Investigation on the integrity of cement annulus during multi-stage horizontal fracturing with FEM/DEM

Abstract

The failure of the cement annulus seriously restricts the process of multi-stage horizontal fracturing, and it exacerbates the failure of casing. The plastic zone of the continuous cement can be determined by FEM with relative accuracy, while the damage process of cement from continuum to non-continuum can be simulated by DEM. Firstly, FEM was employed to simulate the stress distribution across cement, and the stress on each node at the boundary of the plastic area was recorded at each time point. Secondly the continuous elements in the plastic area were transformed into the discrete particles. A microscopic bond contact model for cement was proposed and integrated into a commercial discrete element code PFC2D. The node stress of the finite elements was equivalently applied on the boundary of the particles through the interpolation method, then the continuous loading and unloading process were applied on the particles area again. It is shown that the cement annulus integrity becomes deteriorated when the internal pressure exceeds the average in situ stress on wellbore cross-section. The cement annulus integrity shifted from rapid decline to smooth rise with the increase of the stiffness ratio between casing and cement annulus. The cement annulus was integrity rapidly weakened when the nonuniformity modulus increased, and the maximum number of multi-stage horizontal fracturing is correlated to the strength of cement annulus.