Experimental Method to Simulate Coal Fines Migration and Coal Fines Aggregation Prevention in the Hydraulic Fracture

A simple and effective experimental method is proposed to simulate coal fines migration through the proppant pack; such migration inevitably occurs during the process of fracturing fluid flowback or dewatering and gas production in coalbed methane (CBM) reservoirs. The damage to conductivity caused by coal fines migration in the pack and the factors affecting such migration are analyzed. A dispersion agent of coal fines applicable to hydraulic fracturing in CBM is optimized, consequently solving the problem of coal fines aggregation and retention in the proppant pack. Discharging coal fines with water or water-based fracturing fluid from the proppant pack can be difficult because of the adsorption and hydrophobicity of coal fines. Thus, coal fines are likely to aggregate and be retained in the proppant pack, thereby resulting in pore throat plugging, which causes serious damage to fracture conductivity. Two percent coal fines can reduce propped fracture conductivity by 24.4 %. The mobility and retention of coal fines in the proppant pack are affected by proppant size, proppant type, flowback rate, and coal fines property. When flowback rate exceeds the critical value, coal fines can be discharged from the pack, consequently reducing damage to propped fracture conductivity. More importantly, the steady discharging of coal fines requires steady dewatering and gas production to avoid flow shock, which causes pressure disturbance to drive coal fines in a remote formation. The optimized dispersant FSJ-02 employed in this paper can effectively change the wettability and surface potential of coal fines to improve their suspension and dispersion in water-based fracturing fluid. The recovery rate of coal fines increased by 31.5 %, whereas conductivity increased by 13.3 %.     

Coal Permeability Evolution and Gas Migration Under Non-equilibrium State

Laboratory test of coal permeability is generally conducted under the condition of gas adsorption equilibrium, and the results contribute to an understanding of gas migration in the original coal seams. However, gas flow under the state of non-equilibrium, accompanied by gas adsorption and desorption, is more common in coalbed methane (CBM) recovery and \(\hbox {CO}_{2}\) geological sequestration sites. Therefore, research on gas migration under the non-equilibrium state has a greater significance with regard to CBM recovery and \(\hbox {CO}_{2}\) geological sequestration. However, most permeability models, in which only one gas pressure has been considered, cannot be used to study gas flow under the non-equilibrium state. In this study, a new mathematical model, which includes both fracture gas pressure and matrix gas pressure, and couples the gas flow with the coal deformation, has been developed and verified. With the developed model, the spatial and temporal evolution of gas flow field during gas adsorption and desorption phases has been explored. The results show that the gas pressures present nonlinear distributions in the coal core, and the matrix gas pressure is generally lower than the fracture gas pressure during adsorption, but higher than the fracture gas pressure during desorption. For gas flow during adsorption, the main factor controlling permeability varies at different points. At the initial time, the permeability is dominated by the effective stress, and at the later time, the permeability in the part close to the gas inlet is mainly controlled by the matrix swelling, whereas that in the part close to the gas outlet is still dominated by the effective stress. For gas flow during desorption, from the gas inlet to the gas outlet, the permeability deceases at the initial time, and when the time is greater than 10,000 s, it shows a decreasing and then an increasing trend. The reason is that at the initial time, the permeability is dominated by the increased effective stress caused by the sharp decrease of the fracture gas pressure. Later, desorption of the adsorbed gas results in matrix shrinkage, which further leads to an increase of the permeability.     

Repeatability of the Contour Method for Residual Stress Measurement

This paper describes the results of a residual stress measurement repeatability study using the contour method. The test specimen is an aluminum bar (cut from plate), with cross sectional dimensions of 50.8 mm?×?76.2 mm (2 in?×?3 in) with a length of 609.6 mm (24 in). There are two bars, one bar with high residual stresses and one bar with low residual stresses. The high residual stress configuration (±150 MPa) is in a quenched and over-aged condition (Al 7050-T74) and the low residual stress configuration (±20 MPa) is stress relieved by stretching (Al 7050-T7451). Five contour measurements were performed on each aluminum bar at the mid-length of successively smaller pieces. Typical contour method procedures are employed with careful clamping of the specimen, wire electric discharge machining (EDM) for the cut, laser surface profiling of the cut faces, surface profile fitting, and linear elastic stress analysis. The measurement results provide repeatability data for the contour method, and the difference in repeatability when measuring high or low magnitude stresses. The results show similar repeatability standard deviation for both samples, being less than 10 MPa over most of the cross section and somewhat larger, around 20 MPa, near the cross section edges. A comparison with published repeatability data for other residual stress measurement techniques (x-ray diffraction, incremental hole drilling, and slitting) shows that the contour method has a level of repeatability that is similar to, or better than, other techniques.     

