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Understanding water distribution behaviors plays an essential role in shale gas development due to the hydraulic fracturing technology. In this study, water vapor adsorption isotherms were measured for Upper Triassic Yanchang and Lower Silurian Longmaxi samples at 288.15 K, 298.15 K and 308.15 K to investigate the adsorption behaviors of water on shale. For a description of adsorption process, a Dent’s model provides an estimate of primary and secondary adsorption sites of water adsorption. The effects of temperature, shale mineralogy, and water distribution were discussed. The results show that temperature has a negative effect on water vapor adsorption. Adsorption capacities of primary and secondary sites decrease with increasing temperature. Water vapor adsorption capacity is tightly associated with clay content, whereas water vapor adsorption has no significant relationship with total organic carbon (TOC). Normalized water vapor adsorption content also has no significant relationship with TOC, which indicates no obvious correlation between water vapor adsorption and TOC content. The inter-crystal pores of clay minerals provide significant specific surface for gas adsorption in shale. At low relative pressure, a large number of water molecules adsorb on the primary sites. At high relative vapor pressure, most of the primary sites have been occupied, so that the secondary adsorption sites will be utilized. Therefore, due to relative weak binding energies in secondary adsorption sites, water cluster are formed in micro-pores of shale. As relative vapor pressure increases, capillary condensation gradually predominates over the adsorption. The study will reveal mechanism of water adsorption and distribution characteristics on shale and provide some foundation for geological reserve estimation and shale gas recovery prediction.

Understanding the behavior of multiple cavitation bubbles and their interactions is important for developing deeper insight into cavitation phenomena. Using a revised single-component multiphase Lattice Boltzmann method, the weak and strong interactions between two neighboring cavitation bubbles and the mutual interaction among a cluster of cavitation bubbles are simulated. The simulated bubble evolutions have satisfying agreement with previous experimental results. Details of the multiple bubble interactions (the pressure and velocity fields) are presented. In the case of two-bubble interactions, typical phenomena such as bubble toroidal deformation, micro-jets, the thinning of the liquid film, and bubble coalescence are successfully reproduced. The bubble radius changes under two-bubble weak interaction are adequately described by a revised 2-D Rayleigh-Plesset equation. The transition from the strong to weak interactions with increasing initial distance between the two bubbles is analyzed. The behaviors of a cluster of bubbles are also simulated, where the shield effect of outer layer bubbles and the micro-jets are accurately captured. The present multiphase Lattice Boltzmann method is a promising tool to investigate complex cavitation problems.

It is a common phenomenon during CBM drilling and production that reservoir damage is not eliminated completely. In view of this, a technical idea of composite pulsating hydraulic fracturing of radial horizontal wells which is conducive to blockage removing and stimulation was put forward in this paper. Speaking of the hydraulic jetting in a multi-branch radial well, it is to conduct pulsating hydraulic fracturing moderately through a high-diversion radial hole, so as to crush and break coal beds near the main hole to the uttermost. Thus, an extensive pressure relief and permeability increase area where high-diversion pathways are combined with fracture networks is formed. Then, to verify its technical principles, laboratory tests on pulsating hydraulic fracturing of radial wells were designed and carried out. Besides, the relationships of the features of acoustic emission (AE) response during the formation of fractures by composite fracturing of radial horizontal wells vs. coal breaking degree and macro fracture morphology were experimentally studied by using a pulse servo fatigue testing machine and an acoustic emission detector. And the following research results were obtained. First, under experimental conditions, fractures initially occur when the pressure of composite pulsating hydraulic fracturing of radial horizontal wells is 1/3–1/4 of the peak pressure of conventional fracturing, and the amount of its AE events is 1.38–7.07 times that of conventional fracturing. Second, when composite pulsating hydraulic fracturing of radial horizontal wells is conducted, AE emission signals respond strongly, the peak pressure for macro fracturing is lower and a larger fracture network can be generated more easily under the same condition. Third, radial laterals amount, borehole length, dynamic loading frequency and amplitude are the important factors affecting the effect of composite pulsating hydraulic fracturing of radial horizontal wells. In conclusion, composite pulsating hydraulic fracturing of radial horizontal wells provides a new idea of removing the blockages in CBM reservoirs and developing CBM efficiently, realizing effective blockage removing and stimulation of CBM wells. Keywords: Coalbed methane, Reservoir damage, Stimulation, Radial horizontal well, Composite pulsating hydraulic fracturing, Blockage removing, Laboratory

Acoustic emission has been recognized as a potentially advanced technique to monitor the efficiency of high-velocity waterjet rock drilling. Identification of AE multi-sources signals becomes fundamental to correlate AE singles and rock drilling efficiency. This paper presents a controlled experimental scheme and frequency analysis to identify different signal sources from waterjet impact and rock failure, respectively. Acoustic signals were decomposed into low, medium, and high-frequency signals by using the variational mode decomposition (VMD) approach. Results indicate that the waterjet impact generates a wideband acoustic signal. The frequency distribution of the high-frequency signals exhibited a strong diversity among four materials because of their different acoustic attenuation. Furthermore, the waterjet pressure did not change the frequency domain distribution of the material. An obvious reduction of the high-frequency signals was observed while waterjet rock drilling. Furthermore, the main frequency of high-frequency signals starts to be time-dependent that causes from the rock failure process. The high-frequency band presented a good correlation with the rate of penetration.

Fluid imbibition is a recognized strong phenomenon in tight shale that could greatly influences the dynamic failure behaviors. An experimental study was carried out to account for the influences of three typical drilling fluids (Air, Oil-based, and Water-based) on dynamic mechanics of tight shale. The collected samples were separated into three groups by the treatment of the distilled water saturation, white oil saturation, and drying, respectively. The dynamic properties involving Young's modulus, Compression strength, and specific energy consuming were experimentally measured by using Split Hopkinson Pressure Bar (SHPB). Results confirm that the fluid imbibition phenomena perform a promising influence on dynamic rock mechanics of tight shale. There are very different responses induced by air dry, oil, and water imbibition on dynamic rock strength, cutting size, and energy consuming. Oil imbibition is capable to enhance the strength, achieve uniform size of cutting, and consume less specific energy to break unit volume of rock. Comparatively, water imbibition sharply reduces the strength and consumes much less specific energy. Air dry condition makes much bigger and non-uniform cuttings and consumes the most specific energy. Those observations are no longer the same with obtained knowledge from sandstone and concrete, which attribute to the nanoscale pore distribution and fluid sensitivity of tight shale.

