In this paper, a domain decomposition based preconditioning method is developed to accelerate the Krylov subspace method for solving the linear system arising from the extended finite element discretization of the dynamic crack problem. Based on the observation that the crack tip area has a significant impact on the convergence of the iterative method while the other mesh points do not, the finite element mesh is partitioned into two types: the regular subdomains and the crack tip subdomains. To construct the additive Schwarz preconditioner, the global matrix is partitioned into submatrices which are solved exactly in the crack tip subdomains by the LU factorization and inexactly in the regular subdomains by an incomplete LU factorization. As the crack propagates, we develop a scheme to update the subdomain problems without resolving of them, in which, in order to save the computational cost, only the subdomains around the crack are updated, and all the other regular subdomains remain unchanged. To further speed up the Krylov subspace method, an auxiliary subproblem including crack tips of the previous and current time steps is constructed and solved to provide a better initial guess. We carefully studied the properties of the linear system and the performance of the proposed algorithm. The numerical experiments indicate that the proposed method works well for the simulation of dynamic crack propagation with multiple crack tips.

Among numerous oil-water separation materials, coalescence materials have received widespread attention. In this work, we prepared a lotus leaf-shaped Janus coalescing material by physically mixing polyethylene (PE) and polypropylene (PP) first, pressing it into sheets, then single-face hydrophilic modifying by chemical oxidation method, followed by pressing into a lotus leaf shape. Finally, holes were physically punched at the upper and lower sides of the coalescing material to let the oil drops pass through freely, greatly reducing the time for oil droplets to float. The shape of lotus leaves could provide more opportunities for oil droplets to collide and coalesce. The coalescing materials had asymmetric wettability, with specific surfaces that were oleophilic on the one hand and hydrophilic on the other. The oil droplets adhered to the oleophilic side of the coalescing material, coalesced and grew, then floated up and passed through the coalescing sheet layer by layer until they reached the oil collecting port to realize oil-water separation. The structural parameters of the coalescing material were progressively optimized. The obtained coalescing material exhibited a high oil-water separation efficiency of 99.40 +/- 0.5% with good stability during a 48 h continuous oil-water separation, which provided a reference for the development of efficient coalescing materials.

In this paper, a parallel overlapping domain decomposition preconditioner is proposed to solve the linear system of equations arising from the extended finite element discretization of elastic crack problems. The algorithm partitions the computational mesh into two types of subdomains: the regular subdomains and the crack tip subdomains based on the observation that the crack tips have a significant impact on the convergence of the iterative method while the impact of the crack lines is not that different from those of regular mesh points. The tip subdomains consist of mesh points at crack tips and all neighboring points where the branch enrichment functions are applied. The regular subdomains consist of all other mesh points, including those on the crack lines. To overcome the mismatch between the number of subdomains and the number of processor cores, the proposed method is divided into two steps: solve the crack tip problem and then the regular subdomain problem during each iteration. The proposed method was used to develop a parallel XFEM package which is able to test different types of iterative methods. To achieve good parallel efficiency, additional methods were introduced to reduce communication and to maintain the load balance between processors. Numerical experiments indicate that the proposed method significantly reduces the number of iterations and the total computation time compared to the classical methods. In addition, the method scales up to 8192 processor cores with over 70\% parallel efficiency to solve problems with more than $2\times10^8$ degrees of freedom.

Mesh membranes have a wide range of applications including fog harvesting and oil-water separation. However, due to the wetting energy barrier, water droplets are usually pinned in the meshes and there are still substantial difficulties in achieving directional transport. In this paper, mesh membranes for directional droplet transport are achieved by controlling the surface wetting state with sub-millimeter structures. The effect of mesh structures on the surface wetting state is studied theoretically. And accordingly, a unique filament stack structure is presented to acquire the desired wetting state, which can be further switched on-demand by incorporating the filament swelling. The well-designed mesh structures can be 3D printed efficiently without post-processes such as surface modification. It provides a facile and effective way to develop scalable mesh membranes for directional and selective droplet transport.

