SPECIAL CORING PROCEDURES FOR H₂S SITES

The following guidelines should be used unless otherwise instructed by the offshore installation manager.

• Maintain circulation while a core barrel is dropped or wirelined into and out of the hole.
• Control wireline speeds to prevent/minimize swabbing of H₂S-bearing fluids into the drill string from the well bore.
• Engage the core barrel with the overshot.
• Stop the pumps, if necessary, while unseating the core barrel from the outer core barrel assembly (OCB).
• Pull the core barrel up slowly above the top of the bottom-hole assembly (BHA).
• Stop the wireline winch once the core barrel is pulled above the top of the BHA.
• Pump three volumes of fluid (equal to the volume required to fill the inside of the BHA) to clear any H₂S-bearing fluid that may have been swabbed into the BHA when the core barrel was unseated.

See "Opening the Drill String on the Drill Floor" for additional operational safety information.

Drill Pipe Safety Valve

A drill pipe safety valve is kept on the rig floor. The valve can be used to prevent excessive backflow of fluid through the drill string. Such backflow could be the result of a hydrocarbon or hydrothermal fluid "kick." Either could possibly carry H₂S to the surface.

Install the valve in the drill string if excessive backflow occurs and the top drive is not readily available (i.e., racked back) for immediate makeup to the drill string.

Baker Float Valve

Once the safety valve is in place and closed, a second sub containing a float valve assembly can be made up on top of the safety valve.

The Baker model G (5F-6R) float valve acts as a check valve allowing drilling fluid to be pumped down the drill string while preventing hot or H₂S-contaminated well bore fluids from backflowing up the drill string.

The drill pipe safety valve and Baker float valve assemblies are to be used under special circumstances only (i.e., when the top drive is not installed or is otherwise inoperable for circulation). Note that if the Baker float valve is installed, no downhole tools can be deployed.

BHA Installed Float Valves

The rotary core barrel (RCB) BHA is equipped with a float (flapper) valve that is compatible with the RCB wireline coring system. The advanced piston corer/extended core barrel/pressure core sampler (APC/XCB/PCS) BHA can be equipped with a lockable float (flapper) valve (LFV).

Each of these float valves serves to prevent excessive drill cuttings from "U-tubing" or being swabbed up into the BHA during core barrel retrieval. When functional, these flappers seal the throat of the core bit and prevent backflow into the drill string via that path. It should be noted, however, that neither of the BHA float valves seal the bit jets, and therefore they do not completely seal off the lower end of the drill string from the well bore.

The 6-in drill collar/BHA used with the advanced diamond coring barrel (ADCB) coring system is not compatible with either of the float valves used with the RCB or the APC/XCB/PCS coring systems. It has been suggested, therefore, that use of the ADCB coring system be avoided in formations where H₂S might be present.