Investigation of thermal convection in water columns using particle image velocimetry

Particle image velocity measurements were applied on thermally driven convection at low Rayleigh numbers. In a model experiment using a water column heated from bottom and cooled from above, the velocity field was studied at different vertical temperature gradients. In the testing facility with high aspect ratio (about 19) representing a 1-m-long column with 5?cm diameter, occurrence of free convection was verified for destabilizing temperature gradients of 0.1–2?K/m. The PIV results revealed that significant flow exists already at low vertical temperature gradients. The velocity of the stable large-scale circulations increased linearly with temperature gradient (<1?K/m) from 8?×?10^?5 to 1?×?10^?3?m/s. At higher temperature gradients (1–2?K/m), a transition from quasi-stationary into time-dependent flow was observed, where convection cells changed position, number, and form temporarily. The motivation of this research was to gain more insight into density-driven convection in boreholes and groundwater monitoring wells.     

The Evolution of Parameters During CBM Drainage in Different Regions

Draining while mining is an important method for preventing gas outburst and achieving clean energy, and understanding the evolution of the parameters (gas pressure, strain and permeability) during coalbed methane (CBM) drainage while mining is important for improving extraction efficiency (which is very low at present). Physical simulations of CBM drainage were conducted, and the gas pressures were achieved. The values of the permeability and strain could be calculated. The parameters (gas pressure, permeability and strain) trends were achieved during the drainage process. In terms of timing, during the initial stages of CBM drainage, the gas pressures declined quickly, the permeability decreased sharply, and the volumetric strain increased quickly. During the later stages, the gas pressures decreased slowly, the permeability recovered slowly over time, and the volumetric strain increased slowly. In terms of space, the gas pressure declined more quickly nearer the effective borehole. The permeability initially declined more quickly nearer the effective borehole and then increased during the later stages. During CBM drainage in the stress relief region, the volumetric strain of the stress concentration region was the largest. The volumetric strain of the stress relief region was larger than that of original region I, but this trend reversed as time progressed. The volumetric strain of original region II was the smallest. The relationship between the total strains during CBM drainage in different regions was as follows: the total strain during CBM drainage in the stress concentration region > in the stress relief region > in original region I > in original region II.     

In Situ Observation of Material Failure in Cast Aluminum Alloy During Monotonic Loading Observed by a High-Speed Camera

In situ observation of the failure characteristics of a cast aluminum alloy has been conducted using a testing system with a high-speed camera. The failure process of the cast Al alloy was captured clearly with an high image resolution (1,024?×?1,024 pixels) at a high frame rate (20,000/s), where the specimen surface for observation was dyed dark using a black oil-based ink. A dark curtain was set behind the test specimen as background. Strong lighting of about 10 klx was used, which was applied to the dark specimen surface for clarification of material failure. The aim of this approach was to detect the failure characteristics or failure objects with bright zone. Using this system, both debris particles flying from the fracture sample and dislocation-like movements were detected. These were observable as tiny bright dots. The flying debris particles of about 35 μm in diameter consisted mainly of Si- and Fe-based eutectic structures. The flying speed of the debris particles was about 1,800 mm/s and their flight distance from the specimen was about 100 mm. The velocity of the dislocation-like movements was found to be less than 1,000 mm/s, and this motion was seen repeatedly before and after sample failure occurred.     

