Nozzles are widely used in the petroleum industry, including cleaning, auxiliary rock breaking, perforation, and other functions. As the most commonly used nozzle type, the cone-shaped nozzle has been widely studied, the flow properties nearwall play an essential role in flow resistance and need to be revealed. In this paper, the large eddy simulation model numerically were used to simulate the flow field in the nozzle. The velocity field distribution, velocity gradient, wall pressure, wall shear stress, skin friction coefficient in the nozzle were analyzed. The following results were concluded: the velocity distribution of the fluid changes from a parabola to an M shape during the acceleration process in the contraction section, and Peaks appear near the wall; boundary layer transitions and vortices appear near at the beginning of the converging section; boundary layer transitions and separations occur near at the end of the converging section and the beginning of straight pipe sections, where the velocity gradient, wall shear force, and skin friction coefficient all have peak values; the flow near the wall of the straight pipe section is vortex-laminar alternately; the wall shear force, velocity gradient, and skin friction coefficient of the straight pipe section show periodic fluctuations, which alternate with the flow state near the wall. Most of the nozzle flow resistance is generated in the nozzle straight pipe section, and the appearance of the vortex reduces the skin friction coefficient. When the nozzle drag reduction is performed, the contour of the transition from the nozzle contraction section to the straight pipe section can be optimized, and the straight pipe section should also be considered when passive drag reduction design is carried out inside. By describing the flow characteristics near the inner wall of the nozzle, the location and reason of the frictional resistance of the internal flow characteristics of the nozzle were analyzed, which aimed to provide theoretical support for further optimizing the nozzle structure and reducing the nozzle flow resistance.

Hot dry rock geothermal energy, as an unique renewable energy, is characterized with abundant reserves, wide distribution, carbon-free, stable and efficient supply. High-efficient development of hot dry rock geothermal energy is of great significance to promote the transformation of China’s energy structure and pursuit of low-carbon development. In this paper the distribution and petrogenesis of hot dry rock was presented, and the endowment and heat source mechanism of geothermal resources in China were introduced. Additionally, the main development methods of hot dry rock geothermal energy were elaborated including drilling and well completion, hydraulic fracturing, and heat extraction. Afterward, a future research outline was proposed aiming at fundamental scientific research. It was suggested that a combined theory of “rock mass characterization-drilling and completion-hydraulic fracturing-heat extraction-integrated regulation” would be worked as the main research route to investigate dynamic damage mechanisms and fracture network connection of hot dry rock, and the multiphase coupling mechanism of the thermal-hydraulic-mechanical-chemical effect in the heat extraction process would be elucidated. Finally, a new method of drilling, fracturing, heat extraction, and synthetical control theory of hot dry rock were formed, which was expected to provide a theoretical foundation for the efficient development of geothermal energy in hot dry rock.

Hydra jet fracturing is an effective stimulation method for unconventional reservoir exploitations. Fracturing fluid migration in annular influences behaviors of multi-cluster fractures initiation and extension. To investigate the fluid transportation in annular, a model was adopted to figure out the flow filed feature around water-jet and the relationship between fluid migration and some key parameters, i.e. annular velocity, jet velocity, fracture extend pressure difference (FEPD) and annular outlet pressure. It demonstrated that flow field variation in annular primarily located around jets. Static pressure in jet-core was higher and pressure-drop zone exists around jets. A point-sink-like flow existed around jets. Simultaneously, the flow field distributed heterogeneously. There was no interference among clusters. In the same situation, maximum flow rate difference between clusters was merely 0.08 kg/s, the variation of referenced cluster flow rate was just 0.05 kg/s, the FEPD among clusters was 4 MPa,and clusters’ flow rate changed according to a uniform law as fracture extend pressure changes. A positive relationship existed between clusters’ flow rate and nozzle velocity or annular outlet pressure, while annular velocity had no impact. All parameters mentioned above had no influence on flow rate difference. FEPD among clusters was a domain parameter of difference of clusters’ flow rate. Flow rate in each cluster increased linearly with FEPD deducing. In addition, the flow rate difference possessed a positive linear trend with FEPD. Utilization of fluid would reach to 100%, when annular velocity was less than 3 m/s or annular outlet pressure surpass 40 MPa in this study. This research may function as a reference in hydra-jet fracturing design.

