Hydrate Phase Transition in Sediments Monitored by Fiber Bragg Grating Sensors: Theory and Methods

Natural gas hydrates play a critical role in energy resource development and geological stability. Their phase transition is accompanied by significant volume expansion and thermal effects. To enable in situ, high-precision monitoring of this process, this study develops a multiparameter sensing system based on fiber Bragg grating (FBG) sensors. Water–ice freezing–thawing experiments were first conducted as a reference system to characterize baseline liquid–solid phase transition-induced strain behavior in sediments. Subsequently, temperature-controlled formation and dissociation experiments of tetrahydrofuran (THF) hydrate were performed to systematically investigate hydrate-induced strain responses. The effects of sediment grain size, hydrate saturation, and the spatial positioning of FBG sensors on phase change responses were analyzed, and the evolution of strain rate and spectral response during phase transitions was thoroughly investigated. Results demonstrate that FBG sensors can sensitively capture strain fluctuations induced by phase transitions, with strain amplitude positively correlated with hydrate saturation. Fast Fourier Transform (FFT) analysis of strain-rate signals successfully identified characteristic frequency bands and energy distribution patterns associated with phase transitions in sediments of different grain sizes. Furthermore, finer-grained sediments and higher hydrate saturation levels are associated with markedly larger strain amplitudes and more pronounced strain-rate responses during phase transition processes. Finally, a quantitative inversion model linking FBG parameters to hydrate saturation was developed, and the predicted values showed excellent agreement with experimental data. This study demonstrates the reliability of FBG-based monitoring for tracking hydrate evolution and provides essential experimental support for geological risk assessment during hydrate extraction.