The objective of this paper is to develop a complete model for determining shale stability which includes thermal and chemical effects in addition to mechanical effects. Thermal effects are found to be important for shale formations because thermal diffusion produces stress alterations which occur more quickly than hydraulic diffusion. Rock temperature and pore pressure are fully coupled for porous media but they can be partially decoupled for shale formations. Chemical effects are treated as equivalent hydraulic potential and are simply added to the hydraulic pressure to consider the effect of mud chemistry. Pore pressure, collapse stress, mud weights, and mud salinities are presented to investigate thermal and chemical effects on shale stability.