![]() ![]() Pore pressure is the pressure at which the fluid contained within the pore space of a rock is maintained at depth. 4-Diagram illustrating the three faulting states based on Andersonian faulting theory (courtesy GeoMechanics Intl. However, local perturbations, both natural and manmade, are important to consider for application of geomechanical analyses to drilling and reservoir engineering ( Fig. Because the largest components of the stress field (gravitational loading and plate driving stresses) act over large areas, stress orientations and magnitudes in the crust are remarkably uniform ( Fig. Human activities such as mining and fluid extraction or injection can also cause local stress changes. These are modified by the locally-acting effects of processes such as volcanism, earthquakes (fault slip), and salt diapirism. Gravitational loading forces include topographic loads and loads owing to lateral density contrasts and lithospheric buoyancy. ![]() Plate driving forces cause the motions of the lithospheric plates that form the crust of the Earth. The processes that contribute to the in-situ stress field primarily include plate tectonic driving forces and gravitational loading (see Table 1). 1.b-The vertical ( S v) and horizontal maximum and minimum stresses ( S Hmax and S Hmin), which are usually, but need not be, principal stresses (courtesy GeoMechanics Intl. Because these horizontal stresses almost always have different magnitudes, they are referred to as the greatest horizontal stress, S Hmax, and the least horizontal stress, S Hmin ( Fig. This requires that the other two principal stresses act in a horizontal direction. It has been found in most parts of the world, at depths within reach of the drill bit, that the stress acting vertically on a horizontal plane (defined as the vertical stress, S v) is a principal stress. Coordinate transformations between the principal stress tensor and any other arbitrarily oriented stress tensor are accomplished through tensor rotations. The magnitudes of the principal stresses are S 1, S 2, and S 3, corresponding to the greatest principal stress, the intermediate principal stress, and the least principal stress, respectively. Inc.).Īt each point there is a particular stress axes orientation for which all shear stress components are zero, the directions of which are referred to as the “principal stress directions.” The stresses acting along the principal stress axes are called principal stresses. 1.a-Definitions of the stress tensor in Cartesian coordinates, tensor transformation through direction cosines, the principal stress axes (courtesy GeoMechanics Intl. In all cases, S ij = S ji, which reduces the number of independent stress components to six.įig. Three of these components are normal stresses, in which the force is applied perpendicular to the plane (e.g., S 11 is the stress component acting normal to a plane perpendicular to the x 1-axis) the other six are shear stresses, in which the force is applied along the plane in a particular direction (e.g., S 12 is the force acting in the x 2-direction along a plane perpendicular to the x 1-axis). The stress tensor has nine components, each of which has an orientation and a magnitude (see Fig. The normals to the three orthogonal planes define a Cartesian coordinate system ( x 1, x 2, and x 3). 5.2 Stress constraints owing to shear-enhanced compactionįorces 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.5.1 Stress constraints owing to frictional strength.2 Relative magnitudes of the principal stresses in the earth.The rake always sweeps down from the horizontal plane. One might also expect to see this used when the particular line is hard to measure directly (possibly due to outcrops impeding measurement). In these cases the rake can be used to describe the line's orientation in three dimensions relative to that planar surface. The rake is a useful description of a line because often (in geology) features (lines) follow along a planar surface. The three-dimensional orientation of a line can be described with just a plunge and trend. ![]() In structural geology, rake (or pitch) is formally defined as "the angle between a line and the strike line of the plane in which it is found", measured on the plane. ( January 2019) ( Learn how and when to remove this template message) Please help to improve this article by introducing more precise citations. This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. ![]()
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