Contingency - Heavy losses

It is recommended not to attempt to drill ahead with heavy losses. LCM pills and/or cement plugs should be set as soon as the losses occurred, so that they can be positioned easily on bottom and across the loss zone.

If losses are not manageable, the hole can be opened to 20” and a 16” contingency casing will be set. The next section will be drilled with 14 ¾” bit and BHA to 13 3/8” casing setting point, and opened to 16” in order to run and cement the 13 3/8” casing.

The alternative will be to drill blind. Some wells had multiple loss zones. In this event, the following guidelines should be followed when losses occur at the interface:

• Rapid return to good fluid loss control is recommended
• Work the pipe regularly. This should involve drilling ½ a single and reaming the same section again before continuing.
• Every third stand, perform a wiper trip back to the depth where losses commenced.
• Watch drilling parameters as described above for warning signs of the hole packing off.
• Maintain normal flow rates. In the event of running out of water on site, pull back to the shoe and wait for water while monitoring for possible gas.
• Check that shakers are totally clean before commencing POOH operations.

Ensure a drilling water line is lined up on the drill string by the 20” casing and drill water is bled at an agreed rate, which will depends on the rates of the mud returns and if a total lost circulation situation had occurred.

Bleeding drill water in the annulus is called low head procedure, the basic idea is to find out the dynamic level of the mud in the annulus and adjust the drill water filling/bleeding rate accordingly.

Guidelines for suspected bit balling

Avoid bit balling (- even to the detriment of ROP).

P/u 0.3 m off bottom, increase flow rate and rpm to maximum possible. This worked on 9806 when ROP increased from 0.7m/hr to 2.7m/hr.

An attempt was also made on Well 9806 to clear bit balling using a nut plug sweep to "scour" the 16" bit. The result of the test was "inconclusive" - but as no mud ring fragments were returned with the nut plug at the shakers it is most likely that this material is not the best medium for providing a "scouring" action. On Well 9808 a nut plug sweep was pumped ( for a different reason ) and this plugged the surface screen which is run in the drill pipe to protect the MWD. It was speculated that the screen plugged because the nut plug sweep was not prepared correctly - however in order to avoid similar problems in future, ( particularly downhole at the MWD), the use of nut plug as a bit balling treatment is not recommended.

When POOH, look for (and report in the DDR) any increase in overpull as the stabilisers and bit enter the casing shoe. This is a good indication of the presence of Bit /BHA balling.