Gas Flow Mechanisms Research Articles

A comprehensive mathematical model considering multiple mechanisms for CO2 displacement of methane in shale nanopores

With the rise in energy demand, shale gas exploitation has gained prominence, particularly through the use of carbon dioxide (CO2) injection for enhanced gas recovery. However, the intricate gas flow mechanisms within this process in shale nano-pores remain poorly understood, which greatly limited the accurate numerical simulation of shale gas production by CO2 injection. This study investigates the flow mechanisms of CH4 and CO2 gases in shale nano-pores, developing an improved apparent relative permeability model for the two-component CH4–CO2 gas, incorporating multiple transport mechanisms. Sensitivity analysis includes factors such as pressure, pore radius, viscosity correction effects, flow mechanisms, and composition fractions. Key findings from this study are as follows: (1) Increasing pore radius or decreasing pressure enhances the apparent permeability of CH4–CO2 gas, with pressure effects becoming negligible above 20 MPa. (2) The impact of viscosity correction on gas permeability diminishes with higher pressure or larger pore radius, becoming negligible for pore radii over 10 nm. (3) Larger pore radii increase slip flow and Knudsen diffusion, while molecular and surface diffusion proportions decrease; surface diffusion and slip flow dominate at high pressures. (4) A higher CH4–CO2 component ratio increases CH4 permeability and decreases CO2 permeability, with minimal influence from pressure and pore radius. These results can improve numerical simulations for shale gas production by using CO2 injection.

Improving optical and electrical stabilities of fluorine-doped tin oxide films in sweat solutions with O2 addition in plasma

Abstract Recently, the use of wearable smart devices has significantly increased; however, sweat can corrode the outer-layer films, thereby decreasing their transmittance, conductivity, and overall functionality. In this study, fluorine-doped tin oxide (FTO) films for wearable smart devices were prepared via magnetron sputtering. The effects and mechanism of O2 gas flow in plasma on the properties of the fabricated films were investigated. Minor changes were observed in the film morphologies, with the preferred orientations shifting from polar (101) to nonpolar (110) and standard positions. As the O2 flow rate increased from 0 to 2 sccm, the transmittance of the film within the visible spectrum increased from 83% to 89%, with sheet resistance values in the order of 102–106 Ω·sq–1. Following immersion in an acidic sweat solution, the film without O2 peeled off, whereas several corrosion pits were observed in the films with 1 or 2 sccm O2. Conversely, following immersion in an alkaline sweat solution, several pits were observed in the films without O2, while the other films exhibited excellent corrosion resistance. The transmittance of the films immersed in different solutions did not significantly differ. Notably, the sheet resistances of the films treated with 1 sccm O2 met the industrial requirement of 3000 Ω. Moreover, the coexistence of polar and nonpolar planes provided transparency and conductive stability to the FTO films treated with 1 sccm O2. Our study aimed to not only enhance the transmittance and sweat-corrosion resistance but maintain the conductivity of the outer screen layer of a wearable smart electronic device.

Impact of gas arising from gas hydrate decomposition on the effective permeability of bottom sediments of the Arctic shelf: a laboratory simulation

Relevance. The necessity to investigate the mechanism of gas (methane) flow occurring during decomposition of gas hydrate formations within the layers of bottom sediments. This process is evident through substantial releases of methane bubbles in extensive shallow regions of the Russian Arctic shelf and has the potential to alter the equilibrium of atmospheric methane, a critical greenhouse gas. Aim. To quantitatively investigate, within the context of gas transport in bottom sediments, the impact of free and bound gas within the pores on the nonlinearity of filtration flows. Methods. The model samples with filtration properties similar to those of the bottom rocks of the East Siberian Arctic shelf. During the experiments, a specific amount of gas was injected into the saturated model sample, followed by the measurement of its effective permeability during a slow decrease in pore pressure gradient. The obtained curve was interpreted within the framework of the threshold gradient model. Results. The experiments yielded curves showing the dependency of the threshold gradient magnitude on gas saturation for several samples. It was found that the threshold gradient linearly increases with the growth in gas fraction within the pore space. Already at a gas fraction of approximately 0.02, this value in the experiments reached 0.01 MPa/m, corresponding to the hydrostatic pressure gradient in water. This suggests that even areas with relatively low gas saturation may be impermeable to convective fluid flow. This possibility should be considered when creating models for bubble gas transporting through the rock layers of bottom sediments. Furthermore, the existence of gas-saturated zones with threshold gradients can significantly impact the vertical profile of pore pressure and lead to a reassessment of the depth of gas hydrate stability zones.

