Well Integrity Technical Section

What's the pressure behind the casing when cement is set after cementation?

Hi Ramanand,

Your question is far from obvious. I presume you want to calculate the casing collapse pressure, but let's first start by defining "pressure".

It could be one of three quantities:

• Stress exerted by cement on the casing. This is determined by the initial stress in cement - what is left of the original hydrostatic pressure after cement has set - and the radial stress applied by creeping formations (salt and shales, mostly). The initial stress is close to zero for proper cement (say, >15.4 ppg for class G cement), as shown by a few studies; if you're interested, you can look up papers by @Al Moghadam (e.g., https://www.sciencedirect.com/science/article/pii/S2352380824000534). As for closure stress by creeping formations, a thorough study has revealed that creep stress is very unlike fluid pressure, and that the API 5C3 collapse formula is way too conservative. Casing can collapse in salt, but it does so in a couple of ways, one probably being destabilization of the stress field caused by mud intrusion.
• Pore pressure in cement. Poromechanics of cement is funny, mostly because it is an impermeable medium (<1 microDarcy for decent cement). You have desaturation during setting and equilibrium with porous formations, but in general it is an afterthought, with deviations of around 10% with respect to a purely elastic solution. At any rate, the casing would see stress from the solid matrix rather than pore pressure. So we're back to the conclusions from the previous point "stress is zero, unless you have creeping formations; in which case don't think about 'pressure'".
• The third type of "pressure" is the one exerted by fluid in a defect, either a large one (free pipe or mud pockets) or a thin one (microannulus). Microannuli are fluid layers or empty spaces, so their pressure will depend on hydraulic connection with a source of inflow. Or to a kind of "annular pressure buildup" if you increase temperature in the casing and have trapped liquids in the annulus. The answer can be anything from zero (for a thin, dry microannulus) to pore pressure in a source of inflow (for a connected, water-filled microannulus).

Is there any specific scenario you'd like to discuss?

Best regards,

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Matteo Loizzo
Well integrity consultant
matteo.loizzo@mac.com
Berlin, Germany
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@Matteo_Loizzo has, as usual, answered the question completely.  The key question is what scenario are you talking about?  During cementing? After cementing? During production?  

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Dan Gibson
aka The Well Doctor
Completion & Well Integrity Advisor
Houston, Texas
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