abrasive supercritical CO2 (SC-CO2) jet perforation is the key procedure in the SC-CO2 fracturing, which will directly affect the exploitation of oil and gas. The properties of the SC-CO2 fluid, such as density, viscosity, diffusivity and so on, change greatly with the variation of temperature and pressure, which directly affect the particle-carrying ability and perforation performance. This paper investigates the influence of key parameters, such as ambient pressure, fluid temperature, jet standoff distance and jet pressure, on the perforation ability of abrasive SC-CO2 jet. The results indicate that the ambient pressure has no significant effect on perforation under the condition of a fixed jet differential pressure. When the confining pressure increases from 5 to 15 MPa, the hole depth and diameter decrease by 5.7% and 18.6% respectively. The hole depth increases slightly with the rising of jet temperature. In addition, with the jet temperature rising per 20℃ within the range from 40 to 100℃ and the standoff distances being 4 to 10 mm, the hole depths increase by 3.8% and 12.0% in average, respectively. Furthermore, the hole depths keep unchanged at first and then decrease rapidly with the increasing standoff distance. However, the hole diameters and effectively impinged areas increase with the standoff distance. The influences of SC-CO2 jet pressure on the perforation performance are similar to that of the conventional jet. Additonally, both effective hole depths and volumes increase linearly in general with the increasing of jet pressure. The hole depths averagely increase by 36.6% when the SC-CO2 jet pressure rising by 5 MPa. Furthermore, the research also shows that perforation performance of pre-mixed jet is better than the post-mixed jet, the volume ratio of the two kinds of perforating holes is 12.02 under these experimental conditions. All the above merits have provided a theoretical foundation and experimental proven for the field application of SC-CO2 jet perforation technology.

An analytical model for gas transport in shale media is proposed on the basis of the weighted superposition of slip flow, bulk diffusion and Knudsen diffusion. The model takes account of slip effect and real gas effect, and is successfully validated by experimental data and Lattice Boltzmann simulation results. The contribution of each transport mechanism to the total flow is investigated. The effect of porosity, diameter and pressure on the apparent permeability is studied and a sensitivity analysis is performed to evaluate the significance of the parameters for gas transport. The results show: (1) the present model can reasonably describe the process of the mass transform of all different gas transport mechanisms; (2) As pressure and pore diameter decrease, the number of molecule-wall collisions gradually predominates over the number of intermolecular collisions, Knudsen diffusion contributes more to the total flow; and (3) the apparent permeability increases with porosity, pore diameter, and decreases with pressure. It is more sensitive to pressure in rarefied gas flow regime, and pore diameter has a significant impact under high pressure. The present model can provide some theoretical support in numerical simulation of shale gas production.

Highlights • A efficient method is proposed to calculate the ambient radius and gas composition for sonoluminescing bubbles. • The method is based on diffusive equilibrium for each species inside the bubble. • On average, only ten acoustical cycles of dynamic simulation are needed. • The validation using existing experimental data shows the robustness of the method., A sound driven air bubble can be transformed into an argon bubble emitting light pulses stably. The very foundation to investigate the sonoluminescing bubble is to accurately determine the ambient radius and gas composition in the interior. The conventional approach is to model the air-to-argon transformation process through a large number of bubble dynamics simulations to obtain the physical parameters of the ultimate argon bubble. In this paper, we propose a highly efficient method to pinpoint this information in a phase diagram. The method is based on the diffusive equilibrium for each species inside the bubble and derives the ambient radius and composition inversely. To calculate the former parameter, the bisection algorithm is employed to consecutively narrow down the searching range until the equilibria is approached. Afterward, several cycles of full dynamics simulations are conducted to refine the composition. The method is validated using published experimental data. The calculated ambient radii deviate from the test results by less than 1 μm, which falls within the margin of measurement error. The advantages of this method over the semi-analytical approach reported by Hilgenfeldt et al. [J. Fluid Mech. 365 (1998)] are also discussed. Our study provides a standard procedure to calculate the ambient radius and composition and is beneficial for the numerical simulation of sonoluminescing bubbles.

Conformance control during waterflooding in an oil reservoir is utilized to redistribute water and increase the sweep efficiency and hence oil production. Using preformed gel particles can effectively redirect the flow by blocking the high-permeability zones and forcing water into low-permeability zones where the oil is trapped. However, the size of such gel particles can limit their applications deeper within the reservoir and can result in shear-induced degradation near the well bore. Here, we fabricate core-shell nanohydrogels with delayed swelling behavior; their volume increases by a factor of 200 after about 30 days in brine under reservoir conditions. We study their effect on the flow behavior in a three-dimensional porous medium micromodel consisting of randomly packed glass beads. Using confocal microscopy, we directly visualize the spatial variations of flow in the micromodel before and after nanohydrogel injection and swelling. The swollen nanohydrogels block some pores reducing the permeability of the micromodel and diverting the water into low-permeability regions. A core flood experiment further confirms that the nanohydrogels can significantly reduce the permeability of a reservoir sample and divert the fluid flow. Our results demonstrate that these core-shell nanohydrogels might be useful for flow control in porous media and can be used as a conformance control agent.

High-density completions prevail in shale oil formation in China due to the difficulty of identifying the sweet spot with high accuracy. Knowing the location of sweet spots benefits the horizontal well drilling and the selection of perforation clusters. Generally, field engineers determine sweet spots from the well logging interpretation. However, a group of prevalent classifiers based on gradient boosting decision trees were introduced to automatically determine sweet spots according to datasets from the well logging. Based on boosted tree algorithms, Extreme Gradient Boosting (XGBoost), Unbiased boosting with categorical features (CatBoost) and Light Gradient Boosting Machine (LightGBM) are utilized to control the over-fitting issues. Compared with linear support vector machines (SVMs) or kernel machine, these robust algorithms can deal with comparative scales of the features and learn non-linear decision boundaries via boosting. Moreover, they are less influenced by the presence of outliers. Another prevailing approach, named Generative Adversarial Networks (GANs), was implemented to augment the training dataset by using a small number of training samples. In terms of the training purpose, we randomly selected 60 horizontal wells. In each well, tens to hundreds of datasets of different formation intervals were collected. Features, such as resistivity, interval transit time, layer thickness, shale content, porosity, permeability, oil saturation, and coordinates in three dimensions, were extracted from well-logging datasets and regarded as inputs for classifiers. Datasets of remaining wells were used for testing. Compared with conventional SVMs, the prediction accuracies of sweet spots by XGBoost and CatBoost were significantly improved to 81.61% and 82.5%, respectively. Additionally, GANs, as an unsupervised machine learning tool, have been attempted to augment the dataset by utilizing a relatively small number of training samples. A generative model is used for capturing the data distribution, and a discriminative model aims at predicting a label to which that data created by the generative model. Without special pre-processing of the input datasets and fine tuning CTGANs model, the fake dataset could still bring 68.58% accuracy for all detections and 59.01% of the label corresponding with oil formation that showing its potential in data augmentation. This paper illustrates a new tool for categorizing the reservoir quality by using gradient boosting decision trees and GANs methods, which further helps search and identify sweet spots. An extensive application has been built for the field cases in a certain oilfield. This tool provides a guideline for covering more sweet spots during the drilling and completion treatment, which immensely decreases the exploration cost.

