Considering the damage caused by conventional fracturing fluid in low-permeability reservoirs, a novel fracturing fluid (FNG) combining hydroxypropyl guar (HPG) and functionally modified nano-silica (FMNS) was prepared. The properties of heat/shear resistance, rheological property, proppant transportation, and formation damage were evaluated with systematic experiments. The results showed that the viscosities of FNG before and after the heat/resistance were 1323 mPa·s and 463 mPa·s, respectively, while that of conventional HPG gel was 350 mPa·s. FNG is a pseudoplastic strong gel with a yield stress of 12.9 Pa, a flow behavior index of 0.54, an elastic modulus of 16.2 Pa, and a viscous modulus of 6.2 Pa. As the proportions of proppant mass in further sections transported with FNG were higher than those transported with HPG gel, FNG could transport the proppant better than HPG gel at high temperatures. Because of the amphiphilic characteristics of FMNS, the surface/interface properties were improved by the FNG filtrate, resulting in a lower oil permeability loss rate of 10 percentage points in the matrix than with the filtrated HPG gel. Due to the considerable residual gel in broken HPG gel, the retained conductivity damaged with broken FNG was 9.5 percentage points higher than that damaged with broken HPG gel. FNG shows good potential for reducing formation damage during fracturing in low-permeability reservoirs in China.
Deep-sea natural gas hydrate is rich in resources and has high energy density, making it a clean energy source with great development prospects, compared with the conventional oil and gas formations, hydrate reservoirs are shallowly buried, low hardness and uniaxial compressive strength, as a result of temperature changes, conventional drilling may cause the decomposition of hydrates, which will lead to wellbore instability. In view of the problem of increasing the ROP, horizontal wellbore formed by high pressure jet is considered an efficient method for natural gas hydrate drilling, and the jet drill bit is the core equipment to form horizontal wellbore in the reservoir, and effectively prevent sand production. Therefore, a self-pulling and rotating jet drill bit is proposed, the basic principle of that is when the drill bit breaks the rock at the bottom of the well, the front nozzle provides jet rock-breaking energy, the rear nozzle generates rotational torque, overcoming the friction torque between the rotating drill bit and the shaft, providing rotational energy for the jet drill bit, realize the movement of self-rotating and move forward while drilling. Therefore, three parameters are mainly evaluated in this paper, the self-pulling force, the angular momentum and jet pressure, the self-pulling force is fore the forward moving of the bit, the angular momentum is for the stable rotating of the bit, and the jet pressure is for the natural gas hydrate drilling. Meanwhile, three different structural bits are compared from the self-pulling force, rotating and jet pressure, and influences of inlet flow, outlet size on the self-pulling force, jet pressure and flow ratio of front and rear nozzles are analyzed. The results demonstrate that self-pulling force and jet pressure of bit with four front nozzles is the best at the same conditions, and the flow ratio of front and rear nozzles is depended on the structure of the bit; with the increasing of the inlet velocity, the self-pulling force and the jet pressure increases nonlinearly, and the angular momentum increases linearly. The equivalent outlet size has remarkable influence on the jet pressure, with increase of the front and rear nozzle size, the self-pulling force and jet pressure is decreases greatly. Moreover, the wellbore hole forming ability under the jet is discussed, shows that erosion hole forming ability is best at the jet distance of 0mm. This research provides an effective and feasible solution for the drilling of the hydrate radial horizontal wells.
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Magnetic resonance imaging (MRI) system is an important medical diagnostic equipment. Based on the principle of magnetic resonance in static magnetic field, it offers imaging of human organs at any level with high definition, thus playing an important role in medical examination and diagnosis. Compared with low-field MRI, high field MRI system can improve proton magnetic susceptibility, increase signal-to-noise ratio, reduce the acquisition time, so that the signal changes of brain imaging is more obvious. However, superconducting magnets used in high field MRI systems usually produce high leakage field, which poses great impact on surrounding equipments, so that a practicable and effective shielding method is needed. Due to its low cost and ease to build, iron shielding is universally used, but mainly in low field environment. In the present paper, some physical factors for designing high-field superconducting magnets are firstly discussed, then a method for shape optimization of iron shield for high field magnets is provided. Our procedure is based on a cooperative Integrated Design Optimization Platform (CIDO), which is suited to analysis of large, complicated nonlinear magnetic problems. At last, the optimal design results of 9.4 Tesla magnet systems with different iron shielding models have been presented.
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