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<p>Fracture initiation and propagation under changing effective stress are essential for safely operating CO<sub>2</sub>-EOR and sequestration in unconventional reservoirs. However, the characterization and quantification of stress-dependent fracture evolution remain challenging for the tight sandstone’s complex pore-fracture networks. To address these challenges, we performed in-situ mechanical experiments at varying horizontal stresses (5–14&#xa0;MPa) on tight sandstone using a novel triaxial apparatus equipped with NMR monitoring. We tracked and quantified the temporal–spatial evolution of fractures at the pore scale and illustrated the interaction mechanism between energy dissipation and fracture growth during rock deformation. Results reveal a progressive fracture development process comprising microcrack closure, stable crack growth, and accelerated crack growth. Diffused tension microcrack initiation and propagation from preexisting pores is the dominant fracture growth mode. Decreased horizontal stress promotes tension crack propagation, leading to more pronounced fracture dilation and a longer accelerated crack growth process. Furthermore, both crack initiation and accelerated growth stresses exhibit a power-law increase with increasing horizontal stress. Energy dissipation analysis shows that dissipated energy per unit crack growth decreases notably with reduced horizontal stress. During this process, a higher proportion of dissipated energy (ranging from 2.9% to 13.9%) is utilized for new fractures, although the majority of energy is dissipated in other forms. These findings provide critical insights for optimizing CO<sub>2</sub>-EOR and storage design in tight oil&#xa0;reservoirs.</p>

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Investigation on Fracture Propagation and Energy Evolution of Ultra-low Permeability Sandstone During Reservoir Pressurization

  • 期刊论文

  • 发表日期：

    2025年5月19日

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  • Ming Tang,
  • Tong Zhang,
  • Yanfang Li,
  • Qilong Wang,
  • Mingchao Wang,
  • Yongqiang Chen,
  • Liang Yuan

摘要

Fracture initiation and propagation under changing effective stress are essential for safely operating CO2-EOR and sequestration in unconventional reservoirs. However, the characterization and quantification of stress-dependent fracture evolution remain challenging for the tight sandstone’s complex pore-fracture networks. To address these challenges, we performed in-situ mechanical experiments at varying horizontal stresses (5–14 MPa) on tight sandstone using a novel triaxial apparatus equipped with NMR monitoring. We tracked and quantified the temporal–spatial evolution of fractures at the pore scale and illustrated the interaction mechanism between energy dissipation and fracture growth during rock deformation. Results reveal a progressive fracture development process comprising microcrack closure, stable crack growth, and accelerated crack growth. Diffused tension microcrack initiation and propagation from preexisting pores is the dominant fracture growth mode. Decreased horizontal stress promotes tension crack propagation, leading to more pronounced fracture dilation and a longer accelerated crack growth process. Furthermore, both crack initiation and accelerated growth stresses exhibit a power-law increase with increasing horizontal stress. Energy dissipation analysis shows that dissipated energy per unit crack growth decreases notably with reduced horizontal stress. During this process, a higher proportion of dissipated energy (ranging from 2.9% to 13.9%) is utilized for new fractures, although the majority of energy is dissipated in other forms. These findings provide critical insights for optimizing CO2-EOR and storage design in tight oil reservoirs.