Research On Relationship Of Gas Kicking And Wellbore Pressure In Carbonate Formation Under Managed Pressure Drilling

With the increase of energy demand and continuous advancement of technology,carbonate reservoirs have occupied an important position oil and gas exploration in China.However,because of it's strong heterogeneity and fracture caves exists,gravity displacement gas kicking happens a lot.But it's difficult to identify it in time and regular well control measures is ineffective to deal with it and can easily lead to complex drilling accidents.It has been seriously affected the development of carbonate reservoirs.In this paper,the critical fracture height is deduced by investigating the gas-liquid flow in fracture under steady-state flow conditions.If the fracture height of carbonate is smaller compare to this value,there exists a safe drilling window;otherwise,it is the gravity displacement window;then analysis was conducted and studies have shown that increasing the viscosity of mud can inhibit the occurrence of gravity displacement.Based on the VOF and gas-liquid two-phase flow model,the gas-liquid two-phase flow model for gravity displacement gas kicking and wellbore flow in carbonate formations is established in this paper.Through coupled solutions method,gravity displacement gas kicking and underbalanced gas kicking are simulated and compared.It shows that the bottomhole pressure rises rapidly and then gradually decreases when gas invasion occurs in both type gas kicking.However,the initial gas kicking volume due to gravity displacement is larger,so the bottom hole pressure changes violently,but the gas kicking volume is rarely affected by the bottom hole pressure and basically keep constant.Underbalanced gas kicking decreases with bottomhole pressure increasing.Based on managed pressure drilling process,this paper comprehensively considers the bottom hole pressure and the increment of mud pools and other parameters,establishes the detection criteria for gas kicking of carbonate formations,so gas kicking can be detected in time and also gas kicking types.Based on mass flow model for wellbore.simulations for two types gas kicking control were performed,the results show that conventional managed pressure drilling has a good result on underbalanced gas kicking control,but it has no effect on gravity displacement gas kicking,and pressured mud cap drilling can effectively control complexed situation caused by gravity replace gas kick.The results of this paper can provide theoretical guidance for the safe and efficient drilling in carbonate rocks.