Performance of Rubberized and Hybrid Rubberized Concrete Structures under Static and Impact Load Conditions

In this study, rubberized concrete samples were prepared by partial substitution (5 %, 10 % and 20 % replacements by volume) of sand by waste crumb rubber, and tested under impact three-point bending load, as well as static load. Three types of specimens (size 50?×?100?×?500 mm) namely, plain concrete, rubberized concrete, and double layer concrete (with rubberized concrete top and plain concrete bottom) were loaded to failure in a drop-weight impact machine by subjecting to 20 N weight from a height of 300 mm, and another three similar specimens were used for the static load test. In both the tests, the load–displacement and fracture energy of each specimen were investigated. Finite-element simulations were also performed to study the dynamic behaviors of the samples, by using LUSAS V.14 software. It was noticed that, the impact tup, and inertial and bending loads increased with the increase in the percentage of sand replacement by crumb rubber. It was interesting to observe that these effects were more significant in the double layer specimen compared to the plain and rubberized concrete samples. The static peak bending load always decreased with increase of rubber in the mix. In general, the strength and energy absorbing capability of rubberized concrete was better under impact loading than under static loading. The simulated load against displacement behaviors of all the samples were validated by the experimental results.     

Low Pressure Gas Percolation Characteristic in Ultra-low Permeability Porous Media

Low pressure gas percolation characteristic in ultra-low permeability porous media is investigated in this article through core flow experiments. The results show that the wall-slip layer covers more than 10% of the average porous channel radius on account of minimum pore size when the permeability is below 0.1 × 10^?3μ m ² order, and seepage behavior is contrasted to that in mid-high permeability pore media. When the gas pressure is not high enough, the flow regime turns into transitional flow instead of slip flow, and nonlinear relationship between the measured gas permeability and the reciprocal of average pressure exists. The gas measuring permeability experiment would be influenced by the non-linear relationship. If Klinkenberg-corrected method is applied to speculate the equivalent liquid permeability, the extrapolated value will become less or minus. Simultaneously, actual gas flow velocity at the outlet is beyond the calculated value with Klinkenberg formula. A new gas seepage model based on the general slip boundary condition is derived from the homogenization technique in this article. At last the flow model is examined to be suitable for representing the gas flow behavior in ultra-low permeability media and estimating the absolute permeability from single-point, steady-states measurements.     

Concurrent calorimetric and interferometric studies of steady-state natural convection from miniaturized horizontal single plate-fin systems and plate-fin arrays

Concurrent calorimetric and interferometric studies have been conducted to investigate the effect that reduction of the base-plate dimensions has on the steady-state performance of the rate of natural convection heat transfer from miniaturized horizontal single plate-fin systems and plate-fin arrays. The effect was studied through comparison of the present results with those of earlier relevant calorimetric, interferometric, or numerical studies. Results shown that a reduction of the base-plate area by 74% increased natural convection coefficient by 1.5 times to 26.0 W m^?2 K^?1 for single fin systems and by 1.8 times to 18 W m^?2 K^?1 for fin arrays in the range of the base-plate temperature excess of 20–50°C. A simple correlation for the Nusselt number of miniaturized horizontal plate-fin arrays is proposed in the range of Rayleigh number divided by the number of fins to the 2.7 power from 2 × 10 to 5 × 10⁵.     

多孔线性控制套筒致裂效果影响因素分析

针对坚硬难垮落顶板弱化问题,在分析多孔线性控制套筒致裂机理的基础上,建立700mm×300mm×400mm试块。通过套筒致裂压力及钻孔孔壁周围应变变化情况,分析孔壁周围应力大小、分布及应力集中现象,研究钻孔直径、孔间距、套筒致裂压力对应力集中现象的影响,同时分析这三者对裂纹扩展方向的影响,得出套筒致裂最优法。试验结果表明:(1)套筒致裂效果受钻孔直径、孔间距和套筒压力等综合因素影响,采用大直径、大间距的布孔方式可获得较好的致裂效果;(2)多孔线性控制致裂试验的张性裂缝扩展方向主要沿着各钻孔中心连线方向;(3)钻孔直径30mm、间距300mm的试块致裂压力为18.6MPa,远大于顶板岩体抗拉强度,将套筒致裂法应用于顶板弱化是可行的。本研究对解决采空区坚硬顶板大面积悬顶具有理论意义和应用价值。     

