Poretrykk

Poretrykk (pore pressure) is the pressure exerted by fluids within the pore spaces of a rock. It acts on the solid mineral frame and influences the effective stress that governs rock strength, deformation, and fluid flow. In subsurface formations, pore pressure typically increases with depth due to the weight of overlying fluids, but it can deviate from a purely hydrostatic gradient.

Normal or hydrostatic pore pressure corresponds to the pressure of formation fluids at rest and is commonly around 9–11 megapascals per kilometer of depth (roughly 0.4–0.5 psi per foot). Abnormal or overpressure occurs when pore pressure exceeds the hydrostatic level, while underpressure refers to pressures lower than hydrostatic. Overpressure can arise from rapid sedimentation with insufficient time for fluids to escape, undercompaction, tectonic loading, hydrocarbon generation, or diagenetic changes in clays.

Pore pressure is measured through direct and indirect methods. Direct measurements come from downhole tests, such as formation pressure tests or modular formation dynamics testers, providing a pore pressure profile along the borehole. Indirect estimation uses well logs and drilling data, including sonic velocity, density, resistivity, and mud-weight analyses, to infer pressure where direct data are sparse.

Implications of pore pressure are significant for drilling and reservoir management. Incorrect assessments can lead to kicks or blowouts if mud weight is insufficient, or formation fracturing and lost circulation if mud weight is excessive. In reservoir engineering, pore pressure also influences compressibility, permeability, and hydrocarbon recovery strategies.