Vast amounts of natural gas hydrate are buried in subseafloor sediments without impermeable boundaries, which is recognized as an essential energy source for the future. Previously, the multi-branch well was proposed to enhance the recovery efficiency of natural gas hydrate, and the gas production rate has been dramatically improved comparing with the vertical well. However, the multi-branch well shows a terrible performance in gas production duration. As a continuation of the previous study, numerical simulations were conducted to investigate the influence of hydrate reservoir properties on the gas production potential. Results indicate that it is hard to extract hydrate commercially for hydrate accumulations without impermeable boundaries. A high initial hydrate saturation leads to a long gas production duration but a low gas production rate. An increase in the intrinsic permeability of isotropic reservoirs would shorten the gas production duration and result in a low gas recovery ratio. Permeability anisotropy shows a noticeable effect on enhancing the gas recovery ratio and the gas production duration due to the improved pressure propagation pattern. Therefore, in the upcoming field tests, reservoir reconstructions that enhance permeability anisotropy are strongly suggested to obtain better outcomes.

Radial borehole fracturing is a technology combing hydraulic fracturing and slender boreholes which are radially drilled from the main wellbore by water jets. This paper aims to investigate how the radial boreholes influence the fracture extension when they are pressurized by the fracturing fluids. Concrete blocks containing radial boreholes were cast, the mechanical properties of such blocks were tested, and these samples were hydraulically fractured through a laboratory tri-axial fracturing apparatus. Experimental results show that increasing the azimuths of the radial boreholes enhances the breakdown pressures, but it will reduce the fracture extension distances (deviation distances) paralleling to the radial borehole axes. Similar trends appear when the horizontal stress differences diminish and the distances between adjacent layers of radial borehole axes increase. When changing the numbers of the radial boreholes, the variations of the breakdown pressures depend on the layouts of the radial boreholes. Besides, increasing the pump rates lifts the breakdown pressures and enhances the deviation distances. Compared to perforation fracturing, radial borehole fracturing has higher breakdown pressures and stronger control to fracture propagation. It is concluded that the extrusion forces exist in the horizontal zones between the radial boreholes. A conceptual model is proposed which can explain the variations of the deviation distances. When the injection time of the fracturing fluids is identical, the higher breakdown pressures will decrease the deviation distances because of the extrusion forces. There is a balance between the lower breakdown pressures and the higher pore pressures both caused by the more penetration of the fracturing fluid, which can either increase the deviation distances or reduce the deviation distances.

The adsorption behavior and underlying mechanism of CO2 and CH4 binary mixture in shale kerogen significantly affect the CO2 sequestration with enhanced gas recovery project (CS-EGR). In this study, we investigated the competitive adsorption behaviors of CO2 and CH4 in shale kerogen nanopores using grand canonical Monte Carlo (GCMC) method. Kerogen model takes into effect of matrix and slit nanopores and moisture content based on Ungerer’s molecular model and scanning electron microscope (SEM) analysis, and is successfully validated against experimental data. The effects of temperature, CO2 and CH4 distribution, moisture content, adsorption selectivity, and optimal formation for injection were discussed. The results show that adsorption amount of CH4 on the kerogen increases with increasing pressure and decreases with increasing temperature. The adsorption selectivity of CO2 over CH4 is 2.53–7.25, which indicates that CO2 is preferentially adsorbed over CH4 under different temperatures. H2O prefers to ads...

A production forecasting model for shale gas reservoirs is proposed based on the continuum medium theory (organic/inorganic matrix) and discrete fracture model (fracture system). The fractal property of the pore size distribution of shale matrix is considered. The stability and accuracy of the model are successfully validated with the field data. Variations of different flow mechanisms during the exploitation of shale gas are investigated. The sensitivities of gas production to the pore size distribution of shale matrix, natural fracture parameters and hydraulic fracture parameters are analyzed. Our results show that: (1) The contribution of the convective flow and surface diffusion to total gas flow increases with the depletion of shale gas, while that of the bulk diffusion and Knudsen diffusion decreases gradually. (2) Among the fractal pore-size parameters, the maximum inorganic pore diameter has the most significant impact on the cumulative gas production of up to 13.9%. (3) After the natural fracture number increases to a certain extent, gas production increases slowly even if more natural fractures exist in the reservoir. The complex fracture network is beneficial to increase shale gas production. (4) The cumulative gas production rises as the increase of the hydraulic fracture half-length, flow conductivity, spacing and number. The rapid production rate in the early production stage may result in a fast reservoir energy depletion, which has a negative effect on production.

Based on bubble dynamics theory, a mathematic model describing the cavitation bubble size variation in the flow field of self-resonating cavitating jet was developed considering the pressure field and mass and heat exchange between cavitation bubble and ambient fluid. With this model, the influence factors on the cavitation intensity are investigated. The results show that the destructiveness of cavitating jet in breaking rocks depends on the bubble's first collapse, with decreasing intensity in the subsequent collapses. The self-resonating effect significantly enhances the cavitation intensity by promoting the collapse pressure and elongating its duration. Hydraulic parameters are proven to be the dominating factors influencing cavitation intensity: while collapse intensity monotonously increases with jet velocity, there exists an optimum ambient pressure where highest collapse intensity can be achieved. Conversely, the fluid properties show minor influences: cavitation intensity only slightly decreases with the increasing of fluid's density and barely changes with the variation of viscosity and surface tension. The results from this investigation help to uncover the mechanism of the enhanced erosion potential of self-resonating cavitating jet. The conclusions can be used to further improve the performance of self-resonating cavitating jet in field applications. Key words: self-resonating cavitating jet, cavitating bubble, collapse intensity, hydraulic parameters, fluid properties

