Why Does Lost Circulation Occur in Drilling? How Is LCM Selected?
Lost circulation occurs when part or all of the drilling mud enters fractures, voids, highly permeable zones or fractures induced during drilling. Lost-circulation material, commonly called LCM, should be selected according to the loss mechanism, opening size and equipment passage limits.
Are lost circulation and filtration the same?
No. In filtration, mainly the liquid phase enters the formation and a filter cake develops. In lost circulation, the drilling mud itself enters larger flow paths. PAC or CMC alone should therefore not be expected to seal a large fracture.
Why does lost circulation occur?
- Natural fractures and fissures,
- vugular or coarse-pore formations,
- gravel and highly permeable beds,
- new fractures created by excessive circulating pressure.
Can high pump pressure create new losses?
Yes. Excessive viscosity, high flow rate, a narrow annulus or unnecessarily high mud weight can increase downhole pressure. Before adding LCM, the reason for elevated pressure should also be investigated.
How does LCM work?
Different materials can bridge a fracture, fill voids, form a fibrous network or swell in contact with water to strengthen the seal. One material does not perform the same way in every loss zone.
How are fine, medium and coarse materials selected?
Material that is too fine may pass straight through a large opening; material that is too coarse may plug a nozzle, pump or screen. The particle-size distribution should be large enough to bridge the loss path while remaining controlled enough to pass through the equipment.
Why are different sizes combined?
Coarse particles can create the main bridge, medium particles fill intermediate gaps and fines fill the smaller remaining spaces. Fibrous or swelling components can support this structure when required.
How does SEALFIBER work?
SEALFIBER uses a fibrous structure to support network and bridge formation at the entrance to a fracture and can help retain granular materials.
How does FRACPLUG work?
FRACPLUG is a swelling LCM component that can increase in volume in contact with water and help strengthen sealing in fractures and voids.
Why is the carrier fluid important?
The LCM must reach the target zone. If the mud is too thin, coarse material may settle; if it is excessively viscous, pump pressure may rise. The carrier fluid must suspend the material while remaining pumpable.
What does staged treatment mean?
If the first treatment is insufficient, the result is assessed before simply adding much more material. A second step may use coarser, fibrous or swelling components as required, reducing unnecessary consumption and plugging risk.
SEALFIBER and FRACPLUG
SEALFIBER is positioned for fibrous bridging; FRACPLUG for swelling-assisted sealing. In suitable field conditions they can complement each other.
Frequently asked questions
If losses occur, is more bentonite enough?
Usually not for large fractures and voids. Bentonite supports filter-cake development and rheology; actual lost-circulation zones may require an appropriate LCM mechanism.
Can LCM plug nozzles?
Yes. Coarse or long-fibred materials should not be selected without knowing the narrowest passage in the circulation system.