Investigation on dynamic mechanism of fault slip and casing deformation during multi-fracturing in shale gas wells.

Fracturing horizontal well casing deformation has become very prominent, particularly in tectonic stress-concentrated shale gas fields, limiting the efficient development progress of shale gas. The main failure mode of casing shearing deformation had been attributed to fault slip caused by multi-fracturing. The current research did not provide a clear picture of the dynamic evolution relationship between hydraulic fracturing, fault slip, and casing deformation. In this paper, the dynamic model of fault slip induced by formation pressure change is established, incorporating the effects of stress drop, physical change of friction, and casing and cement-sheath resistance loads. The discontinuous displacement approach and explicit/implicit coupling iteration methods are used to reveal the relationships between the effective normal stress, shear stress, friction coefficient, and sliding velocity during the fault slip process. Furthermore, the microscopic process of casing deformation sheared by fault slip is investigated using static equilibrium theory, and a characterization method for determining the amount casing deformation caused by real-scale fault slip is proposed. The results show that three stages exist in the process of casing deformation sheared by fault slip, including trigger activation stage, accelerated slip stage, and deceleration slip stage. Fault slip is clearly influenced by fault strike. To reduce the amount of fault slip, the fault direction with the maximum in-situ stress should be avoided as much as possible. Serious casing deformation still occurs for large-scale activated faults even though the optimization measure of wellbore structure has been well taken. To fundamentally reduce the possibility of casing shear deformation, it is necessary to prevent fault slip through optimizing the design of hydraulic fracturing. This study lays the theoretical groundwork for the casing deformation control method in shale gas wells.

Numerical investigation of fluid flow behavior in steel cord with lattice Boltzmann methodology: The impacts of microstructure and loading force.

Steel cord materials were found to have internal porous microstructures and complex fluid flow properties. However, current studies have rarely reported the transport behavior of steel cord materials from a microscopic viewpoint. The computed tomography (CT) scanning technology and lattice Boltzmann method (LBM) were used in this study to reconstruct and compare the real three-dimensional (3D) pore structures and fluid flow in the original and tensile (by loading 800 N force) steel cord samples. The pore-scale LBM results showed that fluid velocities increased as displacement differential pressure increased in both the original and tensile steel cord samples, but with two different critical values of 3.3273 Pa and 2.6122 Pa, respectively. The original steel cord sample had higher maximal and average seepage velocities at the 1/2 sections of 3D construction images than the tensile steel cord sample. These phenomena should be attributed to the fact that when the original steel cord sample was stretched, its porosity decreased, pore radius increased, flow channel connectivity improved, and thus flow velocity increased. Moreover, when the internal porosity of tensile steel cord sample was increased by 1 time, lead the maximum velocity to increase by 1.52 times, and the average velocity was increased by 1.66 times. Furthermore, when the density range was determined to be 0-38, the pore phase showed the best consistency with the segmentation area. Depending on the Zou-He Boundary and Regularized Boundary, the relative error of simulated average velocities was only 0.2602 percent.

Assessment of wellbore integrity failure risk and hazardous zones in depleted reservoirs underground gas storage during the operation processes.

Depleted reservoirs are widely used for underground gas storage due to their advantages of low construction cost and easy development. Under the influence of complex geological conditions and frequent operations, the integrity of the wells in depleted reservoirs is prone to failure, which would potentially lead to gas leakage. In this study, by using a finite element-based computational fluid dynamics model, we have developed evaluation criteria for assessing the severity of the occurred wellbore integrity failure and the risk of the un-occurred wellbore integrity failures respectively to identify hazardous zones potentially prone to wellbore integrity failure. The study results indicate that the gas storage wellbore integrity failure is prone to occur inside the wellbore structure in the direction of the minimum ground stress near the lower boundary of the formation interlayer. The wellbore integrity failure hazardous zones are mainly concentrated at the formation interlayer boundaries. The practical guidelines and solutions derived from current research results can provide an accurate direction for monitoring and protecting work of wellbore integrity and avoid environment pollution problems caused by natural gas leakage.

Pore-throat structure characterization of carbon fiber reinforced resin matrix composites: Employing Micro-CT and Avizo technique.

Micro-CT technique poses significant applications in characterizing the microstructure of materials. Based on the CT three-dimensional(3D) reconstruction technology and "Avizo" 3D visualization software, the microscopic pore-throat structure of porous media can be quantitatively characterized. This paper takes the carbon fiber reinforced resin matrix composites as an example to introduce the operation process of "Avizo" in details, which mainly covers the following modules: Volume Edit, Interactive Thresholding, Fill Holes, Mask, Separate Objects and Generate Pore Network Model, then further discuss the difficult problems when the "Avizo" is employed to analyze. The microstructures of carbon fiber reinforced resin matrix composites illustrate that pores in the upper part of sample are dramatically dispersed, and mainly concentrated in the lower part of sample. The porosity of adopted cuboid is 3.6%, accordingly the numbers of pores and throats reach 268 and 7, respectively. The equivalent radius of pores seems mainly distributed in the range of 0.7-0.8µm, accounting for 28.73% of the total pore number. The surface area of pore ranges from 5 to 10µm2, accounting for 14.16% of the total pore number. The pore volume concentrates in the range of 1-20µm3, accounting for 57.46% of the total pore number. In addition, the equivalent radius of throat mainly concentrates in the range of 1-5µm, the overall length of throat is distributed in the range of 37-60µm, and the equivalent area of throat is distributed non-uniformly in the range of 5-75µm2. This work provides a basis for the further investigation of fluid migration mechanism and law in the composite materials by the numerical simulation methodology.

Predicting ultrasound tongue image from lip images using sequence to sequence learning.

Understanding the dynamic system that produces speech is essential to advancing speech science, and several simultaneous sensory streams can be leveraged to describe the process. As the tongue functional deformation correlates with the lip's shapes of the speaker, this paper aims to explore the association between them. The problem is formulated as a sequence to sequence learning task and a deep neural network is trained using unlabeled lip videos to predict an upcoming ultrasound tongue image sequence. Experimental results show that the machine learning model can predict the tongue's motion with satisfactory performance, which demonstrates that the learned neural network can build the association between two imaging modalities.