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SICP and SIDPP
SICP = Shut- In Casing Pressure
SIDPP = Shut- In Drill Pipe Pressure
Both the terms are self-explanatory that both the pressures
are the property to be measured when the well is Shut-In i.e when BOP is closed
(mostly at instances of KICK). SICP is the pressure build up in the annular
area between casing and drill pipe. While SIDPP is the pressure build up in the
Drill Pipe.
After shutting the well vital Well control information can
be obtained from both SICP and SIDPP that can be measure from the casing and drill
pipe pressure gauge located at the surface.
Both the pressures are giving information of the Bottom Hole
conditions.
To understand even clearly refer the following Figure 1
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| Figure 1 |
Visualize the bottle of Coke with open cap (do not shake the
bottle and open cap slowly). Press your hand tight on the mouth of the bottle
and shake the bottle hard. You will feel the pressure on your hand and inside
fluid will exert pressure trying to come out of the bottle. Though there was no
pressure initially, but pressure was because of dissolved gas in coke.
Same is the concept of SICP and SIDPP. Though formation
pressure is balanced, during influx of formation fluid they will exert the pressure
against the HP of mud column. This excess pressure is what we call it SICP and
SIDPP. Its necessary to close well so as to know the increased pressure.
This vital information will help in knowing what amount of
mud weight shall be increased to balance the increased Formation Pressure ( now
onwards FP).
Mostly SIDPP is less than SICP
- The reason is kick fluids will either be oil or gas (in almost cases). Both are lighter than mud (i.e water in general) . So HP in casing (because kick fluids migrate through casing) by fluids (Drilling Mud + Kick Fluid) will be less than HP in drill pipe by fluids (pure drilling mud )
As,
SICP =
FP- HP (By fluid column in annular area between casing and Drill Pipe)
SIDPP =
FP- HP (By Fluid column in Drill Pipe)
HP casing < HP drill pipe
So, SICP >
SIDPP
- For the case when the weight
of mud cuttings in the casing surpasses the reduction in HP in casing
due to kick fluid, in that case HP of casing is greater than Drill pipe and
hence
HPcasing ( Mud + Kick + Heavy Cuttings) > HPdrill
pipe (pure Mud)
SIDPP > SICP
It is important to know which type of Kick Fluid has got in.
That can easily be found out from SICP, SIDPP and known weight of Drilling Mud
and from Pit Gain from Mud Pit
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| Calculation is done by Karan Patel for sake of understanding |
Effect of Gas Migration in SICP
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Image taken from the site "drillinghandbook.com" as a reference to understand the topic |
[ Try to understand the highlighted part (red color) from the image]
As shown in the figure above, when the well is closed the
Gas migration provides increasing Casing Pressure and Simultaneously it
increases Bottom Hole Pressure.
It is necessary to change Mud weight so as to counter
increased formation pressure which caused Kick to enter well bore. But as SICP does
not provide the correct information because of Kick fluids, Formation Pressure
is calculated from SIDPP.
And on the basis of known increased FP weight of Kill Mud is
decided and is circulated through out the well.
Note : AS Gas Migrates SICP keeps on increasing, but it should not exceed predetermined pressure handling capacity for particular Grade of Casing or else will result in failure of casing by bursting. And increased Bottom Hole pressure also results into Formation Damage and resulting into lost circulation. So while migrating the Gas Kick or any Influx out of the well, SICP should be held constant with the opening/closing of valve in return line which goes to choke manifold.
