SAGD / ES-SAGD
Pushing the limits of heavy oil recovery in the Canadian oil sands.

The Challenge
As technology has moved forward, different innovative ideas have been presented for the exploitation of heavy oil deposits. The concept of the Steam Assisted Gravity Drainage (SAGD) process started in the late 1970s and finally culminated in a field pilot in the mid-1980s. In the SAGD process, a pair of horizontal wells is drilled at the base of the bitumen reservoir, vertically separated by about 5 meters. Steam is injected in the top well and the hot bitumen with lower viscosity drains by gravity to the bottom well and is produced to the surface. Advancement of the horizontal drilling technology has helped make this process successful. Five large commercial projects in Alberta, Canada, are operational or being built based on SAGD technology, including ConocoPhillips' Surmont project. By 2020, this technology is expected to produce over a million barrels per day of Canadian bitumen.

The major drawback of a pure steam-based exploitation process is the high-energy intensity. In SAGD, the measure of the energy intensity is the steam oil ratio (SOR), or barrels of steam (cold water equivalent) needed per barrel of oil recovered. In terms of energy values, a quarter barrel of bitumen needs to be burnt to produce a barrel of bitumen. Prior to the natural gas price increases of the last few years, a SOR of 3.0 was considered acceptable. Under the current pricing structure, a SAGD project may have to operate close to a SOR of 2.0 to be attractive. Many innovative technologies have been explored to improve the energy efficiency. For example, cogeneration, where steam is produced as a part of the electricity generation scheme, has been applied. To reduce the dependence on the natural gas supply, steam generation using alternative fuels such as produced bitumen also is being investigated.

The Answer?
The viscosity of oil also can be reduced by dilution with solvent. Several solvent-based recovery techniques have been proposed and are being evaluated. It is perceived that these processes are less energy intensive and reduce water-related issues such as limited supply and expensive water treatment. However, the solvent-based processes alone are generally slower than thermal processes. A possible solution is then a combination of solvent and steam in order to reap the benefit of both processes. One of these thermal-solvent techniques shows the potential for enhancement of SAGD performance and was developed under a joint industry consortium at the Alberta Research Council. In this process, called Expanding Solvent SAGD (ES-SAGD), a small volume of solvent is co-injected with steam in a SAGD operation.

Solvent is required for dilution of the produced bitumen for transportation to the upgrader or refinery. Instead of adding the solvent at the plant gate, the solvent is added downhole in the ES-SAGD process to achieve the improvement in reservoir performance. For a 100,000 barrels-per-day SAGD operation, the amount of the lighter hydrocarbon requirement would be 10,000 barrels per day if the solvent injection is limited to 5 percent of the steam volume. Depending on the type of solvent and the long term operating pressure, less than 10 percent of the injected solvent is retained in the formation and 90 percent is produced back with the bitumen. The produced solvent meets, in part, the solvent requirement for transportation. In addition, about 75 percent of the solvent retained in the reservoir may be recovered during the blow-down phase.
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