has a different pressure effect
7
9 @ 10,000’ MD/TVD
0.3 ppg over-balance = 156 psi
10
11
0 1 2 3 4 5 6 7 8 9 10 11 12
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Keeping the Hole Full
1
@ 10,000’ MD/ 4,000’ TVD
2 Pull 5 stands wet = 1.4 ppg EMW reduction
0.3 ppg reduction if pulled dry
3
5 What is the effect of not keeping the hole full on trips?
6 • Effect is different, depending on TVD
7 • Consider Vertical vs. ERD for 8½” hole with 5½” drillpipe
8
9 @ 10,000’ MD/TVD
10 Pull 5 stands wet = 0.6 ppg EMW reduction
0.1 ppg reduction if pulled dry
11
0 1 2 3 4 5 6 7 8 9 10 11 12
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Surge and Swab Basics
The typical perception of “what causes swab” is the following …
1. That the bit creates most of the swab (piston effect)
2. That the swab suction is not felt all the way to TD (i.e. it is local
to just below the bit)
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Surge and Swab Basics
This explains the practices & logic used on many
horizontal wells:
The bit is pumped out while in open hole …(to
prevent swabbing an influx)
But normal tripping commences when the
shoe is reached
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Surge and Swab Basics
Surge & Swab ECDs are made up of 2 components
1. Bit creates a piston / plunger effect
• This is what we normally think of for the surge & swab
2. But there are also overall drillstring effects…
• This is the more critical portion
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The Drillstring “Pump”
When drillpipe moves down, fluid moves up
• The fluid moves along the entire length of the string
The effect is exactly the same as circulating at the same point
The same applies to swab, when tripping out
• Fluid moves down, along entire length of string
Except this creates a “negative” pressure
• Think of the string as a suction pump
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The Drillstring “Pump”
Surge loads in this
well up to 1 ppg EMW
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Surge and Swab Basics
Swabbing is related to ECDs
• Many of the same issues apply
– TVD amplification of swabbing magnitudes
– Most problems are predictable
• If you don’t know what normal looks like
• But there are misconceptions
– Is the swab mostly due to the bit / BHA?
– Or the drillstring?
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Swab Case Study
Consider this well:
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Swab Case Study
Drilling 8¾” Hole
• 8.9 ppg mud, with 8.5 ppg pore pressure
– Can not trip conventionally until ±6,000’ MD
• Have to pump out >15,000’
– Mud weight is limited, because of ECD limitations
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• This shows the drilling ECDs
• Shows that ECDs are right up against the limit
… mud weight is therefore limited to ≤8.9 ppg
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• This is the predicted swab that the bottom of the hole
would feel on the trip out…
•With NO allowance for the bit, stabilizers or
mud gelling up
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The shaded area is the swab load for
the original situation
So, if the swab at TD is more a function of ECD than
“the plunger effect”
• How does a more aggressive ECD solution affect this?
• In this case, how would drillstring optimization help?
• If a tapered string of 5”x4½” drillpipe was run, initial
swab is somewhat less
• But it still swabs, even for the first several thousand
feet inside cased hole
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The shaded area is the swab load for
the original situation • So the swab is because of ECD reasons, but the
ECD solutions don’t make much difference!
• What’s the point then?
• Are we doomed to pump out?
• K&M argue that ECD solutions must be designed to
aggressively reduce ECDs, to solve this problem
• Because reducing ECDs buys you mud weight
• In this case:
•Can accommodate more MW (0.8 ppg) without
inducing losses and
•Need less MW to prevent swabbing
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• So how does a slimmer drillstring help?
• Shows the tripping situation with the increased
MW now possible
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Swab Case Study
In Summary, swab is all about ECDs
• And ECDs are all about the wellpath
– Shallower & longer wells have higher ECDs
• A side-effect of this is more MW margin is necessary
to prevent swabbing (compared to vertical wells)
– Shallower & longer wells require more MW margin
• And the swab is felt all the way to TD
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The colored curves
are for a vertical well
• Like normal ECDs, swabbing is exaggerated
in ER wells
The shaded area is the swab load for
the original situation (ERD Well) • Tripping margin requires more over-balance
MW than for a vertical well
End result is that swab margins MUST be
calculated for a well, and NOT assumed
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Surge and Swab Basics
Swab ECDs also affect wellbore instability
• Remember, this “drillstring ECD” effect is felt throughout the wellbore
• The effect on wellbore instability is exactly the same as if the mud engineer
reduced the mud weight by this amount for the length of time of pulling
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Surge and Swab Basics
Consider the 12¼” section of this
deepwater ER well
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Like most ER wells, the mud weight is driven by
wellbore stability reasons
• Mud weight for stability is > 1.1 ppg EMW
over-balance to pore pressure
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With this mud weight, tripping
swab at TD is below collapse MW
for a long portion of the trip out
With this mud weight, swabbing an
influx is not a concern
But the swab ECD at TD is below the collapse
MW for long time while pulling out
• More MW is necessary (11 ppg minimum)
Have you ever noticed cavings after a trip, but
not while drilling & cleaning before the trip?
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Surge and Swab Basics
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Surge and Swab Basics
Key message: Swab planning is critical to MW selection
This is especially critical in “big hole” situations
(such as 12¼” hole)
• Where pumping out has a huge stuck-pipe risk
that is unacceptable
• For small hole sizes, it’s less critical (but remember the swab
ECDs are much stronger)
•Stuck pipe risk is more manageable
•Impact is just time & cost (much slower trip times)
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Surge and Swab Basics
Swab ECDs also affect wellbore instability
• Perhaps the most destructive form of swab is when picking up a casing
string or a liner
• Swab is particularly strong, picking up a 7” liner in 8½” hole
• This should be considered when
– When working pipe
– Picking up to break static friction
– Or pulling the string out
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Consider this well …
• 7” liner run (4000’ long)
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And this is the swab as seen at TD
Picking up with 5” or 5½” running string
Swab at TD is ≥ 1.0 ppg EMW EMW
• Each time that string is picked up at connections
• Even if just to take a “pick up load” measurement
And this is a “normal “ well …
• It’s much worse if it’s an ERD well
Recommendations:
1. Don’t pick up unless necessary
2. If necessary to pick up casing or liner, do so with slow
circulation
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Surge and Swab Basics
Weatherford Autofill Float Equipment
Weatherford Mudmaster Filter
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Surge and Swab Basics
Allamon ATC Diverter Sub Baker Hyflo Valve
Weatherford SurgeMasterII
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• Autofill equipment greatly reduces surge.
• It also reduces drag, due to reduction in “piston force”.
• Unfortunately, this benefit is lost as soon as the floats plug
with cuttings or debris.
All is going well on this 13⅜” casing run (through
1500m of 16” liner) to 11,800’ MD
…until the floats plug.
• FF’s jump from 0.30 to 0.45+.
• This is only 500m of “tight clearance” geometry
o 220 psi = 30 kips piston force
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