Research on the law of mechanical damage-induced deformation of cement sheaths of a gas storage well

Abstract

Although the technique of micro-sphere low-density cement slurry has been effectively applied in solving cementing problems of gas storage wells, little research has been conducted on the mechanical damage and failure of micro-sphere low-density cement sheaths subjected to alternating load in the well. In this paper, a simulative wellbore was used to obtain the experimental phenomena of cement sheaths under periodic loading. Based on such phenomena, the law of mechanical damage-induced deformation of micro-sphere low-density cement sheaths was studied via triaxial cyclic loading, the classic theory of fatigue damage mechanics, low-field nuclear magnetic resonance, the permeability and porosity test and the SEM test. The experimental results indicate that temperature has the greatest impact on the mechanical integrity of micro-sphere low-density cement sheaths and that the damage-induced deformation quantity of the cement sheath at a curing temperature of 90 °C is greater than that at a curing temperature of 60 °C. At a curing temperature of 60 °C, the cement sheath suffered fatigue damage after certain periods of cyclic loading, and the irreversible deformation suddenly increased. For a curing temperature of 90 °C, the cement sheath was less likely to be subjected to elastic deformation because of the many harmful pores within and thus corrupted quickly after loading. The following three damage failure modes of micro-sphere low-density cement sheaths occurred after triaxial cyclic loading and unloading micro-sphere ruptures, cracks between pores, and connected cracks among cement matrices. Therefore, the selection of micro-spheres and the mechanical modification of cement sheaths are key contributors to the integrity of micro-sphere low-density cement sheaths.