Practical considerations with Cesium Formate brine
27 May 2017
We just held a fantastic 2-day CWOP for a deepwater, 'marginal HPHT' well completion campaign.
The intention is to use Cesium Formate (CsFo) brine during this activity.
The topic was thoroughly discussed: potential hazards to personnel, impact of elastomers, avoiding dilution (due to the ultra-high cost), the degreasing effect, ensuring maximum recovery (especially after gravel packing operations), filtration etc etc
But, beating the drum "you don't know what you don't know", what other practical considerations and solutions have our learned forum members encountered; those that fall into the category of "crikey, we never thought about that!".
Looking forward to your input.
We always advise selection of a suitable supply boat for cesium formate fluid operations. We also provide supervision at the LMP to brief the crew and advise best practice for minimizing dilution, contamination and losses. Supply boats tanks are cleaned and flushed with fresh water, and residual water is removed before a final check to make sure boat tanks are clean and empty. Residual cesium formate brine in the boat tanks after delivery to the rig is recovered when the boat returns to port.
We have left cesium formate brine in wells with BHST as high as 205 deg.C. and on those occasions the small amount of hydrogen detected when circulating bottoms-up indicates that a small amount of decomposition has occurred. As Ian said, you need a hydrogen-specific gas detector to confirm that the gas is hydrogen as opposed to reservoir gases.
Mostly the same concerns you would have for any high weight brine. But in addition - what is the temperature the exposure time at BHCT and the brine weight?
You can observe decomposition and/or corrosion with cesium formate.
Both mechanisms will lead to the generation of H2 and CO. if you are at high T you will need a sampler that will tell you you have H present as opposed to explosive gas ( e.g. methane ). You don't want too think you have a leaking liner lap when it is just your brine decomposing.
You may want to circulate to limit the time the brine is exposed to high T.
A key weakness in the supply chain is vessels. The brine supplier, mud company and rig crew should all be fully aware of the logistics plan relating to cleaning & drying pits & lines. You really need to get the same message to the actual crew on the actual boat(s) that will handle the brine. As far as they are concerned the CsFo is no different to diesel or drill water. Your challenge is to motivate the crews to clean & dry the vessel tanks, lines & transfer hoses scrupulously; to ensure that the same boat delivers & returns the brine and that it doesn't use the tanks & lines for anything else in the meantime. You need to decide if you can live with renting the CsFo in the ullage on the vessel tanks or if you need a plan to pump the boat tanks dry. You also need to be confident that the boat's pumps can actually pump this dense fluid up to the rig. It would be worth auditing the vessel(s) for cleanliness, pump capability and crew awareness prior to the actual loadout.
I am sure the delegates will have discussed the appropriate drill-in fluid, filter cake & breakers to ensure that there are no losses to the reservoir while the CsFo is in the well. Plenty of lab testing for breaker compatibility will be required.