Ultrafast laser-resolved fracture behavior of laminated shale under CO2 influence
YANG Liu
YU Zhehan
ZHANG Zhengyan
LU Yunhui
CHU Zhanyu
MENG Siwei
Abstract：The Gulong shale in the Daqing Oilfield exhibits diverse millimetre-scale lamina-tions,yet the fracture characteristics and crack propagation laws after CO2 preconditioning re-main unclear.This study investigates mixed-laminated and felsic-laminated Gulong shale sam-ples through three-point bending experiments before and after SC-CO2 treatment,revealing the mechanisms behind fracture toughness,energy release rate,and crack evolution under water-SC-CO2 interaction.High-speed picosecond-pulsed laser imaging(15 ps pulse width)was em-ployed to capture fracture penetration across laminae with ultrahigh temporal resolution.The results show that SC-CO2 exerts significantly different effects on the mechanical properties of the two shale types.Both mixed-layer and felsic laminated shales exhibit reductions in Mode-Ⅰfracture toughness,elastic modulus,absorbed energy,and energy release rate,with the mixed-layer shale showing a more pronounced decline.respectively.SC-CO2 weakens tensile crack resistance,shortening propagation time.Post-treatment,mixed-laminated shale displays reduced maximum/mean crack velocities with intensified fluctuation,while felsic-laminated shale exhibits higher mean velocity and denser fluctuation.Fractures in both types preferen-tially propagate along pre-existing notches post-SC-CO2,as weakening of lamination bonding and mineral interfaces shifts failure modes:Mixed-laminated shale develops stepwise dissolu-tion-swelling coupled cracks,whereas felsic-laminated shale forms brittle intergranular bran-ches with minimized deviation.SC-CO2 reduces crack initiation pressure by lowering critical fracture toughness at the tip,promoting rapid matrix-penetrating failures with diminished en-ergy absorption.Mechanical degradation stems from mineral dissolution,grain detachment,and pore erosion,which reduce KIC and homogenize fracture paths.As crack deflection events decrease,fractures smoothen(lower fractal dimension),facilitating propagation.This study elucidates lithology-dependent fracturing behaviours,supporting stimulation strategy optimiza-tion for laminated shale reservoirs.
Keywords：rock mechanics laminatedshalefracture toughnessSC-CO2crack propagation
Publication Date：2025-07-30
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：19( 850-868 )