Remediation of Set Cement in Heavy Oil Thermal Production Wells with CO₂ under Cumulative Working Conditions

In heavy oil fire flooding, the downhole set cement was experiencing steam huff and puff and steam flooding at first, and then heavy oil in-situ combustion. Based on these actual working conditions, an ultra-high-temperature set cement curing device and high temperature high pressure corrosion test kettle were used to study the changing pattern of the compressive strength and corrosion depth of set cement affected by CO₂ under cumulative working conditions. Also, XRD and SEM were used to investigate the effect of CO₂ on the chemical structure and micromorphology of set cement. Experimental results showed that set cement after curing at normal and high temperatures has high porosity and permeability, and low strength. The same set cement, when curing in CO₂ corrosion test kettle under cumulative working conditions, the compressive strength increases to the contrary; the compressive strength of the set cement was increased to 53.4 MPa when curing for 28 days, increasing by 54.87% compared with the compressive strength of the set cement after steam flooding. With the corrosion of the set cement by CO₂ going on, the corrosion depth on the set cement was gradually deepened, and the structure of the set cement was becoming denser, and after 28 days the set cement was fully carbonized. The reason for this phenomenon is that the corrosion product CaCO₃ has low solubility, it precipitates and crystalizes inside the pores in the set cement. The crystal plugs the capillaries in the set cement, or just divides the big pores into smaller pores, thereby increasing the packing density of the cement particles. The results of this study further enrich the industry’s understanding of CO₂ corrosion, and also provide reference to the performance evaluation and composition optimization of the heavy oil in-situ combustion cement slurries.