Page 82 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 63

Osmosis

Glasstone (1946) described osmosis as a spontaneous flow of water from a more
dilute to a more concentrated solution, when the two are separated from each other by a
suitable membrane (see Fig. 1-6).
According to Jones (1969), the pressure difference across a clay bed could exceed
3500 psi (246 kg/cm2). Thus, stepwise increments of osmotic pressure through a series
of interbedded sands and clays could (as a multistage pump) produce AHFPs.
Some experts, e.g., Poston and Berg (1997) and Swarbrick and Osborne (1998),
however, believe that the osmotic phenomenon would not greatly contribute to higher
levels of overpressures. Thus, further research work (both field and laboratory) is
necessary to reach definite conclusions.

Density contrast

Differences between the density of hydrocarbons (oil and/or gas) and that of water
in a reservoir can create an overpressure (Fig. 2-25). Obviously in the case of gas, the
overpressure will be more pronounced.

Po,: P.i + (15-- l$o)(Zl -z') zl

9 ,
Z2
,:.:,!i:

,,

i,71-w-~iE~-kll,$

Fig. 2-25. Cross-sectional view of an anticlinal reservoir sandwiched between two impervious shale bodies,
showing abnormal pressures in hydrocarbon accumulation in hydrostatic water environment. Yo = specific
weight of oil (e.g., in lb/ft3); Yw - specific weight of water (e.g., in lb/ft3); z = elevation (e.g., in ft); p =
pressure (e.g., in lb/ft2); overpressure (Ap = (Pol - Pwl) = (Yw - Vo)(Z2 - zl)] in lb/ft 2. (Modified after
Hubbert and Rubey, 1959, p. 150; and Gretener, 1969, p. 267; in Rieke and Chilingarian, 1974, fig. 264, p.
310.)