Study on the Influence of Salt Rock Creep on the Integrity of Cement Sheath Gas Seals

Salt rocks are prone to plastic deformation or creeping flow during drilling and cementing due to their solubility and plasticity. This may result in irregular wellbores and deformed or even collapsed casings, which influences air tightness of wellbore in salt rock interval and thereby threatening the normal production and safety of oil (gas) wells. This study aimed to provide a theoretical basis for air tightness evaluations of wellbores in salt rock intervals. For this purpose, the influences of salt rock intervals on the air tightness of the first and the second interfaces in cementing were analyzed by means of petrophysical experiments and three dimensional (3D) finite element simulations. The following results are obtained: The first interface is superior to the cement stone itself in gas sealing ability, and the ability of the second interface can be enhanced from salt rock creep. The gas sealing ability of salt rock intervals is mainly depends on the sealing ability of the second interface and cement stone itself. The gas sealing ability of salt rock intervals has a complex positive correlation with interface contact pressure. A model for quantitative evaluation of gas seal pressure at the second interface in cementing was built based on the analysis results. With this model, the air tightness of salt rock intervals in target areas can be evaluated, and it can provide reference for the cementing in similar intervals in other areas.