Application of Electrokinetics for Enhanced Oil Recovery - Electrokinetics for Petroleum and Environmental Engineers - page 106

3.2.1 PetroleumReservoirs
Unfortunately, some jurisdictions refer to petroleum reservoirs as pools ,
which conjures up images of vast underground lakes of petroleum. In real-
ity, most petroleum reservoirs consist of porous rocks, which more closely
resemble fine-grained sponges, rather than caverns filled with gas or lakes
of liquid petroleum.
3.2.2 Porosity
Reservoir rocks may be though of as being made up of solids (matrix) and
voids (pores). The percentage of rock volume occupied by pores is called
porosity, Ø.
3.2.3 ReservoirSaturations
The pore spaces in reservoir rocks are usually filled with water, as well as
oil and/or gas. Economic gases are light (C 1 - C 3 ) hydrocarbons. Common
non-economic gases are air, N 2 , and CO 2 . Toxic gases include H 2 S. Liquid
petroleum is made up of the heavier (i.e., C 4 and above) hydrocarbon
chain and ring molecules. Liquid contaminants are sulfur and heavy metal
hydrocarbon compounds
Oil and water are immiscible, fluids, with neither particularly soluble in
the other. Gas is soluble, to a point, in oil. Beyond that point, however, it
becomes a separate phase. The same is true for gas and water.
Oil is (usually) less dense than water and gases are less dense than either
oil or water. Because of these density differences, if all three are present in
a given reservoir, they will segregate on the basis of density, with gas at the
top and water at the base (i.e., much like Jell-O 1-2-3®). Pore throat sur-
face effects at the contact between the fluids and rock matrix, however, will
insure that these contacts are not always very sharp.
The types and amounts of fluids, which will co-exist at any given depth,
will depend upon reservoir temperature and pressure, as well as pore
space capillarity, surface tension between the immiscible fluids, wettability
between the rock matrix and fluids, and the contact angle between the dif-
ferent immiscible fluids and the matrix. Clastic (sand and shale) reservoir
rocks tend to be preferentially water wet , because of the polar nature of
water and the net negative charges on the pore throat surfaces, due to cat-
ion substitution in silicate minerals. This results in some water, coating the
pore throats, throughout the reservoir, making it preferentially water wet.
Carbonate reservoirs tend to be preferentially non-water wet. If, however,