The aim of the present study is to help understand hydraulic fracture propagation in shale formations through numerical simulations. The hydraulic fracture propagation regime in shale is analyzed considering the anisotropic nature of the shale rock formation and slick-water fracturing fluid. It is determined that the dominant mechanism of hydraulic fracture propagation is the so-called transitional regime that is characterized by a negligibly small fluid lag region and zero fluid front pressure. For the modeling of the hydraulic fracture evolution over time, we assume orthotropic linear-elastic rock media and that the flow of the fracturing fluid is governed by the Reynold's lubrication equation. For the discretization of the coupled solid-fluid equations within the 2D plane-strain context we use the extended finite element method for the rock media and the finite volume method for the lubrication equation. The problem of the hydraulic fracture evolution over time is modeled as stable quasi-static crack growth where time is the result of upholding the mass conservation principle between the fluid inflow and the crack volume. The Picard iterative approach is used to solve the discrete non-linearly coupled solid-fluid equations. Our model is verified against several analytical solutions. Subsequently, a five-stage hydraulic fracturing problem is simulated to study the interactions between the different fractures. Results show that the on-going hydraulic fractures are attracted by the pre-existing hydraulic fractures as a result of the change of the local stress field relative to the initial in-situ stress field. For the cases considered, fracture deflections are found to be most extensive with decreasing fracture spacing and in-situ stress difference, but insensitive with the increasing the ratio of Young's moduli.

A full assessment of the cavitation erosion potential of a cavitating jet necessitates the determination of the cavitation impact field. However, a detailed description of the impact intensity and spatial distribution of the large numbers of cavitation impacts has not been fully established. Here, we report such a map of cavitation impact field constructed with an inverse finite element method based on pitting analysis. The map presents the distribution of each impact on the impingement surface with its peak impact pressure, impact size, impact force, and impact energy. From this map, it is seen that cavitation impacts are preferentially localized in a ring region and a circular region at relatively small and large standoff distances, respectively. A statistical analysis of the impact data reveals those dominating impacts which contribute most to the overall intensity of the cavitating jet. To elucidate the underlying mechanism for the different impacts distribution patterns appearing on the impingement surface, the flow structures of the impinging jet are investigated by conducting a flow field simulation. The results show that the stagnation region and the recirculation zone above the impingement wall are the two key factors to the formation and variation of impact distribution patterns. Compared with the conventional measurement with pressure transducers, the present investigation is assumed as a further step towards depicting and interpreting the erosive intensity of a cavitating jet.

Cavitation is a ubiquitous phenomenon in ocean engineering. In order to figure out the relationship between cavitation cloud and cavitation erosion, high-speed visualization and erosion test of cavitation jet were simultaneously performed. The temporal and spatial distributions of cavitation cloud was analyzed using Proper Orthogonal Decomposition (POD) and the morphology of eroded specimen was analyzed from both macroscopic and microcosmic perspectives. The results show that POD modes of side view images are able to estimate the maximum radial distribution of cavitation pits, while the energy fraction of the first POD mode can serve as an indicator of specimen mass loss. The intensity of cavitation erosion is decided by both the bubble concentration and collapse intensity at specimen surface. At a medium standoff distance, where collapse intensity and bubble concentration are well compromised, maximum cavitation erosion is achieved. The aggravation of cavitation erosion is the combined consequences of the growth of single cavitation pits and the connection of adjacent pits. These results suggest that the collapse of cavitation bubbles is a necessary, but not sufficient, condition for severe cavitation erosion. The findings are expected to improve the physical understanding of the relationship between cavitation phenomenon and cavitation damage.

The adsorption of carbon dioxide and methane binary mixture in shale kerogen nanopores and the underlying mechanism significantly affect the supercritical carbon dioxide enhanced shale gas development project. In this study, we investigated the adsorption properties of carbon dioxide and methane in shale kerogen using grand canonical Monte Carlo (GCMC) method. Shale kerogen was fabricated based on Ungerer molecular model and its parameters were validated. The effects of temperature, pressure, mole fraction on the adsorption isotherms, average isosteric heat, potential energy distribution, and adsorption selectivity of binary mixture were discussed. The results show that the absolute adsorption capacity of methane in binary mixture decreases as temperature increases, but increases as mole fraction increases. Compared with methane, carbon dioxide is in lower energy absorption sites, which indicates the adsorption capacity of carbon dioxide in shale kerogen is stronger than that of methane. The adsorption selectivity of carbon dioxide over methane first decreases as pressure increases until pressure reaches critical pressure (7.38 MPa for carbon dioxide), and then stays at around 3.8 as pressure continues to rise. Adsorption selectivity and desorption quantity are used to reveal that the optimal injection depth for supercritical carbon dioxide enhanced shale gas development project is 1000–2500 m. This study will reveal the mechanism of the adsorption of methane in kerogen and provide some fundamental data for supercritical carbon dioxide enhanced shale gas development project.

Cavitating jet has been widely used in underground drilling to enhance the rock erosion efficiency. In this circumstance, the high ambient pressure directly influences the bubble traveling process and cavitation impact pressure. To quantitatively assess the erosion intensity of the cavitating jet under the high ambient pressure conditions, a combined numerical and experimental investigation was conducted. A convergent-divergent nozzle was chosen to generate the cavitating jet in a closed test cell. A bubble transportation model was used to simulate the bubble traveling in the water jet through the nozzle and investigate the effects of ambient pressure and flow rate on the transportation efficiency. Pitting analysis with specimen 7075 aluminum alloy was performed to measure the magnitude and distribution of the cavitation impact pressure at different standoff distances. The results reveal the dynamic bubble traveling process and shed light on the cavitation impact field. The bubble transportation capability of the cavitating jet depends on the jet velocity. Given certain ambient pressure and nozzle size, the flow rate must be larger than a certain threshold value for allowing the bubbles to be transported out before collapsing inside the nozzle. The magnitude of cavitation impact pressure is of the order of 1 GPa and shows little dependence on standoff distance. However, the overall impact rate and the distribution pattern are significantly influenced by the standoff distance. It is concluded that the variation of the erosion intensity of cavitating jet correlates more closely with the change of the impact rate than with the absolute impact magnitude.

