Dynamic Fracture Monitoring Technique in Deep Coal Seams Based on a Time-Varying Electric Field Method.

The efficient development of deep coalbed methane (CBM) is constrained by unclear hydraulic fracture propagation mechanisms and the limited resolution of existing monitoring techniques. Conventional methods such as microseismic monitoring and static resistivity imaging are unable to provide real-time, high-resolution characterization of dynamic fracture growth and fluid migration, thereby impeding accurate evaluation of stimulation effectiveness. To address these limitations, this study introduces a novel dynamic fracture monitoring technique based on a time-varying electric field method. By establishing a coupled electric field-fracturing fluid flow model and deploying a high-density downhole electrode array integrated with real-time data processing, the technique enables millisecond-level signal acquisition and dynamic inversion of fracture geometry. Field validation demonstrates that the method dynamically tracks fracture propagation with a spatial accuracy of 0.5 m and temporal resolution better than 5 min. The monitoring results quantitatively delineate a three-stage fracture behaviorcharacterized by nonuniform initiation, differential extension, and dynamic reorientation among clusterswith the maximum single-cluster fracture length reaching 214.95 m. The interpreted fluid distribution and fracture evolution show strong consistency with independent high-frequency pressure-wave analysis of cluster-level fluid intake, confirming the accuracy of the method. Furthermore, comparison with fracture inversion reveals the presence of stress-extended zones not penetrated by fracturing fluid, highlighting the discrepancy between the actual stimulated volume and the fluid-swept region. This reflects the pronounced heterogeneity and corresponding stress-propagation characteristics inherent to deep coal reservoirs. The method offers a robust technical approach for evaluating stimulation effectiveness and optimizing fracturing design parameters in deep coalbed methane reservoirs, further contributing to the large-scale development and production breakthroughs in deep coal seams.