Influence of Cement Defection on Casing Stress Under Cyclic Loading During Multi-stage Hydro-Fracturing

Abstract

The casing integrity problem presents severe challenges for shale gas wells regarding the multi-stage fracturing operation. Multiple factors during fracturing process may have individual or combined influence on it. However, most of the researchers only studied one fracturing stage. The influence of cyclic fracturing loading on casing of shale gas well has been rarely studied. This paper presents a finite element model approach to simulate the multi-stage fracturing operations. Based on the theory of elasto-plasticity, the nonlinear isotropic/kinematic hardening material was used to simulate the response of the casing under cyclic loading. The transient temperature-pressure coupling model of casing-cement sheath-formation (CCF) was established. Multiple analysis steps were used to simulate the multi-stage fracturing processes. The cement channel angle, pump rate, and fracturing fluid temperature were the variables taken into account. An attempt was made to reveal how the factors affect the casing stress. Sensitivity analyses showed that the cement channel had the greatest influence on casing stress. The casing had the highest Von Mises stress and equivalent plastic strain (PEEQ) at the channel angle between 60° and 90°. The stress could be easy to exceed the casing yield stress, causing the casing failure. The more the fracturing stage was, the larger the Von Mises stress and PEEQ of casing were. Larger pump rate tended to dramatically reduce the downhole temperature. The lower the temperature of the fracturing fluid was, the greater the reduction of the downhole temperature was. When the cement channel angle was smaller than 60°, the casing had the higher Von Mises stress and PEEQ for the larger pump rate and lower fracturing fluid temperature. The results indicated that good cement sheath was the basic requirement to ensure the safety of casing. The pump rate and fracturing fluid temperature should be in a reasonable range. During multi-stage fracturing construction, the pressure should be not too high to reduce the casing stress, avoiding the risk of casing deformation.