Page 95 - Geology and Geochemistry of Oil and Gas

TABLE 3.5 68

Types of mechanisms responsible for generating abnormally-low formation pressures (ALFP) (after Chilingar et al., 2002)
Type of changes Description of process

Changes in the rock pore volume
Rock dilatancy Increase in pore volume. During erosion of shallow-buried, clay-rich lithology, dilation of the pores can occur
Tectonic movements Local and regional faulting, earthquakes, etc. With increase in tension of the formation, the pore volume may
increase
Increase in pore volume Dissolution of cementing material. Dissolution of cementing materials such as CaCO 3 can increase the pore volume
Changes in the volume of interstitial
fluids
Temperature change Thermodynamic effects. Cooling of the formation (e.g., due to uplift or erosion) can cause the contraction of fluids TEMPERATURE
and, thus, decrease the fluid pressure
Changes in fluid pressure (hydraulic
head); movement of fluids
Osmosis Osmosis. Contrast in the brine concentration of formation fluids can result in the transfer of fluids across a
semipermeable membrane. This can result in a loss of fluids across a semipermeable membrane with a resultant drop AND
in the fluid pressure, in the upstream side of the system
Production of fluids (gas, oil, and/or Depressuring of reservoir rocks. Can occur as a result of massive fluid production from the formation that is not
water) replaced by an influx of fluids from the adjoining formations, i.e., massive water depletion of producing formations PRESSURE
Migration of gases Gas migration. During uplift, gas is often able to come out of solution as the temperature and confining pressure is
reduced. The freed gas may then escape toward the surface by diffusion or along faults, reducing the pore pressure
of the rock IN
Groundwater movement Fluid movement. Difference between the permeabilities of discharge and recharge areas, as more fluids are leaving
the system than entering it THE

SUBSURFACE