Greetings Priyaranjan,
I'll open with the caveat that I'm not a cementing expert (I cannot help with your fluid engineering) but have done exactly this in past wells.
First of all, do you have an open interval below, where bullheading is an option, or do you need to circulate to place a plug? What casing size are you working in? Inclination?
As you know, CaCl2 brine and cement don't play well together and you need to keep them apart. In order to prevent a flash set, and possibly to provide some supporting rheology, you'll want mud or similar as spacer fluid ahead of, and following, the cement. Key thing is that the spacer needs to be compatible with both the brine and the cement, ideally is water-based, and must be denser than your brine. It doesn't have to be a lot denser, but you want it to sit below the brine and not float upwards. I'd also suggest a solids-free spacer if possible, but that's not always easy. Water-based muds with very fine grind barite can work well.
There are a couple ways to do this: if you can squeeze, you could set a cement retainer at the top of the proposed plug. Sting in and verify injectivity. Then sting out and circulate as needed to spot your spacer to the end of the work string. Place plenty of spacer in the drillpipe annulus immediately above the retainer: do your best to get as close to the retainer as you can and pump plenty. Then sting in and bullhead. Continue with plenty of spacer to flush the brine away and follow with your cement. You can then sting out of the retainer and reverse-out, using that spacer you left above the retainer to keep the brine away from any cement that's still in the string, and end end up with clean CaCl2 in the wellbore above.
Once stung-in, you may want to "soft close" an annular preventer and maintain some back-pressure on the casing. This can help offset some of the tubing movement to help keep you stung in (depending on how much weight you can set down) and also ensures that flow is from annulus to casing when you first sting out at the end of the job. Apply just enough closure pressure to the annular BOP to allow you to maintain this annulus pressure, and you should be able to strip the pipe upwards as needed to sting out. We do this routinely when placing sand control and I usually practice this "soft close" and stripping procedure with the crew before the actual job. It helps to space-out the tool joints as far below the BOP as possible, though these can also be stripped through the the BOP.
This procedure is pretty common when moving uphole and recompleting in a new zone. The only worry is if the retainer leaks while you're reversing ... check your pumped and returned volumes carefully while reversing and don't get too aggressive with your applied pressure till after the cement has set.
If you cannot squeeze, you could set a cast iron bridge plug (CIBP) at the base of the proposed cement plug. Tag the plug with your work string and pick up just a bit, then pump the lead spacer, and then set a balanced plug above the CIBP while picking up the work string. You follow the cement with more spacer and then reverse-out above the cement, leaving some spacer behind.
Once again, you'll probably want to "soft close" an annular and maintain some back-pressure on the casing while doing this, to prevent the denser cement from "U-tubing" out of the workstring. The closure pressure has to be low enough to allow you to strip the workstring upwards.
With careful planning both options can work (and have worked in deepwater wells in the past) and leave you with a wellbore full of uncontaminated brine, without having to displace the whole well from brine to mud and back.
Good luck!
R
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Ron Nelson
Subsea Completion Consultant
ron@deep-blue.ca
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