The propagation of hydraulic fracture network in naturally fractured shale reservoirs remains a complicated and challenging issue. To reveal the propagation principle of the complex fracture network, a numerical fracturing model is presented on the basis of the extended finite element method (XFEM) in this work. The intersection criterion containing five intersection patterns is studied. True triaxial fracturing experiment is conducted to verify the fracture geometry obtained from our model. After that, hydraulic fracturing in naturally fractured formations is studied to investigate the influences of natural fracture azimuth on fracture network geometry. The numerical results show that, hydraulic fracture is inclined to open the obtuse angle side of natural fracture in uniform azimuth model. In random azimuth condition, occurring of various intersection patterns enables complex fracture network to generate easier. And hydraulic fracture is harder to propagate if it is closer to the middle domain due to the squeezing effect. Meanwhile, the fracture network can penetrate deeper shale layer under large in-situ stress difference. In addition, the stimulation effect (e.g., Stimulated Reservoir Volume) in random azimuth condition is much stronger than that in uniform azimuth condition. The findings in this work are of benefit to provide us a better understanding of complex hydraulic fracture network and develop high-efficiency fracturing technology.

水力压裂实践表明,压裂停泵水锤现象会导致压降曲线出现周期性波动,这使得停泵压降的分析变得困难.鉴于此,提出了基于数字滤波的压裂停泵压降分析新方法,即首先采用井筒垂直管流方程将地面压力折算到井底压力,然后采用FIR滤波处理停泵后的压降数据,消除停泵水锤等噪声干扰,从而获得真实反映地层渗流的压降数据.以某低渗透油田的1口水力压裂井为对象,利用新方法开展了停泵压降分析,结果表明诊断结果与其他反演手段的解释结果相近,这在一定程度上验证了该方法的可靠性.本次提出的新方法可用于开展考虑水锤效应的压裂停泵压降分析,对于高效水力压裂实践具有一定指导作用.

The production performance of horizontal well with complex fracture networks remains a challenging issue. To more accurately evaluate the stimulation effect of fracture network on the reservoirs, an equivalent method is presented based on the extended finite element method (XFEM) and the Peaceman well model. We discretize the governing equations with finite volume method and solve the linearized equations with fully implicit scheme. The reliability of the equivalent method is validated against the available numerical method in the literature. After that, two cases of different fracture network structures are investigated. Results show that the equivalent method is feasible to assess the effect of fracture network on horizontal well production. And more broadly distributed fracture network has better enhancing effect. Meanwhile, fracture tips have little stimulation effect on the production. The findings are of benefit to provide us a simplified way to investigate the flow in reservoirs containing complex fracture network.

Mechanical properties such as strength, toughness and anisotropy are significant in 3D printing technology. Unfortunately, the strength and toughness are often mutually exclusive, and are therefore difficult to be enhanced simultaneously. Here, a bioinspired parallel-scan path is proposed to increase both the strength and toughness with in-plane isotropy. In this method, a Bouligand structure is adopted to increase the fracture deflection and energy absorption. Result shows that at the optimal rotational angle 15°, the ultimate strength and toughness can be improved over 12% and 100%, respectively, along with good in-plane isotropy. To investigate the mechanics of the improvement, the fracture surface is analyzed and the finite element (FE) simulation is performed. Our results suggest that the optimal mechanical properties can be obtained at a certain rotational angle by controlling the stress under a moderate level, and maximizing the fracture surface during its propagation. The method is simple and highly adaptive, and may offer a good mechanical reinforce capability in extrusion based 3d printing.