Knowledge of mixed-gas adsorption behavior on shale is important for reserve estimation, production forecast, and carbon dioxide sequestration. In this work, we combined the multicomponent potential theory of adsorption (MPTA) with the hybrid perturbed-chain statistical associating fluid theory equation of state (hPC-SAFT EOS) to describe mixed-gas adsorption on shale. In the hPC-SAFT-MPTA model, the fluid-solid interaction is modeled using the Dubinin-Radushkevich-Astakhov (DRA) potential and the fluid - fluid interaction is described by the hPC-SAFT EOS. Extending the hPC-SAFT-MPTA model to excess adsorption is straightforward and has no need to estimate the adsorbed phase density. Experimental data on the pure-component and mixture adsorption of CH 4 , C 2 H 6 , CO 2 , and N 2 on activated carbon and shale were used to test the hPC-SAFT-MPTA model. Activated carbon was employed as a reference adsorbent because its pore structure is simpler than that of shale. The hPC-SAFT-MPTA model can predict the mixture adsorption with average relative errors of 10.34%–17.12% on the basis of the pure gas adsorption data. The hPC-SAFT-MPTA model provides an alternative approach to modeling mixed-gas adsorption on shale and complements sophisticated methods such as density functional theory and the grand canonical Monte Carlo simulation method. The hPC-SAFT-MPTA model is anticipated to be useful in simulations of shale gas production and CO 2 sequestration in shale.

Shale gas is a promising alternative energy due to the advantages of large reserves and high energy efficiency. The amount of adsorbed gas in shale nanopores is a significant reason that shale gas maintains long-term stable production. Molecular simulation is an important tool for analyzing the adsorption mechanisms and understanding the gas adsorption behavior on shales at the microscopic level intuitively and accurately. In this review, we briefly summarize the molecular models of shale inorganic minerals, organic matter and composite shale models with organic and inorganic regions. We also provide a comprehensive overview of shale gas adsorption and analyze in detail its microscopic mechanism on these molecular models. Finally, we discuss the existing challenges and perspectives to promote the future development of shale gas production. An accurate description of shale compositions and structures, as well as adsorbates, is crucial for molecular simulation of shale gas adsorption.

Summary Two-phase flow is generally significant in the hydraulic-fracturing design of a shale-gas reservoir, especially during the flowback period. Investigating the gas- and water-production data is important to evaluate stimulation effectiveness. We develop a semianalytical model for multifractured horizontal wells by incorporating the two-phase flow in both shale matrix and fracture domains. The complex-fracture network, including both primary/hydraulic fractures and secondary/natural fractures, is modeled explicitly as discretized segments. The node-analysis approach is used to discretize the networks into a number of fracture segments and connected nodes, depending on the complexity of the fracture system. The two-phase flow is incorporated by iteratively correcting the relative permeability to gas/water for each fracture segment and capillary pressure at each node with the fracture depletion. The accuracy of the proposed model is confirmed by the numerical model. Subsequently, the early-time gas- and water-production performance is analyzed by use of various fracture geometries and network configurations. The model was also used to history match an actual multistage hydraulically fractured horizontal well in the Marcellus Shale during the flowback period. The research findings have shed light on the factors that substantially influence the gas- and water-production behavior during the flowback period. We also investigate the effects of fracture-network geometries and complexities on the gas/water-ratio (GWR) diagnostic plots. The results depict that the GWR behavior on the diagnostic plots is highly dependent on fracture-network geometry, configuration, and connectivity, which could assist in deriving the critical fracture properties affecting the production performance. This work extends the semianalytical approach previously proposed for modeling single-phase to two-phase flowback problems in unconventional reservoirs with various fracture-network geometries. The method is easier to set up and is less data-intensive than use of a numerical reservoir simulator, and is capable of providing a straightforward and flexible way to model complex-nonplanar-fracture networks in a multiphase-flow environment.

The success of waterjet drilling technology requires further insight into the rock failure mechanisms under waterjet impingement. By combining acoustic emission (AE) sensing and underwater sound recording techniques, an online system for monitoring submerged waterjet drilling has been developed. For four types of sedimentary rocks, their AE characteristics and correlations to the drilling performance have been obtained through time–frequency spectrum analysis. The area under the power spectrum density curve has been used as the indicator of AE energy. The results show that AE signals from the fluid dynamics and the rock failure are in different ranges of signal frequency. The main frequencies of the rock failure are within the higher range of 100–200 kHz, while the frequencies of the fluid dynamics are below 50 kHz. Further, there is a linear relationship between the AE energy and the drilling depth irrespective of rock type. The slope of the linear relationship is proportional to the rock strength and debris size. Furthermore, the AE-specific energy is a good indicator of the critical depth drilled by the waterjet. In conclusion, the AE characteristics on the power density and dominant frequency are capable of identifying the waterjet drilling performance on the rock materials and are correlated with the rock properties, i.e., rock strength and cutting size.

The primary objective of this study is to investigate the adsorption and surface diffusion of supercritical methane in shale. An adaptive Dubinin–Astakhov (ADA) model, with a term taking the adsorbed phase density, was introduced to interpret measured excess adsorption isotherms and obtain temperature-independent characteristic curves. The ADA model can also predict adsorption isotherms at different temperatures. Combining with the ADA model and the Maxwell–Stefan equation, a new model was developed to describe the surface diffusion of supercritical methane in shale. The model was successfully validated against experimental data. We also studied the effect of fugacity and temperature on the surface diffusion. With increasing temperature, the surface diffusion flux reaches a maximum and then decreases, which is the result of a trade off between the amount adsorbed and the effective Maxwell–Stefan surface diffusivity. The driving force for surface diffusion is the chemical potential gradient, which can be r...

According to the fact that gecko-inspired vertically aligned carbon nanotubes (VA-CNTs) exhibit ultrastrong adhesion, dopamine is utilized to make a modification to this traditional biomimetic material. The composite material is tested for adhesion performance under different environmental conditions by an atomic force microscope. The adhesion force of the modified VA-CNTs does not show obvious fluctuation during the gradual heating process; however, the material gains improved adhesion when increasing the ambient humidity. In addition, the modified CNTs show a stronger adhesion force than the original CNTs in their performance tests. The dopamine polymer has a good combination with CNTs, which is responsible for the aforementioned excellent performance. Overall, this modification method is simple, convenient, efficient, and environmentally friendly, which all indicates a promising future in its application. The modified CNTs are expected to be used for super-adhesion in harsh environments, as well as in the field of microelectronics.

