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External coating of surface casing
14 June 2009
Here in Oman we have a highly saline shallow aquifer (7000-10000 mg/l chlorides) down to around 700 m which has also contained H2S in some of our appraisal wells. We currently drill this in 17-1/2" hole and case with 13-3/8" L-80 casing. Best practice here is to ensure cement to surface, but the zone is also prone to severe or total losses, and top-up jobs are routine so an inadequate cement sheath is more or less inevitable, raising the possibility of long-term corrosion of the 13-3/8".

We are contemplating mitigating this risk on our development wells by using externally coated casing so we were wondering if anyone know what options are available, if there are any limitations, price to apply, ease of application etc and also experiences people have with using coated casing to protect from shallow aquifers. We would also like to hear if anyone who has a similarly corrosiove shallow aquifer has had any corrosion issues - timescale, consequence etc - because on our appraisal well the 13-3/8" casing is not protected so we would like to understand the long term integrity risk.

Doug Noble
SDE, BG-Oman
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2 answer(s)
Sr. Drilling Engineer
Shell E&P
Total Posts: 1
Join Date: 22/05/09

Many moons ago, I worked for Chevron on Barrow Island, NW Shelf, Australia in the early '90's. For the onshore infill wells, we had issues with the first 5-6 joints of conductor being above the water table, and it was managed by externally coating the conductors prior to running (the coating sounds very much like the FBE that Sean mentioned). Things to watch out for - this extra pipe OD then becomes a very tight fit for the Single Joint Elevators (get stuck "on", and/or won't close around the pipe), and after slips/tongs have been used for makeup and running, you need to apply "repair" epoxy coating as the coating gets chipped/worn off at the load points... and if run in-hole un-covered/exposed, this would become a concentration point for accelerated external corrosion and completely defeat the purpose of the coating in the first place.

Good luck.
Rgds, Blake Stephenson (Shell-Rijswijk, Netherlands)
Roc Oil Company Limited
Total Posts: 4
Join Date: 24/01/08
PDO (and others) have been fighting this issue for years and have tried many different ways to resolve it (including just ignoring it!). This was seen to be a major hazard in project I was involved with for four years (Harweel) where the high pressure and sour nature of the wells required significantly more thought with respect to well integrity. The final verdict was that to obtain cement to surface was almost impossible (read expensive) given the cavernous nature of the shallow formations. Coated pipe in combination with cathodic protection was implemented. I'm not sure now of the type and cost, but we didn't put it on all joints, just at the aquifer level. I am fairly sure it was around $1500 per joint, but don't quote me on that. If you happen to bump into John Grieve or Matt Smith at one of the drinking holes in Muscat, they may be willing to part on a significant amount of data, including shallow logs, calipers etc.

I understand that some success was achieved with the early Mukhaizna wells were the heat generated by steam injection meant cement to surface was extremely important. I believe it was through a combination of cement plugs and foam cementing of the casing. Again it's expensive so depends on the criticality.

One rumour flying around PDO from many years ago is that they installed cathodic protection on their wells. Many years later, in a cost cutting environment, they decided to turn it off (because no wells had failed). You know where this one is going.......

Sean - How you doing?

Andy Mellor
Roc Oil, Beijing
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