Study On Gas Distribution Coefficient In Non-Newtonian Fluid And Its Influence On Multiphase Flow In Wellbore

The drilling fluid used in the oil drilling process is a Non-Newtonian fluid.Compared with Newtonian fluid,it has the characteristics of yield stress and shear thinning.The existence of yield stress will make the bubbles suspend in the drilling fluid;the shear thinning property will cause the initial size distribution of the bubbles generated when the gas penetrates the drilling fluid to change,and it will also cause the radial distribution of the rising bubble velocity and the gas content,and the bubble size distribution in the drilling fluid change.The difference in gas distribution status makes the gas distribution coefficient in Non-Newtonian fluids different from those in Newtonian fluids.When the Non-Newtonian fluid properties of drilling fluids are strong,the wellbore pressure calculated by using the gas distribution coefficient in Newtonian fluids is not accurate,so it is necessary to develop research on gas distribution coefficient in Non-Newtonian fluids.In this paper,the influence of gas flow rate and rheology of xanthan gum solution on the initial size distribution of invading bubbles was studied by experiments.The experiment found that the larger the gas flow rate,the larger the bubble diameter distribution range,and the larger the bubble diameter value corresponding to the maximum probability;the stronger the shear thinning of the xanthan gum solution,the larger the minimum diameter of the initial bubble generated by the porous medium,and the largest probability of bubble diameter The corresponding bubble diameter value is also larger.On this basis,considering the radial seepage of the borehole wall and the high pressure at the bottom of the borehole,a prediction model of the initial diameter of the bubbles generated at the bottom of the borehole during gas invasion was established.The error between the experimental data and the model is within 10%.Through the gas distribution characteristic experiment in the wellbore,the characteristics of the shear thinning and apparent gas velocity of the xanthan gum solution on the size distribution,velocity distribution and gas content distribution of the gas in the xanthan gum solution were analyzed.The bubble size distribution satisfies the lognormal distribution model;the radial distribution of bubble velocity satisfies the form of a quadratic function;the radial distribution of gas content is in the form of a power law function.Based on the characterization methods of bubble size distribution,bubble radial velocity distribution and gas content radial distribution,combined with the definition of gas distribution coefficient,the gas distribution coefficient in the Non-Newtonian fluid is calculated.Under the experimental conditions,the gas distribution coefficient in Non-Newtonian fluids can reach up to 3.5,which is much larger than the gas distribution coefficient in Newtonian fluids(usually 1.2).The effect of yield stress on the gas distribution coefficient is that the yield stress will cause some bubbles to exist in the liquid in a suspended state,which affects the distribution characteristics of the gas content;shear thinning increases the degree of coalescence of the bubbles,resulting in an increase in the gas distribution coefficient.Considering the influence of the above factors on the gas distribution coefficient,a calculation model for the gas distribution coefficient in a NonNewtonian fluid is proposed for the first time.Based on the gas distribution coefficient calculation model and the initial bubble diameter prediction model generated at the bottom of the well during gas invasion,combined with the basic data of the example well,the wellbore pressure after gas invasion was simulated under shut-in conditions.The results show that the greater the gas distribution coefficient,the greater the gas rising speed,and the greater the increase in bottom hole pressure compared with Newtonian fluid;when the flow index is 0.2 and the well is shut in for 3 hours,compared to Newtonian fluid drilling fluid,the bottom hole pressure increase has increased by about 35.7%,and the possibility of formation leakage has increased.By analyzing the influence of gas distribution coefficient on wellbore pressure calculation,it provides a theoretical basis for shutin wellbore pressure calculation.