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The study of the gas occurrence mechanism in a microscale coal matrix is the basis of coalbed methane (CBM) reservoir formation mechanism analysis and its exploration and development scheme design, which has important scientific and engineering significance. Currently, many researchers are focusing on a specific coal type to explore the macroscopic adsorption characteristics of gas occurrence. However, the research on the microscale gas-solid coupling mechanism is relatively rare and the electrical control mechanism of gas occurrence is not reported in detail. This study focuses on the electrical mechanism of microscale gas occurrence using physical simulation experiments and molecular dynamics analysis. This study clarifies the "gas adsorption-electrical properties-functional group" linkage mechanism and explores the macroscopic performance of the microscale gas occurrence mechanism using electrical properties. The study reveals the following: (1) the coal reservoirs exhibit a weak negative potential at the nanoscale, and the trends of surface potential (SP) and surface electrical charging density (SECD) are fluctuated with the degree of coal rank increases; (2) there is a good correlation between the SP, SECD values, and the relative content of functional groups; and (3) the charge density on the coal's microscopic surface influences their gas molecule attraction capacity, affecting the gas adsorption capacity of coal reservoirs at the macroscale. This study presents a theoretical foundation for establishing the molecular force field superposition mechanism of gas occurrence in microscale coal matrix and has broad application prospects in the macroscale numerical simulation of CBM development.

Cinnamon essential oil with many bioactivities is an important raw material for the production of various chemicals, and the conventional hydrodistillation (HD) for cinnamon oil extraction always require a longer extraction time. In this work, ultrasound-assisted hydrodistillation extraction (UAHDE) technique was employed to enhance the extraction efficiency of essential oils from cinnamon barks. The parameters with significant effects on the essential oil extraction efficiency (ultrasound time, ultrasound power, extraction time, liquid–solid ratio) were optimized, and the proposed UAHDE was compared with the conventional HD extraction in terms of the extraction time, extraction yield, and physicochemical properties of extracted oils. Compared to the HD extraction, the UAHDE resulted in a shorter extraction time and a higher extraction yield. Using GC–MS analysis, the UAHDE provided more valuable essential oil with a high content of the vital trans-cinnamaldehyde compounds compared with the HD. Scanning electron micrograph (SEM) confirmed the efficiency of ultrasound irradiation for cinnamon oil extraction. In addition, the analysis of electric consumption and CO2 emission shows that the UAHDE process is a more economic and environment-friendly approach. Thus, UAHDE is an efficient and green technology for the cinnamon essential oil extraction, which could improve the quantity and quality of cinnamon oils.

Geothermal energy has been in rich in both onshore and offshore conditions. Air can be used as working fluid at offshore platforms for geothermal energy development. However, existing models cannot...

With the development and progress of carbon dioxide (CO2) capture and storage technology, the study of the flow assurance of CO2 in transmission pipelines needs to be further deepened. In this study, aiming at the gas‐phase transportation mode of CO2, first, a new prediction model of temperature, pressure as well as hydrate formation risk in buried CO2 transmission pipelines is established. Second, the model solving methods of differential discretization and piecewise iteration are introduced in detail. Third, model validation and sensitivity analysis of typical transmission parameters are performed. The results show that: (a) there is good agreement between the model prediction results and software simulation results. (b) The subcooling and the length of the hydrate formation region increase with the increase in the transmission rate. Appropriate increase in the CO2 transmission rate can reduce the risk of hydrate formation in CO2 transmission pipelines. (c) The subcooling and the length of the hydrate formation region decrease as the transmission temperature increases. In order to prevent hydrate risk in the CO2 transmission process, low transmission temperatures should be avoided. (d) The subcooling and the length of the hydrate‐formation area increase first and then decrease as the transmission pressure increases. Appropriate increase in CO2 transmission pressure is conducive to avoiding hydrate formation risk. This study provides basic theoretical guidance for optimizing transmission parameters and risk assessment of hydrate formation in CO2 transmission pipelines. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

