When bentonite is used in drilling the borehole for constructing a monitoring well, which method is best for removing the bentonite from the screen area?

• A. Using deflocculant agents and surging
• B. High-pressure jetting
• C. Mechanical scraping
• D. Backfilling with gravel

Answer: (A)

When bentonite mud remains in the screen area, the goal is to loosen and flush it out without damaging the screen or clogging the formation. Deflocculant agents work by dispersing the clay particles, reducing their tendency to clump into a gel that sticks to the screen and fills the slot openings. This makes the bentonite more suspensible in water. Surging then uses rapid in-and-out water pulses to physically dislodge material from the screen surface and surrounding grout or filter pack, carrying the loosened bentonite up and out of the borehole. Together, deflocculation and surging clean the screen while preserving the integrity of the screen, preserving open slots, and minimizing re-deposition of fines.

High-pressure jetting can erode or damage the screen and can push bentonite into the formation or create uncontrolled turbulence. Mechanical scraping may remove surface material but can scratch or damage the screen and often fails to reach material packed into the socket or slot openings. Backfilling with gravel is a filter-pack approach around the screen and does not effectively remove bentonite from the screen area itself.