预制裂缝页岩焖井流体赋存核磁表征

NMR Characterization of Fluid Occurrence in Pre-Fractured Shale during Shut-In

摘要: 预制裂缝页岩焖井阶段的含液响应影响压裂液滞留与返排认识，但T₂谱结果不宜直接外推为定量滤失模型。为识别通道构型和流体类型对核磁响应的影响，本文制备长10 cm、直径3.8 cm页岩岩心，设置单裂缝和双裂缝预制通道，在围压10 MPa、注入液压5 MPa、常温条件下注入去离子水或0.3%压裂液并焖井，采用LIME-MRI-D2采集T₂谱。结果显示，单裂缝去离子水12~48 h长T₂不超过2.90%；单裂缝压裂液长T₂为13.80%~14.56%；双裂缝去离子水长T₂由0.07%升至15.24%；双裂缝压裂液24~36 h维持约20%长T₂响应，48 h为估算点，仅作趋势辅助。0.3%压裂液和双裂缝构型更易增强中、长T₂含液信号。本文为探索性核磁实验，不建立滤失预测模型，不讨论裂缝开启或扩展。

Abstract: The liquid-bearing response of pre-fractured shale during the shut-in period affects the understanding of fracturing-fluid retention and flowback; however, T₂-spectrum results cannot be directly extrapolated to establish a quantitative fluid-loss model. To identify the influences of channel configuration and fluid type on nuclear magnetic resonance (NMR) responses, shale core samples 10 cm in length and 3.8 cm in diameter were prepared in this study. Pre-fabricated single-fracture and dual-fracture channels were set up. Deionized water or 0.3 % fracturing fluid was injected under confining pressure of 10 MPa, injection pressure of 5 MPa and ambient-temperature conditions, followed by shut-in treatment. T₂spectra were acquired using the LIME-MRI-D2 system. The results show that for the single-fracture sample with deionized water, the long-T₂ fraction was no more than 2.90% over 12~48 h; for the singlefracture sample with fracturing fluid, the long-T₂ fraction ranged from 13.80% to 14.56%. For the dual-fracture sample with deionized water, the long-T₂ fraction increased from 0.07% to 15.24%. For the dual-fracture sample with fracturing fluid, the long-T₂ response remained at approximately 20% during 24~36 h; the value at 48 h is an estimated point and is used only for trend reference. The 0.3 % fracturing fluid and dualfracture configuration tend to enhance intermediate- and long-T₂ liquid-bearing signals. This work presents exploratory NMR experiments; no fluid-loss prediction model is constructed, and fracture initiation or propagation is not discussed.