Geological effects can impact the design and successful completion of oil, gas, and geothermal wells. Understanding the stresses and pore pressures within the subsurface are important to development of a geomechanical model that can guide well design as part of an integrated process to minimize cost and maximize safety. Forces in the Earth are quantified by means of a stress tensor, in which the individual components are tractions (with dimensions of force per unit area) acting perpendicular or parallel to three planes that are in turn orthogonal to each other. The normals to the three orthogonal planes define a Cartesian coordinate system (x1, x2, and x3). The stress tensor has nine components, each of which has an orientation and a magnitude (seeFigure 1.a). Three of these components are normal stresses, in which the force is applied perpendicular to the plane (e.g.,S11 is the stress component acting normal to a plane perpendicular to thex1-axis); the other six are shear stresses, in which the force is applied along the plane in a particular direction (e.g.,S12 is the force acting in thex2-direction along a plane perpendicular to the x1-axis).