Study On Multiphase Flow After Gas Kick In Deep Water Drilling

Although a lot of researches have been done on the multiphase flow in the wellbore,but there are a few researches on multiphase flow in large diameter annuli,dissolution and precipitation of gas in oil based drilling fluid,unsteady multiphase flow solving method.In order to provide support for deepwater drilling wellbore pressure control technology and well control hydraulic parameter design,this paper mainly studies four aspects,respectively,quantitative calculation of diffusion gas influx,flow pattern independent void fraction correlation,unsteady multiphase flow model considering gas dissolution/precipitation,pressure based numerical solution method for unsteady multiphase flow model.Diffusion gas influx is easy to happen in deepwater oil based mud.The gas intrusive channel is divided into five parts,namely the interface between gas and filtrate,mud invasion zone,inner filter cake,outer filter cake,interface between filter cake and mud.Based on Fick's law of diffusion,the expression of total mass transfer coefficient is derived.Considering concentration changes with time,concentration changes caused by drilling fluid flow,concentration changes caused by gas diffusion in axial direction,and concentration changes caused by gas diffusion from reservoir to wellbore,the gas diffusion influx governing equation is established.Based on the analytic solutions of the governing equation,the calculation equations of diffusion gas influx variables are obtained.Effects of wellbore length,porosity,drilling fluid leakoff on diffusion gas influx are analyzed.The total intrusive gas volume in the wellbore is linear with the length of the reservoir.The increase of the porosity of the reservoir promots the diffusion of gas,but this promotion is very weak.The drilling fluid leakoff has a significant inhibitory effect on gas diffusion.Two equations are proposed for the parameters of flow pattern independent void fraction correlation,namely,the distribution parameter and the drift flux velocity.In the distribution parameter equation,mean void fraction and two phase flow Reynold number are used to characterize the distribution of void fraction and velocity profile.In the drift velocity equation,the Harmathy formula and Dumitrescu formula are combined.The accuracy of the proposed correlation is compared with 19 frequently used correlations in literature.Based on the comparison,it is found that the proposed correlation predicts the void fraction consistently with a better accuracy.Based on dissolution experiment and theoretical analysis,using the gas concentration changes in time and space to describe dissolution/precipitation of gas,the unsteady multiphase flow model is established.To overcome poor efficiency of explicit method and convergence problems of implicit method,a new pressure based method is proposed,in which the staggered grid finite volume method and the fully implicit backward Euler method are adopted for the spatial discretization and time integration schemes.The proposed method combines iteration and traversal to ensure convergence of the algorithm.Test case comparison with Roe solver proves that the proposed method has a good performance and a good adaptability to different space and time steps.A full scale experiment is conducted to demonstrate the successful implementation of the pressure based method and drift flux correlation.It is shown that the mean relative difference between experimental data and simulation data is 1.18% and maximum relative difference is 7.09%.Combined with the basic data of a deepwater well drilled in South China Sea,the law of multiphase flow in wellbore is analyzed under three situations of displacement gas influx,negative pressure gas influx and diffusion gas influx when using oil based drilling fluid.In the situation of displacement and negative pressure gas influx,gas dissolution reduces the void fraction,gas migration velocity and pit gain.The gas kick cannot been detected effectively by monitoring pit gain and out flow rate through compound logging.In the situation of diffusion gas influx,when the drilling fluid is circulating,there exists free gas at the wellhead,however the gas only exists in the wellbore near the wellhead.The existence of gas does not reduce the bottom hole pressure obviously.If drilling fluid circulates after a long period of rest,when the high concentration of dissolved gas at the bottom hole reaches the wellhead,the precipitation will cause the pit gain increase significantly,and cause the bottom hole pressure decrease significantly.The significant increase of pit gain may lead to the wrong judgement of the negative pressure or replacement gas influx.The significant decrease of bottom hole pressure may cause the negative pressure gas influx happen.To avoid above problems,It is recommended to apply 3MPa pressure back to restrain the excessive precipitation of gas in the process of circulating dissolved gas.