The influence of solute concentration and temperature of drilling fluids on wellbore failure in tight formation

Abstract

The performance of drilling fluids influences the efficiency of drilling operations. The near-wellbore pore pressure and stress distribution will change with time due to the hydraulic, chemical and thermal potential gradients between the drilling and formation fluids, resulting in wellbore instability during drilling a tight formation. The pore pressure diffusion equation, which considers the effects of hydraulic diffusion, thermal diffusion, and solute diffusion, is presented and the analytic expressions of the near-wellbore pore pressure and stress distribution induced by pore pressure change and temperature stress are presented. An example is used to study the influence of the temperature and solute concentration on the near-wellbore pore pressure and effective stress and the degree of wellbore failure with time. The results show that increasing the solute concentration of drilling fluid reduces the pore pressure but increases the radial and tangential effective stress. In addition, decreasing drilling fluid temperature reduces the pore pressure and tangential effective stress but increases the radial effective stress, both of which are beneficial to wellbore stability. When wellbore failure occurs, increasing the drilling fluid temperature will cause a slight increase of wellbore failure and decreasing the solute concentration may cause a more serious failure in early time.