| Caused by the complex formation pressure system and the difficulty of wellbore pressure control,the formation fluid is easy to invade the wellbore in deepwater drilling.Due to the low temperature and high pressure in shallow formation,and high temperature and high pressure in bottomhole formation,the fluid flow pattern and the law of wellbore pressure variation are complicated when the gas migrates upward along the wellbore,which leads to inaccurate prediction of the bottomhole pressure and causes lost circulation or blowout.Based on the initial form and influencing factors of gas influx,this paper reveals the characteristics of gas suspension and migration in the drilling fluid,hydrate phase transformation and its influence on gas migration velocity,establishes a gas migration model considering the characteristics of yield power-law drilling fluid and hydrate phase transition,and develops a wellbore multiphase flow model for pressure prediction in the well shut-in period.The purpose of this study is to promote the progress of safety control theory of wellbore pressure when gas invasion occurs in deepwater drilling.The growth and detachment of bubble and its influencing factors in non-Newtonian yield pseudoplastic fluid were studied experimentally,and the characteristics of the flow field were analyzed by numerical simulation.Based on the analysis of bubble formation and flow field in non-Newtonian fluid,it was found that the gas bubble growth in the yield pseudoplastic fluid is different with that in the Newtonian fluid.The two-stage of gas bubble growth and detachment wasfound in Newtonian fluid to three-stage in yield pseudoplastic fluid,and the distribution characteristics of fluid velocity,pressure,and viscosity fields around bubbles were obtained.The effects of gas flow rate,pore size,pressure and solute concentration on the gas bubble growth and detachment process were analyzed,and the effects of fluid physical parameters such as static shear force,consistency coefficient and rheological index on the gas bubble formation and detachment morphology were obtained.The results showed that the bubble separation volume increases with increasing the pore diameter;the increase of pressure can promote both the gas bubble separation time and the bubble separation volume;the solute concentration affects the gas bubble growth and separation process via altering the rheological characteristics such as viscosity,and the gas bubble separation time delays with the increase of the static shear force,consistency coefficient and rheological index.Based on the experiment,the empirical formula of the bubble equivalent radius considering yield stress,inertial force,viscous force and surface tension was established with an average error of 0.80%.With regard to the characteristics of gas migration in the deepwater wellbore,bubble suspension and migration characteristics in yield pseudoplastic fluid,hydrate phase transformation,and gas-liquid flow pattern transformation under high pressure were analyzed.The critical suspension criterion of gas bubbles in the yield pseudoplastic fluid was established.The influences of initial bubble size,solution viscosity,solution density and surface tension on the bubble migration velocity were analyzed.The deformation,motion trajectory and rising velocity of the gas bubbles covered by a hydrate film during the rising process were investigated.Moreover,the effects of XG,CMC,hydrate concentration and flow rate on hydrate formation rate were studied.The experimental results showed that the increase in XG,CMC and hydrate concentrations decreased the hydrate formation rate.Furthermore,the critical void fraction increases with increasing the pressure.A new criterion of flow pattern transition in annular was developed,providing better prediction results under high pressure compared to the previous models.Considering the influence of drilling fluid rheology,wellbore temperature and pressure field changes,and hydrate phase transition,a model for velocity prediction of single bubble migration process in wellbore was established.For developing the velocity model of a clean bubble without hydrate shell,a method to determine the viscous drag coefficient and the equivalent radius of gas bubble was experimentally proposed;then a new drag coefficient model was developed considering the bubble deformation,bubble dissolution,and temperature and pressure field changes.Regarding a gas bubble with hydrate shell,a method for determining the bidirectional mass transfer coefficients of water permeation and gas diffusion was proposed,and the pore renewal rate in porous hydrate was quantitatively characterized.Considering the fluid phase transition,gas-liquid flow in wellbore,and mass and heat transfer process between wellbore and reservoir,a theoretical model for predicting the temperature and pressure fields in a shut-in wellbore was established,and the corresponding fully implicit numerical algorithm was proposed.With the simulation case of a deepwater field well in the South China Sea,the fluid migration pattern in a wellbore after gas invasion was analyzed,and the pressure change process was predicted.The simulated results showed that the complicated flow fields such as gas-liquid countercurrent would appear in the wellbore after shut in.Affected by the fluid phase transition,and wellbore-reservoir heat transfer,the fluid temperature in the wellbore will gradually recover to the ambient temperature,which change the thermophysical properties of the fluid and affect the wellbore pressure buildup process.The compressibility of drilling fluid is an important factor affecting the pressure change in wellbore after shut in.Overall,the growth rate of fluid pressure increases with the liquid compressibility in the first and second stage of a well shut-in process;the reservoir permeability mainly affects the flow process in the first stage of a well shut-in process.With the permeability increasing,the rates of gas influx and the wellbore pressure recovery increase;the leakage characteristics of drilling fluid mainly affect the wellbore flow process in the second stage of a well shut-in process.The stronger the leakage of drilling fluid,the slower the change of fluid pressure in wellbore.The pressure underbalance at the bottomhole has no obvious influence on the recovery and buildup trend of wellbore pressure. |
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