Recovery Mechanisms and Drive Energy

How a Reservoir Gives Up Its Oil

An oil reservoir is rock under pressure, with oil, gas, and water sharing the pore space. When a well is opened, the pressure drop drives fluids toward the wellbore. The energy that does this work is the drive mechanism, and which one dominates sets how much oil comes out on its own.

• Solution gas drive: as pressure falls below the bubble point, gas comes out of solution and expands, pushing oil to the well. Common but inefficient, with recovery often only 5 to 30 percent.
• Gas cap drive: an overlying gas cap expands downward as oil is produced, sweeping oil ahead of it.
• Water drive: an underlying or flanking aquifer expands and encroaches, supporting pressure and displacing oil. The most efficient natural drive.
• Gravity drainage: in steeply dipping or high permeability reservoirs, oil drains downdip under its own weight.

Even the best natural (primary) drive leaves most of the oil in the ground. That is why operators inject water or gas to add energy and sweep more oil. Injecting water, a waterflood, is the workhorse of secondary recovery.

The Recovery Ladder

Primary recovery rides the natural drive energy and typically recovers a small fraction of the oil in place. Secondary recovery, mostly waterflooding, adds energy and sweep and often doubles the recovery factor. Tertiary or enhanced recovery attacks the oil that water leaves behind. Each rung adds energy the reservoir no longer supplies on its own.