Sustained casing pressure (SCP) in gas wells brings a serious threat to worker safety and environmental protection.According to geological conditions,wellbore structure and cement data of gas wells in the Sichuan-Chongqing region,China,the position,time,environmental condition and the value of SCP have been analyzed.On this basis,the shape of the pressure bleed-down plot and pressure buildup plot were diagnosed and the mechanism of SCP has been clarified.Based on generalized annular Darcy percolation theory and gas-liquid two-phase fluid dynamics theory,a coupled mathematical model of gas migration in a cemented annulus with a mud column above the cement has been developed.The volume of gas migrated in the annulus and the value of SCP changing with time in a gas well in Sichuan have been calculated by this model.Calculation results coincided well with the actual field data,which provide some reference for the following security evaluation and solution measures of SCE