Radial swirling impeller abrasive water jet drilling technology is a potential cost-effective way to develop carbonate reservoirs. Swirling impeller abrasive water jet carbonate rock breaking experiments are utilized to study the feasibility of drilling radial branches in carbonate reservoirs. Besides, the influence low of process parameters and optimal process parameter combination on the swirling impeller abrasive water jet carbonate rock breaking performances are studied and illustrated. Finally, the carbonate rock breaking mechanisms of swirling impeller abrasive water jet are discussed and revealed. The results of this study showed that the swirling impeller abrasive water jet could create a regular smooth circular hole with a conical bulge at the hole bottom. The cross-section of the swirling impeller abrasive water jet drilled hole presented a “W”-shape. The jet drilled hole diameter was greater than the maximum outside diameter of the jetting pipe string and the conical bulge at the hole bottom could be eroded by the subsequent jet, which proofed that it was feasible for the swirling impeller abrasive water jet to drill radial branches in carbonate reservoirs. The jet drilled hole diameter increased with the increase in the jetting pressure, the standoffdistance and the exposure time. And, it increased firstly and then decreased with the increase in the abrasive mass concentration. The jet drilled hole depth and hole volume increased with the increase in the jetting pressure and the exposure time, and with a decrease in the standoff distance. And, they increased firstly and then decreased with the increase in the abrasive mass concentration. The optimal process parameter combination under our experimental conditions was the jetting pressure, standoffdistance, abrasive mass concentration and exposure time were the 30 MPa, 0, 9.0%~10% and 60 s respectively. In the process of swirling impeller abrasive water jet carbonate rock breaking, the swirling impeller abrasive water jet not only through the tension force that was created by the high angle abrasive particle impact, but also through the shear force that was created by the low angle abrasive particle impact to break the carbonate rock. Since the tensile strength and shear strength of carbonate rock material was far small than the compressive strength, the swirling impeller abrasive water jet possessed a high rock breaking ability and efficiency on carbonate rock. The research results were expected to provide a theoretical basis and experimental foundation for utilizing the swirling impeller abrasive water jet to drilling radial branches in carbonate reservoirs.

The oil and gas stored in deep and ultra-deep carbonate reservoirs is the focus of future exploration and development. Conical PDC (Polycrystalline Diamond Compact) cutter, which is a new kind of PDC cutter, can significantly improve the rate of penetration (ROP) and extend PDC bit life in hard and abrasive formations. However, the breakage characteristics and failure mode of the conical PDC cutter cutting carbonate rock is still masked. In this paper, a series of single-cutter cutting tests were carried out with the conical and conventional PDC cutters. The cutting force, rock-breaking process, surface morphology of cutting grooves and cuttings characteristic were analyzed. Based on the derived formula of the brittle fracture index, the failure model of carbonate rock was quantitatively analyzed under the action of conical and conventional cutter. The results show that the average cutting force of the conical cutter is less than that of the conventional cutter, which means greater stability of the cutting process using the conical cutter. Carbonate rock with calcite as the main component tends to generate blocky rock debris by conical cutter. The height of the cuttings generated by the conical cutter is 0.5 mm higher than that generated by the conventional cutter. The conical cutter exhibits enhanced penetration capabilities within carbonate rock. The accumulation of rock debris in front of the conventional cutter is obvious. Whereas, the conical cutter facilitates the cuttings transport, thereby alleviating drilling stickiness slip. At different cutting depths, the conical cutter consistently causes asymmetric jagged brittle tensile fracture zones on both sides of the cutting groove. Calculations based on the brittle fracture index demonstrate that the brittle fracture index of the conical cutter generally doubles that of the conventional cutter. For carbonate rock, the conical cutter displays superior utilization of brittle fracture abilities. The research findings of this work offer insights into the breakage process and failure mode of carbonate rock by the conical cutter.

Hydraulic fracturing is considered the main stimulation method to develop shale gas reservoirs. Due to its strong heterogeneity, the shale gas formation is typically embedded with geological discontinuities such as bedding planes and natural fractures. Many researchers realized that the interaction between natural fractures and hydraulic fractures plays a crucial role in generating a complex fracture network. In this paper, true tri-axial hydraulic fracturing tests were performed on polymethyl methacrylate (PMMA), on which pre-existing fracture was pre-manufactured to simulate natural fracture. A cohesive model has been developed to verify the results of the experimental tests. The key findings demonstrate that the experimental results agreed well with the numerical simulation outcomes where three main interaction modes were observed: crossing; being arrested by opening the pre-existing fracture; being arrested without dilating the pre-existing fracture. Crossing behavior is more likely to occur with the approaching angle, horizontal stress difference, and injection rate increase. Furthermore, the higher flow rate might assist in reactivating the natural fractures where both sides of the pre-existing fractures were reactivated as the injection rate increased from 5 to 20 mL/min. Additionally, hydraulic fractures show a tendency to extend vertically rather than along the direction of maximum horizontal stress when they are first terminated at the interface. This research may contribute to the field application of hydraulic fracturing in shale gas formation.

