Page 29 - Origin and Prediction of Abnormal Formation Pressures

1 ~ E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV

Zone of decreasing Zone of increasing
pore pressure pore pressure

H20 -------~I i i~i ili ii!iiii iEi ili iiiii.iiiiiiiiiiii~l H20

H20 ~ H20
i.._
0 ""
..-. 0
-4,.,,.
e" N~O ~ H~O ~ .0
9 o
'*- H20 ~ H20 ~
N),.,,- NI,,,,-
O O
(I) H~O ~ H~O ~ (])
r- t-
O O
N N
H20 H20 ' '
H20 "'~iiHiiiiiiiiiiii!iiiiiiiiiiiii~iiii iii iiii iiil H20 i,,

Fig. 1-6. Schematic diagram of osmotic flow through semipermeable clay membrane (without fractures).

The pressure will be abnormally high in the water influx side of the membrane, and
the water will contain a considerably lower concentration of electrolytes. Formations
with a large lateral continuity and high permeability would probably dissipate the
osmotically induced high pressure, whereas the formations which are surrounded by
rocks having low transmissibility would exhibit higher pressure. Young and Low (1965)
conducted experiments that illustrated the behavior and effectiveness of argillaceous
sediments as semipermeable membranes (Hanshaw and Zen, 1965).
Lomba et al. (2000) discussed a model for calculation of transient pressure profiles
and solute diffusion through low-permeability shales applied to the calculation of pore
pressures near a wellbore. They found that the osmotic potential contributes to the
generation of a high hydraulic pressure gradient near the wellbore that controls the flow
of water from the formation.
Swarbrick and Osborne (1998), on the other hand, calculated that in the North Sea
rocks, an osmotic pressure of only about 3 MPa (435 psi) can be generated even
with salinity contrasts as high as 35 wt% NaC1 equivalent. They also stated that if
shale contains microfactures, osmosis is impossible. Thus, this possible mechanism for
creating overpressures should be thoroughly investigated.

Salinity of interstitial water

Often on approaching formations with abnormally high pressure, there is a freshening
of interstitial water (e.g., see Rieke and Chilingarian, 1974). Yet, the reasons for this