Page 46 - Geochemical Remote Sensing of The Sub-Surface

Geoelectrochemistry and stream dispersion 23
Studies of the behaviour of water in capillaries (Derjaguin et al., 1980) reveal that in
a capillary of diameter 4 x 10 -3 ~tm water still remains as a Newtonian fluid, that is to
say, the start of water movement m the capillary does not demand an initial pressure
gradient. Therefore it is possible for gaseous bubbles in capillaries of diameter up to 4 x
10 .3 ~tm to migrate by Archimedes force. In addition experimental data show that for
rocks of low porosity, for instance, limestone, having porosity 1.14% and permeability
1.1 x 10 .5 ~tm 2, the diameter of the pores in the rock ranges from 0.016 - 0.2 lam (mainly
0.020-0.032 ~tm) (Kalinko, 1987). Consequently, gaseous bubbles of corresponding
diameter possibly penetrate this kind of rock.
The following points are of great significance m evaluating the possibility of
migration of gaseous bubbles in the Earth's crust.

9 The first super-deep drilling to 12.8 km in the Kola peninsula of Russia verified the
effect of de-consolidation (the reverse of consolidation) in rocks at a depth >5 km
(Dortman, 1992). This effect results from the increase of fracture content and
porosity of rocks at depth.
9 Modem analysis techniques reveal in the upper crust, concealed loose structures,
which can not be observed macroscopically (Favorskaya and Tomson, 1989). These
may serve as channels for penetration of gaseous bubbles with diameters of the order
of microns.
9 Isotope analysis in gas fields in northeast China has revealed the presence of biogenic
He, H2 and CH4 from depth (Go and Wang, 1994). It is reported that R- 0.19RA-
0.5RA (where R = 3He/4He in natural gas, RA = 3He/4He in the atmosphere), which
means that some gases come from depth in the crust and even from the upper mantle.
9 Experimental measurements have determined the great speed of development of gas
anomalies over man-made underground gas reservoirs.
9 Many field and experimental measurements have shown that gas flow can penetrate
the cap of gas reservoirs.

These observations suggest that, at least up to several kilometres in the crest, gas
flow (i.e., flow of gaseous bubbles in a water-saturated porous system) exists. The
quantity of gases depends on their solubility at the temperature and pressure at depth
(Fridman, 1970; Kalinko, 1987).
Experimental studies show that, within a large area, there exists at depth in
considerable concentrations many different soluble gases, including N2, CO2, CH4, H2,
H2S, He and others (Shvets, 1973; Kalinko, 1987; Kiriukhin et al., 1988). These gases
may be divided into poorly soluble and highly soluble gases. At a temperature of 20~
and a pressure of 1 atmosphere the highly soluble gases (CO2, H2S) have solubilities of
878-2588 ml/1 whilst the poorly soluble gases (He, H2, N2, CH4) have solubilities of 9.3-
33.1 ml/1. Laboratory modelling under high temperature and pressure conditions
demonstrates that H2, CO: and CH4 at temperatures of 600-800~ and pressure of 20-30