Cement Sheath Integrity: Simulation and Measures for Improvement at High Temperature Deep Well Conditions

One of the most important purposes of cement sheath study is to establish experimental apparatus and procedure for evaluating the integrity of cement sheath. The working load of cement sheath in a wellbore can be simulated on an artificial cement sheath using equivalent stress method. With the help of core holder, downhole temperature and stress environment can be simulated. By pressurizing the artificial sheath with gas to see if there is gas channeling, the cement sheath integrity can be verified. CT scanning can be used to probe the internal structure of cement sheath, hence to bring to light the failure mechanism of cement sheath. Based on these ideas, a whole set of equipment was developed and a procedure established to evaluate the integrity of cement sheath. Simulation experiments have been carried out using the equipment and the procedure on cement sheaths found in high temperature high stress deep wells in an oilfield in west China. The results of the experiments showed that, at a certain formation pressure in high stress environment, when casing pressure was first increased to a certain level and then decreased, the cement sheath was damaged and even became failed. Once the channels for gas channeling were formed, no matter how low the pressure was inside the casing, gas bubbles can be detected. It was observed that no visible fractures were found on both ends and the side surface of the cement sheath, but the casing string that was tightly bonded with the cement sheath can now fall out freely, indicating that it was the lack of toughness of set cement that resulted in the failure of the bonding between the casing and the cement sheath, producing a micro annular space for the casing to fall. Data acquired with CT software showed that the interface pore volume of the cement sheath sample was reduced by 67.97%, the volume of the micro annular space was 3061 mm³, 9.51% of the total sheath volume, and the total volume of cement sheath was reduced by 2.21%, proving the existence of micro annular space. By modifying the material added to cement slurry, the toughness of the set cement can be improved, and a cement slurry with 0.2% carbon fiber was formulated. The method described in this paper can be used to evaluate the quality of cement sheath, and as a helpful supporting means in modifying cement slurry additives.