The self‐propelled swirling jet bit was designed to ensure the rock‐breaking efficiency of radial horizontal drilling and meet the roundness of borehole diameter and borehole shape. The jet bit provides swirling jet through internal rotating impellers and increases the drilling depth by jetting through the central holes on the impeller. Based on numerical simulation method,RNGk−εturbulence model was employed to analyze the three‐dimensional flow characteristics of flow field inside and outside of the two kinds of jet bits. Moreover, the rock‐breaking efficiency of the two jet bits was also compared and analyzed in accordance with the laboratory experiments. The results show that the simulated bottom‐hole flow field for the two jet bits can be both divided into bottom‐hole overflow area, forward jetting area, and backward jetting area. The forward jet is a jet beam which consists of a central jet and four circumferentially equispaced jets, which generates more complex overflow area at the bottom hole. The maximum jet speed and the jet impact energy of forward jetting from the combined swirling and round multijet bit are larger than that of the swirling multijet bit. The speed attenuation law of forward jet nozzle from the two kinds of jet bits is similar. They both increase slowly in the mixing zone, while increase remarkably when approach to the nozzle. Then decrease linearly in the impact zone, and decline straightly when get close to the impact area. The simulation results of flow field characteristics are in coincidence with the actual rock‐drilling features. Therefore, the simulation results of flow field characteristics can provide guidance for jet bit design and structure parameter optimization for radial horizontal drilling.

This paper studied the fracability of tight sandstone reservoirs by means of incorporating geomechanics properties and surrounding in situ stresses into a new model. The new fracability evaluation model consists of variables such as brittleness index, critical strain energy release rate index, horizontal stress difference, and minimum horizontal principal stress gradient. The probability of interconnection of a complex fracture network was quantitatively studied by the brittleness index and horizontal principal stress difference index. The probability of obtaining a large stimulated reservoir volume was evaluated by the critical strain energy release rate index and minimum horizontal principal stress gradient which also quantifies conductivity. This model is more capable of evaluating fracability, i.e., it agrees better with the history of production with a high precision and had correlation coefficients (R²) of 0.970 and 0.910 with liquid production of post-fracturing well testing and the average production of six months of post-fracturing, respectively. It is convenient that all model inputs were obtained by means of loggings. Using this model, tight sandstone reservoirs were classified into three groups according to fracability: Frac ≥ 0.3 MPa⁻¹·m for Type-I, 0.22 MPa⁻¹·m ≤ Frac < 0.3 MPa⁻¹·m for Type-II, and Frac < 0.22 MPa⁻¹·m for Type-III.

The fracture toughness of shale is a basic parameter that can provide effective theoretical support for wellbore stability and hydraulic fracturing of a shale reservoir. Due to the composition and microstructure, there are many problems in evaluating the mechanical properties of shale in a macroscopic test. In this paper, the composition and pore distribution of shale were studied by X-ray diffraction and nuclear magnetic resonance. Scanning electron microscopy was used to characterize the pore structure. The setting of experimental parameters and the selection of the indenter were discussed. Micro-indentation technique was proposed and applied to fracture toughness analysis of shale. The results show that Berkovich indenter is more suitable for shale indentation test than Vickers indenter. Fracture toughness of shale indentation is obviously affected by surface roughness and indentation position. Fracture toughness of shale decreases slightly with the increase of the indentation load. The energy analysis result presents that the effect of cracking on the ratio of total/unloading work is minimal when there is no significant stripping on the shale surface. Compared with the experimental method, energy methods can obtain all the analysis parameters from a single indentation test. The results of comparative analysis with macroscopic experiments display that micro-indentation test can effectively predict the macroscopic fracture toughness of shale.

High-efficiency rock breaking by hydraulic jetting is the key to radial horizontal drilling technology. In order to improve the drilling efficiency of hydraulic jet rock breaking in radial horizontal wells, based on LS-Dyna display dynamics, a numerical simulation model of single-nozzle jet rock breaking was established to analyze the influence of different nozzle parameters on the rock-breaking effect. Then, the numerical simulation model of the spin multi-nozzle jet bit was established, and the influence of different rotation speeds on the rock-breaking effect of the jet bit was analyzed. Finally, the rock-breaking drilling characteristics of the spin multi-nozzle jet bit and the conventional multi-nozzle jet bit were compared and analyzed. The results show that when the jet impacts the rock surface, the larger the inclination angle is, the larger the rock-breaking width formed by the jet is. The smaller the dip angle, the greater the rock-breaking depth. When the inclination angle is greater than 60°, it is difficult to meet the needs of reaming. The width and depth of the nozzle gradually increase with the increase of the diameter. When the nozzle diameter is greater than 1.3 mm, the growth rate of rock-breaking depth begins to decrease. The optimum nozzle diameter is 1.3 mm. When v = 50 m/s, the damage caused by the jet to the rock surface is very small, because the condition of rock fracture is not reached with this velocity. This shows that there is a critical value of the water jet impact velocity, and only when the velocity is reached, will the rock break. When the velocity is v = 150 m/s, v = 200 m/s, v = 250 m/s, v = 300 m/s, the rock breaks. At the same time, the higher the speed, the higher the degree of rock fracture, the greater the fracture depth, the greater the fracture area, and the better the fracture effect. The tangential and radial velocity of the jet increases the shear and tensile failure rate of the sample, and improves the rock-breaking efficiency of the jet, which has a certain guiding significance for improving the rock-breaking drilling efficiency of radial horizontal well drilling.

CO₂ reservoirs and high CO₂ content gas reservoirs in marine sediments generally have complex pressure system, which usually encounters supercritical CO₂(SC‐CO₂) invasion into the wellbore during drilling process. The wellbore transient flow and phase transition along the wellbore during the drilling process has not been thoroughly investigated, because the CO₂ physical properties and the reservoirs petrophysical properties could affect the transient flow of mixed fluid and phase transition of CO₂ along the well. A numerical model is presented in this research to analyze the transient flow in wellbores and to predict phase transition of CO₂ during formation SC‐CO₂ invasion in the drilling process. The main considerations were the following: multiphase transient flow in wellbores was coupled with the non‐Darcy flow through the formation; wellbore temperature, pressure and the mixture of drilling fluid and SC‐CO₂ were calculated in the method of coupling; the invaded SC‐CO₂ could affect the mixed fluid physical properties in the annulus; the change of physical properties of CO₂ (eg, solubility, phase transition) was considered and integrated. The research results indicate that as the mixture of drilling fluid and SC‐CO₂ flows from the bottom to the surface, the invaded CO₂ is difficult to be detected at the early invasion stage because of high CO₂ density and solubility; the CO₂ would change from supercritical to gas phase and the dissolved quantity and density of CO₂ rapidly decrease near the wellhead, and further cause well‐control issues. Furthermore, the bottom‐hole pressure (BHP) would decrease and could not provide sufficient balance to the formation pressure, which could significantly increase the risk of well blowout. Sensitivity analysis was utilized to evaluate the effects of the choke pressure, drilling fluid pumping rate, initial pressure difference, reservoir permeability, and drilled reservoir thickness on the BHP and the depth of phase transition of CO₂.