Formation characteristics and penetration rate will dictate proper fluid make-up and flow. With an increase in penetration rate, fluid flow should be increased to properly clean the bit. The cuttings generated by an impregnated bit abrade the metal matrix around the diamonds, which plays an important role in keeping the bit sharp. Conversely, if the cuttings are washed away too quickly, the metal matrix will not wear away fast enough to properly expose new diamond surfaces, and the bit will polish, reducing performance.
In general, impregnated diamond bits should be run at higher rotational speeds (rpm) than are similar size surface set diamond bits. Maximum bit performance is achieved by running the impregnated bit as fast as conditions allow, without vibration caused by high rotation speed.
Optimum weight-on-bit, combined with proper rotation speed, fluid flow and penetration rate provide for a balanced system. The least possible weight-on-bit that maintains efficient penetration rates should be used. Applying insufficient weight results in polishing the diamonds, while too much weight, over a prolonged period, may damage the bit. When the bit fails to penetrate after adjusting weight-on-bit, rpm, or fluid flow, a different bit should be selected which is designed for drilling harder rock.
1): Stable, smooth, vibration free rotation optimizes bit cutting efficiency. Reaming shells and stabilized core barrel outer tubes should provide improved bit performance.
2): Experience indicates that optimum rotation speeds for impregnated bits in wireline applications should be 400 rpms or greater for N sized bits.
3): While bit lubrication with a polymer or water-soluble oil mixture is helpful, high fluid viscosity can create a cushion between the bit and formation, resulting in poor performance.