The formation of hydrate will lead to serious flow assurance problems in deepwater submarine natural gas transmission pipelines. However, the accurate evaluation model of the hydrate blocking risk for submarine natural gas transportation is still lacking. In this work, a novel model is established for evaluating the hydrate risk in deepwater submarine gas pipelines. Based on hydrate growth-deposition mechanism, the mathematical model mainly consists of mass, momentum and energy conservation equations. Meantime, the model results are obtained by finite difference method and iterative technique. Finally, the model has been applied in the production of deepwater gas field (L Gas Field) in China, and the sensitivity analysis of relevant parameters has been carried out. The results show that: (a). The mathematical model can well predict the hydrate blockage risk in deepwater natural gas pipelines after verification. (b). Hydrate is easily formed at the intersection of horizontal pipeline and vertical riser, and the maximum blocking position often occurs in middle of the riser. (c). The hydrate blockage degree and length of hydrate formation region (HFR) decrease with the increase of gas transport rate. (d). The hydrate blockage degree and length of HFR decrease with the increase of gas transport temperature. (e). The hydrate blockage degree and length of HFR increase with the extension of horizontal pipeline. (f). Injecting inhibitors can effectively inhibit hydrate formation and blockage, but the improvement of transmission measures can significantly reduce the dosage of inhibitor. It is concluded that measures such as increasing gas transportation rate and temperature, shortening horizontal pipeline length, optimizing inhibitor injection point and injection rate can play a safe, economic and efficient role in hydrate preventing and controlling.

The study on shale gas transport mechanisms plays an important role in shale gas production simulation. The majority of previous works neglected the body gas diffusion mechanism, and the multiple effects of stress sensitivity and multi-component of gas mixture system. In this paper, first, a model is built considering the multiple transport mechanisms of continuum flow, slippage flow, body gas diffusion and Knudsen diffusion. Then, the multiple effects of stress sensitivity and multi-component of gas mixture system are coupled into the model. Results show that: (a). For the body gas flow, both of the two weighted coefficients for items of body gas diffusion and slippage flow should be equal to one. And the ratio of one to Knudsen number is belong to the equation of body gas diffusion. (b). For a nanoscale pore of shale, the contribution of body gas diffusion on total mass flux cannot be neglected. The conductivities of continuum flow, slippage flow, modified slippage flow, Knudsen diffusion and weighted average flow decrease with decreasing of pressure due to shrinkage of nanopore radii and decrease in molecular concentration. (c). The decreasing of effective radii leads to the increase of the value of Knudsen number. The dominant transport mechanism is converting from modified slippage flow to Knudsen diffusion when the increase of stress sensitivity. (d). For methane confined in nanopores with extremely small effective transport radii, the injection of CO2 can increase the mobility of methane. After a period of CO2 injection, the methane molecules stored in nanopores can be flooded to fractures and then produced to the ground.

The modeling of shale gas transport through nanopores is the basis for shale gas production simulation. The real shale gas always consists of a series of gases, including ethane, propane and hydrogen sulfide etc. Previous models neglected the multi-component effect on the shale gas transport mechanisms. In this paper, a novel model is presented for simulating the real gas mixture transport through nanopores of shale formation. Firstly, a model is presented for ideal shale gas transport through nanopores considering the multi-component effect, then, the real gas effect is coupled into the model. Simulation results show that: (a) When it is under low pressure level condition, the conductivities of slippage flow and Knudsen diffusion increase with decreasing methane fraction. (b) When it is under medium pressure level condition, the conductivities of slippage flow and Knudsen diffusion increase with decreasing methane fraction. (c) Under high pressure condition, the conductivities of different flow patterns increase with decreasing methane fraction.

Geothermal energy is becoming more attractive nowadays. Compared with enhanced geothermal recovery method, developing geothermal energy using a closed loop heat exchanger possesses unique advantage...

