Reservoir Drives
Oil and gas in petroleum reservoirs are often subjected to tremendous amounts of pressure and energy. When wells are drilled, the pressure and energy can assist in producing and lifting the oil to the surface.
Contents
What are reservoir drives?
Oil cannot lift itself to the surface, but pressure and energy in the gas and saltwater that often shares the reservoir can assist in lifting the oil.
Because of this, reservoirs are also classified by the type of natural energy, or drive, available to produce formation fluids.
There are three (3) classifications of natural drive mechanisms. They are:
- Water-drive
- Dissolved gas-drive
- Gas-cap-drive
How do drive mechanisms work?
Water Drives
Water is able to compress and expand about 1/2,500th for every pound per square inch (psi) change in pressure. This doesn’t seem like very much –and it isn’t – but if you have an enormous quantity of water, such as in a petroleum reservoir, this can turn into a great deal of energy. For example, let’s say you have a reservoir of oil surrounded by a great quantity of saltwater. As pressure within the formation is reduced by the removal of hydrocarbons, the saltwater will expand. The expanding water moves and displaces the petroleum upward within the formation. The pore spaces vacated by the oil are filled with water and the hydrocarbons are moved towards the producing wells as a result of the “water-drive.”
Dissolved Gas Drives
In nearly all cases, oil in an underground reservoir contains some amount of dissolved gas. As pressure within the formation decreases, the dissolved gas will break out of the oil and expand. This expansion of the dissolved gas will drive oil to the producing wells and assists in lifting it to the surface.
Reservoirs that produce oil with the aid of expanding dissolved gas are called dissolvedgas-drive reservoirs. With only 15-25 percent of the oil in place being recoverable, dissolved gas drives are typically the least effective in getting oil out of the reservoir.
Gas Cap Drives
In many instances there exists more gas in the reservoir than the oil can hold under the conditions of reservoir temperature and pressure. The excess gas, being lighter than oil, rises to the top of the reservoir and forms a gas cap over the oil. This "gas-cap" is an additional source of energy which can assist in the production of formation fluids. As oil is produced from the formation the gas expands and fills the pore spaces vacated by the oil as pressure lessens within the reservoir.
Also, under favorable conditions, dissolved gas in the oil will come out of solution and rise into the cap thus maintaining or expanding the gas-cap. Gas-cap drives will normally recover 25-50 percent of the oil in place in the reservoir, thus they are generally more efficient than dissolvedgas- drives.
The effective recovery of oil from reservoirs by either a saltwater drive or a gas drive is dependent on three factors:
1. The physical nature of the reservoir rock
2. The care exercised in drilling, completing and working over the well.
3. The rate of oil and gas production as a whole.
These factors greatly affect the water-drive and gas-cap-drive production processes. However, the rate of production seems to have only a minor effect on oil recovery from dissolved-gasdrive type reservoirs, except where conditions favor gas-cap formation.
In many cases reservoirs possess the possibilities for either water-drive or gas-drive. When this occurs, the kind of production and the rate of production will determine which type of drive will be most effective and yield the greatest recovery of oil from the formation, In reservoirs where water-drive is effective, recovery yields of 50-75 percent can be attained, making water-drive the most effective recovery mechanism.
Reservoirs which utilize the processes of gas-cap drive in conjunction with water-drive are classified as "combination-drive" reservoirs
Depending on the type of water-drive present (partial or complete) each barrel of withdrawal from the reservoir may not be replaced by a barrel of saltwater. When the influx of water is insufficient the gas-cap may expand along with the encroachment of water into the voided pore spaces.
Another type of drive mechanism is "gravity flow", the least effective mechanism for recovery of oil in most instances. Gravity flow may not push oil to the producing well. Where gravity flow does exist the downhole pumping equipment is moved to the lower depths as the fluid level in the reservoir drops. Gravity flow is normally the predominate drive mechanism in shallow, steeply dipping reservoirs.
References
Van Dyke, Kate. Fundamentals of Petroleum, Fourth Edition. University of Texas Petroleum Extension Service, (1997). Pp. 15 - 20.
