Design and application of a triaxial hydraulic fracturing experiment combining distributed optical fiber strain and acoustic emission monitoring

Understanding the morphology of hydraulic fractures is fundamental to the scientific design of hydraulic fracturing treatments. Because natural rock cores are opaque, it is often difficult to observe the complete fracture propagation process in laboratory-scale physical simulation experiments on hydraulic fracturing. This study aims to combine multiple conventional monitoring techniques to clearly visualize fracture propagation dynamics, further elucidate the underlying experimental mechanisms, and thereby enhance the teaching of hydraulic fracturing.

To clearly reveal fracture morphology and the fracture propagation process, a method integrating distributed optical fiber strain monitoring and acoustic emission monitoring was developed. Distributed fiber optic strain monitoring based on optical frequency domain reflectometry (OFDR) was used to capture fracture propagation dynamics. This technique offers high spatial resolution, typically on the order of millimeters, and strain measurement accuracy up to 1 με. A fiber strain waterfall plot was generated from the acquired data, and tensile and compressive strain convergence zones were identified to analyze post-fracturing fracture morphology. Specifically, the presence of a tensile strain convergence zone flanked by two compressive strain convergence zones in the waterfall plot indicates fracture initiation and propagation. Acoustic emission monitoring was employed to characterize the stimulated reservoir volume by counting microseismic events generated during fracture propagation. First, grooves were cut in the coal rock surface around the drilled wellbore, and optical fibers were embedded in the grooves. After securing the optical fibers, the coal rock was cast with cement, thus yielding an embedded fiber configuration. Following the hydraulic fracturing experiment, the sample surface was observed and photographed, and rock breaking was performed along the surface fractures for post-fracture characterization. A fluorescent tracer was added to the fracturing fluid to accurately identify the fractures generated by hydraulic fracturing. The study also investigated the effects of different fracturing fluid injection rates on coal rock fracture propagation dynamics and fracture morphology. Pressure–time curves were used to characterize rock breakdown and fracture propagation pressures.

The combined monitoring method achieved the detailed characterization of fracture propagation dynamics during hydraulic fracturing in natural rock cores. The results corresponded well with those obtained from post-fracture rock breaking. A comparative analysis of the physical simulation results of coal rock hydraulic fracturing under different fracturing fluid injection rates revealed the following: (1) A higher injection rate results in a higher breakdown pressure. (2) A lower injection rate tends to produce more complex and more heterogeneous fracture networks. (3) A higher injection rate yields a larger stimulated reservoir volume, indicating more effective reservoir stimulation.

This study designed a true triaxial hydraulic fracturing experimental system and methodology that combines OFDR-based distributed optical fiber strain monitoring and acoustic emission monitoring. The proposed approach can visualize the dynamic fracture propagation process, including fracture initiation, propagation, and intersection with the optical fiber. The post-fracturing fracture morphology can be analyzed using tensile and compressive strain convergence zones in the fiber strain waterfall plot. By integrating acoustic emission to characterize the stimulated reservoir volume, the study provides a comprehensive evaluation of the hydraulic fracturing fracture propagation mechanism. Thus, it holds remarkable value for understanding and teaching fracture propagation dynamics and post-fracturing morphology in hydraulic fracturing.