Study on the Mechanism of Expansive Self-Healing Additives for Oil Well Cement

Micro-fractures developed inside set cement in an oil or gas well seriously affect the isolation integrity between borehole wall and casing string, thereby shortening the life of the well. Self-healing of micro-fractures in set cement is an effective way of resolving this problem. Studies on the mechanisms of expandable self-healing agents have seldom been conducted until now and the self-healing mechanisms lack theoretical support. This paper presents fill factor as an indicator to evaluate the self-healing of set cement, builds a mathematical model describing the sealing of micro-fractures in set cement by expandable self-healing agents, and explains main factors affecting the self-healing of set cement. A simple quantitative method for simulating the micro-fractures developed in set cement was designed based on Brazilian disk test. Several self-selective healing agents were evaluated for their self-healing ability using set cement micro-fracture self-sealing evaluation device, verifying the relationship between the self-healing effect and the amount of liquid adsorbed by self-healing agent, concentration of self-healing agent, as well as the width of micro-fracture. It was found that the selfselective healing agent, when mixed with water, had critical self-healing fill factor of 0.473, and the healing agent is, at most, able to seal fractures of 234 μm in width. The self-selective healing agent when mixed with oil, had critical self-healing fill factor of 0.490, and it is, at most, able to seal fractures of 150 μm in width. The studying results lay a theoretical foundation for the use of expandable self-healing agents in sealing micro-fractures developed inside set cement.