Page 24 - Petroleum and Gas Field Processing

through the reservoir and the pressure conditions are controlled
to create a pressure that is lower than the bubble point
pressure, the liberated gas expands and drives the oil out of the
formation and assists in lifting it to the surface. Reservoirs with
the energy of the escaping and expanding dissolved gas as the
only source of energy are called solution-gas-drive reservoirs.
This drive mechanism is the least effective of all drive
mechanisms; it generally yields recoveries between 15% and
25% of the oil in the reservoir.
II. Gas-Cap-Drive Reservoirs: Many reservoirs have free gas
existing as a gas cap above the oil. The formation of this gas
cap was due to the presence of a larger amount of gas than
could be dissolved in the oil at the pressure and temperature of
the reservoir. The excess gas is segregated by gravity to occupy
the top portion of the reservoir.
In such a reservoirs, the oil is produced by the
expansion of the gas in the gas cap, which pushes the oil
downward and fills the pore spaces formerly occupied by the
produced oil. In most cases, however, solution gas is also
contributing to the drive of the oil out of the formation.
Under favorable conditions, some of the solution gas may
move upward into the gas cap and, thus, enlarge the gas cap
and conserves its energy. Reservoirs produced by the
expansion of the gas cap are known as Gas-cap-drive
reservoirs. This drive is more efficient than the solution-gas
drive and could yield recoveries between 25% and 50% of
the original oil in the reservoir.
III. Water-Drive Reservoirs: Many other reservoirs exist as huge,
continuous, porous formations with the oil/gas occupying only
a small portion of the formation. In such cases, the vast
formation below the oil/gas is saturated with salt water at very
high pressure. When oil/gas is produced, by lowering the
pressure in the well opposite the petroleum formation, the salt
water expands and moves upward, pushing the oil/gas out of
the formation and occupying the pore spaces vacated by the
produced oil/gas. The movement of the water to displace the
oil/gas retards the decline in oil, or gas pressure, and conserves
the expansive energy of the hydrocarbons.
Reservoirs produced by the expansion and movement of
the salt water below the oil/gas are known as water-drive
reservoirs. This is the most efficient drive mechanism; it could
yield recoveries up to 50% of the original oil.

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