Simulation Study of Cement Plug Micro-Annulus in Milling Section of Abandoned Wells

When the integrity of the annular barrier outside the casing is degraded, parts of the casing in the abandoned well must be milled to place a permanent barrier. Considering the micro-annulus induced by accumulation fluid, this paper developed a 3D finite element model of cement plug in the milling section. Monte Carlo methods evaluated the fluid accumulation rate as a part of the model. The model simulated the micro-annulus propagation at the cement-formation interface under the fluid-driven force. The influence of milling position and reaming radius was analyzed. Meanwhile, the influence of the Poor Bonding Zone (PBZ) was analyzed based on a new method of simulating bonding quality. Simulation results show that the accumulation fluid affects the interface stress state and induces the micro-annulus. The initial stress of the interface is non-uniformly distributed under non-uniform in-situ stress, and the micro-annulus tends to expand along the axial direction. The selection of low Young's modulus, high Poisson's ratio, and low permeability formation region for milling is beneficial. The micro-annulus is highly sensitive to PBZ, and reducing the proportion of PBZ on the bonding face is significant in preventing barrier leakage. The results of this paper are essential for permanent barrier design and prevention of leakage.