| Formation gas will invade the annulus once a kick occurs during the deepwater drilling practice.The invading gas in the drilling fluid will exist in two states:dissolved gas and free gas.The dynamic distribution of the two kinds of gas would cause a complex response of the annulus flow pattern,which further leads to the erratic change of wellbore pressure.To ensure a safe deepwater well control operation,it is necessary to understand the gas dissolution and diffusion behavior and its influence on the annulus flow pattern and wellbore pressure.In this work,different methodologies were used,including experimental test,molecular simulation and numerical analysis,to study the dissolution characteristics of methane in drilling fluid.The experiments were carried out with different solvents under a wide range of temperature and pressure conditions.In addition,it also investigated the solubility difference of supercritical methane and vapor methane in drilling fluid.According to the components of drilling fluid in the experiments,it established the molecular models for methane/drilling fluid mixed systems.Molecular dynamic simulations were conducted with those models under different temperature and pressure conditions to discuss the dissolution mechanism in micro level.Based on the results of experiment and molecular simulation,the phase equilibrium thermodynamics theory was used to establish a prediction model of phase behavior for methane/drilling fluid system.The equation of state and mixing rule were optimized to fit methane/drilling fluid system.According to the regressed binary interaction parameters from experiments,a high precision model was developed for the binary interaction parameter in methane/drilling fluid system.Based on the above research,considering the influence of dynamic distribution of dissolved gas and free gas in the annulus,formation production and multiphase flow parameters,a transient annulus flow model was established to describe the process of deepwater well control.The heat transfer relations between formation,seawater,wellbore and drilling fluid were discussed by using of Fourier's Law of Heat Conduction,Newton's Cooling Law and energy balance law,and a no-steady temperature model was also presented.The influence of circulation time,flow rate and shut-in time to transient wellbore temperature was investigated.Coupled with corresponding initial condition and boundary condition,the annulus transient flow characteristics and wellbore pressure behavior during deepwater kick,shut-in and conventional killing processes were studied by using the above-mentioned annulus flow model and transient temperature model.The experimental results show that the solubility of methane in oil-based and synthetic-based drilling fluids is much greater than that in water-based drilling fluids.The solubility of methane in drilling fluid increases with the increase of pressure:the higher the pressure,the greater the increment.Methane solubility appears to be insensitive to temperature at low pressures.However,at high pressures,gas solubility increases with the temperature,and the higher the pressure,the greater the influence of temperature is observed.From the microscopic point of view,the dissolution and diffusion behavior of methane in drilling fluid is actually the filling and transition process of methane molecules in the solvent gap.Such a process is restricted by three factors:free volume,interaction energy and dissolution free energy.The results demonstrate that oil-based drilling fluid and synthetic-based drilling fluid can provide more free volume and interaction energy than water-based drilling fluid,and the dissolution free energy is lower than that of water-based drilling fluid.As a result,the methane solubility in oil-based and synthetic-based drilling fluids are much greater than that's in water-based drilling fluid.When the temperature or pressure condition changes,the effect of the three microscopic parameters can be arranged in the following order:interaction energy>free volume>dissolution free energy.The results of phase equilibrium study of methane/drilling fluid system show that the PR equation of state with two-parameter mixing rule can predict the phase behavior of methane-oil based/synthetic based system very well.During the kick process in deepwater drilling,the influx gas exists as dissolved gas in the oil-based drilling fluid and free gas in water-based drilling fluid.Therefore,it is more difficult to detect a gas kick in oil-based drilling fluid.After shut-in,the gas influx stops until the bottom hole pressure increases to the formation pressure in oil-based drilling fluid,and the wellbore pressure remain unchanged.In the water-based drilling fluid,the gas kick stops after the bottomhole pressure returns to the formation pressure,but the free gas continues to slip up and the wellbore pressure continues to rise.Therefore,it cannot maintain long-term shut-in in water-based drilling fluid during well control operation.In the conventional well killing process,the saturated solubility at different well depths in the oil-based drilling fluid is always much greater than the actual solubility in the discharge kick stage,due to the existence of choke pressure.As a result,the annulus is actually under single-phase flow,and the distribution of wellhead pressure during the well killing process is relatively simple.Through the above research,an overall theory for prediction and control was proposed to study the characteristics of the annulus transient flow and the dynamic response of the wellbore pressure during deepwater kick,shut-in and conventional killing processes.The research results can not only deepen the understanding of the dissolution and diffusion behavior of gas in drilling fluid,but also have important theoretical and practical significance for solving the problem regarding deepwater drilling safety. |
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