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Description
Pore-fracture connectivity and nanoscale pore accessibility are critical factors influencing gas occurrence, transport, and fluid migration in shale reservoirs. However, due to the extremely low permeability and high heterogeneity of shale components, accurately characterizing these properties remains challenging. This study integrates advanced experimental techniques to investigate the controlling mechanisms of pore-fracture connectivity and pore accessibility in overmature marine shales from the Wufeng-Longmaxi and Niutitang Formations in South China.
To evaluate pore-fracture connectivity, small-angle neutron scattering (SANS) under vacuum and high-pressure conditions, repeated mercury intrusion capillary pressure (MICP), and field-emission scanning electron microscopy (FE-SEM) imaging after Wood’s metal (WM) impregnation were employed. The results revealed that the sealing of pore system by brittle minerals significantly reduces overall connectivity within the shale matrix, resulting in isolated pore networks. While brittle minerals preserve pores within organic matter and clay minerals, they hinder the connectivity between pore systems. This isolation effect has important implications for methane transport, as only 38–78% of pores within 100 nm were accessible to methane in the studied samples. Furthermore, confinement effects were observed to increase methane density in nanopores smaller than 20 nm. This phenomenon results in the formation of nanoscale methane clusters, with densities exceeding those of ideal gas states under equivalent conditions. The novel integration of repeated MICP measurements and FE-SEM imaging after WM impregnation provides a robust framework for evaluating pore-fracture connectivity in shale systems.
In parallel, pore accessibility was systematically investigated using contrast-matching small-angle neutron scattering (CM-SANS) and supplementary experiments, including air-liquid contact angle measurements and spontaneous imbibition. A novel accessibility index was developed to quantify the interaction of fluids with varying wettability in nanoscale pore networks and their temporal dynamics. CM-SANS results indicated that pores larger than 7 nm were predominantly filled with toluene, attributed to the development of organic pores and the connectivity between organic and inorganic pore systems. Conversely, smaller hydrophilic pores (<7 nm) were associated with clay minerals or clay swelling, making them accessible primarily to water. The integration of CM-SANS and MICP further demonstrated that pore accessibility to water and toluene is largely controlled by pore surface wettability and connectivity.
The combined insights from these methodologies link pore-fracture connectivity and pore accessibility, offering a comprehensive understanding of their roles in methane transport and hydrocarbon fluid migration. Pore-fracture connectivity determines the transfer of gas from matrix pores to fracture systems and significantly influences gas storage and transport pathways. Simultaneously, pore accessibility governs fluid migration within the pore network, impacting fracturing fluid imbibition and hydrocarbon recovery efficiency. Understanding the interaction between pore connectivity and wettability offers new perspectives for improving hydraulic fracturing strategies and unconventional reservoir stimulation.
| Country | China |
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