•A basic drilling fluid function is to seal permeable formations and control filtration (fluid loss). •Adequate filtration control and the deposition of a thin, low-permeability filter cake are often necessary to prevent drilling and production problems. •Potential problems from excessive filter-cake thickness: –Tight spots in the hole that cause excessive drag. –Increased surges and swabbing due to reduced annular clearance. –Differential sticking of the drillstring due to increased contact area and rapid development of sticking forces caused by higher filtration rate. –Primary cementing difficulties due to inadequate displacement of filter cake. –Increased difficulty running casing.
•Potential problems from excessive filtrate invasion:–Formation damage due to filtrate and solids invasion.–Invalid formation-fluid sampling test.–Formation-evaluation difficulties caused by excessive filtrate invasion, poor transmission of electrical properties through thick cakes–Oil and gas zones may be overlooked because the filtrate is flushing hydrocarbons away from the wellbore, making detection more difficult.