Abstract: Gas accumulation and migration are common in marine sediments, impacting the safety of subsea infrastructure, the evolution of submarine morphology, and potential greenhouse gas leakage. This study developed a dual-cylinder plunger pump for precise long-term gas injection to investigate gas breakthrough, migration, and associated deformation in saturated sands. Injection tests were conducted on soil samples with particle sizes of 0.15-0.3 mm (Group SG) and 0.3-0.6 mm (Group LG). Results show a strong linear correlation between breakthrough pressure and flow rate for the same grain size. Analysis indicates that the breakthrough pressure difference between the two grain sizes approximates the theoretical minimum capillary force calculated from linear interpolation of the coordination number for different particle arrangements. Regarding deformation patterns: In SG and LG under high injection rates, gas initially invades via capillary invasion, followed by fracture invasion. At low injection rates, LG exhibits only capillary invasion. Two dimensionless parameters, ΧC and ΧE, were introduced to quantify the influence of capillary forces and excess pore pressure. ΧC governs flow direction during the capillary invasion and the gas saturation of the soil. ΧE>1 is a necessary condition for fracture invasion and influences the height of the initial fracture onset. At high injection rates in SG, a downward trend in injection pressure suggests soil failure and fluidization. V-shaped features seen in layered soil in long-term gas injection tests can provide evidence for hypotheses related to stratigraphic deformation caused by repeated eruptions of fluid in conduits.