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Casing Pressure Test & String Weight
16 December 2020
Dear All,

This might seem an elementary query, but need some understanding of the physics of the mechanism.

It has been observed that during pressure testing the casing, the weight of the string on the martin decker/ weight indicator reduces.

Any fundamentals reason as to why this happens? Since it is a closed system and the pressure that is applied, ideally is exerted in all directions, so there should be no changes in the weight of the string.
7 answer(s)
Drillling Engineer
Pakistan Petroleum Ltd
Total Posts: 5
Join Date: 03/08/19
Hi Umair,

I don't see any reason for hook load to decrease during the pressure test of casing. However, if it is case of pressure test after liner hanger setting, then any axial load due to piston effect would be transmitted to the hanger slips, and the circumferential area difference in liner and setting tool might face an uplift force due to pressure exertion. Just a random thought.
Drilling Consultant
SPREAD Associates
Total Posts: 46
Join Date: 11/02/09
While the casing will increase in volume during the pressure test, this has no effect on buoyancy.  The volume increase = the amount of mud pumped in to the casing which increases the weight inside the pipe.  Overall there is no change; the weight of additional fluid displaced = the weight of the fluid pumped in.

Pressure acts downwards on the top plug, creating a force which increases the axial stress in the casing.  The pressure also acts upwards on the same cross sectional area so I don't see how the pressure itself translates into an overall force acting upwards.

If there is sufficient friction between pipe and casing and it's lower down in the casing string, I think the overall increase in length of the casing will tend to cause a small decrease in weight.

Happy to hear other theories!

Regards, Steve
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 502
Join Date: 10/01/05
Good posts all. 

My MLT i.e. Mass length and Time, suggestion, in regard to these physical core Drilling elements, to better 'Manage Lost time', would be,

First and Always, Hand Draw a wellbore schematic of situation, scenario, operating, and note ably the physical conditions (of mass, length and time) that evidently exists. e.g. As taught by our peers many moons ago. 

If you cant then evaluate the MLT physical things for yourself. Please afford the SPREAD members the wellbore schematic, (suitably populated with all the evident facts that you can*) with your post.

*This avoids having to make numerous assumptions and fill in all the evident gaps generally needed.

From your hand drawn wellbore schematic. Timeline and Piece together to better understand the evident MLT physical (parts) relationships and then consider other people, paperwork and oddity evident factors that may also be going on.

What results? should far more evident from within your schematics and evidence gathering afforded. This is key.

Why do this?, is because the more you do this the more enabling this will become. That in conjunction with your evident data and often quite unique well and specific case conditions / situations / operations gathered. You and your teams should be able to start figuring these day to day 'safety' observational things, for ones-self.

When drillers can better understand all aspects of MLT from the drill floor, at the rig-site. Far more proactive and better problem-solving, decision-making, dynamic risk assessment, and management of change, outcomes, benefits and value shall result.

Festive greetings to all. 

fishing tool hand / consultant / advisor / coordinator / tech. rep.
SPREAD Associates
Total Posts: 45
Join Date: 13/03/12
It is called hydraulic force, multiply the force x inches squared and it will give you the string weight decrease. So multipy your pressure x the sq. In. In the rams or hydril.. I use .7854 x od of item in rams or hydrl. Apply too much force and out the hole it comes...just ask any old school driller who near hung the kelly over the monkey board with his big, new high horse power mud pump....
Drilling Consultant
SPREAD Associates
Total Posts: 46
Join Date: 11/02/09
Hello Umair.  When you apply pressure inside a casing, the stress in the casing wall increases.  This leads to two opposing effects.  The hoop stress in the wall makes the pipe increase in diameter which will shorten the casing.  The axial stress in the wall makes the pipe increase in length.  The increase in length from axial stress will be greater than the decrease in length from the hoop stress so overall the effect will be to slightly increase the length of the casing.  Friction of the casing against the wall of the hole will mean that the shoe will get a tiny bit deeper and the hook load will decrease at the top. 

Mathcad is a great tool to do these kinds of calculations.  I've attached a screen shot of a Mathcad calculation I did recently for 9-5/8" casing having a test pressure of 8943 psi applied.   The decrease in length calculates to 3.214 ft, the stretch calculates to 5.357 ft so overall the pipe length increases by 2.143 ft.

Regards, Steve

Documents uploaded by user:
Mathcad calculation.png
Regional Wells Manager – Continental Europe
Vysus Group
Total Posts: 9
Join Date: 08/04/08
Hi Umair,
Assuming that the casing is hanging free in the well in mud/brine or wet cement, this indication can be induced by the ballooning effect of the casing when applying pressure during the test. When the size of the submerged balloon (casing) increases, the buoyancy force increases and cause the reduction in effective hookload.

Workover Engineer
SPREAD Associates
Total Posts: 21
Join Date: 01/08/18

Hi Umair,


Any bended pipe is straightened out once it is internal pressurized (Bourdon Effect).

If you are familiar with Fisher Actuator you can see the application of this effect on valve position controller. What is happening is not weight lost but length increase along the vertical direction of the well. Since the pipe is free on one of its ends (upper end - Rig Hooke) the MD Weight Indicator is simply showing the lifting effect on the Rig Hook.


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