Liquid nitrogen (LN₂) fracturing is a kind of non-aqueous fracturing technology, which is expected to provide a new and efficient way for coalbed methane (CBM) development. The mechanical properties of coal under LN₂ freezing are very important for studying the mechanism of LN₂ fracturing. However, most of the current research is limited to studying mechanical properties of rocks after being frozen by LN₂ and returned to room temperature. In this paper, the effect of LN₂ freezing on the mechanical properties of coal was studied. Uniaxial strength tests and Brazil tests were carried out for dry and water-saturated coal samples with different types and bedding directions. In addition, standard electron microscopy (standard SEM) and cryo-electron microscopy (Cryo-SEM) were used to compare the fracture morphology of coal samples at room temperature and LN₂ temperature. The results showed that LN₂ freezing can damage and improve the mechanical properties of coal simultaneously. The strength of saturated coal under freezing is higher than that of dry coal, and the filling of ice can enhance the mechanical strength of coal. In addition, the mechanical properties of coal with higher porosity are enhanced more than that of coal with lower porosity under LN₂ freezing. The main findings of this study are the keys to the research of LN₂ fracturing mechanisms in CBM reservoirs.

The Stinger PDC cutter has high rock-breaking efficiency and excellent impact and wear resistance, which can significantly increase the rate of penetration (ROP) and extend PDC bit life for drilling hard and abrasive formation. The knowledge of force response and mechanical specific energy (MSE) for the Stinger PDC cutter is of great importance for improving the cutter's performance and optimizing the hybrid PDC bit design. In this paper, 87 single cutter tests were conducted on the granite. A new method for precisely obtaining the rock broken volume was proposed. The influences of cutting depth, cutting angle, and cutting speed on cutting force and MSE were analyzed. Besides, a phenomenological cutting force model of the Stinger PDC cutter was established by regression of experimental data. Moreover, the surface topography and fracture morphology of the cutting groove and large size cuttings were measured by a 3D profilometer and a scanning electron microscope (SEM). Finally, the rock-breaking mechanism of the Stinger PDC cutter was illustrated. The results indicated that the cutting depth has the greatest influence on the cutting force and MSE, while the cutting speed has no obvious effects, especially at low cutting speeds. As the increase of cutting depth, the cutting force increases linearly, and MSE reduces with a quadratic polynomial relationship. When the cutting angle raises from 10° to 30°, the cutting force increases linearly, and the MSE firstly decreases and then increases. The optimal cutting angle for breaking rock is approximately 20°. The Stinger PDC cutter breaks granite mainly by high concentrated point loading and tensile failure, which can observably improve the rock breaking efficiency. The key findings of this work will help to reveal the rock-breaking mechanisms and optimize the cutter arrangement for the Stinger PDC cutter.

Producing complex fracture networks in a safe way plays a critical role in the hot dry rock (HDR) geothermal energy exploitation. However, conventional hydraulic fracturing (HF) generally produces high breakdown pressure and results only in single main fracture morphology. Furthermore, HF has also other problems such as the increased risk of seismic events and consuption of large amount of water. In this work, a new stimulation method based on cyclic soft stimulation (CSS) and liquid nitrogen (LN₂) fracturing, known as cyclic LN₂ fracturing is explored, which we believe has the potential to solve the above issues related to HF. The fracturing performances including breakdown pressure and fracture morphology on granites under true-triaxial stresses are investigated and compared with cyclic water fracturing. Cryo-scanning electron microscopy (Cryo-SEM) tests and X-ray computed tomography (CT) scanning tests were used for quantitative characterization of fracture parameters and to evaluate the cyclic LN₂ fracturing performances. The results demonstrate that the cyclic LN₂ fracturing results in reduced breakdown pressure, with between 21% and 67% lower pressure compared with using cyclic water fracturing. Cyclic LN₂ fracturing tends to produce more complex and branched fractures, whereas cyclic water fracturing usually produces a single main fracture under a low number of cycles and pressure levels. Thermally-induced fractures mostly occur around the interfaces of different particles. This study shows the potential benefits of cyclic LN₂ fracturing on HDR. It is expected to provide theoretical guidance for the cyclic LN₂ fracturing application in HDR reservoirs.