Downhole Steam Generation Pushes Recovery Beyond Conventional Limits
Authors: Wilson Adam 
Published: February 2015

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 150515, ’Advancing Thermal and Carbon Dioxide Recovery Methods Beyond Their Conventional Limits: Downhole Innovation,’ by Laura Capper, SPE, CAP Resources; Myron Kuhlman, SPE, MK Tech Solutions; George Vassilellis, SPE, Gaffney Cline & Associates; M.J. Schneider, Global Marine Drilling Company; and Nick Fitzpatrick, World Energy Systems, prepared for the 2011 SPE Heavy Oil Conference and Exhibition, Kuwait City, Kuwait, 12-14 December. The paper has not been peer reviewed.

Enhanced oil recovery (EOR) is a general term used for processes that seek to improve recovery of hydrocarbons from a reservoir after the primary production phase. EOR is designed to increase the mobility of the oil, with an objective of improving oil sweep efficiency or enabling lower residual-oil saturation. Downhole steam generation (DHSG), which combines thermal and nitrogen or CO2 EOR, offers several benefits to accelerate the production of oil. Moreover, the CO2 that is generated in situ can be used elsewhere.

Conventional-EOR Limitations

Surface-Steam-Injection Limitations. Steam that has 100% quality consists entirely of vapor, whereas 0% steam quality is liquid water. Higher-quality steam, therefore, contains much more energy than lower-quality steam, and higher steam quality (generally greater than 60%) results in more-effective oil recovery. The steam vapor transfers heat to the oil, lowering its viscosity, and, when it condenses, the resulting water drives the oil toward the producer well.

A simulation and field economic model for a 2,000-ft-deep reservoir in California showed that steam quality drops to 50% or below at approximately 700 m (approximately 2,300 ft). There-fore, surface steam injection is generally limited to reservoirs no deeper than 800 m and, more typically, is applied in reservoirs less than 500 m deep.

CO2 Limitations. CO2 use is largely dependent on the reservoir’s proximity to large natural sources or large refineries, chemical plants, or gas plants that produce nearly pure CO2. Because oil fields are not necessarily near these sources, pipelines have been the main source of CO2, and their availability and cost limit applications of CO2 EOR. When reservoir pressure is too low or oil gravity is too high, much of the CO2 does not dissolve in the oil. However, at high pressure, enough CO2 (40–60 mol%) dissolves in the oil to improve heavy-oil recovery by causing the oil to swell, reducing the oil’s density and improving mobility.

Conventional In-Situ-Combustion Limitations. In both light- and heavy-oil reservoirs, burning some of the oil in situ creates a combustion zone that moves through the formation. In deep light-oil reservoirs, in-situ combustion (ISC) (or high-pressure air injection) is a method of generating an inert gas from spontaneous combustion of the oil at temperatures above 75°C. This provides pressure maintenance to the reservoir. In heavy-oil reservoirs, combustion creates a steam drive (whose size depends on connate water or the amount of injected water) and an inert-gas drive for the recovery of oil.