Wellbore Stability in Oil and Gas Drilling with Chemical-Mechanical Coupling

Wellbore instability in oil and gas drilling is resulted from both mechanical and chemical factors. Hydration is produced in shale formation owing to the influence of the chemical property of drilling fluid. A new experimental method to measure diffusion coefficient of shale hydration is given, and the calculation method of experimental results is introduced. The diffusion coefficient of shale hydration is measured with the downhole temperature and pressure condition, then the penetration migrate law of drilling fluid filtrate around the wellbore is calculated. Furthermore, the changing rules of shale mechanical properties affected by hydration and water absorption are studied through experiments. The relationships between shale mechanical parameters and the water content are established. The wellbore stability model chemical-mechanical coupling is obtained based on the experimental results. Under the action of drilling fluid, hydration makes the shale formation softened and produced the swelling strain after drilling. This will lead to the collapse pressure increases after drilling. The study results provide a reference for studying hydration collapse period of shale

China’s 10-year progress in DC gas-insulated equipment: From basic research to industry perspective

The construction of the future energy structure of China under the 2050 carbon-neutral vision requires compact direct current (DC) gas-insulation equipment as important nodes and solutions to support electric power transmission and distribution of long-distance and large-capacity. This paper reviews China's 10-year progress in DC gas-insulated equipment. Important progresses in basic research and industry perspective are presented, with related scientific issues and technical bottlenecks being discussed. The progress in DC gas-insulated equipment worldwide (Europe, Japan, America) is also reported briefly

The Ninth Visual Object Tracking VOT2021 Challenge Results

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Porothermoelastic Response of a Borehole in Fluid-Saturated Medium Subjected to Thermal Osmosis Effect

With the thermo-hydro-mechanical coupling process considered, this paper derives a set of analytical porothermoelastic solutions to field variables including the stress, displacement, and pore pressure fields to evaluate the wellbore stability around a vertical borehole drilled through an isotropic porous rock. The thermal effect on the wellbore stability of the low-permeability saturated rock also introduces the thermal osmosis term. The wellbore problem is decomposed into axisymmetric and deviatoric loading cases considering the borehole subjected to a nonhydrostatic stress field. It obtains the time-dependent distributions of field variables by performing the inversion technique for Laplace transforms to the porothermoelastic solutions in the Laplace domain. The results suggest that the thermal osmosis effect should not be neglected on the premise that a lower permeability porous rock is characterized by the substantially large thermal osmotic coefficient and the small thermal diffusivity values. The case that the thermal osmosis effect reduces the undrained loading effect leads to the decrease of the mean shear stress that is determined by the effective maximum and minimum stress around a borehole, since, and accordingly contributes to the wellbore stability to resist the shear failure

Fracturing Pressure of Shallow Sediment in Deep Water Drilling

The shallow sediment in deep water has weak strength and easily gets into plastic state under stress concentration induced by oil and gas drilling. During drilling, the formation around a wellbore can be divided into elastic zone and plastic zone. The unified strength theory was used after yielding. The radius of the plastic zone and the theoretical solution of the stress distribution in these two zones were derived in undrained condition. The calculation model of excess pore pressure induced by drilling was obtained with the introduction of Henkel’s excess pore pressure theory. Combined with hydraulic fracturing theory, the fracturing mechanism of shallow sediment was analyzed and the theoretical formula of fracturing pressure was given. Furthermore, the influence of the parameters of unified strength theory on fracturing pressure was analyzed. The theoretical calculation results agreed with measured results approximately, which preliminary verifies the reliability of this theory

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Borehole Stability Analysis in Deepwater Shallow Sediments Deepwater shallow sediment is less-consolidated, with a rock mechanical behavior similar to saturated soil. It is prone to borehole shrinkage and downhole leakage. Assume the deepwater shallow sediments are homogeneous, isotropic, and ideally elastoplastic materials, and formation around the borehole is divided into elastic and plastic zone. The theories of small deformation and large deformation are, respectively, adopted in the elastic and plastic zone. In the plastic zone, Mohr-Coulomb strength criterion is selected. The stress and deformation distributions in these two zones, and the radius of plastic zone are derived. The collapse pressure calculation formula of deepwater shallow sediments under the control of different shrinkage rates is obtained. With the introduction of excess pore pressure theory in soil mechanics, the distribution rule of excess pore pressure in these two zones is obtained. Combined with hydraulic fracturing theory, the fracture mechanism of shallow sediments is analyzed and the theoretical formula of fracture pressure is given. The calculation results are quite close to the practically measured results. So the reliability of the theory is confirmed

Piston-like plugging of fuzzy-ball workover fluids for controlling and killing lost circulation of gas wells

During well-killing operations for the workover of low-pressure gas wells, formation pressure should be balanced so as to guarantee well control safety by preventing natural gas overflow. In this paper, a laboratory evaluation was conducted with fuzzy-ball fluids as killing fluids. The results show that, the fuzzy-ball fluid, with a density of 0.5–1.5 g/cm3 and a viscosity up to 78,50,000 mPa·s at a low shear rate, realizes controllable performance and forms piston-like plugging slugs of solid-free high structural strength on natural gas wellbore after bonding. During well workover, multiple fluid column pressures were set up by injecting fuzzy-ball fluids with different densities at various rates. Owing to high structural strength of the fluids at a low shear rate, natural gas breaks through only inside the piston-like slug and cannot flow upwards to the ground, so the pathways of natural gas in the wellbore are isolated from the ground surface. Moreover, the fluid can wholly move up and down like a piston-like plug, with the change of formation pressures or the tripping of pipe strings. Like the conventional operations, the production can be restored after the workover, so long as the fluid in wellbore is cleaned by means of gas lift. In a natural gas field in NW China, where the formation pressure coefficient dropped to 0.60–0.82, three wells were fully filled with fuzzy-ball workover fluids for 7 days and another three wells were treated with the piston-like plugs of fuzzy-ball workover fluids for only 3 days. They all presented better technical results. The technology provides a new way for low-pressure gas well workover