Some aspects of casing / liner flotation
On extreme ERD wells casing flotation could be the only option for long high angle casing strings. In basic form the casing is run empty and filled with mud when on depth (making the casing lighter in the tangent, reducing drag, through by increasing buoyancy). Technique can be applied to casing or liners.
· Air filled – casing is run in hole empty (filled with air).
· Light mud filled – casing is run in hole filled with lighter mud than in the annulus.
· Mud over air – lower casing section is empty, upper section mud filled. Selective Flotation Collar (SFC) has to be used to compartmentalize the string.
Basic strategy for casing flotation is to use buoyancy to reduce pipe weight to minimize the side force, which, in turn, reduces drag (and torque) making the casing easier to run (or rotate) into the hole. In a fluid, buoyancy is produced by an amount equal to the magnitude of the weight of fluid displaced by the body.
Casing flotation appears simple but requires careful planning and a good understanding of risks and that management fully understands them, as well.
Risks associated with casing flotation:
- Inability to circulate through any cuttings while running in hole
- Casing collapse or float equipment failure may result in a well control event and/or collapse of previous string:
o Annulus barriers are mud and BOP.
o Internal barrier is full opening safety valve if it can be installed or shear rams if they have the capacity to shear the pipe. Tested float equipment is not an effective barrier if the casing collapses above the equipment.
Well control risk mitigations:
- High collapse casing with premium connections (account for surge and order the best quality pipe and connections available!).
- Ensure prior casing string is designed for partial evacuation (one string of collapsed casing is bad enough!).
- Monitor backside and have ability to fill at high rate.
- Differentially tested float equipment with redundancy (2-3 float valves).
- Fill shoe joints of air filled casing with mud or base oil.
- Ensure casing running tool (CRT) pack-off pressure rating is adequate.
- Allow enough time to fill casing safely (air swap).
- Develop a plan for stabbing the XO and safety valve on top of the casing when it is high in the derrick.
Rotating floated casing:
Casing running tools / drive systems (CDS / CRT) are the best tools for casing rotation. A few companies provide this type of service (i.e. Weatherford, Franks, Tesco and Volant).
- High torque connections suitable for bending are recommended.
- Ensure the break-over torque is accounted for when selecting the system. Plan to minimize pipe stationary time to reduce the break-over torque.
- The higher the RPM and the slower the running speed the more axial friction will be broken and the closer the drag will be to the modelled free rotating weight.
- Keep RPM as low as possible to reduce wear of CDS and keep frictional heat low (collapse resistance reduces as the pipe temperature increases).
- Consider placement of float collars, stab-in type casing running system cannot be used if the float collar is made up to the top of a joint.
- If casing must be rotated all the way to TD use a slip-type hanger to avoid the stationary pipe time spent laying down the CDS to pick up the hanger and landing joints, also the mandrel hanger cannot be rotated into the wellhead.
Casing Flotation – Other Considerations
- Consider proactively back-reaming the wellbore prior to floated casing run as no circulation is possible while running. This is still standard practice across the industry.
- Closely monitor the actual run to optimize SFC placement and record data and trends.
- Maximize displaced and contingency fluids storage.
- Exercise caution tagging bottom to ensure shoe does not become plugged.
- PU slowly to avoid swabbing in a kick or inducing wellbore instability.
- Ensure Casing Stretch Calculation caused by introducing dense mud is accounted for when filling the pipe.
- Allow time for mud and air to swap before breaking circulation.
- Condition the mud prior to tripping out and consider thinning and reducing gel strength to keep surge and circulating ECD as low as possible.
- Centralization:
o Solid body, particularly if planning to rotate.
o Minimal number of centralisers to provide an adequate cement job and to mitigate differential sticking if a concern. Centralisers in open hole will only increase drag due to stiffness and extra weight.
o Flexible shoe track to avoid ploughing through build and to help navigating ledges or other obstructions.
