Analysis on cement sheath integrity under transient thermo-mechanical coupling effect

The practical shale gas well engineering shows that casing fracturing can easily destroy the integrity of cement sheath, leading to sustained annular pressure (SAP). In this paper, the wellbore temperature feld calculation model and the fnite element model of casing-cement sheath-formation assembly were established based on the actual casing fracturing engineering. The effect laws of transient thermo-mechanical coupling on the magnitude and distribution of cement sheath stress were calculated by using analytical method and numerical method comprehensively. It is shown that the temperature difference between inside and outside the wall of cement sheath increases frstly and then decreases in the process of fracturing. Steep temperature gradient occurs near the inside wall at the initial fracturing and it tends to result in obvious increase of stress on the inside wall. Due to the effect of transient thermo-mechanical coupling, the stress on the inside wall of cement sheath is increased signifcantly, and the risk of cement sheath integrity failure is aggravated.The initial fracturing stage is the "risk period" when cement sheath failure tends to happen easily. As time goes, the maximum stress on the inside wall of cement sheath results in multiple cracks easily and makes the SAP more serious. The research results can provide the reference for the design and control of cement sheath integrity during the fracturing of shale gas well.