Giant Effect of CO2 Injection on Multiphase Fluid Adsorption and Shale Gas Production: Evidence from Molecular Dynamics.

Carbon dioxide (CO2) injection in unconventional gas-bearing shale reservoirs is a promising method for enhancing methane recovery efficiency and mitigating greenhouse gas emissions. The majority of methane is adsorbed within the micropores and nanopores (≤50 nm) of shale, which possess extensive surface areas and abundant adsorption sites for the sequestration system. To comprehensively discover the underlying mechanism of enhanced gas recovery (EGR) through CO2 injection, molecular dynamics (MD) provides a promising way for establishing the shale models to address the multiphase, multicomponent fluid flow behaviors in shale nanopores. This study proposes an innovative method for building a more practical shale matrix model that approaches natural underground environments. The grand canonical Monte Carlo (GCMC) method elucidates gas adsorption and sequestration processes in shale gas reservoirs under various subsurface conditions. The findings reveal that previously overlooked pore slits have a significant impact on both gas adsorption and recovery efficiency. Based on the simulation comparisons of absolute and excess uptakes inside the kerogen matrix and the shale slits, it demonstrates that nanopores within the kerogen matrix dominate the gas adsorption while slits dominate the gas storage. Regarding multiphase, multicomponent fluid flow in shale nanopores, moisture negatively influences gas adsorption and carbon storage while promoting methane recovery efficiency by CO2 injection. Additionally, saline solution and ethane further impede gas adsorption while facilitating displacement. Overall, this work elucidates the substantial effect of CO2 injection on fluid transport in shale formations and advances the comprehensive understanding of microscopic gas flow and recovery mechanisms with atomic precision for low-carbon energy development.

Gas state equation and flow mechanism of gas–liquid two-phase flow in airlift pump system

Airlift pumps (ALPs) have a simple structure and significant application potential. However, previous studies on ALPs have generally assumed that the gas density remains constant. In this paper, the gas state equation (GSE) for ALP is established based on the van der Waals formula, which explicitly considers the density of gas. An electrical resistance tomography system is used to collect the gas void fractions at different heights under different gas flow rates, and an empirical formula for the gas void fraction is established. To verify the effectiveness of the proposed model, high-precision pressure sensors and a high-speed camera are used alongside an electrical resistance tomography system to determine the realistic ALP flow parameters. The results of 409 sets of experiments show that: (1) the gas in ALP cannot be regarded as ideal gas, because the ideal GSE cannot distinguish between different gas flow rates; (2) the state change of gas in ALP is a quasi-equilibrium process, whereby the GSE of ALP can be obtained from the pressures under several locations; (3) the axial pressures predicted by the proposed GSE for ALP are in good agreement with experimental data; and (4) a single parameter of the GSE uniquely determines the state process. The proposed model and the experimental data provide a new methodology and comprehensive references for studying the working mechanism and efficiency of ALPs.

Experimental Study of the Effect of Molecular Collision Frequency and Adsorption Capacity on Gas Seepage Flux in Coal

