Carbonate reservoirs, characterized by extensive fractures and cavities, are prone to gravity displacement during drilling when the bottom-hole pressure approaches equilibrium. This phenomenon, driven by density differences between drilling and formation fluids, can result in simultaneous overflow and leakage, posing significant well control risks such as kicks or blowouts. The occurrence of gravity displacement downhole makes its timely detection through conventional annular flow monitoring techniques challenging. This study investigates the triggering conditions and safe density window for gravity displacement in fractured and cavernous formations. Through theoretical analysis and experimental simulation, we examined the displacement mechanisms in both fractured and cavernous conditions. Computational fluid dynamics (CFDs) simulations were used to validate critical fluid column heights for fractured formations and the proposed safe density window. Based on these findings, practical methods to mitigate the hazards associated with gravity displacement overflow are proposed. The results offer valuable guidance for the field identification and mitigation of such incidents, contributing to managed pressure drilling and enhancing drilling safety in complex carbonate reservoirs.
Tao et al. (Sat,) studied this question.