A difficult P&A Challenge

Dear members

Time to put your thinking caps on again!

This is being submitted on behalf of a member....

"We’ve been abandoning a series of ~15yr old shallow onshore wells with a large diameter 10¾" OD surface casing set at say 120-150m and cemented to surface. TD is approx 250-350m.
There’s also an uncemented partly slotted 6⅝"  liner from TD to 30m above the top of the liner.

The outside of the liner is gravel packed in most wells from TD to just below the top of the liner.

Pressures are sub hydrostatic. Work over fluids basically water.

We’re using a major cementing contractor.

We’ve been setting plugs inside the liner from TD to 30-50m above the top of the liner, WOC overnight then test as required by the regulator next day (12 hrs rig).

The regulator requires a tag and p/test of the plug above the top of the liner and has approved a reduced value p/test value (<500 psi) based on the age, low spec and condition of the non API casing.

Anyway the P/test of this “shoe  plug” has failed in several wells in almost every case.

We’ve set plugs on top on this plug and also getting failures after WOC (say 15hrs). 500 psi UCA time is ~ 8 hrs. Coming back a week later we’ve got a p/test of the surface plug in 2 cases but not sure how valid this is.

We’ve been trying to get some 10¾" bridge plugs but having difficulty getting here in this size.

The slurry has fluid loss and friction reducer additives. The slurries in the 14.6 FAB - 15.6 ppg neat range with an LCM additive (low conc of polyester flake).

We did lose a sacrificial FG stinger in one case. Historically had other losses.

The samples look good. The Pre job tests are good. It’s got everyone stumped.

We’ve also run  a packer and the casing holds pressure but the plug doesn’t. Repeatedly applying pressure doesn’t reduce the losses.

From the pressure drop and fluid compressibility we seem to be losing only 1 litre or so in 10 mins in each test.

No surface pressure after plugs set.

Initial thoughts were of a path down the casing, some sort of non bonding.

Any thoughts on what’s happening.

- could the plug be slipping downhole slowly until the cement gets to a certain lateral strength  (1 litre is ~ 1 inch of movement) rather than losing through the plug. The force on the plug is something like 30,000 lb (P x area).

- anyone seen anything similar?

- other thoughts

- know of 10¾" BPs off the shelf (10.192 set) 


Thanks

Dave
15 Answer(s)