Page 39 - Drilling Technology in Nontechnical Language

30 Drilling Technology in Nontechnical Language Second Edition

Gas drive

The reservoir is partially or completely isolated from the pressure
regime in the surrounding rock. As oil is produced, the gas cap above
it expands. As the gas cap expands, it loses energy. The temperature
and pressure in the reservoir drop until eventually there is not enough

energy left to drive the oil out. Gas drive is not an efficient long-term
production driver.
At the stage when there is insufficient pressure left, there are several

possibilities:
1. Inject more gas into the reservoir to increase the pressure.
2. Ignite oil underground by injecting air. The gas from the burning
oil increases reservoir temperature and pressure and drives more
oil out.
3. Install a downhole pump to pump the oil to the surface. This may
be a mechanical pump (with a “nodding donkey” or “horsehead”
on the surface providing the power via rods connected to the
pump) or a downhole hydraulic or electric pump.
4. Inject gas into the well. This mixes with the oil, which makes the
column of oil lighter, allowing the reduced pressure to drive the
oil out. This is called gas lift.

5. Inject water and chemicals in part of the reservoir to drive the oil
towards the producing wells.

These techniques are called secondary recovery.

Water drive

Chapter 1 explained the principle of hydrostatic pressure. Liquids have

a pressure gradient—the heavier the fluid, the higher the pressure gradient.
In a sedimentary rock sequence, there is a local water table; that is, the
rock pore spaces contain salt water, and this exerts a pressure at depth (as
was explained in chapter 1).
In a reservoir that has water drive, the reservoir is connected
hydraulically to the area pressure regime (such as an aquifer that is open
to the atmosphere). Water from the local water table pushes the oil to the

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