α is basically a function of the quotient of the drained bulk modulus Kb and the bulk modulus of solid grain only Kg:
α = 1 - Kb/Kg 0 ≤ α ≤ 1 (Biot 1962).
Incorporation of the Biot coefficient considers the pore pressure influence on the effective stress: A solid rock without pore space such as a quartzite has an α » 0 and shows consequently no pore pressure influence. Weak and porous rocks have α » 1 and display, therefore, the maximum pore pressure influence (Zoback 2007).
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
Pcc 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°).
σ0 is the shear strength of a rock at the absence of normal stress (σn = 0 MPa) caused by the adhesive forces of the mineral grains.