Summary The differences in the transport behavior and adsorption capacity of different gases in coal play crucial roles in the evolution of coal permeability. Previous studies of coreflooding experiments failed to explain the mechanism of gas flow and have attributed the variation in gas seepage flux (flow rate) at the beginning of the experiment to the change in effective stress, while the differences in the microscopic properties of different gases, such as molar mass, molecular diameter, mean molecular free path, and molecular collision frequency, were ignored. To research the effect of these gas properties on seepage flux while circumventing the effective stress, coreflooding experiments with helium (He), argon (Ar), nitrogen (N2), methane (CH4), and carbon dioxide (CO2) were designed. The results show that the gas transport velocity in coal is affected by the combination of molecular collision frequency and dynamic viscosity, and the transport velocities follow the order of ν (CH4) > ν (He) > ν (N2) > ν (CO2) > ν (Ar). A permeability equation corrected by the molecular collision frequency is proposed to eliminate differences in the permeabilities measured with different gases. The adsorption of different gases on the coal matrix causes different degrees of swelling, and the adsorption-induced swelling strains follow the order of ε (CO2) > ε (CH4) > ε (N2) > ε (Ar) > ε (He). The reduction in seepage flux and irreversible alterations in pore structure caused by adsorption-induced swelling are positively correlated with their adsorption capacities. The gas seepage fluxes after adsorption equilibrium of coal follow the order of Q (He) > Q (CH4) >Q (N2) > Q (Ar) > Q (CO2). Like supercritical CO2 (ScCO2), conventional CO2 can also dissolve the organic matter in coal. The organic molecules close to the walls of the cleats along the direction of gas flow are preferentially dissolved by CO2, and the gas seepage flux increases when the dissolution effect on the cleat width is greater than that on adsorption swelling.

Characterization of the Macroscopic Impact of Diverse Microscale Transport Mechanisms of Gas in Micro-Nano Pores and Fractures

The objective of this study is to construct a refined microscopic transport model that elucidates the transport mechanisms of gas flow within micro-nano pores and fractures. The collective impact of various microscopic transport mechanisms was explained through the apparent permeability model, specifically related to gases such as methane and carbon dioxide, within the shale matrix. The apparent permeability models, taking into account microscopic transport mechanisms such as slippage flow, Knudsen diffusion, transition flow, and surface diffusion, were established individually. Subsequently, the influencing factors on apparent permeability were analyzed. The results demonstrate that the apparent permeability of the shale reservoir matrix is significantly influenced by pore pressure, temperature, pore size, and total organic carbon (TOC). As pressure decreases, the apparent permeability of Knudsen diffusion and surface diffusion increases, while the apparent permeability of slippage flow decreases. In addition, the apparent permeability of the reservoir matrix initially decreases and then increases. With increasing temperature, the apparent permeability of slippage flow, Knudsen diffusion, and surface diffusion all increase, as does the apparent permeability of the reservoir matrix. As pore size increases, the apparent permeability of surface diffusion and Knudsen diffusion decreases, while the apparent permeability of slippage flow and the reservoir matrix increases. Furthermore, an increase in TOC leads to no change in the apparent permeability of slippage flow and Knudsen diffusion, but an increase in the apparent permeability of surface diffusion and the reservoir matrix. The model presented in this paper enhances the multi-scale characterization of gas microflow mechanisms in shale and establishes the macroscopic application of these micro-mechanisms. Moreover, this study provides a theoretical foundation for the implementation of carbon capture, utilization, and storage (CCUS) in shale gas production.

Gas transport model and numerical solution in roof rock based on the theory of free gas diffusion

Accurately identifying the source of coalbed methane is of great practical significance in preventing gas concentration overruns and improving extraction efficiency. At present, it is widely believed that underground gas mainly comes from coal desorption, and the mechanism of gas flow inside the coal seam is explored in depth, while the gas released inside the rock is often neglected, and there is insufficient understanding of its internal transportation law. In this study, the density gradient theory (FGDGD) in the coal matrix was used as a guide to characterize the adsorption and transport properties of gases within the rock. The anthracite and mudstone from the same roadway in Yuxi mine were selected for the experiments, and coal rock samples with particle sizes of 0.425 mm–0.25 mm were ground. The adsorption constant test and adsorption process experiments were carried out under the constant volume condition, and the transport behavior of gas within the rock was simulated and numerically solved, and the content and flow behavior of gas in the rock were discussed in detail. The results are as follows: (i) The simulation curves based on the density gradient model fit well with the experimental data for both coal and rock downholes, indicating that the gas transport law inside the rock downhole is also in accordance with the density gradient theory, which broadens the scope of application of the theory. (ii) The free gas diffusion coefficients Dks (rock) are smaller than Dks (coal) for the gases in the rock matrix and coal matrix, which may be due to the difference in the pore structure of the rock matrix, resulting in lower adsorption sites than those of the coal. (iii) Although the gas content inside the rock is relatively low compared with that of coal, it is significant for reducing the prediction error of gas influx in engineering practice. Therefore, it is necessary to consider the storage of gas in rocks and its migration behavior when studying the sources of gas emissions in engineering fields.