A Combined Theoretical/Experimental Approach for Reducing Ringing Artifacts in Low Dynamic Testing with Servo-hydraulic Load Frames

One of the most challenging tasks facing computer-aided engineering (CAE) analysis is the acquisition of accurate tensile test data that spans quasi-static to low dynamic (10^?5/s?≤ $ \overset{.}{\varepsilon } $ ≤5?×?10²/s) strain rates ( $ \overset{.}{\varepsilon } $ ). Critical to the accuracy of data acquired over the low dynamic range is the reduction of ringing artifacts in flow data. Ringing artifacts, which are a consequence of the inertial response of the load frame, are spurious oscillations that can obscure the desired material response (i.e. load vs. time or load vs. displacement) from which flow data are derived. These oscillations tend to grow with increasing strain rate and peak at the high end of the low dynamic range on servo-hydraulic tensile test frames. Common practices for addressing ringing are data filtering, which is often problematic since filtering introduces distortion in smoothed material data, or trial-and-error design of test specimen geometries. This renders techniques for reducing ringing based upon the mechanics of the load frame and optimization of tensile specimen geometry quite attractive. In the present paper, relationships between load, stress wave propagation, and specimen geometries are addressed, to both quantify ringing and to develop specimen designs that will reduce ringing. A combined theoretical/experimental approach for tensile specimen design was developed for reducing ringing in flow data over the low dynamic range of strain rates (10^?5/s≤ $ \overset{.}{\varepsilon } $ ≤5?×?10²/s). The single camera digital image correlation (DIC) method was used to measure the displacement fields and strain rates with specimens resulting from the combined theoretical/experimental approach. While the approach was developed on a specific commercial load frame with a TRIP steel subject to a two-step quenching and partitioning heat treatment (Q&P980), it is readily adaptable to other servo-hydraulic load frames and metallic alloys. The developed approach results in a 90 % reduction in ringing artifact (with no filtering) in a tensile flow curve for Q&P980 at $ \overset{.}{\varepsilon}\kern-4pt $ = 5?×?10²/s. Results from split Hopkinson bar tests of Q&P980 were performed at $ \overset{.}{\varepsilon } $ = 500/s and compare favorably with the test data generated by the developed testing approach. Since the Q&P980 steel represents a new generation of advanced high strength steels, we also evaluated its strain rate sensitivity over the low dynamic range.     

Dehydration characteristics and mathematical modelling of lemon slices drying undergoing oven treatment

In this study, the effect of drying temperature on drying behaviour and mass transfer parameters of lemon slices was investigated. The drying experiments were conducted in a laboratory air ventilated oven dryer at temperatures of 50, 60 and 75 °C. It was observed that the drying temperature affected the drying time and drying rate significantly. Drying rate curves revealed that the process at the temperature levels taken place in the falling rate period entirely. The usefulness of eight thin layer models to simulate the drying kinetics was evaluated and the Midilli and Kucuk model showed the best fit to experimental drying curves. The effective moisture diffusivity was determined on the basis of Fick’s second law and obtained to be 1.62 × 10^?11, 3.25 × 10^?11 and 8.11 × 10^?11 m² s^?1 for the temperatures of 50, 60 and 75 °C, respectively. The activation energy and Arrhenius constant were calculated to be 60.08 kJ mol^?1 and 0.08511 m² s^?1, respectively. The average value of convective mass transfer coefficient for the drying temperatures of 50, 60 and 75 °C was calculated to be 5.71 × 10^?7, 1.62 × 10^?6 and 2.53 × 10^?6 m s^?1, respectively.     

