Mud Shale Wellbore Stability Study

The study on shale wellbore stability is the engineering problem which is fully concerned about and not solved well in drilling region both home and broad, and it is also a grand problem of hindering the exploring and exploiting velocity. The problem of shale hydration plays a very important role in the study on shale stability, which is the true reason of shale instability. Therefore, it is extremely important to study the mechanisms of shale hydration and search for efficient restraining measures.The dissertation's main purpose is to study and analyze the shale wellbore stability, especially the effect of hydraulic difference, chemical potential and temperature after the drilling fluid coming into contact with shale formations. Through studying and analyzing, following are the major points and achievements presented in this dissertation: (1) systematically analyzing the internal factors of shale hydration by studying shale compositions and physical properties; (2)determining the pore pressure and horizontal stresses by means of logging method; (3) determining the rock mechanics parameters with experimentations and logging data, simulating water absorption diffusion with the thermal diffusion model, studying the moisture content of rock around the wellbore with the equation of mass conservation, studying the effect of shale hydration on rock mechanics parameters; (4)studying the distribution of rock stresses around the deviated hole, establishing the mathematical model and analyzing the wellbore stability; (5)studying the coupling flow of water and ions into and out of shale formations due to the difference in chemical and hydraulic potentials with the no ideal semi-permeable membrane and continuity equation, studying the change and distribution of ion density and pore pressure, quantitatively analyzing the influence of coupling flow on the rock stresses, and analyzing the shale stability; (6) quantitatively analyzing the effect of temperature difference between drilling fluid and formation on the shale stability.