4. SEAL PERMEABLE FORMATIONS
Permeability refers to the ability of fluids to flow through porous formations;
formations must be permeable for hydrocarbons to be produced. When the mud
column pressure is greater than formation pressure, mud filtrate will invade the
formation, and a filter cake of mud solids will be deposited on the wall of the wellbore.
Drilling fluid systems should be designed to deposit a thin, low‐permeability filter cake
on the formation to limit the invasion of mud filtrate This improves wellbore stability
and prevents a number of drilling and production problems. Potential problems
related to thick filter cake and excessive filtration include “tight” hole conditions, poor
log quality, increased torque and drag, stuck pipe, lost circulation, and formation
damage. In highly permeable formations with large pore throats, whole mud may
invade the formation, depending on the size of the mud solids. For such situations,
bridging agents must be used to block the large openings so the mud solids can form a
seal. To be effective, bridging agents must be about one‐half the size of the largest
opening. Bridging agents include calcium carbonate, ground cellulose and a wide
variety of seepage‐loss or other fine lost‐circulation materials. Depending on the
drilling fluid system in use, a number of additives can be applied to improve the filter
cake, thus limiting filtration. These include bentonite, natural and synthetic polymers,
asphalt and gilsonite, and organic deflocculating additives