Separate height from matrix grade
Height describes the amount of cutting material. Matrix grade describes how that material wears in the formation. A taller matrix that polishes or loses gauge will not deliver the expected benefit. Start with rock hardness, grain size and abrasiveness.
Account for the time spent changing tools
A bit change can involve pulling and returning the entire string. As hole depth increases, this interruption becomes more significant. Compare productive cutting time with handling and trip time, rather than looking only at purchase price.
Compare matrix heights
13 mm is shown as the standard comparison; Rabbit options are 18, 23 and 28 mm. Height alone does not establish meters per bit.
Keep the hole and core diameters stable
Inspect the outer and inner cutting edges, not just the remaining matrix height. Abrasive intervals, vibration, loose core and an undersized reaming shell can shorten useful service before the matrix is consumed.
Plan circulation throughout bit life
Waterways must continue to clear the face as the cutting section wears. Monitor pressure and actual returns. Changing pressure can reflect restrictions, losses or reduced passage depth; interpret it with penetration and wear observations.
Compare runs on a common basis
Log the same formation interval, bit specification and operating data. Evaluate meters per bit, core recovery, cutting hours, trip hours and the reason the bit was removed. A longer life is valuable when the overall drilling cost and sample quality also improve.