Mud Related Drilling Problems Packing Off-Under gauge Hole

Packing Off

•Drilling-fluid systems with poor suspension characteristics exhibit strong packing-off tendencies
•Factors that can lead to caving of the formation include:
–Pressure imbalance
–Shale hydration
–Bottom hole assembly striking the wall
Massive particle caving sticks the drill bit.•The Solution is to increase the suspension characteristics of the mud

Mud Related Drilling Problems
Unde gauge Hole

•Under gauge hole is a condition where the borehole is smaller than the bit diameter used to drill the section.
•Under gauge hole can result from any of the following causes:
–Plastic flowing formations
–Wall-cake buildup in a permeable formation
–Swelling shales
•A plastic flowing formation is a formation that is plastic (easily deformable when stressed) and can flow into the borehole.
–When these types of formations are penetrated by the bit, the hole is at gauge.
–However, when the hydrostatic pressure exerted by the column of drilling fluid is less than the hydrostatic pressure of the formation, under balance results, the formation flows, and hole diameter decreases.
•Undergauge hole is a common problem when drilling a thick salt section with an oil mud.
–The salt can flow into the borehole and make the section undergauge.
–When plastic salt formations exist, they are usually below 5,000 feet.
–Spotting fresh water is the best way to free the pipe from a plastic salt formation.
•Wall-cake buildup occurs when the drilling fluid has poor filtration control across a permeable zone.
•Excessive wall-cake buildup can also be caused by:
–High percentage of low-gravity solids
–High differential pressures (excessive mud weights)