Experimental study on conglomerate-breaking mechanism by rotary cutting with triple-ridged PDC cutters and planar PDC cutters

With a rapid development of global economy, the dependence on oil and gas resources has been increasing year by year. Being the largest conglomerate oilfield around the world, Mahu Oilfield is wellknown for its abundant reservoir resources but characterized by complex lithology, substantial particle size variation, and high heterogeneity, which result in severe wear and damage to conventional PDC bits. Relevant researches have shown that the triple-ridged PDC cutter (TRC) exhibit an excellent ability of conglomerate-breaking. However, there is a paucity of research on the method of hybrid-cutter layout of the TRC particularly in the context of conglomerate formations, and a dearth of high-performance PDC bit. Therefore, in this paper, the multi-cutters synchronous rotary cutting conglomerates (compressive strength about 200 MPa) experiments with triple-ridged PDC cutters and planar PDC cutters (PLC) were innovatively carried out. The cutting angle in the experiment was 20 degrees and the rotational speed was 65 rpm. The effects of TRC-PLC cutters layout parameters on the cutting force, rockbreaking volume, and mechanical specific energy (MSE) are analyzed. Meanwhile, a high-speed photography is employed to record the rotary cutting process, and a 3-D scanning profiler is utilized to observe the cutting grooves to reveal the conglomerate-breaking mechanisms from a microscopic perspective. The results indicate that the lateral thrust generated by TRC effectively removes the cuttings away from the front of cutter, facilitating the rapid evacuation. Furthermore, TRC can induce numerous micro-cracks in the rock, thereby weakening both the matrix strength of the conglomerate and the bonding strength of the gravel particles. Thus, properly configuring the cutter spacing distance (CSD) along the same cutting plane and the cutter protusion height differential (CPHD) between the front and the rear row cutters can enhance the rock-breaking efficiency of the coupled cutting elements. Specifically, the optimal CSD and CPHD were determined to be 4 mm and 0.75 mm under the experimental conditions, respectively, resulting in a minimum MSE. Following these experimental findings, a hybrid-cutter PDC bit tailored for such a conglomerate formation was developed. The field test showed the bit increased the footage by 109.43%-370.12% and the rate of penetration (ROP) by 7.88%-63.37% when compared to previous bits in the neighboring wells at the same conglomerate formations. These findings can provide significant theoretical support for personalized hybrid-cutter PDC bit design in high-heterogeneity formations.