Page 40 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 23

TABLE 2-1 (continued)
Type of changes Description of process
Oilfield production operations Repressuring of reservoir rocks. Can occur as a result of massive fluid
injection or fluid influx into the formation, i.e., massive water
injection programs (secondary recovery). Pressure increase may
occur across faults, or behind casing.
Permafrost environment Formation of frost heaves (pingos). Permafrost encroachment: trapping
of unfrozen zone in practically closed system. Freeze-back pressures
around shut-in arctic wells. Gas hydrate reservoirs (e.g., Mackenzie
Delta, Canada).
Differences in specific weights Density difference. Difference between the weight of a gas column and
that of a fluid (oil or water) column.

TABLE 2-2
Types of mechanisms responsible for generating abnormally low formation pressures (ALFP)
Type of changes Description of process
Changes in the rock pore volume
Rock dilatancy Increase in pore volume. During erosion of a 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 CaCO3 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 and, thus, decrease the
fluid pressure.
Changes in fluid pressure (hydraulic head); movement of fluids
Osmosis Osmosis. Contrasts 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 in the fluid pressure, in the upstream side of
the system.
Production of fluids (gas, oil Depressuring of reservoir rocks. Can occur as a result of massive fluid
and/or water) production from the formation that is not replaced by an influx of
fluids from the adjoining formations, i.e., massive water depletion of
producing formations.
Migration of gases Gas migration. During uplift, gas is often able to come out of solution
as the temperature and confining pressure are reduced. The freed
gas may then escape toward the surface by diffusion or along faults,
reducing the pore pressure of the rock.
Groundwater movement Fluid movement. Difference between the permeabilities of discharge
and recharge areas, as more fluids are leaving the system than
entering it.