The design prototypes of four kinds of channel structures for three fluid recirculation ways, i.e. without reverting microchannels, with rim reverting microchannels and with central reverting microchannels, respectively, for the hexagonal heat sink are put forward. The influences of the fluid recirculation way, the microchannel branch number and the microchannel distribution on the maximum thermal resistance and the equivalent thermal resistance based on entransy dissipation of the hexagonal heat sink are studied based on thermo-fluid-stress-deformation multi-physics field coupling calculations and the thermal stress and strain analyses are carried out. The results of numerical calculations show that the fluid recirculation way with central reverting microchannels is the best among the three fluid recirculation ways with the objectives of minimizing the maximum thermal resistance and the equivalent thermal resistance. The construct of the hexagonal heat sink with six branch microchannels and central reverting microchannels, in which the upper microchannels distribute along the radius of the hexagonal inscribed circle and the lower microchannels distribute along the hexagonal circumscribed circle, is optimal, and its heat transfer performance is the best. Its maximal thermal strain is 0.28 GPa, and is an order of magnitude smaller than the yield strength of silicon. Its maximal deformation is 1.4 μm. The results can provide some theoretical supports for multidisciplinary design of practical microchannel heat sinks.

Hydrate formation and blockage in gas-dominated pipelines has always been an important issue in research of pipeline flow assurance. In this work, a novel hydrate deposition prediction model consid...

Constructal theory has been widely used in performance optimizations of various problems. Among these problems, engineering problem is a hot topic for constructal theory. The developments of constructal theory about engineering problem in China over the past decade are reviewed in this paper. Multi-disciplinary, multi-objective and multi-scale constructal optimizations of various transfer processes in engineering, such as heat conduction and thermal insulation processes, fluid flow processes, convective heat transfer processes of fins, heat sources, cavities, cooling channels, vascular networks and heat exchangers, mass transfer processes of porous mediums, heat and mass transfer processes of solid-gas reactors and solid oxide fuel cells, iron and steel production processes and generalized transfer processes, have been conducted since 2006. Performance improvements are realized, new design requirements are satisfied, and more practical and generalized results are obtained after new research objects, model improvements, optimization objectives, boundary conditions, constraint conditions and transfer extensions are considered. It shows that the developments of constructal theory are still being made in China, which will provide more design guidelines not only for engineering problem, but also for natural and social problems.

Second-law-based minimum operation cost optimization is performed based on constructal theory for in-line cylindrical pin-fin heat sinks (PFHS). With the minimum operation cost which is based on the entropy generation rate (EGR) as the objective, the optimal values of diameter, height to diameter aspect ratio and quantity of cylindrical pin-fin are achieved by combining the analytical and numerical method, the effects of fin-material fraction (FMF), heat transfer load (HTL) of the PFHS, fluid velocity (FV) and cost ratio on variations of the operation cost of the PFHS with the diameter of pin-fin are analyzed. The results indicate that when the single freedom degree optimization is performed, the minimum operation cost and the optimum constructs are various under various FMF, the HTL, the FV and the cost ratio in the research range of parameters; when the double freedom degree optimization is carried out, the operation cost of the PFHS increases with the height to width aspect ratio of the PFHS increasing.

In recent decades, horizontal well has been widely used for heavy oil recovery. Besides, new developed thermal carriers, such as multi-component thermal fluid and superheated steam, have been proved effective in some oil fields. However, when the length of the horizontal wellbore is quite long or when the reservoir is of serious heterogeneity, the steam fingering phenomenon can be serious. The existence of horizontal heterogeneity leads to the low recovery efficiency and serious steam fingering phenomenon. In this paper, a numerical model is built to study the effect of horizontal heterogeneity on the productivity of production well during the cyclic superheated steam stimulation process. Simulation results show that (a) the effect of permeability distribution pattern in the horizontal direction on the recovery degree can be neglected. This may be caused by the infinity of fluid fluidity in the wellbores. (b) The effect of permeability distribution pattern in the horizontal direction on the cyclic oil production can be neglected. (c) The temperature is the highest in the high permeability section, while the temperature is the lowest in the low permeability section. However, given the fact that the wellbore conductivity is assumed to be infinity, the effect of permeability distribution pattern on the performance of the production well can be neglected.

