Is There Life After SAGD?

Abstract

The answer depends on what you call "Steam-assisted Gravity Drainage" or SAGD. In its original, pristine form, it may be the ne plus ultra of the entire repertoire of the EOR methods. In the current usage, just about any field project involving steam injection and a horizontal well, or two, is called "SAGD." Furthermore, from the published literature one gets the impression that the application of SAGD is more the rule than the exception. This would naturally lead to misapplications of the SAGD process, with less than optimal results. Our purpose in this article is to show what the original concept is, what additional factors may distort it, and what are some of the limitations of the process as applied. We will not discuss variations of SAGD, such as Enhanced SAGD and Single Well SAGD. Gravity flow and segregation are an integral part of all oil recovery processes. The role of gravity in steam injection processes was first recognized by Doscher(1) for California reservoirs, typically depleted (-0.5 MPa), with high vertical permeability and gas saturations at the top. Recently, Vogel(2) provided a lucid comparison of drive and gravity, in the context of such reservoirs. FIGURE 1: Conceptual diagram of the steam-assisted gravity drainage process. (Courtsey R.M. Butler(6)). Illustrations available in full paper. The Original Concept Figure 1 illustrates the original SAGD concept(3). Two horizontal wells, an injector above a producer, are drilled in the lower part of a formation. Both wells are at first heated by means of steam circulation. When communication is established between the two, bitumen and condensate drain along the sides of the "steam chamber." The rise of steam and the downward flow of oil and condensate are unsteady state processes. However, once the steam chamber is formed, the pressure (and so the temperature) in the chamber remain constant, with steamflood residual oil saturation in the chamber. The sideways growth of the chamber is responsible for oil production. Details have been given in several papers (in particular, see Butler(4, 5). Two definitions of SAGD are worth mentioning: "In the Steam-assisted Gravity Drainage (SAGD) process, heated oil drains from around growing steam chambers, driven by gravity to lower horizontal wells. "Butler(6) "SAGD is counter-current override, where oil moves in a direction opposite to that of steam front advance. "Edmunds(7) Gravity provides the drive in the processes described above, otherwise the processes are quite different. Whereas in the first case steam is the only flowing phase inside the steam chamber, in the second, countercurrent flow of steam, oil and steam condensate occurs-something like the segregation drive in conventional oil recovery. Edmund's description is closer to the observations in numerical simulations. In an earlier paper, Edmunds, Haston, and Best(8) identified two types of drainage processes: ceiling drainage nd slope drainage. Butler developed the flow equation for the above concept, as given in Reference (3), as well in previous publications in somewhat different forms. The key variables are: steam chamber height, permeability to oil, displaceable oil saturation, and oil viscosity at steam temperature.