Dayu Ye

Shale gas permeability is jointly controlled by fracture and pore characteristics, yet their coupled effects under multifactor conditions remain insufficiently quantified. This study proposes a fractal flow model integrating gas transport, shale deformation, and fracture–matrix interaction, introducing four fractal parameters (tortuosity, roughness, fracture quantity, and length) to quantify structural effects. The study results show that: (1) Fracture geometry controls pore structure evolution—as the roughness increases from 0.04 to 0.08, and both the tortuosity and fracture fractal dimensions increase from 1.2 to 1.6, the pore fractal dimension rises by 0.0019, 0.0028, and 0.0007, respectively. (2) Fracture complexity enhances gas flow—these structural changes reduce fracture gas pressure by 23%, 40%, and 32%, expanding diffusion pathways and improving connectivity, thereby enhancing permeability and gas transport efficiency. (3) Pore complexity promotes adsorption and impedes discharge—in contrast, higher pore fractal dimension (from 1.2 to 1.6) increases gas pressure by 20%, reflecting higher adsorption capacity and more tortuous flow paths. The findings quantitatively elucidate fracture–pore coupling mechanisms and their combined impacts on shale gas extraction, offering a predictive framework for optimizing shale gas recovery.

During the process of oil and gas extraction, the microscopic topological structure of pores, the slippage effect, and the tortuosity of the migration channels play a crucial role in the oil and gas production and permeability. In this study, an innovative research method that couples these three factors is proposed and a new hydro-mechanics coupling model is established. This model is established by using the power-law method, and sufficiently considers the effects of these three factors on gas seepage. The research results show that: (1) Compared to the base case without considering both, considering slippage effect increases the permeability by 34.29%, while considering tortuosity decreases the permeability by 63.90%. When the two are coupled, the permeability decreases by 25.50%, which clearly indicates that the inhibitory effect of tortuosity on gas seepage dominates; (2) the microscopic structure has a significant impact on permeability, the slippage effect, and tortuosity, among which the maximum fracture length has the most prominent influence; (3) compared with traditional models, the model constructed in this study can more accurately reflect the influence of the microscopic structure. The establishment of this model can provide solid theoretical support for improving extraction efficiency and reducing extraction costs.

In shale gas extraction projects, an investigation into the mechanisms of energy/mass transfer associated with shale gas adsorption/desorption in organic matter (kerogen) microstructure under high temperature and stress condition is crucial for improving the efficiency of shale gas production. This study presents a coupling thermo-hydro-mechanical model based on an improved fractal method that could explain the microstructural evolution of the kerogen system and the resultant alterations during the gas adsorption/desorption process under varying thermal conduction, gas seepage, and stress conditions. The influence of porosity, diameter, and tortuosity on the abundance, length, and complexity of kerogen networks under coupled multi-field effects is evaluated. The significance of this study is it could address the following aspects quantitively: (1) the spatiotemporal evolution of kerogen fractal dimensions following various extraction timelines; (2) the influence of shale temperatures on kerogen structures; (3) the influence of the kerogen fractal dimension on the shale gas desorption capacity and production efficiency; and (4) under different temperatures, when the fractal dimension/tortuosity fractal dimension of kerogen changes due to extraction disturbances, the volumetric deformation induced by gas adsorption increases by a maximum of 26.1%/decreases by 28.1% and in the later stages of extraction, the maximum gas pressure decreases by 44.7%/increases by 47.1%. The proposed fractal method adeptly reveals shale gas desorption behaviors under multi-field coupling conditions from a microscopic perspective, which cannot be found in the literature.