Steam-assisted-gravity-drainage (SAGD) has been proved effective in heavy oil recovery. Preheating of the wellbore-surrounding reservoir is to circulate steam in the injector and producer so that heat can be conducted into surrounding oil layer. At this stage, the amount of steam injected into the reservoir is neglected. As a result, creating a large temperature difference between wellbore and annuli is key during the preheating process. A model is established for estimating steam properties in the wellbores so that the highest steam temperature in wellbores can be achieved. The model is comprised of mass, energy and momentum balance equations and the model is solved with numerical method. It is found that: (a) rich heat energy reflected in high steam quality does little effect on heat absorption rate of oil layer. The only effective method for temperature increase in oil layer is to increase the steam temperature in wellbores; (b) in order to increase the heat conduction rate to oil layer, a lower steam quality, a higher steam pressure and a lower mass flow rate is recommended.

Abandoned oil and gas wells are proved to be effective in geothermal energy extraction, especially when the wellbore is located at a perfect geothermal reservoir. In this paper, CO2 is selected as the working fluid and a model is proposed to simulate the CO2 flow in the retrofitted wellbores. The CO2 is injected into the inner tube (injector) and then flows out through the annuli (producer) to surface. The proper length of the insulation layer installed outside the casing is predicted. Results show that: (a) when the CO2 flows from well-bottom to well-head in the producer, its temperature has an increase at first and then turns to decrease. An insulation layer outside the casing is an efficiency way to prevent CO2 from transferring heat to the stratum. (b) From the temperature point of view, a smaller mass flow rate is recommended to obtain a higher CO2 temperature at outlet of the producer. (c) A higher mass flow rate leads to a higher geothermal energy extraction rate but a lower temperature. (d) A small increase in temperature or the geothermal energy extraction rate need a large pressure increase, which can be high-cost for field practice.

Shale oil reserves are becoming more important with the development of oil & gas industry. However, the transport mechanisms of oil in nanopores of kerogen remain a mystery. The understanding of the multiple transport mechanisms of oil in nanopores is crucial in the successful development of shale oil reservoirs. In this paper, a new model for flow enhancement of oil transport in nanopores of kerogen is proposed. Both boundary slip and physical adsorption are taken into consideration in the analytical model. Based on the previous experimental and theoretical studies, the model is validated. Results show that: (a) Covering the Hagen–Poiseuille flow, specific oil flow and the water flow, the new model is more universal in practice; (b) When the radius of the nanotube is equal to 1 nm, the value of slip factor is between 24081 and 48161 (the corresponding correction factor is between 0.7 and 1.4); (c) the flow enhancement is negligible when the radius is larger than 10 nm under various values of correction factor conditions. Besides, the flow enhancement increases with the increasing of the correction factor; (d) when the correction factor is small, the section of the curve higher than zero is characterized with short and low; (e) when the correction factor is smaller than 0.007, the normalized velocity increases slowly with increasing of the correction factor. When the correction factor is higher than 0.07, the normalized velocity increases rapidly when the transport length in nanopores is increased from 10 nm to 200 nm.

Hydrate generation and pipe blockage in long-distance natural gas transmission pipelines has always been a major issue affecting the transmission safety. Although considerable progress have been made in recent years, there is still a long way to go in the study of the hydrates generation and plugging prediction in the gas pipelines. Hydrate plugging and accumulation is a gradual process. However, the previous studies have focused only on the prediction of whether hydrates are formed. Actually, the research on hydrate formation rate and pipelines blockage degree at different times is also important for the hydrate prevention and control. In this work, the authors proposed a novel risk assessment method for hydrate blockage in long-distance natural gas transmission pipelines. Firstly, considering the hydrate formation process, a new model consist of mass, momentum and energy balance equations was established. Secondly, the model results are solved by the iterative method and finite difference method. After comparison, the calculation results and the field data are in good agreement. Finally, the sensitivity analysis were performed on the important parameters of our model. According to the sensitivity analysis, transmission rate, inlet temperature, and natural gas dew point temperature have different effects on the hydrate generation, the location of the largest plugging point, and the degree of blockage. Meanwhile, considering the actual gas transmission process and the hydrate plugging characteristics, the proposed hydrate prevention and control measures can be taken to achieve safe and efficient natural gas transmission.

