Ledges and Doglegs
What is the definition of Ledges And Doglegs And How They are a cause for Mechanical Stuck Pipe?
ledges at formation changes
Ledge: The well bore passes through rock of varying types and ledges develop at the interfaces between layers of differing hardness.
Closely spaced ledges decreases the effective diameter of the hole and produce a “shouldered” hole surface. The stiff casing string requires a smooth, more in-gauge+ hole diameter for free passage and not to stuck. Doglegs: While drilling a well bore, the characteristics of the rock cause the bit to be deflected and can result in a change in Azimuth. Likewise when drilling with a directional BHA, sudden changes in angle can cause a kink in the well bore direction. Sharp deviations in wellbore direction are called doglegs.
When this may happen during drilling:
High doglegs stick the BHA while POOH.
- When an unsuitable Bottom Hole Assembly BHA is run.
- After a change in Bottom Hole Assembly BHA.
- Prognosed hard soft interbedded formations.
- Prognosed fractured / faulted formations.
- After direction changes.
- While POOH.
Preventative Action To Avoid Ledges and Doglegs Stuck Type
- Ledging can be reduced by using a packed hole assembly.
- Minimize direction changes in the well bore.
- Try to keep Bottom Hole Assembly BHA configuration constant.
- Reaming also is important.
- Record depths of ledges.
- you can bring a large scale printout from the mud loggers and to draw a scale Bottom Hole Assembly BHA on a separate piece of paper. The paper Bottom Hole Assembly BHA can be positioned at the depth of any Overpulls and it is easy to see if any of the Stabilizers are hanging up at the same point. By using this technique it is simple to keep track of multiple problem zones and to communicate expected problem depths clearly to the driller (Check also Driller Job Descriptions).
- Survey with sufficient frequency. Increasing the well bore survey frequency will:
- assist in evaluating/reducing well bore tortuosity.
- reduce the number of Bottom Hole Assembly BHA changes.
- Slow tripping pipe speeds before Bottom Hole Assembly BHA enters the suspected ledges zone or dog legs. Avoid prolonged circulation across soft inter bedded formations.
ledges and doglegs
- Limit initial set down weight to less than 50% of down Drag to minimize momentum effects when running into a tight zone. Do not start angle building operations too close to the shoe (start at least 30m below old hole
Ledges and Doglegs Stuck Indications Tips
- If you encountered Sudden erratic overpull or setdown.
- Problems are at fixed depths.
- Full circulation is possible.
How To Free ledges and doglegs stuck pipe?
- If moving up when ledges and doglegs sticking occurred, apply torque and jar down with maximum trip load.
- If moving down, jar up with maximum trip load. Do not apply torque.
- If able to, backream or ream very slowly past problem as rotation will assist the Stabilizers and/or other Bottom Hole Assembly BHA Components to roll past the ledge.