Modeling of Coal Fine Migration During CBM Production in High-Rank Coal

During CBM (coalbed methane) production, the interaction of coal fracture surface with water flow commonly generates and starts coal fine flow. Part of flowing coal fines deposit in coal fracture system due to water production reduction and methane production increase. The fine sedimentation results in the reduction of coal permeability and well productivity. Despite the increasing awareness of the importance of fine migration, limited research has been carried out on the flow model of coal fine coupled with water and gas. In this paper, a flow model of coal fine is established coupled with water and gas flow, taking coal fine generation, migration and sedimentation process into consideration. Then, case simulations are conducted to illustrate effects of water production schedule, permeability performance and gas content on production performance in flow model. The simulation results indicate that methane rate with the lowest initial water rate is observed to have the highest production in late production period. This is mainly due to the reason that the low water flow cannot generate and start the flow of coal fine. Further, the case with high initial water production has faster gas and water flow rate, thus higher coal fine generation rates, which can improve well productivity at earlier production period. As water production declines quickly, both permeability and production performance decrease, which leads to the loss of well productivity. Meanwhile, higher gas content will lead to a faster water production decline at late production period. This indicates that a portion of coal fines plugged in the fracture as water production deceases and the CBM reservoir with high gas content should not adopt a high initial water production schedule.     

Evaluation of two RANS turbulence models in predicting developing mixed convection within a heated horizontal pipe

The developing weakly turbulent regime of mixed convection in a uniformly heated horizontal pipe was first studied experimentally, by means of heat transfer measurements in the following ranges of dimensionless numbers: 3.19 < Re × 10^? 3 < 6.39, 1.80 < Gr _h  × 10^? 8 < 4.20. The working fluid was FC-72?, with Pr = 12.4.

In order to gain a better insight into the thermo-fluid dynamics involved in the phenomenon and obtain the velocity and temperature fields at every point of the fluid domain, numerical simulations were performed by means of commercial software. Turbulence was modelled by using the Reynolds averaged Navier–Stokes equations (RANS) approach. Two closures of the governing equations were evaluated: realizable κ–? (RKE) model and renormalization-group κ–? (RNG) model.

Both models were capable of reproducing the observed physical trends. However, deviations from the experimental data lower than 20% were obtained only in the entry-zone with the RKE model, while the RNG model gave fair predictions only in developed or quasi-developed flow.     

An experimental investigation of the ELAC 1 configuration at supersonic speeds

Supersonic flight of aerospace planes is of marked interest since several flow regimes characterized by different local flow structures have to be flown through. This problem was investigated experimentally for the hypersonic research configuration ELAC 1. The aim of the study was to detect the influence of the rounded leading edge, of the thickness distribution prescribed, and of the Reynolds number, especially on the flow on the leeward side of the configuration. The experiments were carried out in the transonic wind tunnel of Aerodynamisches Institut of RWTH Aachen, at a freestream Mach number Ma _∞=2, a unit Reynolds number of Re _∞=13×10⁶, angles of attack between ?3°?α?10°, and in a wind tunnel of the Institute for Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk. The freestream Mach numbers covered in these experiments were varied between 2?Ma _∞?4, freestream Reynolds numbers per unit length between 25×10⁶?Re _∞?56×10⁶ and angles of attack between ?3°?α?10°. Flow visualization studies, measurements of surface pressure distributions and of aerodynamic forces were used to analyze the flow. The results, which will also be compared with numerical data, clearly indicate marked differences in the location of the separation and reattachment lines, and the formation of the primary, secondary and tertiary vortices, for the flow regimes investigated.     

Effect of Water Saturation on Pressure-Dependent Permeability of Carboniferous Shale of the Qaidam Basin,China

Permeability is the most important parameter that describes gas flow characteristics in shale. Water saturation and effective pressure have a considerable effect on shale permeability. This paper presents the results of a laboratory study of the effects of water saturation and effective pressure on gas permeability in Carboniferous shales of the Qaidam Basin, China. The permeability of shale samples with varying water saturation (0–33 wt%) was measured at effective pressure of 6.9 to 27.59 MPa and at low mean pore pressure (<?6.89 MPa) at room temperature, using a pressure pulse decay permeameter. The results indicate that the water saturation and the effective pressure are the main factors affecting the shale permeability. Permeability of sample C034, which has a high clay content and is dominated by nanoscale slit-shaped pores, shows a large decrease (up to 90%) with increasing water saturation (from 0 to 31.7 wt%), depending on the effective pressure. A much larger permeability reduction with increasing water saturation fraction is associated with the swelling of clay minerals. For each sample with varying water saturation, our analyses revealed a consistent line relationship between log permeability and effective pressure variation. The impact of effective pressure on the measured permeability becomes more significant as water saturation increases. With increasing water saturation, the gas slippage factor decreases and calculated effective pore size increases, and gas–water flow in the shale samples occurs as channel flow. This study provides practical information for further studies of stress-dependent permeability of shale with water and the gas slippage effect in two-phase, gas–water flow.     

