Page 93 - Origin and Prediction of Abnormal Formation Pressures

74 A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON AND E AMINZADEH

. . . . . . . . . . . . '
7. "
Flow Ko Flow

. . . . .
Fig. 3-2. Two extreme cases of beds orientation with respect to fluid flow direction. (Modified after Gurevich
et al., 1994, fig. 2, p. 69.)

obtain any direct quantitative evaluations of their manifestation and their influence on
fluid pressure and fluxes in nature. The list presented above does not include such rather
'exotic' mechanisms as pressure increase due to osmotic processes. For such a process
to be operative, water with higher salinity must be located in a sand lens among clays
surrounded by low-salinity water, and the permeability of the clay must be zero. Clearly,
such situations are rather exotic and do not play any role in regional processes.
It is useful to notice that the role of each of the mechanisms listed above is
determined by the rate at which the process takes place and the rate at which pressure
change is dissipated.

Role and distribution of formation permeability

Permeability is a tensor value and depends on space distribution of lithologic
heterogeneity. There are two extreme cases of distribution of lithologic bodies relative
to the direction of fluid flow: alternating layers of different permeability lie (1) parallel
and (2) perpendicular to the flow direction (Fig. 3-2). In the first case the average
permeability k~ of the formation for the flow will be:
Hi
k,i-y (3-1)
-
where kli and Hi are lateral permeability and thickness of the layer i, H is the total
thickness of the formation. In the second case, the average permeability k2 will be:
~H'
k2 -- (3-2)
Hil/k2i
where H' is the length of the formation (in the direction of flow) along which
permeability of rocks is being averaged, H i' is the thickness of layer i, and k2i is the
permeability of this layer along the flow line.
Gurevich (1972) analyzed the problem of averaging permeability of geological
formations for block sizes up to a sedimentary basin. It was shown that regional
deep formation water flow velocities, calculated on the basis of regional permeability
averaged as the arithmetic average from permeability values measured in wells in oil
and gas fields, are in contradiction with hydrogeochemical zonality of groundwater.
Calculated (arithmetically averaged permeability values) velocities are at least two or
three orders of magnitude higher than it is possible at the existing geochemical zonality.
The method of averaging permeability may be the only cause of this discrepancy.