The Carbon Dioxide Enhanced Coalbed Methane (CO2-ECBM) technique significantly enhances clean energy extraction and mitigates climate change. Central to this process is the dynamic evolution of rough fracture networks within coal seams, influencing the migration of CO2 and natural gas. However, existing research lacks a comprehensive, quantitative approach to examining the micro-evolution of these fractures, including fracture roughness, fracture density, fracture touristy, and fracture size, particularly under thermo-hydro-mechanical effects. Addressing this gap, our study introduces an innovative, fractal model for quantitative analysis. This model intricately characterizes fracture networks in terms of number, tortuosity, length, and roughness, integrating them with fluid dynamics affected by external disturbances in CO2-ECBM projects. Upon rigorous validation, the finite element method analysis reveals significant impacts of micro-parameters on permeability and natural gas extraction. For instance, increasing CO2 injection pressure from 4 to 6 MPa changes fracture network density by up to 6.4%. A decrease in fracture density (Df) from 1.6 to 1.5 raises residual gas pressure by 2.7% and coal seam stress by 9.5%, indicating crucial considerations for project stability. Applying the proposed interdisciplinary model to assess CO2 emissions in Australia, it is can be obtained that when Df decreases from 1.6 to 1.5, the total amount of CO2 storage reduces by 17.71%–18.04%. Our results demonstrate the substantial influence of micro-fracture behaviors on CO2-ECBM projects, offering a ground-breaking solution for efficient greenhouse gas reduction and clean energy extraction, with practical implications for the energy sector's sustainability.

This study explores the efficiency of using carbon dioxide (CO2) to extract shale gas, highlighting its potential to enhance extraction while mitigating environmental CO2 pollution. Given the intricate microstructure of shale, CO2 injection inevitably induces deformation within the shale reservoir's internal microstructure, thereby impacting gas displacement efficiency. The organic matter (kerogen) network and fracture network in shale, serving as primary spaces for gas adsorption and migration, exhibit complex microstructural characteristics. Thus, we developed a dynamic coupled hydro-mechanics permeability model for binary gas displacement, and three novel, interdisciplinary fractal power-law parameters are proposed to represent the distribution of shale fractures, considering the adsorption–desorption strength of the kerogen network. Numerical simulations analyzed the changes in gas seepage, diffusion, shale stress, permeability, and factors influencing displacement efficiency during the CO2–EGR (enhanced gas recovery) projects. Key findings include (1) CO2 injection leads to a nonlinear increase in the number of shale fracture networks, thereby enhancing the CH4 output efficiency. (2) Compared to traditional fractal theory, the proposed power-law model is applicable to a wider range of reservoir fracture distributions and effectively characterizes the density (by α), size (by r), and complexity (by n) of the fracture network during the CO2–EGR process. (3) Changes in the proposed interdisciplinary power-law parameters significantly alter CO2 and CH4 adsorption capacities and, in turn, significantly affects displacement efficiency and shale deformation. According to calculations, these parameters have the greatest impact on the CO2–EGR process, ranging from 16.3% to 68.1%.

Coal bed methane, a crucial clean energy source, has attracted extensive research attention. Characterized by intricate and rough fracture systems, coal seam is vital for gas migration, which will be influenced by the in situ stress, coal temperature, adsorption–desorption effect, solid deformation, and gas pressure. This paper introduces an innovative, interdisciplinary fractal model that addresses the limitations of current computational models in accurately representing the complex fractures under the coupled multi-field effects. Four novel fractal micro-parameters are introduced to capture the dynamics of rough networks. And rigorous validation against field extraction data reveals that the proposed micro-parameters outperform existing methods in analytical efficacy. Notably, those micro-parameters significantly influence fracture behavior and gas seepage. For instance, a DT increase from 1.2 to 1.8 and an ε rise from 0.06 to 0.18 lead to a respective 29.8% and 22.7% increase in gas pressure. Moreover, alterations in these fractal micro-parameters under coupled multi-field effects markedly impact coal bed stress, raising safety concerns in engineering projects, with a potential increase in coal stress by up to 2.62%. This research offers innovative insights into the complex coupled mechanisms governing rough fractures and significantly advances the understanding of the efficiency and safety in clean energy extraction processes.