Combining TOUGH2 and FLAC3D to Solve Problems in Underground Gas Storage

This paper describes modeling studies assessing the feasibility of increasing the maximum storage pressure in several underground natural gas storage reservoirs. This required an assessment of the potential for gas transport in the caprock and the geomechanical response to pressure change in the storage reservoir. To solve this problem in an efficient manner, two-phase flow (TOUGH2) and geomechanical (FLAC3D) models were combined in series. The TOUGH2 model was calibrated to fit pressure data collected on-site, from both the reservoir and caprock. The mechanical response of the caprock to increased storage pressure was modeled using FLAC3D, allowing assessment of the induced stresses in formations surrounding the reservoirs. We focused on two sites. In the first, field data were obtained from a deep borehole above the gas reservoir, which provided indirect observations of the geomechanical response of the caprock to pressure changes in the reservoir. In the second, open boreholes intersecting two thin caprock units immediately above the reservoir allowed gas flow to a shallower unit, significantly impacting the modeled fracture gradient.     

Load-Inversion Device for the High Strain Rate Tensile Testing of Sheet Materials with Hopkinson Pressure Bars

A high strain rate tensile testing technique for sheet materials is presented which makes use of a split Hopkinson pressure bar system in conjunction with a load inversion device. With compressive loads applied to its boundaries, the load inversion device introduces tension into a sheet specimen. Two output bars are used to minimize the effect of bending waves on the output force measurement. A Digital Image Correlation (DIC) algorithm is used to determine the strain history in the specimen gage section based on high speed video imaging. Detailed finite element analysis of the experimental set-up is performed to validate the design of the load inversion device. It is shown that under the assumption of perfect alignment and slip-free attachment of the specimen, the measured stress–strain curve is free from spurious oscillations at a strain rate of 1,000 s^?1. Validation experiments are carried out using tensile specimens extracted from 1.4 thick TRIP780 steel sheets. The experimental results for uniaxial tension at strain rates ranging from 200 s^?1 to 1,000 s^?1 confirm the oscillation-free numerical results in an approximate manner. Dynamic tension experiments are also performed on notched specimens to illustrate the validity of the proposed experimental technique for characterizing the effect of strain rate on the onset of ductile fracture in sheet materials.     

Influence of rotating magnetic field strength on three-dimensional thermocapillary flow in a floating half-zone model

The effects of rotating magnetic field (RMF) on the three-dimensional thermocapillary flow of semiconductor melt (Pr?=?0.01) in a floating half-zone model under microgravity are investigated numerically by the finite volume method. The results indicate that the thermocapillary flow without magnetic field is a steady three-dimensional convection for Ma?=?40 in a floating half-zone model with As?=?1, and the convection evolves to an oscillatory three-dimensional flow by applying 1–6?mT RMF with 50?Hz rotating frequency. Based on the fast Fourier transform spectrum, the convection is confirmed to be a periodically oscillating flow, the oscillatory main frequency, 1.59?×?10^?3?Hz for 1?mT RMF and 5.84?×?10^?2?Hz for 6?mT RMF, increases with the magnetic strength. However, with increasing the magnetic field strength up to 7?mT, the three-dimensional thermocapillary flow is effectively controlled and the convection turns into a steady axisymmetrical one.     

大孔径静态破碎膨胀压力特性及布孔参数分析

大孔径静态破碎与传统静态破碎有着显著的不同。利用电测法测量了直径40和100 mm钢管中的破碎剂膨胀压力和温度,对比分析了两种工况下的不同现象。实验表明,孔径的增加能够提高膨胀压力,加快反应速度。基于实验的数据,利用有限元方法计算了静态破碎时钻孔周围岩石介质中的应力分布。基于实验数据和有限元数值计算结果,使用数据拟合方法对静态破碎时岩石中的应力分布弹性模型进行修正,得到了应力分布方程。利用该方程推导的布孔参数计算公式,适合运用于实际工程之中。