Baryte plugs to cure losses.Drilling 12 1/4" hole through a fault at 13,700 ft. Thereafter, gas levels continued to increase (80%). Despite weighing up the mudf, high gas levels were measured. Two gasblok cement plugs were set across / above the fault, but were unsuccessful (80% gas levels monitored after setting the plugs).
Consequently, it was decided to drill out the cement and set baryte plugs above the fault. The objective of the plugs being to contain the gas so that a cement plug could then be set without being contaminated by gas.
A 1,196 pptf 170 ft. baryte plug was spotted from 13,300 - 13,130 ft, resulting in the gas levels dropping to 23% on BU. A second 1,196 pptf 170 ft. baryte plug was then spotted from 13,105 - 12,935 ft. The maximum gas reading on BU after this plug was 6%. Cement plugs were then successfully set on top of the baryte plugs.
Hazards :
1. Danger of getting the string plugged => keep fluid moving
2. Danger of getting the string stuck => keep string moving when plug leaves muleshoe.
3. Possibility of backflow.
Plug formulation :
To make 60 bbls : 26 bbls of base oil
5 Gls water
1 mud cup Hymul
Add baryte to 23 PPG.
During mixing, add 40 lts of OW (oil wetting) to reduce thickening. Immediately prior to pumping, add 3 Gls of Defloc.
Plug settling time :
Settling occurs after 1 min, hard settling after 10 mins (static conditions).
Job outline :
A 170 ft plug was set using a 3 1/2" stinger, with a Kelly cock placed 90 ft below the cement head. Pumped 10 bbls of base oil spacer ahead. Pumped 25 bbls of baryte plug and 1 bbl of base oil behind (10 bbls of base oil in annulus gave same height as 1 bbl of base oil in the stinger). Spotted the plug at 430 gpm. As the baryte plug left the muleshoe, the pumps were reduced to 50 gpm and the stinger POOH (such that the bottom of the stinger was always just below the top of the baryte plug). After pulling the first stand, the Kelly cock was closed (in the unlikely event of backflow!!) and the stand laid out. The Kelly cock was opened and the second stand POOH without pumping (allowing the plug to fall out, +/- 3 bbls underdisplacement). After pulling the second stand, the bottom of the stinger was 10 ft above the top of the baryte plug (20 ft. if crown-o-matic was disabled).
Recommendations :
1. Baryte plug must be underdisplaced.
2. Advisable to set a maximum plug length of +/- 170 ft. As a result the pipe will be stationary only once while spotting the plug.
3. Ideally, plug should be mixed in a batch tank to maximise the mixing rate.
4. Minimise surface rig-up, to minimise the time that the string is stationary while spotting plug