In gas-bearing coal seam mining projects, the pivotal considerations encompass the assessment of gas migration, emission trends, and coal seam stability, which are crucial for ensuring both the safety and efficiency of the project. The accurate evaluation of the nonlinear evolution of the fracture network, acting as the primary conduit for gas migration and influenced by mining disturbances, coal seam stress, overlying strata pressure, and gas pressure, emerges as a key determinant in gauging coal seam stress and safety. To address the industry challenge of quantitatively assessing the complex behaviors of fracture networks during gas-bearing coal seam extraction, this study introduces a novel, interdisciplinary fractal analysis model. Drawing upon fractal theory for classical porous media, four fractal parameters capable of quantitatively characterizing the microscopic behaviors of fractures are proposed and defined as functions of permeability. Subsequently, the gas pressure in gas-bearing coal seams, coal seam deformation and stress, in-situ stress, overlying strata pressure, and adsorption–desorption effects are comprehensively coupled and applied to the classic gas-bearing coal seam at the Jianxin Coal Mine’s 4301 working face in Shaanxi, China. Upon the robust validation of the proposed model, the present computational results reveal: (1) the proposed micro-parameters adeptly characterize the number, roughness, tortuosity, and length of fractures in gas-bearing coal seams; (2) a larger fractal dimension of fractures leads to increased coal seam stress and strain, while the fractal dimensions of fracture tortuosity and roughness are inversely proportional to coal seam stress and strain; (3) these fractal parameters directly induce evolutionary changes in gas seepage behavior, leading to varying degrees of mechanical property evolution in the coal seam. When [ ] and [ ] increased from 1.2 to 1.8, the maximum change in coal seam deformation was 16.9% and 13.8%, respectively, and when [ ] increases from 0.03 to 0.12, the coal seam deformation changes by 15.1%. This represents a quantitative characterization unattainable by previously published coal seam analysis models, including mainstream fractal computation models.

Changes in the internal microstructure of shale reservoirs have a significant effect on gas production. In this research, based on the double power law model and the triple medium theory, a triple medium model of shale multi-scale power law fracture distribution is constructed. The accuracy of the model is verified by comparing with the actual test results. Then, the model is used to analyze the relationship between shale stress, permeability, and three kinds of pore media as well as the influence of power law parameters and external force on reservoir microstructure and macroscopic gas production. The findings indicate that (1) the microstructure of shale reservoirs is affected by power law parameters and external forces; (2) gas exchange can cause the evolution of reservoir structure; and (3) the stress change of a single system will cause stress rearrangement of the whole system. This model provides a basic and comprehensive understanding for considering the various transport mechanisms of shale matrix and the microstructure change law of gas production process and proposes a new method for optimizing shale permeability simulation.

As one of the most significant clean energy sources, the migration characteristics and extraction processes of coalbed methane (CBM) have been extensively studied. The structural distribution and evolutionary characteristics of reservoir fractures, as the main conduit for gas migration, significantly affect the permeability and gas production. However, few models have been able to quantitatively and accurately explore reservoir micro-macro interactions under coupled thermal-fluid-solid effects. This work develops a new highly coupled model based on the widely-adopted power-law function to quantify reservoir thermal conduction effect, gas pressure evolution, reservoir deformation, in-situ stress, adsorption-desorption effect and reservoir microstructure evolution. Three parameters are adopted to quantitatively characterize the reservoir structure: (1) fracture power index (to characterize the fracture density), (2) fracture length ratio r_f (to characterize the fracture size) and (3) the maximum fracture length l. The results demonstrate that the fractal network is a special kind of network in the power-law length distribution. And the proposed power-law seepage model is able to accurately characterize the evolution of reservoir microstructure and the impact of microevolution on extraction under multi-field coupling effects, compared to the traditional power-law model. The proposed model can provide a good theoretical and practical support for the study of CBM migration and extraction.

The pores in rocks are multi-scale and highly complex. Based on complex network theory, the topological properties of a sandstone flow network are studied. The results show that the sandstone pore scale network is a small-world network with an average shortest path of 5.720, and 80% of the network node degree is less than 3. This network structure can improve the security of the network and ensure the permeability of the pores under the action of external forces. In addition, nodes with appropriate degree distribution have a significant influence on permeability. Nodes with a larger degree will significantly reduce the average shortest path of the network, while nodes with a smaller degree will expand the coverage of the seepage network.