Self-adaptive gas flow and phase change behaviors during hydrate exploitation by alternate injection of N2 and CO2

Since hydrate resources play a part of the stratigraphic framework structure in sediments, establishing a safe and economic method for hydrates exploitation remains the primary challenge to this day. Among the proposed methods, the spontaneous displacement of CH4 from hydrate cages by CO2 seems to be a perfect mechanism to address gas production and CO2 storage, especially in today's strong demand for carbon reduction and replacing clean energy. After extensive lab researches, in the past decade, injecting a mixture of CO2 and small molecule gas has become a key means to enhance displacement efficiency and has great potential for application. However, there is a lack of in-depth research on gas flow in the reservoir, while the injected gas always passes through low-saturated hydrate areas with high permeability and then occurs gas channel in a short term, finally resulting in the decreases in gas production efficiency and produced gas quality. Therefore, we explored a new injection-production mode of alternate injection of N2 and CO2 in order to fully coordinate the advantages of N2 in enhanced hydrate decomposition and CO2 in solid storage and heat compensation. These alternate “taking” and “storing” processes perfectly repair the problem of the gas channel, achieving self-regulation effect of CH4 recovery and CO2 storage. The 3-D experimental results show that compared to the mixed gas injection, CH4 recovery is increased by >50% and CO2 storage is increased by >70%. Additionally, this alternate injection mode presented a better performance in CH4 concentration of produced gas and showed outstanding N2 utilization efficiency. Further, we analyzed its self-adaptive gas flow mechanism and proposed an application model of “one injection and multiple production”. We look forward to this study accelerating the application of CO2-CH4 replacement technology.

FRACTAL ANALYSIS FOR PERMEABILITY OF MULTIPLE SHALE GAS TRANSPORT MECHANISMS IN ROUGHENED TREE-LIKE NETWORKS

In this work, a new gas transport model for shale reservoirs is constructed by embedding randomly distributed roughened tree-like bifurcation networks into the matrix porous medium. We constructed apparent permeability models for different shale gas flow mechanisms based on fractal theory, taking into account the effects of relative roughness and surface diffusion. The effects of bifurcation structure parameters as well as shale gas parameters on different apparent permeabilities are systematically analyzed. It is found that the permeability generally shows a decreasing trend with the increase in pore pressure, but the effect on viscous flow is inconsiderable. In addition, larger porosity, fractal dimension and bifurcation levels lead to increased permeability of different mechanisms. Whereas, the increase in the bifurcation levels implies a greater flow resistance, resulting in a decreased permeability. In addition, The relative roughness hinders the development of total permeability but favors surface diffusion permeability. Moreover, larger length ratios and diameter ratios are beneficial to the shale gas flow.

Influence Mechanism of Gas Pressure on Multiscale Dynamic Apparent Diffusion-Permeability of Coalbed Methane.

The permeability and diffusion coefficient of coal show multiscale characteristics due to the influence of multiscale pore sizes. The gas pressure will continuously decrease during the coalbed methane (CBM) extraction. However, there are contradictory perceptions in the effect of gas pressure on the diffusion coefficient and permeability. Therefore, it is essential to clarify the influence mechanism of gas pressure on multiscale diffusion-seepage. Diffusion-seepage experiments are carried out using particle coal and cylindrical coal without stress loading. Meanwhile, seepage experiments measured by the steady-state method are conducted under stress loading. The results show that the apparent diffusion coefficient is dynamically attenuated with time in the experiments of particle and cylindrical coal. A new model of multiscale dynamic apparent diffusion is proposed. The mechanism of gas flow in multiscale pores is elucidated. The multiscale pores determine the attenuation of the diffusivity and permeability of coal. The initial apparent permeability decreases and then increases with the increase of gas pressure, which is caused by the effect of gas pressure stretching and multiscale flow regime. Three patterns of permeability with gas pressure, monotonically increasing, monotonically decreasing, and U-shaped changes, will occur.

