The low rock breaking efficiency of conventional polycrystalline diamond compact (PDC) bits in hard abrasive formations prompts the development of PDC cutting elements from the planar structure to the non-planar structure. As an innovative non-planar cutter, the design and research of the three-ridged diamond element (3-RDE) cutter is still in its infancy, and its rock breaking mechanism and laws are not yet clear. In this paper, a three-dimensional (3D) finite element model of dynamic rock breaking with 3-RDE cutter has been established. The accuracy of the numerical model was verified by experimental data. Then, the difference of rock breaking mechanism between 3-RDE cutter and conventional cutter was studied. The effects of back-rake angle, cutting depth, rotational angle, and rock properties on rock breaking efficiency were also analyzed. The results show that, unlike the conventional PDC shear rock breaking cutter, the 3-RDE cutter breaks rock mainly by crushing and shearing, and the rock breaking efficiency is higher. A small back-rake angle and reasonable cutting depth contribute to improving the rock breaking efficiency; the existence of rotational angle is not conductive to the rock breaking. The field application shows that compared with the conventional cutter, the 3-RDE cutter is easier to penetrate into the formation, and is more stable with less torque required. The research results can be of benefit to the design and manufacture of 3-RDE PDC bits.

Axial cracking in threaded joints of rotary steerable tools is a critical but under-investigated failure mode that can severely disrupt shale gas drilling operations. Understanding its root cause is essential for prevention. This study aims to determine the cause of an axial cracking failure in an S35150 austenitic stainless steel threaded joint from a field operation. A comprehensive analysis was conducted, integrating physicochemical characterization of the failed joint. The stress corrosion behavior of the threaded joint in a simulated corrosive environment was evaluated via four-point bend (FPB) and double cantilever beam (DCB) stress corrosion tests. The results showed that the material exhibited high susceptibility factors: a hardness of 38.5 HRC, a yield-to-tensile ratio near 1, and a P content exceeding the standard. Fracture surface analysis revealed an intergranular morphology with substantial chlorine (0.78%) and sulfur (0.93%) contents, indicative of stress corrosion cracking (SCC). The laboratory tests results demonstrated that the threaded joint had poor crack resistance: the fracture toughness value of the specimen measured by the DCB test was 24.14 MPa·m^0.5, and all specimens fractured during the FPB.

Cracking occurred in the surface coating of a screw rotor during shale gas well operations. To determine whether the coating cracks could contribute to the failure of the 42CrMo substrate, the microstructure and morphology of surface cracks and local corrosion pits were examined and analyzed using a metallographic microscope, an SEM, and an EDS. To investigate the cross-sectional morphology and elemental distribution of corrosion pits, EDS mapping was performed. The composition of the corrosion products was characterized using Raman spectroscopy and XPS. In addition, four-point bend stress corrosion tests were conducted on screw rotor specimens under simulated service conditions. The results indicate that the P and S contents in the screw rotor substrate exceeded the specified limits, whereas its tensile and impact strengths satisfied the standard requirements. The microstructure consisted of tempered sorbite and ferrite, along with a small amount of sulfide inclusions. The corrosion products on the fracture surface were primarily identified as FeOOH, Fe₃O₄, and Cr(OH)₃. All specimens failed during the four-point bend tests. The chlorine (Cl) content in the corroded regions reached up to 8.05%. These findings demonstrate that the crack resistance of the 42CrMo screw rotor was markedly reduced under the simulated service conditions of 130 °C in a saturated, oxygenated 25% CaCl₂ solution. The study concludes that stress concentration induced by sulfide inclusions in the screw rotor, together with the combined effects of chloride ions, dissolved oxygen, and applied load, promotes the initiation and propagation of stress corrosion cracking. Therefore, it is recommended to strictly control the chemical composition and inclusion content of the screw rotor material and to reduce the oxygen content of the drilling fluid, thereby mitigating the risk of corrosion-induced cracking of the rotor.