ABSTRACT:

This work presents a cement integrity evaluation experiment to study the failure behavior of cement sheath under cyclic loading. The CT scanning and Nuclear Magnetic Resonance (NMR) are used to investigate the fracture morphology and the changes in pore structure in terms of macroscopic and microscopic views. The results show that the stress presents a nearly linear relationship with strain in each cyclic loading period. However, there is a hysteresis loop between adjacent periods, which means that the plastic deformation occurs after each cyclic loading period. And the plastic strain increases as the number of the loading period increases. The CT scanning indicates the increase of the sample density, but no major damage is observed after the cyclic loading. There is also no major changes in pore size distribution according to the NMR results at the medium peak load. However, the pore size distribution changes significantly at the high peak load. The percentage of larger pores decreases substantially, while the smaller pores increases substantially. The purpose of this experiment is to simulate the stress state of cement sheath during staged fracturing and to provide guidance in assessing the impact of staged fracturing on cement sheath integrity.

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