Effects of thermal alternating cycle number on the elastic parameters of set cement

Shale gas reservoirs are commonly developed by means of multi-stage fracturing of horizontal well. In the process of fracturing, the mechanical performance of cement sheath is impacted by the cyclic thermal stress. As a result, the borehole integrity is deteriorated and the casing is deformed or damaged. To solve this problem, the thermal alternating cycle experiment was conducted on set cement to investigate the influential laws of fracturing stage number on casing stress. Firstly, the compressional and shear velocities of set cement after thermal alternating cycle were measured by using an oscilloscope, and the elastic parameters of set cement were calculated. Then, the casing stress was simulated and calculated. Finally, the change laws of set cement elastic parameters and casing stress after different thermal alternating cycle numbers under different cooling conditions were analyzed. It is indicated that the elastic modulus of set cement increases firstly and then decreases gradually to a constant value with the increase of thermal alternating cycle number when low-displacement fracturing is carried out while it decreases continuously when high-displacement fracturing is carried out. The Poisson's ratio of set cement and the casing stress in both fracturing modes increase with the increase of thermal alternating cycle number, and the increase amplitude of casing stress is greater in the mode of high-displacement fracturing. The research results can provide the basis for analyzing the casing stress in the process of multi-stage fracturing of shale gas horizontal wells.