Page 38 - Drilling Technology in Nontechnical Language

Chapter 2 – OIL AND GAS RESERVOIR FORMATION 29

Fig. 2–3. Secondary migration within reservoir

Some (very few) reservoirs are single phase; that is, they contain only

a single fluid type, either all gas or all oil. Oil is rarely found without some
gas or some water. Gas generated from coal seams, as described previously,
can be single phase. Mostly, however, reservoirs are multiphase—they
contain mixtures of gas, oil, and water. Secondary migration will tend to

separate these fluids out by gravity so that the gas sits at the top (known
as a gas cap), oil under the gas, and water under the oil (lightest fluids at

the top to heaviest fluids at the bottom). The oily part of the reservoir may

contain a mixture of oil and water within the pore spaces, in which case
the reservoir rock may have a layer of water adhering to the surface of the
rock grains (water wet) or it may have a layer of oil adhering to the rock
(oil wet). These factors and others must be considered by the reservoir
engineers when deciding how to exploit the reservoir.

Reservoir Drives

What provides the energy to drive the hydrocarbons to the surface

through a hole drilled into the reservoir? Most oil reservoirs, when first

drilled into and produced, have sufficient pressure in the reservoir to push
the oil to the surface. This energy can come from different sources.

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