Assessment of hydraulic fracture propagation in layered rock using optical frequency domain reflectometry technology

Hydraulic fracturing remains a pivotal technology for the efficient development of unconventional oil and gas reservoirs. However, the intricate bedding planes and natural fractures within these reservoirs hinder the accurate prediction of fracture propagation behavior, necessitating the development of high-precision monitoring methods. In this study, true triaxial fracturing experiments were conducted on multilayered rock masses using optical frequency domain reflectometry (OFDR) to systematically investigate the correlation between fracture propagation and strain response. The results reveal that thick encasing layers promote the formation of high-tensile zones and associated compression zones, whereas thin encasing layers induce broad low-strain bands. Furthermore, a dual-criterion fracture identification index based on strain and strain-rate reversal was established, enabling the quantitative identification of layered fractures with 100% accuracy. The experimental data indicate that high-strength bedding planes trigger multiple microfractures and jagged pump pressure fluctuations, while thick encasing layers facilitate the formation of complex fracture networks. Bedding plane strength exhibits a negative correlation with crack opening displacement (COD), a maximum COD of 35.6 μm was observed in samples with medium-strength bedding planes, which decreased to 22.4 μm under stronger bedding conditions. The proposed COD inversion model effectively accounts for the strain disturbance effects of adjacent fractures. Notably, the inversion accuracy exhibits a positive correlation with COD magnitude. Specifically, the average error rates are 4.2% and 3.0% for larger CODs, whereas these rates increase to 8.9% and 8.7% for smaller CODs. This research provides a robust experimental basis and theoretical framework for the monitoring and evaluation of hydraulic fracturing in layered reservoirs.