With the large-scale commercial exploitation of shale gas around the world, the multi-interval fracturing technique in horizontal well has played an important role to stimulate shale gas reservoirs. The commonly used fracturing methods in reservoir stimulation are the sequential fracturing, the alternate fracturing, and the latest proposed modified zipper fracturing (MZF), which has improved the shale gas production significantly. However, the mechanism of stimulation has not been well understood yet. This paper presents some numerical simulation results for the three different fracturing patterns by use of extended finite element method (XFEM). The numerical solution mainly considers the influences of the in-situ stress difference and the fracturing spacing on fracture propagation. The analytical parameters include the maximum principal stress, the principal stress direction and the fracture width distribution. The numerical results indicate that the induced stress from adjacent fractures is the key factor affecting the fracture configuration. And the stress interference becomes significantly serious when fracture spacing decreases or fracture number increases. Moreover, the in-situ stress difference can counteract the effect of stress interference on the fracture deviation and reduce the extent of deviation. Compared with the other two fracturing techniques, MZF generates larger maximum induced stress, but less change of the principal stress direction, which ensures a desired propagation path. Therefore, the optimal fracture spacing of MZF is smaller than that of sequential fracturing and alternate fracturing. Under the same stress difference and fracture spacing, MZF generally achieves better formation fracturing effects. The results obtained in this paper are of benefit to guide the high-efficient practices of hydraulic fracturing in horizontal wells.

Viscous fingering in porous media during CO2 flooding is investigated experimentally as well as numerically in this study. Experiments were accomplished in real cores saturated with simulated oil in thermostat, and the effect of CO2 flooding is studied in homogeneous cores with differing permeability and viscosity under various pressures. Furthermore, a numerical model called diffusion-limited aggregation (DLA) is adopted to investigate viscous fingering in homogeneous and heterogeneous porous media and porous media with interior crack. The simulation of viscous fingering in cores with permeability contrast shows that heterogeneity can reduce displacement effect. When the permeability contrast ratio of each layer is larger than 10, the effect of gas channeling cannot be ignored and recovery efficiency decreases sharply. Meanwhile, for flooding in fractured porous media, it is found that CO2 breaks through preferentially along the crack and later CO2 advances along the flow path. Gas channeling is more serious in the fractured porous media than homogeneous media. According to experiments and simulation results, gas channeling is the primary cause for significant decreasing of oil displacement efficiency and shall be avoided in CO2 displacements. That is, CO2 flooding is more appropriate for homogeneous and low-permeability porous media.

水力压裂是目前开采致密油气的主要方式,压裂施工期间测量的地面压力及流量数据包含地层压力、渗透率和裂缝半长等重要信息,这些参数是判断压裂施工成功率及压后开采制度制定的重要依据.提出了基于数字滤波压裂停泵数据反演方法,首先采用井筒垂直管流方程将变密度和流量的地面压力折算到井底压力,其次采用FIR滤波处理停泵后压降数据,消除停泵水锤等噪声干扰,获得反映地层渗流的压降数据,最后采用垂直裂缝井试井分析方法解释滤波后的压降数据,获得地层渗透率、原始地层压力及裂缝半长等重要参数,实现了对裂缝及地层参数的实时分析及评价.通过与关井压力恢复分析结果比较,证明了所提方法的可靠性.

Under the conditions of high temperature and high pressure, viscous fingering phenomenon occurs during CO2 flooding because of viscosity differences between CO2 and crude oil, which leads to premature breakthrough of displacement fluid and the decrease of sweep efficiency. Therefore, oil production cannot reach the expected yields. In this paper, we investigate the effect of viscosity ratio, injection rate, and pore structure on viscous fingering in 3D digital cores obtained by CT experiments based on a diffusion-limited-aggregation (DLA) model. The results show that viscosity difference is the dominating factor that influences viscous fingering. With the increase of viscosity difference between the two displacement fluids, sweep efficiency decreases dramatically. In a lower permeable reservoir and at an appropriate injection rate, the viscous fingering can be decreased. The numerical simulation is validated against the CO2 flooding experiment.

Viscous fingering phenomenon happens during CO2 flooding because of viscosity differences between CO2 and crude oil. It leads to premature breakthrough of displacement fluid and decrease of sweep efficiency. In this paper, a three dimensional Diffusion-Limited-Aggregation model based on pressure-oriented rule is presented to account for the effect of viscosity ratio, injection rate on viscous fingering. Furthermore, Digital cores in our simulation are obtained by CT experiment from real rocks. The results show that viscosity difference is the dominating factor that influences viscous fingering. With the increase of viscosity difference between the two displacement fluids, sweep efficiency decreases dramatically. At appropriate injection rate, the effect of viscous fingering can be decreased.

