Experimental and numerical investigations of accumulated plastic deformation in cement sheath during multistage fracturing in shale gas wells

Abstract

Sustained Casing Pressure (SCP) has become a serious problem in the development of shale gas fields, which is believed to relate to the multistage fracturing. However, few researches were on this, especially the study of micro-annulus at the interface between the cement sheath and casing, which was one of the mechanisms of the cement sheath integrity failure, and always caused by accumulated plastic deformation. This paper presented experimental and numerical investigations of the emergence and development of plastic deformation occurred at the interface between the cement sheath and casing during multistage fracturing. Mechanical tests and physical simulation experiments were carried out so as to analyze the plastic deformation variation characteristic of the cement sheath under cyclical loading and unloading. 3-D numerical models were established to analyze the emergence and development of the accumulated plastic deformation based on the test results, considering the actual engineering and geological conditions during multistage fracturing in shale gas wells, which presented a new method to calculate the width of the micro-annulus between the cement sheath and casing. The research showed that the numerical results were in accordance with the experimental results. With the increase of cycle times, the accumulated plastic deformation increased, which was the greatest for the first time, and then increased linearly. Decreasing elastic modulus and increasing Poisson's ratio, cohesive force and internal friction angle could facilitate the protection of the cement sheath integrity. Finally, a new cement slurry system was proposed based on the results of study and applied in 5 wells with good effects acquired, none of them showed SCP after fracturing.