Petroleum Reservoir - Drive Mechanisms - Solution Gas Drive

Solution Gas Drive

This mechanism (also known as depletion drive) depends on the associated gas of the oil. The virgin reservoir may be entirely liquid, but will be expected to have gaseous hydrocarbons in solution due to the pressure. As the reservoir depletes, the pressure falls below the bubble point, and the gas comes out of solution to form a gas cap at the top. This gas cap pushes down on the liquid helping to maintain pressure.

This occurs when the natural gas is in a cap below the oil. When the well is drilled the lowered pressure above means that the oil expands. As the pressure is reduced it reaches bubble point and subsequently the gas bubbles drive the oil to the surface. The bubbles then reach critical saturation and flow together as a single gas phase. Beyond this point and below this pressure the gas phase flows out more rapidly than the oil because of its lowered viscosity. More free gas is produced and eventually the energy source is depleted. In some cases depending on the geology the gas may migrate to the top of the oil and form a secondary gas cap.

Some energy may be supplied by water, gas in water or compressed rock. These are usually minor contributions with respect to hydrocarbon expansion.

By properly managing the production rates greater benefits can be had from solution gas drives. Secondary recovery involves the injection of gas or water to maintain reservoir pressure. The gas oil ratio and the oil production rate are stable until the reservoir pressure drops below the bubble point when critical gas saturation is reached. When the gas is exhausted the GOR and the oil rate drops, the reservoir pressure has been reduced and the reservoir energy exhausted.