The utilization of acid has been approved as a potential approach to enhance Stimulated Reservoir Volume (SRV) by creating secondary pores and microfractures in shale reservoirs. This paper proposed an acid soaking technique to quantify the generation of microfracturing behaviors. Two types of core samples from distinct shale depositional environments, namely Longmaxi carbonate-rich, and Yanchang clay-rich shale were emphasized. Hydrochloric acid (15%+ 3%KCL) was utilized to treat the shale samples for a period of 2 hours. Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) experiments were performed to correlate the individual mineral phases. Inductively coupled plasma-optical emission spectrometry (ICP-OES) and X-ray fluorescence tests (XRF) were utilized, to quantify the elemental analysis of the distinct mineral phases. For post acidizing test conditions, three different types of microfractures, namely stress-induced micro-fractures, acid dissolution micro-fractures, and acid shrinkage micro-fractures were observed. For the clay-rich shale, the stress-induced microfractures were more prominent within the Kaolinite and the boundary edges of the Chlorite/Biotite regions. Whereas, shrinkage micro-fractures dominated the chlorite regions. The Longmaxi carbonate-rich shale, on the other hand, revealed more evidence of the creation of acid dissolution microfractures within Anhydrite, calcite, illite, and organic matter regions. According to the ICP-OES and XRF test results, the clay-rich shale sample underwent excessive ionic imbibition, as well as an ion dislodgment mechanism. However, the carbonate-rich shale experienced only the ion dislodgment mechanism from the shale interface. No ions reverted to the shale sample as the electric equilibrium was established. This paper would contribute immensely to the classification of different micro-fracturing features, and their characteristics for the distinct shale mineralogies.

Natural gas hydrate, a potential energy source, is clean and occurs abundantly in nature. Enhancing the gas production efficiency from gas hydrate-bearing sediments (GHBS) is of great significance to promote its industrial development. In this study, the radial jet drilling (RJD) technology is innovatively proposed to stimulate oceanic hydrate reservoirs. With an open-source simulator HydrateResSim, a 3D model is constructed based on the geological data from the SH7 site in the South China Sea. The hydrates exploitation performance using the combination of radial wells and depressurization is studied for the first time. Results indicate that radial wells can significantly enhance the gas production rate in the early stages of production. The hydrate recovery factor and the radial branch length are linearly related. However, the application of radial wells seems incapable of prolonging the gas production life cycle. By analyzing the multi-physical response of GHBS induced by depressurization, the stimulation mechanisms of radial wells are characterized as enlarged drainage area, enhanced pressure drop propagation, and increased geothermal heat flow. This work verified the capability of RJD in promoting hydrate extraction. Also, it provides insights into the potential applications of multi-branch wells in field trials of hydrate production.

Hydraulic fracturing is widely applied to develop tight oil reservoirs, and it is significant to diagnose fracture hit to determine optimal well spacing. Pressure test is an effective method to determine fracture hit in a multi-well system. In this study, we developed a semi-analytical model to quickly identify fracture hit in multiple vertical well system in tight oil reservoir. This semi-analytical model involves several steps, i.e., prior production step of parent wells, pressure build-up step, and interference test step. The model was verified by a numerical reservoir simulator for single well and multiple wells. Then, based on the model, the well interference by fracture hit in a three-well system (two parent wells and one child well) is discussed. Results show that the developed semi-analytical model could efficiently simulate the entire production process of a multi-well system, quickly diagnose fracture hit by using pressure interference test data and determine the optimal shut-in time of parent well. The practicability of model lies in that the reservoir pressure depletion by prior production of parent wells is considered. Besides, it is found shutting down the parent well is very necessary to diagnose fracture hit when the child well is infilled. When the shut-in parent well is connected to producing child well, the pressure build-up process of parent well is terminated with the bottom hole pressure reducing continuously. What is more, the shut-in time of parent wells should be sufficient to better detect the fracture hit, especially under the condition that parent wells have produced for a long time or the connectivity among wells is weak. This work can provide critical insights into understanding the mechanism of well interference by fracture hit in a multi-well system.

A model for real gas flow in shale gas matrices is proposed and consists of two main steps: (a) developing a microscopic (single pore) model for a real gas flow by generalizing our previously reported Extended Navier-Stokes Equations (ENSE) method and (b) by using fractal theory concepts, up-scaling the single pore model to the macroscopic scale. A prominent feature of the up-scaled model is a predictor for the apparent permeability (AP). Both models are successfully validated with experimental data. The impact of the deviation of the gas behavior from ideality (real gas effect) on the gas transport mechanisms is investigated. The effect of the structural parameters (porosity Ф, the maximum pore diameter Dmax, and the minimum pore diameter Dmin) of the shale matrix on the apparent permeability is studied and a sensitivity analysis is performed to evaluate the significance of the parameters for gas transport. We find that (1) the real gas transport models for a single pore and porous shale matrix are both reliable and reasonable; (2) the real gas effect affects the thermodynamic parameters of the free gas and the adsorption and transport capacity of the adsorbed gas; (3) the real gas effect decreases the effective permeability for convective flow and surface diffusion; i.e., the derivation degree of the effective permeability for bulk diffusion and Knudsen diffusion increases with increasing pressure but presents a bathtub shape when the pore diameter is smaller than 10 nm; and (4) the apparent permeability increases with Ф, Dmax, and Dmin. It is more sensitive to Dmax, followed by the porosity. Dmin has a minor impact. © 2016 American Institute of Chemical Engineers AIChE J, 2016

Supercritical carbon dioxide (CO 2 ) jet-assisted radial drilling is regarded as a potential alternative drilling method because supercritical CO 2 jet has higher efficiency in rock-erosion efficiency than water jet. Although researchers have investigated how the rock-erosion performance by supercritical CO 2 jet impingement correlates with the influencing parameters, there is little on the rock failure mechanism and change of pore structure. Scanning electron microscopy (SEM) analysis has been an important means in mineral identification, material damage detection, and aiding in water jet research. Therefore, rock-erosion experiments and SEM observations and analyses were carried out on different rocks including sandstone, marble and so forth. Secondary electron (SE) images show obvious grain shapes on the complex impinged surface of sandstone and marble by supercritical CO 2 jet, and cleavages on the mineral grains, and cracks and holes between the grains in different sizes. It suggests that cementations in weaker rocks are firstly removed and then mineral grains broken, generating the intergranular and intragranular cracks and micro-fractures and fissures. Supercritical CO 2 jet cannot bring obvious changes to hard limestone and shale at the same condition because the rock substances are tightly packed and filled with large cementing forces and ultra-low permeability and porosity, but can produce a few micro-cracks vertical to bedding planes. The features show that supercritical CO 2 jet broke rock substances mainly in the brittle tensile failure mechanism and made the rock much easier to further break, accompanied with shear failure mechanism in particular locations of the perforation hole. In contrast, water jet impingement forms flat cutting sections in marble with continuous parallel breaking traces, indicating the shear failure mechanism. Low viscosity and high diffusivity of supercritical CO 2 are primarily attributed to the rock tensile failure mechanism, they facilitate supercritical CO 2 penetrates into deep into rock pores and produces static pressure on the inner wall and at the tips of the fractures, besides exerting jet impinging pressure. Moreover, the energizing effect caused by the phase change from supercritical to gas leads to the energizing effect and intensify rock breaking into more shattered pieces and, concurrently, improve general rock connectivity and porosity in the surface layers. As a consequence, supercritical CO 2 jet appears to be more efficient and suitable than water jet in slim-hole radial drilling and hydraulic fracturing, particularly in unconventional reservoirs with low permeability.

