Wellbore Breathing Experiment And Safety Density Window Analysis Of Deepwater Drilling

Wellbore breathing is a phenomenon often encountered in deepwater drilling,but it is often overlooked or mistakenly considered to be a surge caused by formation fluids invading the wellbore.It is especially important when the safety density window of the formation is narrow,which can easily cause complex drilling conditions.Therefore,it is of great practical significance to study wellbore breathing.This paper systematically expounds the basic characteristics of wellbore breathing,analyzes its mechanism,and studies its identification,prevention and suppression methods.A set of indoor wellbore breathing experiments was designed and established.The wellbore breathing was simulated in the laboratory,and the characteristics of wellbore breathing under different formation conditions and under different operating conditions were studied.In this paper,a mathematical model of wellbore breathing for a single isolated deformable rectangular fracture is established,a calculation method for safety margin of wellbore breathing is proposed,and the influence of safety margin of wellbore breathing on the safety density window of deep water drilling is studied.Wellbore breathing simulation experiments well simulate the characteristics of wellbore breathing.The results are as follows.The drilling fluid throughput produced by wellbore breathing increased significantly with the increase of drilling fluid circulation pressure,and decreased significantly with the increase of fracture closure pressure.Wellbore breathing is very sensitive to the types of rocks that make up the fracture.Under the same pressure conditions,the denser the rock,the more obvious the wellbore breathing and the longer the duration.A coefficient for assessing the severity of wellbore breathing is proposed,namely the wellbore breathing coefficient?_b,whose size is directly proportional to the severity of wellbore breathing.The analysis of the safety density window of deepwater drilling based on wellbore breathing shows that:due to the poor cementation,shallow structure and low strength of shallow rock formations in deep water,wellbore breathing mainly affects the upper limit of the drilling fluid density window in deep water shallow formations.Wellbore breathing will reduce the upper limit of the drilling fluid density window by0.01-0.15g/cm~3,and the smaller the depth of the well,the more the upper limit of the drilling fluid density window decreases,and the effect is most obvious near the mudline.Considering the safety margin of wellbore breathing in deepwater drilling can more accurately determine the safety density window.