Steam-assisted gravity drainage (SAGD) is an in-situ recovery process used to extract bitumen from the Athabasca oil sands in Northern Alberta. It uses two horizontal wells drilled at the bottom of the reservoir. Steam is injected into the top well and produces a steam chamber that grows upwards and outwards. The heated, less viscous, oil and condensed water at the edge of the chamber flow downward through the formation and are collected by the producing well. As the oil drains, the chamber expands farther into the bitumen reservoir.

The success of this technique depends on being able to propagate steam throughout the reservoir, but heterogeneity can prevent this. Instead of a balloon shape (Fig. 517), the steam chamber may develop irregularly (Fig. 518). This affects the amount of oil that can be produced and illustrates the need to monitor the growth of SAGD steam chambers. Successful monitoring can help optimize production efforts by further understanding the reservoir, decreasing the steam-to-oil ratio (i.e., the amount of water needed to produce a unit of oil), locating missed pay, identifying thief zones (i.e., steam that escapes via unexpected paths such as faults), and more efficiently using resources.