The flow law was obtained by combining the mechanics of continuous media and molecular kinematics method for shale gas reservoirs with nano/micropores. The new non-linear seepage model considering the diffusion, slippage and desorption effects was established and simplified. The steady and transient governing equations were constructed on the basis of the non-linear model. The pressure characteristic analytical solution was then obtained. The permeability adjustment factor changes largely when the pore throat radius is less than 100 nm. Therefore, the flow law in nano/micropore departs significantly from Darcy's law. Numerical simulation results show that the flux of gas changes significantly with the permeability of the pore throat radius, and the flow in nano/micropore reservoir shows the microscale effect. Compared with the pressure distribution calculated by Darcy's law, the pressure distribution calculated by the new model decreases slowly because of the diffusion and desorption effect. The results provide a theoretical foundation for developing shale gas reservoirs. (C) 2015 Elsevier B.V. All rights reserved.

苏里格气田气井具有低压、低产、产水、携液能力差等特点,由于井筒积液严重,部分气井出现压力和产量下降过快的现象,制约了气井的正常生产,因此有必要选择合适的排水采气措施来清除井筒积液.然而,排水采气井筒多相流体流动的机理较复杂,目前,排水采气措施的参数(如气举的注气量)设计多是依靠经验或利用较简单的临界携液流量等参数确定的,针对整个排水采气井筒气液流动规律的变化及能量损失的研究较少.文中通过采用数值模拟和实验模拟研究相结合的方法,对苏里格气田低产积液气井气举前后整个井筒气液流动规律进行分析,并根据注气量对井筒压降和气举效率的影响,确定适用于苏里格气田气井气举复产的最优注气参数,为选择合适的排水采气措施提供了理论指导.

WUTONG coal mine is adjacent to an auxiliary dam of the Yuecheng Reservoir. In this paper, mining-induced surface subsidence prediction is conducted by means of the finite difference method (FDM) to judge whether the extraction of the coal seam will have a negative impact on the dam. First, the initial values of the rock mass mechanical parameters are estimated using the available literature that relates intact rock and discontinuity properties to rock mass parameters. Then, based on available surface subsidence monitoring data on WUTONG’s mined areas, the main mechanical parameters of coal and rock masses are determined by a back analysis procedure that combines an experimental design technique with numerical simulations. Finally, the surface subsidence results in the mining area are numerically predicted for four different mining scenarios (S1 through S4). Scenario S3 emerged as the best choice of these four scenarios. The predictions are: (1) the maximum surface subsidence within the mining area is 2.14m, with the maximum settlement point located in the mid-west area of the coalfield, and (2) the nearest distance from the boundary of the surface movement area to the edge of the dam foundation is 35m. Therefore, mining the coal seam will not cause damage to the dam.

An impacted mini-air-motor is studied and developed to be applied in the combustible,explosive and antimagnetic conditions.The operating principle and the structure of the mini-air-motor are discussed.The discharge characteristics of the nozzle and the mechanical characteristics of the mini-air-motor are analyzed.The effect of main structure parameters on the output torque of the mini-air-motor is simulated.In order to study the mechanical characteristics of the mini-air-motor,a measurement instrument is designed.The experimental results indicate that the output torque of the motor has good linear relation with the rotational speed,and the range of the speed regulation is very wide.The impacted mini-air-motor is suitable to the application condition.

The leakage was considered of dimethyl ether (DME) and liquefied petroleum gas (LPG) with much lower viscosities from high pressure fuel pumps. Mathematical model and computing method are presented to analyze quantitatively the effect of low viscosity liquefied fuel on leakage flow characteristics in the clearance between plunger and plunger sleeve. A fluid-solid coupling finite element analysis (FEA) model is developed by FEA software, ANSYS, to investigate the deformation of plunger pinion and the fuel leakage from the tolerance clearance of the fuel pump. Some suggestions are also given to solve fuel leakage from conventional pumps in diesel engines fueled with much lower viscosity fuels.