Page 77 - Petrophysics

MIGRATION AND ACCUMULATION OF PETROLEUM 51

at a temperature of about 200°F. This temperature is well within the
temperature range for generation of petroleum and thus may assist in
the primary migration of oil when smectites are present in the shale
body [6].

SECONDARY MIGRATION

Inasmuch as petroleum reservoirs exist in a water environment, the
migration of hydrocarbons from the point of release from a source
rock to the top of the trap is intimately associated with capillary
pressure phenomena and hydrology. The pore size distributions,
tortuosity of continuous channels, porosity, permeability, and chemical
characteristics of reservoir rocks and their interstitial fluids differ widely.
Nevertheless, because of the ubiquitous presence of water, capillarity,
buoyancy, and hydrology apply in all cases [ 141.
The migration of oil as distinct droplets in water-saturated rock is
opposed by the capillary forces, which are functionally related to pore
size, interfacial tension between oil and water, and adhesion of oil to
mineral surfaces (wettability). This is expressed through a contact angle
for a capillary of uniform size as:

where: P, = capillary pressure, Pa.
o = interfacial tension, (NX 10-3)/m.
8 = contact angle.
r, = radius of the capillary, m.

The more usual case is one in which the oil droplet exists within
the confines of a large pore containing several smaller-sized pore throat
exits (Figure 2.9). Under these conditions, the pressure required to
displace the droplet from the large pore through the constriction of
a pore throat (the displacement pressure) is the difference between
the capillary pressures of the leading (l) and trailing (t) pores [61:

(2.5)

where: Pd = displacement pressure, Pa.
61 = contact angle of the leading edge.
et = contact angle of the trailing edge.
rl = radius of the leading pore, m.
rt = radius of the trailing pore, m.