As the reservoir pressure in a solution-gas system falls, liberated gas may migrate to the top of the reservoir, producing a secondary gas cap. If the oil is produced rapidly, gas bubbles may form and expand in the pores of the rocks, which can impede oil flow through the reservoir. As with a primary gas cap, depleting the secondary gas cap can cause rapid pressure decrease and reduce recovery. Solution gas drive system recovery factors range from 5% to 30% of the OOIP. The average recovery factor is around 15%.
Gravity drives are found in reservoirs that have both gas cap and water drives; these types of reservoirs can be very effective producers. The energy comes from two directions: upward from the hydrostatic pressure in the oil column and downward from an expanding gas cap. This reservoir drive mechanism may also be referred to as combination drive. For many oil reservoirs, the production of fluids is controlled by more than one drive mechanism, and reservoir engineers use the term combination drive to describe them. Gravity drive system recovery factors range from 5% to 85% of the OOIP. The average recovery factor is around 50%. Producing slowly from formations with these drive mechanisms may greatly improve ultimate recovery.
The rock matrix in some reservoirs contracts as fluids are produced. This contributes energy to the system and is referred to as compaction drive. Although compaction drives are rare, the presence of compaction improves recovery factors compared to the recovery rates from formations that don't contract from production. These reservoirs frequently have solution gas, water or gas cap drive mechanisms.
Consequences of Drive Mechanisms
Reservoir engineers study downhole conditions to optimize completion strategies. After production begins, they monitor the changing conditions to avoid situations that could negatively impact reservoir performance and recovery factors. Reservoir management may involve avoiding early onset water breakthrough and coning, pressure management to avoid dropping below the bubblepoint pressure, higher gas production in solution drive systems and patience with gravity drainage systems to allow fluids to equilibrate.
Eventually, primary recovery mechanisms become ineffective, and reservoir engineers may turn to enhanced recovery techniques to extend the life of producing fields. During primary production, secondary recovery techniques such as waterflood and gas reinjection, may improve recovery, improve economic viability and extend the life of a reservoir. Proper reservoir management can help operators increase recovery and improve asset performance.