Page 323 - Geology and Geochemistry of Oil and Gas

284 APPENDIX A

Fig. A.13. Movement of gas globule through a constriction (after Muravyov et al., 1958). r 1 and r 2 ¼
R 1 and R 2 (radii of curvature).

and water in a reservoir, however, results in the formation of water–oil and
gas–water–oil mixtures (see Muravyov et al., 1958). The amount of gas coming
out of solution during migration is greater with increasing amount of dissolved
active substances, with increasing surface area of porous medium (i.e., with
decreasing permeability), and with decreasing temperature. As the oil–water–gas
mixture moves through pores, the gas bubbles and water droplets are deformed
on passing through constrictions (see Fig. A.13; see Muravyov et al., 1958). In order
to move, the gas globule as shown in Fig. A.13 must overcome the capillary pressure
equal to

1 1
Dp ¼ p p ¼ 2s (A.31)
1
2
R 2 R 1
Although the Dp may be very small for a single globule, the cumulative resistance
of many bubbles may be large (Jamin effect). Additional resistance to flow is created
by the polymolecular layers of oriented molecules of surface-active components in
the oil, which are adsorbed on the rock surface and may be quite thick
3
(10 –10 4 cm). At a constant pressure differential, the rate of oil filtration through
porous media diminishes with time and is more pronounced in the case of higher
content of polar compounds in the oil.
In water-wet carbonate rocks, vugs are ‘‘bad news’’ because during the
waterflooding operations oil is trapped in the vugs. On the other hand, in oil-wet
rocks, vugs are ‘‘good news’’ because water (non-wetting phase) will displace oil
from the vugs. It should be remembered that the non-wetting phase preferably flows
through larger pores.

A.3.1. Water block

The minimum pressure, p cwb , required for oil to displace a globule of water stuck
in a pore throat between the rock grains, providing the oil is the wetting phase, is