Hydraulic fracture propagation characteristics of deep coal based on acoustic emission and distributed optical fiber sensing

Hydraulic fracture propagation in deep high-stress coal reservoirs is affected by bedding planes and natural fractures. In this study, true triaxial hydraulic fracturing tests were conducted on natural coal specimens using integrated acoustic emission (AE) and distributed optical fiber sensing (DOFS). The effects of horizontal stress difference ( Δ σ ), injection rate, fluid viscosity, and self-suspending proppant (SSP) on fracture propagation and post-fracturing flow capacity were investigated. When Δ σ increased from 5 to 13 MPa, the breakdown pressure increased by up to 22.30%, whereas the fractal dimension of the main hydraulic fracture decreased by up to 9.55%. AE events were concentrated mainly near the dominant main fracture, indicating that a larger stress difference promoted directional main-fracture propagation but reduced fracture complexity. Under Δ σ = 13 MPa, increasing the injection rate from 5 to 30 mL/min increased the breakdown pressure by 3.93% and the apparent permeability coefficient by 76.56%. DOFS revealed more continuous and relatively symmetric fracture-induced strain responses. Increasing fluid viscosity from 10 to 60 mPa·s reduced the overall fracture-network fractal dimension to 2.338, and reduced the COD asymmetry coefficient to 9.22%. The higher-viscosity fluid favored stable main-fracture development but limited bedding-related branching. After the addition of 70–140 mesh SSP, the apparent permeability coefficient was approximately 36.75% higher than that of the 60 mPa·s high-viscosity case. This result suggests that the proppant may contribute to improving hydraulic-fracture flow capacity in coal. These findings provide experimental guidance for optimizing hydraulic fracturing parameters in deep coal reservoirs.