This paper presents the development of a model for determining wellbore stability for oil and gas drilling operations. The effects of mechanical forces and poroelasticity on shale behavior are included, as well as chemical and thermal effects.

Chemical effects are caused by the imbalance between the water activity in the drilling mud and the shale water activity. The magnitude of this contribution depends on the effectiveness of the mud/shale system to perform as a semipermeable membrane. Experimental results show that osmotic pressures develop inside shales when they are exposed to different drilling fluids. This osmotic pressure is treated as an equivalent hydraulic potential, and is then added to the hydraulic wellbore and pore pressure as time progresses.

Thermal diffusion inside the drilled formation induces additional pore pressure and rock stress changes and consequently affects shale stability. Thermal effects are important because thermal diffusion into shale formations occurs more quickly than hydraulic diffusion and thereby dominates pore pressure changes during early time.

Rock temperature and pore pressure are coupled for most porous media studies; however, we have found that they can be partially decoupled for shale formations by assuming that convective heat transfer is negligible. The partially decoupled temperature and pore pressure effects can therefore be solved analytically under appropriate initial and boundary conditions. Experimental data for shale strength alteration, which occurs when shales are exposed to different fluids, are also included for the determination of cohesion strength decay.

Pore pressure, collapse stress, and critical mud weights are variables investigated for determining poroelastic, chemical, and thermal effects on shale stability. The most important factors, which affect wellbore stability, are clearly identified.