Study on Creep Characteristics and Constitutive Model of Damaged Shale: Based on the Impact of In-situ Methane Explosion

s Methane In-Situ Explosion Fracturing (MISEF) is an innovative technique for enhancing production in unconventional oil and gas reservoirs. It operates by precisely detonating pre-desorbed methane gas within targeted perforation zones, generating high-intensity shock waves that significantly modify the compact structure of shale reservoirs and improve fluid transport capacity. Although MISEF is increasingly applied in shale gas extraction, the influence of explosion-induced damage on shale creep behavior under different load conditions remains insufficiently understood. In this study, a custom-designed MISEF experimental system is employed to conduct graded creep tests on shale samples under six explosion load levels. High-frequency pressure sensors record real-time pressure–time ( p –t) curves, and computed tomography (CT) combined with three-dimensional reconstruction characterizes the evolution of internal pore structures. Strain curves and creep rates under varying stresses are analyzed to clarify the relationship between shale failure strength and both initial and peak explosion pressures. Acoustic emission (AE) monitoring further identifies fracture modes in damaged shale. A nonlinear viscoelastic–plastic damage creep constitutive model is developed, which demonstrates excellent agreement with experimental results and surpasses classical models in describing the time-dependent mechanical behavior of shale. Based on variations in P- and S-wave velocities, the model quantitatively evaluates damage degrees. The constitutive model is further embedded into FLAC 3D to simulate a shale–cement sheath–casing wellbore system, enabling investigation of post-explosion creep behavior and its impact on casing Mises equivalent stress evolution. Results show that higher explosion loads substantially promote pore development and connectivity, accelerate damage accumulation, and reduce the creep failure stress threshold. Shale samples subjected to stronger shocks enter the accelerated creep stage earlier and become unstable under the same stress conditions. The calculated damage degrees range from 0.0361 to 0.188. Numerical simulations reveal that under strong explosion loads, casing reaches the steel yield strength (650 MPa) within 2.8 years, whereas under weaker shocks this occurs after 4.8 years, indicating a service life reduction of approximately 41.67% and a significant acceleration of creep failure.