Regularities of the process of crack formation in clay filter cake during well cementing

The object of this research is the process of crack formation in the clay filter cake upon its contact with hardening cement slurry / stone. During well cementing, it is impossible to completely remove the clay filter cake formed on the borehole walls. This creates prerequisites for poor wellbore sealing due to the formation of channels for fluid migration at the contact boundary and directly within the clay filter cake. Studying the processes that occur during cementing makes it possible to better understand their nature and to propose technological measures to ensure the tightness of the wellbore seal. It has been established that the process of crack formation is characterized by three periods: induction, cracking, and stabilization. The duration of each period is determined by the state of the “clay filter cake – hardening cement slurry / stone” system. The process proceeds most intensively under conditions corresponding to the near-surface (wellhead) part of the well. This is explained by the dehydration of the cement slurry during its pumping in the annular space and the loose structure of the clay filter cake. In this case, the area of the clay filter cake affected by cracks exceeds 80%, while in conditions of the bottom hole part of the borehole, it does not exceed 30%. The effect of aqueous electrolyte solutions on the crack resistance of the clay filter cake has been investigated. It was found that with a decrease in the concentration of CaCl₂ and an increase in the concentration of NaCl, the area of the clay filter cake affected by cracks decreases. No crack formation was observed in the clay filter cake after its treatment with 2% and 5% solutions of Na₂CO₃. It was established that with a decrease in the thickness of the cement sheath, the induction and cracking periods increase, while the overall area of the clay filter cake affected by cracks decreases. The obtained results will serve as a basis for developing a comprehensive approach to ensuring high-quality wellbore sealing. This may include optimizing well design and improving the formulation of drilling fluids for specific geological and technical conditions.