Co-optimization of multi-stakeholders in multi-heterogeneous energy system considering gas flow dynamics

Multi-heterogeneous energy system aims to exploit the advantages of coupling between electricity, heat, and gas. However, the common approach utilized to obtain the overall optimum cannot consider the existence and interests of all stakeholders in energy sectors such as electricity, heat, gas, and energy hubs. In contrast, this paper develops collaborative optimization and solution methods after clarifying the interaction mechanism of multi-stakeholders. Firstly, the operational models of natural gas system, electric power system, heating pipeline system, and energy hub are built with emphasis on the implementation of dynamic modeling of the gas network. Secondly, the multi-stakeholder interface constraints are established, while an upper-mid-lower optimization mechanism is proposed. Then, to solve the structured dynamic model, a microelement-integral approach for solving dynamic gas flows is proposed and extended to entire gas network. Moreover, a set of methods including Karush–Kuhn–Tucker optimality conditions, the big-M method, the quasi-accurate method, and the stepped-price extension method are employed to solve multi-stakeholder optimization problems with multiple nonlinear terms. Finally, the simulation case with six comparison scenarios are constructed to validate the proposed model and methods. The obtained results show that the gas network dynamic model can reflect the actual transmission mechanism of gas flow, and the proposed multi-stakeholder model can effectively balance the interests of various stakeholders. Consequently, all stakeholders have more potential to jointly promote the application and construction of multi-heterogeneous energy systems.

A Review of Macroscopic Modeling for Shale Gas Production: Gas Flow Mechanisms, Multiscale Transport, and Solution Techniques

The boost of shale gas production in the last decade has reformed worldwide energy structure. The macroscale modeling of shale gas production becomes particularly important as the economic development of such resources relies on the deployment of expensive hydraulic fracturing and the reasonable planning of well schedules. A flood of literature was therefore published focused on accurately and efficiently simulating the production performance of shale gas and better accounting for the various geological features or flow mechanisms that control shale gas transport. In this regard, this paper presents a holistic review of the macroscopic modeling of gas transport in shale. The review is carried out from three important points of view, which are the modeling of the gas flow mechanisms, the representation of multiscale transport, and solution techniques for the mathematical models. Firstly, the importance of gas storage and flow mechanisms in shale is discussed, and the various theoretical models used to characterize these effects in the continuum scale are introduced. Then, based on the intricate pore structure and various pore types of shale gas reservoirs, this review summarizes the multiple-porosity models in the literature to represent multiscale gas transport, and discusses the applicability of each model. Finally, the numerical and analytical/semi-analytical approaches used to solve the macroscopic mathematical model governing shale gas production are reviewed, with a focus on the treatment of the complex fracture network formed after multistage hydraulic fracturing.

Prediction of gas drainage changes from nitrogen replacement: A study of a TCN deep learning model with integrated attention mechanism

Accurate prediction of pure gas volume changes in nitrogen injection replacement coal seams is of great significance to increase coalbed methane (CBM) production and prevent underground gas disasters in coal mines. However, the mechanism of coal seam gas flow during nitrogen injection replacement is complex, and it is extremely difficult to achieve quantitative prediction of the pure volume of gas in nitrogen injection replacement coal seams. In this study, we propose a new model for predicting the change of pure amount of gas in coal seam of nitrogen injection replacement in coal mines (Attention-TCN). Meanwhile, we believe that a stable and reliable predictive model should have the ability to capture the underlying patterns and relationships of regular and noisy datasets during the learning process, rather than memorizing training data. Therefore, in this paper, the Attention-TCN model is trained and tested using the results of nitrogen injection and replacement methane double diffusion test (regularity dataset) and the results of nitrogen injection and replacement experiment in coal mine underground engineering (noisy dataset), respectively. The difference between the Attention-TCN model and the baseline models of TCN, gated recurrent neural network (GRU) and long and short-term memory neural network (LSTM) was also compared. The validity of the model was demonstrated using ten-fold cross-validation results. The results show that the proposed Attention-TCN prediction model achieves good prediction performance on both regular and noisy datasets, with prediction R2 greater than 0.99, and is highly flexible and robust. The model is better able to extract the basic features and structures in different types of datasets, enabling it to perform well on unseen data. The ten-fold cross-validation results clearly show that the Attention-TCN model is more accurate and all outperforms the TCN, GRU and LSTM prediction models. In addition, the prediction results of all models for different mine underground nitrogen injection replacement coal seam pure gas volume datasets further validate that the Attention-TCN model has better ability to generalize to new datasets and is more effective and practical for predicting the variation of pure gas volume in underground nitrogen injection replacement coal seams in coal mines.