A model for gas flow in nano-pores was developed based on the extended Navier–Stokes equations with the assumption of neglecting adsorption and desorption. The model describes multiple flow regimes, including continuum regime, slip flow regime, transition regime, as well as molecular regime. The total mass flux includes a convective motion term and a diffusion mass transport term. The latter was obtained as a weighted superposition of bulk and Knudsen diffusion. The mass flux, contributed by different transport mechanisms, was analyzed by varying the Knudsen number (ratio between mean free path of the molecules and the pore diameter). The effect of the ratio coefficient (the power-law exponent in the relation between bulk and Knudsen diffusion), the pore size and the pressure on the gas transport was investigated using the proposed model. The predictions of the newly developed model are in good agreement with the Direct Simulation Monte Carlo (DSMC) method and with the results of the experiments. Our results show that: (1) As the Knudsen number increases, the contribution of viscous flow to gas transport decreases monotonically, bulk diffusion increases to a peak, and then decreases, whereas the Knudsen diffusion increases monotonically. (2) For a larger ratio coefficient, the bulk diffusion changes more rapidly and the peak is higher. The changing trend of the bulk diffusion is opposite to the Knudsen diffusion. (3) The pressure range in which the viscous flow dominates becomes larger and the peak of bulk diffusion is smaller in larger pores. When the pore pressure is higher, the viscous flow and the bulk diffusion tend to dominate.

Natural gas flow in shale matrices consisting of both organic and inorganic components was modeled using fractal concepts. The expression of the apparent permeability (AP) of such shale systems was derived following three main steps: (a) modeling real gas flow in a single pore by generalizing our previously reported Extended Navier-Stokes Equations method, then (b) using fractal theory concepts to obtain the apparent permeability for both the organic and inorganic-matter cells, and finally (c) upscaling the AP model to the sample scale, accounting for the heterogeneous distribution of the organic matter. The up-scaled AP model is more realistic, because it considers not only the varying cross-section shapes and tortuosity of the pores, but also the characteristic flow mechanisms and multi-scale pore size distribution in heterogeneous shale matrix systems. The model was successfully validated with experimental data from real samples. The effects of the organic matter and shape of the pores on the AP are investigated. The sensitivity of the AP to the total organic carbon (TOC), porosity, pore shape, and structural parameters of both organic and inorganic systems is studied. The results indicated that the AP would be overestimated by up to 24.1% if the characteristics of the organic matrix are ignored, and the AP proves more sensitive to the effect of the pore shape in the inorganic matrix. In addition, the top three key parameters affecting the AP are pore shape, maximum pore diameter in the inorganic matrix, and porosity. The AP is more sensitive to the pore size within the inorganic matrix than within the organic matter. The proposed model provides some theoretical and technical support for shale gas simulations.

Understanding spontaneous imbibition of fracturing fluid in shale is critical for hydraulic fracturing design and optimization. In this paper, we present an analytical model for spontaneous imbibition including a hysteretic relative permeability-saturation-capillary pressure (k-S-P) relation and evaluate the relevance of hysteresis effect when modelling fracturing fluid imbibition. In the hysteretic formulations, capillary pressure and relative permeability depend not only on the current saturation but also on the history of saturation in the invaded zone. Herein, we concentrate on fracturing fluid imbibition into shale matrix during shut-in time after refracture treatment, which is driven by the strong capillary forces present in the tight matrix. We demonstrate the importance of accounting for the hysteresis in capillary pressure and relative permeability for predicting the imbibed volume of a fracturing fluid. For a problem that involves drainage and imbibition cycles, a hysteretic k-S-P relation is required to accurately assess the distribution of fluid saturation in the invaded zone. We also demonstrate that although the difference in viscosity between the fracturing fluid and gas is large, the viscosity of the gas cannot be neglected in modelling fracturing fluid imbibition in shale. The results of this investigation are expected to provide a better understanding of fracturing fluid imbibition in shale.

Unconventional shale gas is rapidly increasing as an available source of natural gas in the worldwild. Horizontal well drilling and multistage fracturing are two principle techniques of shale gas resources production. Benefiting most from these two techniques, the United States, Canada, and China have become only three countries to produce shale gas in commercial quantities. The key technical issues emerged in the horizontal drilling and fracturing involve the rock fragmentation issues, wellbore trajectory optimization and control, wellbore stability, cementing and its quality control for long horizontal lateral, and hydraulic fracturing design. Although some progress associated with those technical issues have been achieved in China, there are still many geologic and engineering challenges: (1) the low rate of penetration and fast bit wearing for both PDC and Cone bits are still the primary issues, which makes hard achieve the “one-trip” drilling; (2) wellbore collapse and lost circulation are serious with a certain collapsing period due to natural fractures system; (3) complex multi-fractures propagation behaviors still need to be understood and the fundamentals of fracturing design should be established after undstanding multi-fractures propagation behaviors.

An analytical model for gas transport in shale media is proposed on the basis of the linear superposition of convective flow and Knudsen diffusion, which is free of tangential momentum accommodation coefficient. The present model takes into the effect of pore shape and real gas, and is successfully validated against experimental data and Lattice–Boltzmann simulation results. Gas flow in noncircular nanopores can be accounted by a dimensionless geometry correction factor. In continuum-flow regime, pore shape has a relatively minor impact on gas transport capacity; the effect of pore shape on gas transport capacity enhances significantly with increasing rarefaction. Additionally, gas transport capacity is strongly dependent of average pore size and streamline tortuosity. We also show that the present model without using weighted factor can describe the variable contribution of convective flow and Knudsen diffusion to the total flow. As pressure and pore radius decrease, the number of molecule-wall collisions gradually predominates over the number of intermolecule collisions, and thus Knudsen diffusion contributes more to the total flow. The parameters in the present model can be determined from independent laboratory experiments. We have the confidence that the present model can provide some theoretical support in numerical simulation of shale gas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2893–2901, 2016

The supercritical carbon dioxide (SC-CO2) jet can break rocks at higher penetration rates and lower threshold pressures than the water jet. The abrasive SC-CO2 jet, formed by adding solid particles into the SC-CO2 jet, is expected to achieve higher operation efficiency in eroding hard rocks and cutting metals. With the computational fluid dynamics numerical simulation method, the characteristics of the flow field of the abrasive SC-CO2 jet are analyzed, as well as the main influencing factors. Results show that the two-phase axial velocities of the abrasive SC-CO2 jet is much higher than those of the abrasive water jet, when the pressure difference across the jet nozzle is held constant at 20 MPa, the optimal standoff distance for the largest particle impact velocity is approximately 5 times of the jet nozzle diameter; the fluid temperature and the volume concentration of the abrasive particles have modest influences on the two-phase velocities, the ambient pressure has a negligible influence when the pressure difference is held constant. Therefore the abrasive SC-CO2 jet is expected to assure more effective erosion and cutting performance. This work can provide guidance for subsequent lab experiments and promote practical applications.

