A Novel Hydraulic Fracturing Apparatus for Hydrate-Bearing Sediment with Multi-Physical Monitoring

Abstract

Marine natural gas hydrate (NGH) reservoirs in deep-sea and permafrost regions are inherently under true triaxial stress states (σ1 ≠ σ2 ≠ σ3) owing to tectonic activities and overpressure fluids. Conventional axisymmetric triaxial tests fail to replicate the in situ conditions, yielding sediment deformation and fracture initiation behaviors that deviate fundamentally from the true reservoir response. Therefore, we present a novel hydraulic fracturing apparatus for hydrate-bearing sediment with multiphysical monitoring, the system features three core innovations: (1) precise high-pressure (70 MPa, ±0.01 MPa) and low-temperature (−10–50 °C, ±0.1 °C) control enabling controlled and reproducible in situ hydrate formation; (2) true triaxial in situ fracture initiation under independently controlled three-dimensional principal stresses; (3) synchronized acoustic-thermal-strain multiphysical monitoring integrating acoustic emission (AE) and distributed fiber optic sensing. This system breaks the “black box” limitation of conventional low-temperature, high-pressure stimulation experiments. Utilizing the system, hydraulic fracturing experiments on specimens with 0% and ∼30% hydrate saturation revealed distinct failure modes: the hydrate-free specimen exhibited ductile failure with a breakdown pressure of 7.44 MPa and a gradual pressure decline, whereas the hydrate-bearing specimen transitioned to brittle failure at 10.52 MPa with a pronounced pressure drop. AE analysis further indicated that high-energy events concentrate along a dominant fracture trajectory in hydrate-bearing specimens, reflecting constrained microcrack coalescence and elevated stored elastic energy. This integrated true triaxial system bridges a critical experimental gap and provides an indispensable tool for unraveling the thermo-hydro-mechanical-chemical coupling mechanisms that govern fracture dynamics in hydrate reservoirs.