Recently, more and more attention has been paid to the development of deep-water gas wells around the world. However, hydrate formation in string has always affected the safety of deepwater gas well production and there is no accurate risk prediction method currently. In this study, a more optimized and practical prediction model for hydrate formation region (HFR) was established. The model has been applied to deepwater gas wells and the influence of typical factors on HFR is analyzed. The conclusions are as follows: (a) the gas temperature decreases gradually from bottom hole to wellhead, and the rate of decline is obviously faster near mud line. (b) under low gas recovery rate, the effect of heat transfer on gas temperature reduction is more obvious, but at high gas recovery rate, the opposite is true. (c) because of the pressure change in wellbore, the hydrate equilibrium temperature gradually decreases with the decrease of well depth and HFR often appears in the upper half of wellbore. (d) the increase of gas production and formation temperature is helpful to shorten the length of HFR in the string, which is due to the increase of gas production and reservoir temperature increases the temperature distribution in the pipe string. (e) the hydrate equilibrium temperature and HFR decrease by increasing inhibitor content. Increasing the use of hydrate inhibitors is the most practical and effective measure to reduce hydrate risk. In the production process of deepwater gas wells, the effective location of HFR is very important to the production safety of deepwater gas wells. This research can be helpful to the research of wellbore flow assurance during the production of deep-water gas wells.

Although considerable advances have been achieved in recent years, there is still a long way to go for hydrate prediction and prevention during deepwater gas well testing. Well hydrate blockage is a time-dependent continuous process, and evaluation of wellbore blockage is critical for hydrates prevention and control. In this work, the authors presented a novel wellbore blocking degree evaluation model for deepwater gas-well testing. Firstly, a new evaluation model consisting of mass, momentum and energy balance equations considering hydrates formation was proposed. Secondly, considering hydrate phase equilibrium, the finite difference method and iterative technique were used to obtain the model results. Finally, the model was applied to the deepwater gas wells in the South Sea of China for verification and was subjected to sensitivity analysis. The predicted results were in good agreement with field test results. According to the sensitivity analysis, gas output, methanol concentrations, water production rate have different degrees of influence on hydrate blocking. The length of the hydrate stability region (HSR), the position of largest plugging point, and the distribution of wellbore inner diameter are the key to hydrate prevention and control. Meantime, combining the model evaluation results, optimizing the testing process can achieve the purpose of preventing-controlling gas hydrates economically and effectively.

The mixture of CO2 and superheated steam (SHS) has been proved effective for heavy oil recovery by field practices. It has been proved that the development effect is closely related to the state parameters (temperature, pressure and superheated degree etc.) of the mixture. As a result, wellbore modeling is an important branch in the study of thermal recovery of heavy oil. In recent years, concentric dual-tube wells (CDTW) coupled with multi-point injection technique (MPIT) is proposed to overcome the fingering phenomenon, induced by serious reservoir heterogeneity, during the steam injection process. In this paper, a wellbore model is developed for simulating CO2 & SHS flow in a full-length CDTW. The model is solved with finite difference method on space. It is found that: (a) An injection pressure, higher than 4.1 MPa, in the integral joint tube (IJT) or an injection pressure, smaller than 4.1 MPa, in annuli are recommended to obtain a uniform steam absorption rate along the horizontal wellbores. (b) The mass injection rate at well-head condition should be close to each other to avoid un-uniform of steam absorption rate along the horizontal wellbores. (c) When phase change occurs in the annuli, the mixture temperature increases with increasing CO2 content. (d) The total length of wet steam flow in the horizontal section of the wellbores increases with increasing CO2 content.

Constructal optimizations of the first to fifth order line-to-line vascular channels (LVCs) with convective heat transfer are carried out in this paper by taking minimum entropy generation rate (EGR) and minimum entropy generation number (EGN) as the optimization objectives, respectively. The angles of the LVCs are taken as the optimization variables, and the fixed vascular channel areas and total volumes of LVCs are taken as the constraints. The optimal constructs corresponding to minimum EGR and minimum EGN are obtained, respectively. The influences of the mass flow rate (MFR) on the optimal results are analyzed. The results show that the EGR of the LVCs decreases when the angle freedom increases with the constant dimensionless MFR, which is more obvious when the order increases. When the order increases, the EGR increases but the EGN decreases. Both the EGR and the EGN increase with the increase of the dimensionless MFR. The results obtained herein can provide some thermal design and management guidelines for the applications of LVCs.