It is reported that supercritical carbon dioxide (SC-CO2) jet can efficiently erode rocks at a comparably low threshold pressure with high penetration rate. However, the influences of bottom-hole pressure and borehole irregularities on SC-CO2 jet cutting efficiency have not been studied. Therefore, in this research, comprehensive methods of numerical simulations and lab experiments were carried out to investigate the influences of the ambient pressure and the nozzle-to-target distance on the jet impinging pressure and perforation performance. Results show that, both the effective jet impinging pressure and the eroded depth of perforation hole notably decrease with the increase of the ambient pressure, when jet inlet pressure is constant. When the pressure difference between inlet pressure and ambient pressure is kept constant, the effective impinging pressure hardly changes with ambient pressure, but eroded depth increases at first and then decreases, bounded by the critical pressure of CO2. As the nozzle-to-target distance extends, both the depth and volume of the perforation hole decrease, and diameter of the perforation hole increases at first and then decreases. Under the research condition of certain pressure and temperature, a distance of 8 times the nozzle diameter is the critical distance that clarifies the different effect of nozzle-to-target distance on the hole diameter. Different features between using SC-CO2 jet and water jet, including the optimal distance for rock-erosion efficiency, are attributed to the difference in fluid properties. Further simulation results show that, unlike water, SC-CO2 fluid is compressible and it leads to specific variations of jet structure, namely the increase of length of SC-CO2 jet potential core, and flow type from non-submerged jet to submerged jet. As a consequence, under the simulated bottom-hole conditions, SC-CO2 jet is proven to be able to acquire high rock-erosion efficiency and the jet-assisted drilling rate at a larger application ranges. This research can promote the future application of SC-CO2 jets.

Natural Gas Hydrate (NGH) develops and exists in pores of soil sediments under deep seabed and permafrost regions. A cavitation jet is an efficient method of rock breaking, especially for soft hydrate sediment erosion. This paper presents an experimental apparatus that was developed to synthesize NGH and hydrate-bearing sediments and analyze the drilling efficiency of the cavitation jet. The visualization study of fluid flow and breaking mechanism can be conducted over a temperature range varying from -20 °C to 100 °C and up to a maximum confining pressure of 20 MPa. This apparatus is mainly composed of the pressure control and injection system, the cooling system, the cavitation system, and the reaction vessels into which the lab-fabricated temperature/pressure/resistivity sensor probe is inserted. The basic principles of this apparatus are discussed, and a series of experiments were performed to verify that the cavitating jet can be practically applied in the exploitation of NGH reservoirs.

Silica sand micro-particles are added into submerged cavitation jet to enhance its cavitation intensity and erosion ability. The micro-particles promote cavitation as additional heterogeneous nuclei and the activated cavitation bubbles can accelerate micro-particles. The synergistic effect of micro-particles and cavitation bubbles helps to improve the erosion ability of jet. High-speed visualization, cavitation noise measurement, and erosion test are performed to validate the improved cavitation intensity and erosion ability of submerged cavitation jet with micro-particles. The high-speed filming shows the disappearance of the periodic shedding of cavitation cloud and the massive expansion of cavitation region when micro-particles are present. The acoustic power of cavitation noise rises by at most 113.24%. The cavitation intensity of jet increases with increasing micro-particle concentration and decreasing micro-particle size. The erosion test of sandstone shows that submerged cavitation jet with micro-particles increases the mass loss and the surface roughness by at most 86.96% and 128.28%. The changes of macroscopic erosion pattern and microscopic surface morphology are also elucidated.

Using a hybrid thermal Lattice Boltzmann method, the dynamics of laser-produced single cavitation bubbles in bulk liquid and the bubble collapse process near a solid boundary are numerically investigated. The simulated bubble evolutions satisfyingly agree with the theoretical calculations and the previous experimental results. The simulated bubble radius changes in bulk liquid are in good accordance with the calculations of a revised 2-D Rayleigh–Plesset equation that incorporates an extra thermal effect term. The maximum bubble radius is linearly proportional to the bubble collapse time and the input laser energy, which is consistent with the experimental data and bubble dynamics theory. Processes of a single cavitation bubble collapse at various distances from a solid boundary are analyzed in detail. The velocity vectors, density, pressure, and temperature fields are presented. The retarding effect of a solid boundary is successfully reproduced in the LBM simulations and leads to bubble elongation, the formation of micro-jet, bubble toroidal deformation, and the attraction of the bubble to the boundary during the collapse phase. The attraction effect, maximum jet velocity, and maximum pressure at the solid boundary all increase with reduced non-dimensional distance. A critical non-dimensional distance of 2.2 is validated by both the simulation and experiment. The hybrid thermal Lattice Boltzmann method is a reliable tool to investigate non-isothermal cavitation bubble dynamics.

The adhesion energy of kerogen in continental and marine shale was innovatively discovered using the colloid probe technique with atomic-force microscopy (AFM). AFM results indicated that the adhesion force of kerogen was higher than the inorganic material in both the continental and marine shale samples. The chemical elements in the two kinds of samples were measured by energy-dispersive X-ray analysis with scanning electron microscopy (SEM). The chemical compositions of kerogen involved C═C bonding, C═O bonding, pyridine nitrogen, and pyrrole nitrogen, whereas the primary constituent involving inorganic matter was Si–O bonding. These results were confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The high percentages of C═C and C═O bonding in kerogen are attributed to the large dipole on the kerogen surface which allowed kerogen to contain liquid and gaseous hydrocarbons.

In article number https://doi.org/10.1002/advs.201902043, Quan Xu, Yu Tian, Zhenhai Xia, and co‐workers present how mussel cuticle is a functionally‐graded material with high stiffness, extensibility, and self‐healing capacity and oxygen absorbs on metal ions, weakening the iron‐catecholate bonds in the cuticle and collagen core, but this process can be reversed by sea water.