Dynamic Instability Characteristics of Sandstone-Mudstone Combinations During Gas Drilling in Tight Sandstone Gas Reservoirs

Summary Gas drilling utilizes compressed nitrogen rather than mud as the drilling fluid to fundamentally eliminate reservoir damage, making it a highly promising technology for tight sandstone gas reservoir development. However, wellbore instability remains a significant challenge, particularly in interbedded sandstone-mudstone formations or zones containing continuous mudstone interlayers. This study investigated the dynamic failure characteristics of sandstone-mudstone composite formations under impact loading. The dynamic strengths of composite specimens (with sandstone-mudstone thickness ratios of 1:4, 2:3, 2.5:2.5, and 3:2, denoted as G1–G4) were evaluated using a 50-mm-diameter split Hopkinson pressure bar (SHPB) system. Crack propagation was analyzed through strain fields captured by digital image correlation (DIC), while combined discrete element method (CDEM) simulations were used to investigate crack initiation and propagation. In addition, strain-rate magnitudes near the wellbore were estimated from gas influx-induced pressure variations to validate the applicability of the SHPB method for simulating transient downhole dynamic loading. Results indicate that the dynamic strength increased with strain rate, and specimens with thicker sandstone layers displayed greater strain-rate sensitivity. For G1 specimens with thicker mudstone, peak strength increased from 33.93 MPa to 59.4 MPa. G3 specimens with equal sandstone and mudstone thickness showed a more pronounced strength increase from 48.21 MPa to 91.93 MPa. When the sandstone thickness exceeded that of the mudstone, the overall strength decreased slightly, indicating a reduced synergistic deformation capability between the layers. The complexity of crack propagation increased with strain rate. At higher strain rates, cracks initiated predominantly at the interface rather than within the mudstone, and propagation paths became more intricate, shifting the failure mode from tensile to mixed tensile-shear. Increasing sandstone layer thickness enhanced interfacial strength, preventing crack penetration into sandstone layers and altering failure patterns. Energy analysis revealed that the dissipated energy ratio initially decreased with increasing sandstone thickness, reaching a minimum of 29.2% at balanced layer thickness. The results provide new insights into the dynamic instability of interbedded sandstone-mudstone formations during gas drilling in tight sandstone gas reservoirs.