Formation Mechanism Of Deep Water Borehole Breathing And Optimization Method Of Safe Density Window

Deepwater formations have the characteristics of complex temperature and pressure environment,narrow safety density window,and weak cementation of shallow formations,which lead to obvious borehole breathing during drilling.Borehole breathing is a phenomenon in which the mud is lost when the pump is turned on for circulation,and the mudflows back after the pump is stopped.Since the phenomenon of mud flowback in borehole breathing can easily be mistaken for well kick,wrong well control measures are taken,which seriously affects deep water’s safe and efficient drilling.Therefore,clarifying the induction mechanism of borehole breathing in deepwater drilling,studying the response characteristics of different types of borehole breathing,and optimizing the drilling safety density window are significant for the reasonable control of wellbore pressure in deepwater drilling.To this end,this paper mainly carried out the following research:（1）Based on the stress-seepage coupling method of poroelastic media,and considering the characteristics of wellbore pressure fluctuations,a loss-return flow calculation model for permeability borehole breathing in deep water shallow formations is established.The model reveals the permeability mechanism that induces borehole breathing in deep water shallow formations with high porosity,high permeability,and no fractures,and clarifies the influence of lithology and drilling fluid rheological parameters on permeability borehole breathing.The study found that the flowback of mud in permeable borehole breathing is less than the leakage.The intensity of borehole breathing increases significantly when drilling into formations with higher permeability and porosity.Using drilling fluids with higher viscosity or yield strength can effectively inhibit the occurrence of borehole breathing.（2）Based on the mechanism of opening and closing the fracture system around the well,a borehole breathing experimental device was developed.Using this device,the physical simulation experiment of the whole process of fracture borehole breathing was carried out.The typical characteristics of the entire process of wellbore fluid loss-hold-flowback when borehole breathing occurs are revealed through experiments.The effects of rock type,fracture opening pressure and mud circulation pressure on the leakage and duration of borehole Breathing were studied.The results show that these three parameters are critical for predicting borehole breathing because they significantly impact the maximum leakage and duration of occurrence.Based on the experimental results,a new parameter that can be used to evaluate the strength of borehole breathing quantitatively is proposed:borehole breathing coefficient₍（7）.The intensity of borehole breathing can be suppressed by reducing the value of₍（7） by reducing the mud density or circulating pressure.（3）Based on the Cohesive cohesive force unit method,a 3D geological model for simulating fractured borehole breathing is established,and the 3D simulation of fractured borehole breathing is realized.The dynamic response mechanism of fractured borehole breathing is studied through the selected model,and the influence of lithology and rheological parameters on borehole breathing is analyzed,which supplements the insufficiency of the experiment.The strength of fractured borehole breathing decreases with the increase of the elastic modulus of the formation and the minimum horizontal in-situ stress,and is not sensitive to the changes of Poisson’s ratio and porosity;the leakage increases with the increase of the initial fracture width.Increasing the viscosity of the mud can effectively reduce the intensity of borehole breathing and prolong the duration of the loss and back phases.（4）Based on the permeability and fracture mechanism of deep-water borehole breathing,considering the law of non-Newtonian fluid seepage and coupling the stress and seepage of the dual-porosity medium,a mathematical model that can calculate the leakage and backflow caused by borehole breathing is established.The model is suitable for deep-water shallow permeable formations and deeply fractured formations,and practical cases verify the model.Based on this model,a calculation method of leakage pressure and safety margin considering formation borehole breathing is established.At the same time,the prediction model of formation pore pressure and fracture pressure suitable for deep water drilling is optimized according to the formation mechanism of abnormal formation pressure and the characteristics of deep water formation,forming a safe density window optimization method of deep water drilling considering borehole breathing.The research shows that the safety margin of borehole breathing is restricted and affected by various factors such as the elastic modulus of the formation,Poisson’s ratio,and fracture stiffness.Considering the safety margin,the safety density window can be more accurately.This paper takes the induction mechanism of borehole breathing in deepwater formations as the mainline of research,clarifies the response characteristics of different types of borehole breathing,realizes the optimization of the drilling safety density window,and lays a foundation for the fine control of wellbore pressure in deepwater drilling.It is important to ensure safe and efficient drilling in deep water.