An improved transport model in shale nanopores based on advection diffusion method

As an unconventional oil and gas resource, shale gas plays a very important role in global energy supply. A breakthrough in efficient exploitation of shale gas is how to accurately describe the percolation mechanism of shale gas in nano-pores. In this article, a new shale gas apparent permeability model is established on the basis of advection diffusion model and considering adsorption stress effect and gas viscosity correction. On this basis, the influence factors of apparent permeability, the comparative difference between stress effect or not and viscosity correction or not, and the contribution of gas flow mechanism are studied. The results show that: the apparent permeability decreases with the increase of pressure, increases with the increase of pore radius, and increases with the increase of temperature; when the pore radius increases, the difference caused by stress correction becomes obvious; when the pressure decreases, the difference caused by viscosity correction becomes obvious. The results can provide some new ideas and methods for the study of gas flow mechanism in microscopic nano-pores.

Explanation of the mechanisms of unsteady gas flow through the turbocharger seal system, including thermal and structural interactions

Gas flow in the seal system can be expected during the operation of a turbocharger and is associated with negative effects on the quality of the lubricant or turbocharger efficiency. Gas flow also affects particulate matter production due to lubricant entrainment in the compressor or turbine. The prediction of gas flow rates depends on many design parameters and the operating conditions of the turbocharger, but sufficiently accurate descriptions of the gas flow mechanisms and their quantification depending on the operating conditions have not yet been presented. The proposed computational approach simultaneously solves the gas dynamics in the seal system, the heat transfer in the turbocharger rotor-bearing system and the dynamics of the seal rings and rotor, including the bearings. The computational model for the turbocharger of a heavy-duty vehicle engine is experimentally validated. Two mechanisms have major influences on gas mass flow: the gas flow through the thin gap between the moving ring and groove and the flow through the ring gap. The results show that the importance of these mechanisms depends on several geometrical dimensions of the seal system and the operating conditions of the turbocharger, with a strong connection to the rotor dynamics and thermal load of the impellers. Influences involving rotor movement or rotor thermal conditions are crucial, and their non-inclusion limits the ability to correctly predict gas mass flow.

The Effects of Pore Wettability on the Adsorption and Flow Capacity of Shale Gas: New Insights from Molecular Dynamic Modeling

Pore wettability plays a key role in controlling shale gas adsorption and flow in marine shales. To reveal the mechanism of shale gas adsorption and flow in pores with different wettability (hydrophobic or hydrophilic), molecular modelling is simulated using the molecular dynamics method. Three pore assemblages, namely, the quartz–illite (Q–I) pores, the quartz–organic matter (Q–OM) pores, and the illite–organic matter (I–OM) pores, are proposed and the adsorption and diffusion behavior in these pores are simulated under different conditions. The results show that matrix compositions in shale greatly affect pore wettability. Water molecules preferentially absorb on the junction of materials and result in more hydrophilicity at the junction than pores. Methane density simulations reveal that free methane molecules form clusters in the Q–I pores, with higher free methane density than adsorbed methane. Methane molecules in the Q–OM pores are primarily adsorbed on organic matter. Free methane molecules extensively exist in the I–OM pores, leading stronger density of free methane than adsorbed methane. The more the proportion of free methane, the larger the methane self‐diffusion coefficient. This study provides new insights to understanding of shale gas adsorption and diffusion behavior, which is important for the effective development of shale gas reservoirs.

Well-scale experimental and numerical modeling studies of gas bullheading using fiber-optic DAS and DTS

