The rock-breaking efficiency and durability of PDC cutters are critical to improving the rate of penetration and drilling efficiency. Previous studies on PDC cutter rock breaking have mainly been conducted at a constant depth of cut, and the penetration-shearing hybrid rock-breaking behavior and associated shear fracture characteristics during cutter penetration into the formation remain insufficiently investigated. In this work, a penetration-shearing hybrid rock-breaking experiment was developed on a vertical turret lathe to investigate the effects of lithology, depth of cut, back rake angle, and cutter geometry on the rock-breaking performance of PDC cutters. Pearson correlation analysis was further introduced to quantitatively characterize the correlations between the influencing factors and rock-breaking efficiency, and shearing fracture tests were also conducted at back rake angles of 30° and 35°. During penetration-shearing hybrid rock-breaking, lithology was the primary factor controlling the rock-breaking efficiency of PDC cutters, with both the cutting forces and mechanical specific energy during granite cutting being significantly higher than those during sandstone cutting. Depth of cut was another key variable affecting rock-breaking efficiency. As the depth of cut increased, the mechanical specific energy decreased rapidly and gradually approached a stable value, whereas aggressiveness increased monotonically and remained independent of lithology. Shaped cutters required less energy for rock breaking, exhibited higher rock-breaking efficiency, and showed stronger resistance to shear fracture. However, the correlations of cutter geometry and back rake angle with rock-breaking efficiency were weaker than those of lithology and depth of cut. At back rake angles of 30° or higher, the rock-breaking mode gradually shifted from shearing to crushing. Meanwhile, cuttings became more difficult to remove, the cutting forces increased, and periodic dynamic impacts intensified, which readily induced shear-fracture failure of the PDC cutter. And rational selection of the back rake angle and proper control of the depth of cut were effective measures for preventing premature failure of PDC cutters. These findings provided theoretical guidance for the design optimization and field application of PDC cutters.

The enhanced drilling parameters and custom-designed polycrystalline diamond compact (PDC) bits have greatly improved both rate of penetration (ROP) and footage. Then how to further improve the bit’s ROP and how to deal with the side effects caused by the enhanced drilling parameters remain a challenge. In this work, the single-cutter rock-cutting tests and full-sized bit drilling tests were conducted to investigate the effects of rock types, drilling parameters, and bit designs on ROP. The results showed that in the easy-to-drill formations, the enhanced drilling parameters had a more pronounced effect on improving the bit’s ROP than the optimizations of bit designs such as changing the cutter shape and size. On the other hand, in the hard-to-drill formations, smaller-sized and shaped PDC cutters combined with high-torque tools offered a promising approach to increase ROP. To further improve ROP and footage, two innovative approaches were introduced: improving the bit durability without compromising ROP to ensure one-trip drilling, and using extended directional nozzles together with enhanced hydraulic parameters. The bit durability was improved by optimizing the cutter shape and diamond materials, which helped complete the single-run footage of 2986 m in the field trial of Shengli Oilfield. It was also found that the extended directional nozzle was less effective under conventional hydraulic parameters, but increased the ROP by 32.1% under enhanced hydraulic parameters because of improving jet impact performance through reduced jet diffusion. The findings provided insights for ROP improvement in the oil and gas drilling operations.

Sand production is one of the main obstacles restricting gas extraction efficiency and safety from marine natural gas hydrate (NGH) reservoirs. Particle migration within the NGH reservoir dominates sand production behaviors, while their relationships were rarely reported, severely constrains quantitative evaluation of sand production risks. This paper reports the optical observations of solid particle migration and production from micrometer to mesoscopic scales conditioned to gravel packing during depressurization-induced NGH dissociation for the first time. Theoretical evolutionary modes of sand migration are established based on experimental observations, and its implications on field NGH are comprehensively discussed. Five particle migration regimes of local borehole failure, continuous collapse, wormhole expansion, extensive slow deformation, and pore-wall fluidization are proved to occur during depressurization. The types of particle migration regimes and their transmission modes during depressurization are predominantly determined by initial hydrate saturation. In contrast, the depressurization mainly dominates the transmission rate of the particle migration regimes. Furthermore, both the cumulative mass and the medium grain size of the produced sand decrease linearly with increasing initial methane hydrate (MH) saturation. Discontinuous gas bubble emission, expansion, and explosion during MH dissociation delay sand migration into the wellbore. At the same time, continuous water flow is a requirement for sand production during hydrate dissociation by depressurization. The experiments enlighten us that a constitutive model that can illustrate visible particle migration regimes and their transmission modes is urgently needed to bridge numerical simulation and field applications. Optimizing wellbore layout positions or special reservoir treatment shall be important for mitigating sand production tendency during NGH exploitation.