Upstream Oil and Gas Production Drivers

Natural (Subsurface) Production Drivers

In The Oil and Gas Production Drivers lesson, we will discuss the types of natural reservoir drive that allow oil and gas to be produced from a newly completed well.

Production always occurs in stages, which depend on the characteristics of the reservoir and the contained fluids. The initial stage of production, discussed here, is called primary recovery. Primary recovery includes the flow created by the natural reservoir drive as well as flow stimulated by artificial lift.

Secondary recovery is that production that is increased using enhanced oil recovery methods.

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First, let’s do a quick review of how a well is drilled and completed.

When an oil company drills a well, a hole (called a wellbore) is drilled to the formation or reservoir that contains oil and gas. A pipe called casing is installed in the hole and cemented into place. Next, the formation needs to communicate with the wellbore. This is done by punching holes through the casing and cement, and back into the formation. The fluids in the formation – a mix of oil, gas and water – can now flow into the wellbore through the holes.

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Again, this lesson focuses on natural reservoir drives. Artificial lift and enhanced oil recovery are addressed in later lessons.

Natural reservoir drive is the ability to get the reservoir fluids moving from the reservoir, up the production tubing, to the surface. During the initial stages of a well’s producing life, the fluids are kept moving as long as the pressure from the natural energy source, or driver, is greater than the weight of the column of flowing oil or gas.

The three most common natural drivers are water, gas and gravity. The reservoir drive is the major factor affecting design of the surface production facilities.

Water Drive

In water drive, the weight of the water in the reservoir is greater than that of the column of oil or gas. The water’s weight literally forces the lighter hydrocarbons to the surface. Water drive is the most effective natural drive. It is possible for a reservoir with water drive to produce approximately 30-75% of it’s hydrocarbons.

Gas Drive

Gas drive occurs in two forms: dissolved-gas and gas-cap. We’ll look at dissolved-gas first.

Dissolved-Gas Drive

Oil in most reservoirs includes dissolved gas. As the oil is produced from the reservoir, the pressure drops which causes the fluid to release gas. As the gas escapes from the oil, it expands. The force of the expanding gas drives the oil through the reservoir, into the wellbore and then to the surface. However, dissolved-gas drives are not very effective at recovering large amounts of oil. Typically only 5-30% of the oil can be recovered in a dissolved-gas drive reservoir.

Gas-cap Drive

The second form of gas drive is a gas-cap. Some oil reservoirs contain a lot more gas than can be dissolved in the oil. When this occurs, the extra gas, being lighter than oil, forms a cap over the oil. This gas-cap is under extreme pressure. As oil is produced, the gas-cap expands and pushes more oil up to the surface. In this type of reservoir, the dissolved-gas and gas-cap work together. With primary recovery, approximately 20-50% of the oil in this type of reservoir can be recovered.

Gravity Drive

The final type of natural drive is gravity. The force of gravity is, of course, always at work in a reservoir. Most reservoirs are not perfectly horizontal. Reservoirs can have highly tilted or even vertical structures. Over time, gravity causes oil to migrate upward by pulling the heavier water down beneath it.

Combination Drive

Many reservoirs actually contain a combination drive with a gas-cap above the oil and water below. Another type of combination drive has dissolved-gas in the oil, again with water below it.