Bullheading is an effective well control technique to pump produced fluids back into the formation during drilling and completion operations to regain well control. However, existing bullheading procedures are often based on a trial-and-error approach due to the complexity of modeling counter-current gas flow mechanisms in well control simulators and the lack of empirical bullheading datasets in full-scale conditions. To address this gap, a 5163-ft-deep experimental wellbore was utilized to conduct bullheading tests using nitrogen and water to simulate a “gas below BOP” scenario. We present a novel application of fiber-optic distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) in facilitating the measurement and monitoring of gas slugs in the wellbore for an improved understanding and modeling of the gas bullheading processes.Full scale tests were performed by injecting a nitrogen slug into the wellbore annulus which was bullheaded with water while taking returns via tubing. A detailed analysis of the bullheading data for two different gas slug volumes and bullheading rates is presented. DTS and DAS data were analyzed using a variety of time- and frequency-domain signal processing techniques to estimate displacement velocities and changing gas slug lengths during injection, subsequent expansion, and displacement in the annulus. Numerical simulations were also performed for an improved understanding of the bullheading processes using a drift flux model (DFM)-based transient multiphase flow simulator modified for counter-current flow situations. Different gas slip models, used in the DFM, were benchmarked for the best match with the DAS and DTS measurements obtained from the bullheading experiments. The data from downhole pressure gauges were also compared to the DAS and DTS measurements as well as the results from numerical simulations and satisfactory agreements was obtained.This study demonstrates a novel application of DAS and DTS in obtaining high-resolution spatial-temporal measurements for the facilitation of improved understanding and modeling of well-scale bullheading hydrodynamic behaviors. The evaluation, selection, and calibration of transient multiphase flow sub-models used in this study improved the understanding of the counter-current flow in a well and provided references for applying the DFM in simulating bullheading operations.

Pore Permeability Model Based on Fractal Geometry Theory and Effective Stress

AbstractA reasonable coal seam permeability model should be established to accurately estimate the extraction effectiveness of coalbed methane (CBM). Existing permeability models typically ignore the influence of pore structure parameters on the permeability, leading to an overestimation of the measured permeability, and consequently, the CBM production cannot be effectively predicted. This paper presents a novel permeability model based on discrete pore structures at the micro–nano scale. The model considers the interaction between the pore fractal geometry parameters, coal deformation, and CBM transport inside these pores. The contributions of key pore geometry parameters, including the maximum pore diameter, minimum pore diameter, porosity, and fractal dimensions, to the initial permeability were investigated. A numerical analysis showed that the influence of fractal dimension on the permeability is finally reflected in the influence of pore structure parameters. The initial permeability is exponential to the minimum pore diameter and proportional to the maximum pore diameter and porosity. In addition, the macroscopic permeability of the coal is positively correlated with the maximum pore diameter, minimum pore diameter, and porosity, with the minimum pore diameter having the most significant influence on the permeability evolution process. This research provides a theoretical foundation for revealing the gas flow mechanism within coal seams and enhancing the extraction effectiveness of CBM.

A lattice Boltzmann model for simulating gas transport in coal nanopores considering surface adsorption and diffusion effects

The gas flow in coal is extremely complicated as a combined results of multiple flow mechanisms and complex pore structures. Comprehensive understanding of gas flow behaviors in coal is of great importance for the prediction of coalbed methane (CBM) production and the elimination of gas-induced disasters in coal mines. Based on the lattice Boltzmann method (LBM), this study constructs a LB model considering surface adsorption and diffusion effects, which involves multiple mechanisms of gas flow, including viscous flow, enhanced gas slippage (gas slippage is enhanced by surface diffusion) and surface diffusion. The results show that in micropores (<2 nm) and mesopores (2–50 nm), under the effect of adsorbed gas, bulk gas flow velocity increases at a low pressure but decreases at a high pressure. Adsorbed gas surface diffusion permeability, enhanced gas slippage permeability and viscous flow permeability decrease with the rise of pore pressure. In nanopores in coal reservoirs, surface diffusion more significantly enhances gas slippage at a low pressure and in smaller pores. Besides, in the flow calculation, gas slippage without the considering effect of adsorbed gas and Knudsen diffusion can replace each other. According to the results from the LB model, the apparent diffusion coefficients of gas in coal particles decreases with the increase of pore pressure in smaller pores(<10 nm) but increases in larger pores(>10 nm). The study further investigates flow mechanisms controlling the CBM extraction yield in different extraction stages. In the early stage of extraction, the CBM extraction yield is controlled by Darcy seepage in fractures; in the middle stage, it is controlled by viscous flow in larger matrix pores; in the late stage, it is controlled by gas diffusion induced by the superposition of surface diffusion and enhanced gas slippage in matrix micropores and mesopores.