The addition of certain amounts of non-condensable gas to the steam assisted gravity drainage (SAGD) process has been known to reduce the steam consumption. The addition of small amounts of such gases (i.e. carbon dioxide) may improve oil recovery as the gas accumulates at the upper surface of the reservoir as a thin insulating layer, limiting the rate of front spreading at the corners of the steam chamber. Since the gas raises the pressure of the oil in the reservoir, it may be looked upon as pushing the oil downwards. This new concept has been called steam and gas push (SAGP).
In order to investigate these phenomena, six experiments with or without carbon dioxide injection were conducted. Experiments were carried out in a scaled physical model packed with crushed limestone premixed with a 12.4 ° API heavy crude oil. Temperature, pressure and production data were continuously monitored during the experiments.
The SAGP mechanism was detected at two different well spacings, when considerable carbon dioxide was added to steam with different steam/carbon dioxide injection ratios. However, as the distance between the injection and production wells increased, the volume needed to observe the SAGP phenomenon increased. In addition, the steam condensation temperature and the steam-oil ratio decreased, and the heavy oil become less mobile in the steam chamber, as the amount of carbon dioxide increased in the injection mixture. Thus, the heating period was prolonged, decreasing the cumulative amount as well as the rate of oil recovery.
The recovery of heavy crude using a special form of steam flooding known as steam-assisted gravity drainage (SAGD) is one of the major thermal enhanced oil recovery (EOR) processes. In this EOR process, the movement of oil to the production well is caused by gravity forces, and the geometry is such that the oil moves approximately parallel to the interface that forms the boundary of a growing, steam-saturated zone known as the steam chamber.
A major limitation of SAGD is that it requires a large amount of steam particularly in thin, low quality reservoirs. This means that it requires high energy for the production of continuous steam. Das and Butler1 proposed that this limitation could be avoided or decreased in two ways: addition of noncondensable gases such as carbon dioxide or methane for increasing the recovery by creating a steam and gas push (SAGP) mechanism, and the Vapor Extraction (VAPEX) process where liquefied petroleum gases such as propane or butane are injected.
Initially proposed by Butler in 1999 and further improved by his co-workers in 2000, the SAGP process can be simply explained as the addition of a non-condensable gas to steam using dual horizontal wells2. The concept of SAGP is basically the same as conventional SAGD: steam not only mobilizes the oil by heating it but also equalizes the pressure vertically and allows the liquid to fall. This can only happen if the steam vapor is cooled. In such cases the steam condenses and is no longer in saturated form3,4.