0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000 0 2000 4000 6000 8000
Tangent Section Length (m)
80 82 84 86 88 90
These equations are provided for insight into the governing issues. For projections, DSS can be used to generate a running weight profile. An example profile is shown which illustrates how the maximum running weight and frictional considerations determine running limits:
DEPTH
CASING WEIGHT
Sub-Critical Well Section
Well Section Above Critical Angle
Maximum Running Weight Frictional Weight Loss
Maximum Running Depth w/o Weight Assistance
Figure 7-3.
Mechanical Weight Loss Considerations
Distinct from frictional losses, mechanical losses occur which reduce casing running weight. Mechanical losses can be caused by cuttings, casings, ledging, differential sticking, and centralizers embedding into formations. ERD casing running experiences showed mechanical losses can occur anywhere in open-hole and can be much larger than frictional losses. Mechanical weight losses may be as high as 100 kips while frictional losses may be much lower, i.e. 25 kips.
Circulation is usually effective in working through mechanical weight losses. As a result, fill- up/circulation tools are mandatory on critical ERD casing jobs to ensure that the string can be quickly circulated when problems appear. These tools, available from TAM, Frank’s, etc., are made up into the top-drive and allow simultaneous circulation and reciprocation of the string. This Wytch Farm casing job shows the effectiveness of circulation in removing excessive drag and restoring the casing running weight to normal levels.
F20 9-5/8" Casing Job.
0 50000 100000 150000 200000 250000 300000 350000 400000 0 1000 2000 3000 4000 5000 Measured Depth (m) . DSS Down DSS Up Act Ave Act Max Act Min Circulate @741m Circulate @2605m Circulate @3851m Wash Down @4106Figure 7-4. Wytch Farm casing job showing the effectiveness of circulation in removing excessive drag and restoring the casing running weight to normal levels.
If local casing running experience indicates that conventional running procedures may pose unacceptable risks in terms of getting to bottom, various modified running techniques and contingency measures are available as discussed below.
Partial Flotation Techniques
Unocal developed techniques for partially floating casings in their Platform Irene ERD operations which are now marketed by Davis-Lynch. Flotation greatly reduces buoyed casing weight and frictional losses. The basis of partial flotation is to float the lower casing section which extends into the long tangent or horizontal section. At some depth, a sub is made-up into the casing which allows filling the casing above the lower floated section with mud. The upper section provides normal buoyant casing weight to push the floated section into the well.
Although early partial flotation subs were mechanical and required tripping in the casing with DP, the tools are now hydraulic. Their operation is designed in two stages, the first being a release of the seal at one pressure, and the second being a shearing of the tool internals as part of the cement job. The seal release allows venting of the air in the floated section and a complete filling of the string with mud. Circulation is then established, and the casing cemented. A lead or tail cement plug is used to fully shear the sub internals and leave the sub with full drift.
Field experience (Hamilton Brothers in Liverpool Bay using Davis-Lynch subs) was, however, that the process was effectively one-step. This was due to the pressure surge from the initial release pressure causing a shearing of the sub internals, with those internals falling to the casing float collar during filling and mud circulation. This outcome had no impact on the operation and the jobs were successful in terms of flotation function of the subs and the cement job. A diagram illustrating the tool and the technique are shown: 14 15 13 9 28 5a 5 7 6 3 30 29 30 23 21 20 19 22 24 21 19 19 19 4 12a 16 17
Design issues for partial flotation include: • Optimum placement of the flotation sub • Shearing pressure margins
The releasing pressure must account for the initial hydrostatic imbalance between the mud-filled upper section and the lower evacuated (air-filled) section, and for pressure surges during running. Partial flotation precludes circulation while running, so selecting partial (or full flotation) relies on the prediction that frictional weight losses will dominate mechanical weight losses. In one Gulf-of-Mexico operation, an independent operator applied the Unocal partial flotation technique successfully only to find the casing moved freely after the sub was released and the casing had been fully filled with mud. This observation contradicted the pre-well prediction that the casing would not have remaining weight at the subject depth and, in this case, indicated that partial flotation may not have been required.
Full Flotation Techniques
ERD casings may also be fully floated. As with partial flotation, casing collapse must be checked and running weights projected. An additional issue is the casing may have little weight or in fact be buoyant. This impacts running equipment such as spiders and back-up tongs. If the projection shows that casing may be buoyant, slips should be acquired which can hold upward loads.
On Wytch Farm Well M3, the 9-5/8 inch 40 lb/ft C-95 casing was run to TD at 17,535 ft (5,346m) fully floated in a 10.4 ppg (1.25 SG) mud. This casing was buoyant and was pushed into the well using the top- drive and swivel. A drive-sub comprised of a drive shoulder welded onto a short DP pup was made-up into the top-drive. The drive-sub was lowered into the casing until the drive-sub shoulder contacted the coupling looking up. Top-drive and swivel weight were then applied to push the casing into the well. The M3 casing flotation was performed as a test to determine if the technique would work and to identify constraints and issues for its use on very long reach wells. The test was successful, although improvements were identified. On subsequent ERD well M5, the 9-5/8 inch was run to greater depths, i.e. 19708 ft. (6007m), with conventional mud filling and periodic circulation. Wytch Farm's experience to date has been that with good lubricity from OBM, their 9-5/8 inch casings can be run to bottom with conventional procedures and patience (pipe working) in the lower 12-1/4 inch section. The M3 casing flotation experience also shows that the procedure could be used for even higher departure wells with higher inclination and longer 12-1/4 inch sections.
Top-Drive Manipulation
As a contingency for unexpected problems while running casing conventionally, or as a planned procedure on very high departure wells (beyond those currently being drilled), procedures can allow top- drive manipulation of the casing to assist running. Top-drive manipulation could include the ability to circulate, reciprocate, rotate, and compress the casing. To provide these capabilities, a cross-over from casing to top-drive, high-torque casing connections and solid-body centralizers is required. The cross- overs should be made of integral stock and have at least matched-strength to the casing. Rotation of the casing by the top-drive would provide mechanical assistance in breaking up cuttings beds (and other obstructions) and removing friction. Rotation of filled casing would be limited due to high torque levels, but rotation of floated casing is predicted to be feasible even in deep sections.
Compression of the casing by the top-drive provides added running weight, and is applicable to filled or floated casing. Use of top-drive and swivel weight to compress the casing should be pursued only after analysis of the involved load path and component reactions. Analysis of the procedure on the Deutag T- 47 rig, which has a Varco TDS-4H top-drive suspended under a National P-500 swivel, resulted in several conclusions. These involved the reaction of the load through the top-drive main shaft and into the swivel cover housing and the fact that the limiting structural factor was the swivel cover bolts. As a result, these bolts were changed to a higher strength type. Enhanced casing shoes should also be considered (i.e., the “Silver Bullet” float shoe developed by BP Colombia). For severe ERD wells, it should be clear that enhanced (ribbed and tapered) float shoes, flotation methods, and top-drive manipulation can be engineered to produce a casing running system capable of flotation and rotation to extreme TDs.