Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.
Waterflooding was first practiced for pressure maintenance after primary depletion and has since become the most widely adopted improved-oil-recovery (IOR) technique. It is now commonly applied at the outset of reservoir development. The reservoir-connate-water composition usually differs significantly from the composition of water available for injection. Parametric laboratory studies of crude-oil recovery showed that, for connate and injected brines of the same or different composition, waterflood recoveries could differ substantially depending on brine composition. However, laboratory tests designed to predict waterflood performance usually have not incorporated the difference in connate and injected brines.
Improved recovery of crude oil by low-salinity waterflooding (LSW), with only modest increase in resistance to flow, was reported by Tang and Morrow (1997). Since then, many laboratories and organizations have grappled with the opportunities and problems associated with identifying, reproducing, and explaining the low-salinity effect (LSE). Various forms of LSE and the necessary conditions for its occurrence are reviewed. The evidence, mounting rapidly from both field and laboratory studies, is examined with the aim of facilitating identification of LSW targets and design of the floods.
Necessary conditions for LSE identified by Tang and Morrow (1999) in Berea-sandstone cores were as follows.
• Significant clay fraction
• Presence of connate water
• Exposure to crude oil to create mixed-wet conditions
Note, however, that while necessary for the types of sandstones investigated, these conditions are not sufficient; many outcrop sandstones meeting these conditions have not shown LSE recovery. The cause of such significant differences has yet to be identified. Identification of promising LSE projects requires specific laboratory tests, followed by pilot tests of increasing scale. It should be noted also that some special cases of LSE have been identified in which dissolution of soluble minerals occurs upon injection of the low-salinity flood water in either carbonates or sandstones.