Laboratory testing was performed to investigate the behavior of sandstone from a high-pressure/high-temperature (HP/HT) reservoir when subjected to realistic in-situ stresses, pore pressures, and stress changes caused by production. Volumetric changes, stress-path effects, plastic deformation, and strength were included. It was determined that the Terzaghi effective stress definition was most suitable to handle the different mechanisms. Relevant material properties were defined.