Bulk modulus

K describes the stiffness of a rock in hydrostatic compression (Zoback 2007). It is expressed as the change of the confining pressure acting on a rock specimen dP in relation to the thereby caused volume change dV/C:

K = (dP/dV)*V.

The compressibility is the reciprocal bulk modulus K^-1.

Capillary threshold pressure

P_c^c is the pressure difference between the injected gas phase and the water phase in the caprock. This pressure is needed to overcome the capillary forces to initiate a flow into the caprock (Egermann et al. 2006).

Coefficient of internal friction

µ is the tangent of the angle of (internal) friction φ

µ = tanφ

and to this effect a material constant which describes the break angle with the main principle (compressive) stress σ₁ in response to its (diagenetic) compaction, grain size and mineralogical composition. As a rule of thumb applies: loose and/or fine grained rocks which consist mainly of platy minerals (poorly consolidated sandstone, clay- and siltstone, shale and schist) have a lower µ (generally in the range of 0.25, φ » 15°) than hard, coarse grained rocks composed of subangular to well-rounded grains (e.g. a silica cemented, coarse grained, mature sandstone with µ approximately 0.6, φ » 30°).