Research and Development Technical Section

Hello everyone,

I am Harsh Sahani from Dehradun, India. I am working on my assignment "Challenges in Cementing". Please post your suggestions both in onshore or offshore conditions.

Thanks,

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Harsh Sahani
Student
Dehradun
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Hi Harsh,

There are indeed many cementing challenges left for the daring (or headstrong). They range from fundamental understanding of cement behaviour once in place, to novel formulations and additives, to technologies to mix and pump cement, simulate its placement and finally non destructive techniques to evaluate its performance (wireline logging, distributed sensing and all that).

Here are some open questions about fundamental understanding:

• C-S-H gel and material science: predicting the behaviour of Portland class G, with or without aggregates, is fairly advanced. But the addition of silicates, especially pozzolanic materials, changes the nanostructure of C-S-H gel and increases the robustness of the resulting cement (e.g. resistance to chemical attack and thermal degradation) but micro-mechanical models of cement + fly ash are still absent.

• Initial stress in cement: the transition from slurry to solid is a complex interplay of poromechanics (including creep) and thermal effects in an "onion" system with steel, cement and rock. Interesting models have been published over the past 5-10 years, but experiments and evidence are still lacking. But as many have remarked, how can we hope to model cement mechanical behaviour (and failure) over its lifetime if we don't understand its state of stress? (And I haven't even started on how initial stress evolves as cement chemistry changes and creep takes place...)

• Cement plug failure modes: abandoned wells are coming into the limelight because of methane emission risk. We're much better at abandoning wells, and balanced plugs have come of age, so much so that pancake plugs (section milling) are touted as a potential universal approach. But abandonment has no return on investment, so costs have to be minimised: especially offshore deepwater there is a big push to have as little plugs are possible, with tubing and casings in place, and rely on coiled tubing at most. How far can we go? How do cement plugs fail? Some people fear about debonding, and the old issue of contamination/slumping is still debated. There is no data whatsoever on the reliability of cement plugs, and this affects our ability to optimise, improve and take rational decisions.

• Scaling in microannuli: debonding and microannuli are annoying leakage pathways (even though their safety and environmental risk is somehow limited). But how does chemical reactions between the leaking fluid, cement and casing affect the microannulus width? Can cement degrade substantially or, conversely, can scaling plug the defect? Mind that there have been some interesting PhD work on the subject, but not nearly enough on practical O&G situations.

• Acoustic signature of pathway flow: every few years people listen to flow through cement. Now it's the turn of the Distributed Acoustic Sensing (DAS). But what is the signature of defects? How (and if) can we quantify flow from the noise it makes? Fluid-structure interaction is interesting, and there is no reason tabletop experiments cannot shed light on this issue, away from proprietary (and obscure) correlations.

• Chimneys: pathways formed by overpressured brine or gas (or a mixture) flowing through setting cement have so far leaked a credible and consistent model, with rules-of-thumb used as a basis of design - with mixed success.

• Foamed cement: conceptual models and lab experiments suggest this is a wonder material, albeit fiendishly difficult to mix and place. But how does foam cement look once set outside the casing? There has been quite some soul-searching after Macondo, and yet harder data may be welcome.

Better stop here. If you're not looking for ultra-technical and offshore, you can add your wits to the vexed question of allowing penetrations through the subsea wellhead to monitor and manage outer annuli (B and beyond). Is it worth it? And yet it's still forbidden...

Best regards,

Matteo

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Matteo Loizzo
Well integrity consultant
Berlin
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Original Message:
Sent: Jun 29,2015 06:59 AM
From: Harsh Sahani
Subject: CEMENTING CHALLENGES

Hello everyone,

I am Harsh Sahani from Dehradun, India. I am working on my assignment "Challenges in Cementing". Please post your suggestions both in onshore or offshore conditions.

Thanks,

------------------------------
Harsh Sahani
Student
Dehradun
------------------------------