Imaging the formation process of cuttings: Characteristics of cuttings and mechanical specific energy in single PDC cutter tests

Polycrystalline Diamond Compact (PDC) bits account for most of the footage drilled in the oil field. Abundant information relating to cutter-rock interactions and rock breaking mechanism are provided by the cuttings and Mechanical Specific Energy (MSE) in single cutter tests, which are crucial for insuring the longevity and rate of penetration (ROP) enhancement of PDC bits. In this paper, a series of linear cutting tests was conducted at different depths of cut, back rake angles and cutting speeds on three kinds of rock. The formation of cuttings was recorded frame by frame with a high-speed camera. Then, the cuttings were collected and divided into five groups by sieving. Finally, the mass fraction change of cuttings in each group was compared with the MSE variation. The results show that a PDC cutter does not move at a constant speed: relatively slower when crushing and faster when chipping, and crushing dominates almost the entire cutting process. The mass fraction of chunklike cuttings generated in chipping is increased with decreasing back rake angle, while increasing depth of cut and cutting speed; and the mass fraction of powderlike cuttings generated in crushing shows an opposite trend. Furthermore, because of the crystalloblastic texture and well-developed cleavages in calcite, far more chunklike cuttings and totally different fracture surfaces are found when cutting marble, compared with sandstone and granite. Finally, a strong correlation between the mass fraction of cuttings and MSE is found. This indicates that the mass fraction of cuttings could be deemed as a simple method to evaluate the efficiency of rock breaking by PDC cutters. The results in this paper are expected to provide guidelines to the selection of drilling practices and PDC bit design.