Wellbore cleanup is an important part of deepwater gas wells completion testing and also a critical stage for hydrate formation. For evaluating hydrate risk and proposing specific hydrate preventio...

At present, geothermal energy is a promising research area but with a series of unknowns waited to be explored. Recently, the U-shaped closed loop geothermal extraction system was proposed to improve the geothermal recovery performance. However, there is a lack of mathematical model to simulate the circulating process of CO2 in the wellbore. In this paper, a model is developed for simulating CO2 flow in the descending, horizontal and ascending sections of the geothermal well. Besides, in order to properly evaluate the geothermal recovery performance, two new concepts of critical position and effective distance are proposed. Simulation results show that: (a) There exists a critical position in the ascending wellbore where the working fluid temperature is equal to the formation temperature. (b) The effect of mass flow rate on pressure drop becomes more obvious when the gravity is neglected, especially under the condition that the mass flow is at a higher value. (c) The rapid increase of working fluid temperature does not mean that the geothermal recovery performance is better. At this time, the effective distance should be adopted as an additional reference.

The pores in tight sandstone gas reservoir are mostly in microscale, and the gas water two-phase flow behavior through those pores is considerably complicated, thus, how to establish a reliable relative permeability model is the urgent problem to be solved. Based on the single fluid continuity equation of the microtube, second-order slip model and the fractal scaling theory, the gas-water relative permeability model of tight sandstone gas reservoir is established. The model considers the gas slippage effect, the pore throat structure and the water saturation distribution. The reliability is validated by a variety of published experimental data of gas-water relative permeability. The results show that: 1) The established model can be reasonably adopted to describe the fluid flow behavior in the tight sandstone reservoir with microscale pores. 2) The increase of pressure and water saturation will reduce the effect of slippage effect on the relative permeability of gas phase. When the pressure is less than 1 MPa, the gas relative permeability is extremely sensitive to the pressure. The impact of temperature on the gas relative permeability is limited and can be neglected. 3) A higher fractal dimension of pore size distribution is beneficial to the relative permeability of gas phase, but it has a negative impact of the relative permeability of water phase. Similarly, a higher fractal dimension of tortuosity results in a higher gas phase relative permeability but a little lower water relative permeability.

Extraction process of CO2 from acidic seawater by using hollow fiber membrane contactor is studied in this paper by using finite time thermodynamics or entropy generation minimization theory. Analytical formulae of the extraction rate of CO2 and entropy generation rate (EGR) caused by mass transfer (MT) and the flow of the process are obtained when the acidic seawater (liquid phase) flows in the lumen side and the H2 (gaseous phase) flows in the shell side conversely. Relations between the concentration of CO2 and contactor length in both liquid phase and gaseous phase, between the MT flux and contactor length, between the pressure of the two phases and the contactor length, as well as between the local EGR and contactor length are obtained by numerical examples. The influences of the volume flow rates of the acidic seawater and the H2 at the inlet, the number and the length of hollow fibers on the extraction rate of CO2 and EGR per MT rate are also analyzed. The results show that the extraction rate of CO2 can reach up to more than 98%. The EGRs caused by MT and fluid flow account for 85.99% and 14.01% of the total EGR, respectively. Furthermore, the total EGR caused by MT and fluid flow is minimized by using nonlinear programming method with fixed extraction rate of CO2 when the gas mixture of H2 and CO2 flows in the shell side and the concentration of CO2 in the gaseous phase is completely controllable. The EGR of the minimum EGR MT strategy can be decreased by 36.57% than that of the MT strategy that H2 flows in shell side, in which the EGR caused by MT is decreased by 42.52% and the EGR caused by the flow remain basically unchanged. The EGR caused by MT is minimized by using optimal control theory and the analytical solution of the optimal concentration configuration of CO2 in the gaseous phase is obtained.