What is Solution Gas Drive in Heavy Oil Reservoirs
In light oil reservoirs, the drive mechanism of solution gas drive comes mainly from the expansion of oil and its dissolved gas. For that reason, the solution gas drive is sometimes called internal gas drive, dissolved gas drive, depletion gas drive, or fluid expansions drive. The solution gas drive is by far the most cost effective production method.
The initial reservoir pressure can be either above or equal to the bubble point pressure. The production process that took place above the bubble point pressure tends to decrease the reservoir pressure rapidly. During this period, all gas in the reservoir remains in the solution. At this stage, forces contributing to the production of the oil are in general: expansion of oil, dissolved gas, connate water, and rock compressibility. On the other hand, below bubble point pressure gas bubbles commence to form and expand throughout the reservoir as the pressure decreases. These bubbles are trapped in the pores because of the domination of the capillary forces. The gas saturation increases in the reservoir as the pressure continues to decrease. As the gas bubbles grow, they will eventually occupy at certain time several pores, and connected with bubbles from different pores and form a continuous phase. The gas saturation at which the gas becomes mobile is called the critical gas saturation. Then, the gas oil ratio increases dramatically when a continuous gas phase exists and the gas saturation becomes more than the critical value. The presence of the gas as a continuous phase decreases the relative conductivity of the oil; consequently, the oil production rate declines. Producing the gas depletes the reservoir energy swiftly.
Some heavy oil reservoirs, such as Lindbergh and Lloyminster in Canada, and Orinoco in Venezuela, are well known for their anomalous behavior under primary depletion. This unusual behavior is characterized by high production rate, high recovery factor, lower gas oil ratio, and abating pressure drop. There are many theories explaining this unexpected behavior. Foamy oil model by Maini, reduced gas mobility (Pooladi-Darvish and Firoozabadi), and pseudo bubble point pressure (Kraus et al.) are among these theories. By means of these theories, progress has been made in understanding this unusual behavior, but still more experimental tests are needed to understand the process.
