Effects of water-based drilling fluid on properties of mud cake and wellbore stability

The challenge of wellbore stability is getting harder in complex formations. A better understanding of mud cake and its effect on stress state around the wellbore is beneficial for the solution of wellbore stability problems. Thus, the testing method and describing function of dynamic mud cake properties were developed with different drilling fluid additives. The microstructure of mud cake was investigated to further illustrate the mechanism of drilling fluid additives. Based on the poroelastic model, the effect of time-dependent mud cake properties on the distribution of pore pressure, effective stress state around the wellbore and safe mud weight window was analyzed. The results indicate that the buildup of mud cake slows down the increase of pore pressure and broadens the safe mud weight window by increasing the effective stress of compression. The significant increase in fracturing pressure illustrates the potential effect of mud cake in wellbore strengthening. Although the pressure isolation effect increases with the addition of bentonite, it has a limited influence on the safe mud weight window. The addition of Xanthan (XC) enhances the pressure isolation effect of mud cake by improving the coalescence stability of fine-grained particles and plugging the pore with hydration membrane. The viscoelastic granular plugging material has the best sealing efficiency with the deformation property. Although the fine-grained walnut shell has a relatively good pressure isolation effect, its effect on borehole cleaning is far greater than that on wellbore stability.