The Rock Fragmentation Mechanism in Rock Cutting with PDC Cutter Under Hydrostatic Pressures

Drilling into deep formation characterized by high formation and hydrostatic pressures often brings great challenges to oil and gas industry and the phenomena, such as bit slip, balling and cutter wear, are easy to be exposed. Therefore, it is very important to conduct rock-breaking experiments under high confining and hydrostatic pressures and understand the cutter–rock interaction behaviour for revealing the rock-breaking mechanism of Polycrystalline Diamond Compact (PDC) cutter. In this paper, granite, sandstone and shale were used to carry out PDC cutter cutting experiments under confining and hydrostatic pressures. Cutting force, mechanical specific energy (MSE), cuttings, cutting grooves and energy change were comprehensively analyzed when rock suffers different failure modes under different cutting conditions. The results show that hydrostatic pressure significantly increases the cutting force and MSE compared with cutting results under atmospheric pressure. In addition, different lithology of rocks leads to the different changing trends of cutting force with hydrostatic pressure and affects the pore pressure and differential pressure under hydrostatic pressures. Besides, the smoother cutting grooves and fragmentary cuttings are obtained with increased hydrostatic pressure during cutting and the mud phenomenon of cuttings is witnessed. Furthermore, the fluctuation of cutting force curve and the number of peaks and valleys in cutting force curve reveal the failure behaviour and energy change of rock during cutting. In general, the distance between peaks or valleys is small and more peaks and valleys are obtained when rock suffers ductile failure. Meanwhile, the energy absorption time of rock is shorter and the small-sized cuttings are obtained. Conversely, the distance between peaks or valleys is large and less peaks and valleys are produced when rock suffers brittle failure and the energy absorption time of rock is longer and large-sized cuttings are obtained. Although hydrostatic pressure tends to change the failure mode of rock from brittle to ductile, the number of peaks and valleys in the cutting force curves obtained under hydrostatic pressures is less. The experimental results provide a new understanding of the failure behaviour of rocks and cutter–rock interaction under different cutting conditions.