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Directly obtaining representative pore structure information is important to understand subsurface shale gas storage and production, while acquiring large-scale and high-resolution images using a single imaging tool is plausible as there is always a trade-off between the resolution and field-of-view. We report a new method to define representative elementary volumes (REV) of a shale pore system and define pore structural parameters of diameter, surface area, porosity, and other features. Automated ultrahigh resolution scanning electron microscopy, integrated with a modular automated processing system (MAPS), was used to image pore distribution in two dimensions. Focused ion beam (FIB) milling was further utilized to construct a true three-dimensional digital image, on which the REV analysis was then carried out. The results show that (i) pores are mainly developed in organic matter (OM) and as interparticle inorganic pores and (ii) the diameter of inorganic pores is slightly larger than those in OM. The pore network coordination number, representing the average number of pores that are connected to a specific pore, indicates that the pores can be either clustered within mainly OM pores or more widely connected by slit-like pores and throats in minerals. Extracting cubic sub-blocks, ranging from 500 to 5000 nm in edge dimension, defines the minimum REV as ∼4000 nm, as measured using minimum and maximum pore sizes, surface areas, and shape factors. Combined FIB and MAPS provide insight into pore morphology and connectivity at multiple scales with the reconstructed digital rock used to determine representative REV sizes. Such results are useful in understanding the pore structure in shales and for the rapid acquisition of pore structure distributions.

Shale reservoirs, characterized by their relatively low permeability and porosity, often employ hydraulic fracturing techniques to increase production during development. However, conventional large-scale hydraulic fracturing encounters challenges such as excessive water consumption, low flowback rates, and environmental concerns. Given the increasing scarcity of water resources and increased emphasis on sustainability environmental sustainability, hydraulic fracturing no longer meets the requirements for green and environmentally friendly extraction. The utilization of supercritical CO2fracturing technology emerges as a promising alternative, offering advantages such as reduced water usage and minimized environmental impact. Additionally, this technology allows for the sequestration of CO2underground, presenting an integrated approach to CO2geological storage and oil-gas extraction. Supercritical CO2possesses many unique physical and chemical properties. However, the micro-mechanisms governing its interaction with rock during fracturing, along with the mechanisms and propagation characteristics of fracture initiation, necessitate further in-depth investigation. This study aims to explore the development mechanisms of supercritical CO2fractures under the influence of single factor variables and natural fractures of different sizes (2 m, 4 m, and 6 m). To explore the interaction mechanisms between natural fractures and supercritical CO2fractures, single factor variable control experiments were conducted with natural fracture angles of 30 °, 45 °, and 60 °, and in-situ stress deviations of 3, 5, 7, and 9 MPa. Our research delves into the impact of natural fracture inclination angle, in-situ stress deviation, and natural fracture size on supercritical CO2fractures development. This comprehensive exploration unveils the intricate interaction mechanisms between natural fractures of different sizes and supercritical CO2fractures. Various control chart were studied, considering different natural fracture under diverse ground stresses and inclinations. The findings of this study bear theoretical significance and engineering application for enhancing efficiency in shale gas production.

Understanding the physical mechanisms of exploitation of such sources that occurs in the shale matrix in the middle and late stages is critical in the world’s energy supply. However, there is a current lack of research on the elusive relationship between mechanisms governing gas diffusion in shale matrices and the efficiency of gas production. In addition, various pores exist in a shale matrix within different diffusion mechanisms, affecting the mass transfer. In this work, we establish a microscopic model that considers the explicit interactions among various pore systems in the gas diffusion processes. The model was first verified with reported stress-dependent diffusion experimental data and then extended to the field scale. A sensitivity analysis was finally conducted to investigate the gas diffusion mechanism in gas production. The evolutions of the gas diffusion coefficient depended on the competition among the interactions, adsorption strain, and effective stress. “Production sensitive range” in which the shale gas production rate could be improved explicitly exists. Larger initial macroscopic and microscopic pore diffusivities can improve the early stage and overall gas-production efficiencies, respectively. Gas depletion is highly sensitive to extraction pressure in the middle and late production stages; as a result, adjusting the extraction pressure in a timely manner can improve the gas yield. In the deformable range, shale reservoirs with a large pore bulk modulus have better gas production rates in the middle stage. This work provides new insights into improving the gas production performance in the field.