We have following types of mud losses while well drilling on drilling rig.
1-Surface mud losses
Surface mud losses mean mud loss due to surface equipments like shale shakers, desander, desilter, mud cleaner and centrifuge. Solid control equipment do not discard dry solid, there is limited liquid portion of mud that is also discharge with cutting or solid. This drilling fluid loss with cutting or solid being exit from solid control equipment is called surface mud or drilling fluid loss.
We can minimize surface loss of mud on oil and gas well drilling by following steps.
- Use proper shale shaker screen. A good shale shaker screen cover 2/3 of shale shaker screens area.
- Take mud return on more shale shaker as per mud flow rate, to prevent over flow from shale shaker.
- Clean and wash shaker screen time to time to avoid screen plugging.
- Ensure spray discharge from desander and desilterhydrocyclone for minimum liquid discharge from apex.
- Add water at required rate by a small hose inside ditch line to compensate for liquid phase of drilling fluid converting into vapour due to temperature.
Hole fill is mud required to fill the drilled hole as well drilling progress. As drilled cutting go out from solid control equipment, the space occupied by drilled cutting is filled by mud in well bore. This is called hole fill.
3-Down hole mud losses
It is the mud lost in the formation while well drilling due to high permeability, fracture or higher mud weight. We are more concerned with these losses, depending on severity down hole losses can be classified as.
- Spurt losses
- Seepage losses
- Partial losses
- Severe losses
- Complete losses.
These are spontaneous mud loss into formation as the formation is exposed to drilling fluid before filter cake is build up on wall of wellbore. Higher the permeability more will be spurt losses.
These are down hole losses ranging from 2 to 10 bbl per hour.
In partial loss most if mud being pumped is return to surface where as part of it lost into formation. Partial losses are easy to manage as drilling rig mud system mixing hopper is capable to build up more mud to continue drilling.
In severe losses, most of mud pumped into hole is being lost and very less amount of mud returning to surface. These losses must be cure before continue further drilling. Mud pits will get empty if continue drilling and mixing hopper is not capable to deliver required mixed mud.
Complete losses also called total losses, in this kind of mud losses, whatever drilling fluid is pumped its being lost into wellbore. Severe and complete losses can be cured by LCM pill or cement plug. It may take several attempts with LCM pill or cement plug to cure these losses to acceptable range. `