Geomechanics glossary

Clear, one-line definitions of the Geomechanics terms used across the OgbonLab textbooks. Each entry links to the interactive sections where the idea is taught.

biot coefficient

The factor alpha weighting how much of the pore pressure offsets the total stress, one minus the ratio of dry-frame to mineral bulk modulus.

byerlee's law

The empirical observation that the friction coefficient of most faulted rock clusters near 0.6, nearly independent of rock type.

See: Byerlee's Law: Friction Without a Rock Name

coulomb failure

Shear failure when the shear stress reaches the cohesion plus the friction coefficient times the effective normal stress.

drilling-induced tensile fracture

A hair-thin fracture on the borehole wall, aligned with the maximum horizontal stress, formed when the hoop stress falls below the tensile strength.

See: Drilling-Induced Tensile Fractures

effective stress

The stress the rock frame actually carries, the total stress minus the pore pressure; it governs failure, friction, and deformation.

See: Effective Stress Moves the Circle

fault reactivation

The renewed slip of an existing fault when a pressure change lowers the effective normal stress until the shear stress overcomes friction.

fracture gradient

The pressure per unit depth at which a formation takes fluid and fractures, set by the minimum horizontal stress; it falls as a reservoir depletes.

See: The Fracture Gradient

hoop stress

The tangential stress around a borehole wall; where it exceeds the strength the wall breaks out, where it drops below the tensile strength it fractures.

induced seismicity

Earthquakes triggered by human activity, chiefly fluid injection or extraction that changes the stress on faults; its maximum magnitude is bounded by the volume moved.

See: Induced Seismicity

kirsch equations

The elastic solution for the stress concentration around a circular borehole; the hoop stress peaks at the minimum-stress azimuth.

See: The Kirsch Equations

leak-off test

A pressure test that fractures a formation to measure the minimum horizontal stress from the fracture closure pressure.

mechanical earth model

A depth-continuous profile of the stresses, pore pressure, rock strength, and elastic moduli along a well, built once and reused for every decision.

See: The Mechanical Earth Model

mobilized friction

The friction coefficient a stress state's own stress ratio implies, measuring how close the rock or a fault sits to frictional failure.

mud-weight window

The safe range of drilling-fluid pressure, bounded below by the pore pressure and collapse and above by the fracture and breakdown pressures.

See: The Mud-Weight Window

overpressure

Pore pressure above the hydrostatic value, generated by undercompaction or fluid expansion, which lowers the effective stress and narrows the mud window.

See: Overpressure or Gas?, Detecting Overpressure

poroelasticity

The coupled mechanics of a fluid-filled porous rock, in which pore pressure and stress deform and load one another.

principal stress

One of the three mutually perpendicular stresses on the planes where the shear stress vanishes; the largest, intermediate, and least stresses.

reservoir compaction

The thinning of a depleting reservoir as the rising effective stress compresses the rock, driving surface subsidence and casing shear.

sand production

The failure of a weak reservoir sand at the perforations, producing rock grains with the fluid; a late-life problem as depletion raises the effective stress.

slip tendency

The ratio of shear to effective normal stress on a fault plane, measuring how close the fault sits to slipping, from zero to the friction coefficient.

See: Slip Tendency

stress path

How the horizontal stress changes as the pore pressure changes, at the coefficient gamma; under depletion the horizontal stress falls at a fraction of the pressure drop.

See: The Stress Path

stress polygon

The set of all minimum and maximum horizontal stresses that friction allows at a depth; its corners are the frictional limits of the three faulting regimes.

See: The Stress Polygon

stress tensor

The full description of stress at a point, three normal and three shear components, which resolves onto any plane into a normal and a shear traction.

See: The Stress Tensor

subsidence

The sinking of the ground surface above a compacting reservoir, spread into a broad Geertsma bowl whose depth is a fraction of the reservoir compaction.

See: Compaction and Subsidence

unconfined compressive strength

The axial stress at which a rock cylinder fails with no confining pressure, written UCS; the intercept of the failure envelope.

vertical stress

The overburden load per unit area at depth, the integral of rock density times gravity from the surface down; also called Sv.

See: The Vertical Stress from a Density Log

wellbore breakout

A stress-induced spalling of the borehole wall at the minimum-stress azimuth, where the concentrated hoop stress exceeds the rock strength.