Cement sheath integrity of shale gas wells: A case study from the Sichuan Basin

Sustained annulus pressure and casing deformation of gas wells are common in several shale gas demonstration areas in the Sichuan Basin. In view of this, the conditions for pressure change inside the wellbore in the process of drilling were analyzed, and then an elastic-plastic mechanical model of casing-cement sheath-surrounding rock system for calculating the whole change process of wellbore pressure in the stress-strain state was established. By virtue of this model, the generation and development of micro-annulus at the first and second interfaces under various internal pressure conditions were analyzed. Then, combined with the actual field data, calculation was performed. Results show that no micro-annulus is generated in the process of loading, but the loaded internal pressure determines the plastic deformation degree of cement sheath. Besides, the decrease of internal pressure during the unloading exerts tension on the interface, leading to the generation of micro-annulus, and consequently the integrity of cement sheath is damaged. Finally, the mechanical performance of cement sheath can be improved by optimizing the slurry formula, and operation technologies are used to guarantee the integrity of cement sheath in shale gas wells. This model was applied in 13 wells for field tests. It is indicated that the average good quality rate of horizontal section cementing is 92.67%, and 11 wells which have been put into production are not subjected to sustained annulus pressure. It is demonstrated that the sustained annulus pressure of gas wells in shale gas demonstration areas is improved effectively. This research achievement can be used as a reference for integrity management of cement sheath in the similar blocks.