A frequently asked question concerning the re-injection of produced water is: 'How clean must the water be to avoid injection well plugging problems?' Over the years, industry has developed several techniques to establish cleanliness specifications.1 These techniques include both on-site coreflood studies as well as laboratory investigations. The specifications derived from these tests often include maximum allowable values for the content of solids and residual or emulsified oil in the water. Recent field and laboratory studies indicate that two of the most widely used specifications, oil-in-water and total suspended solids, may not provide the best indication of whether a produced water will or will not plug a target formation.
Field data suggest that the plugging characteristics of a produced water may better be defined by the term Total Filterable Solids. This term treats suspended oil as a solid rather than a liquid. The field data also shows that there was no increase in plugging frequency even after an unexpected and significant increase in the oil content of a produced water system at Prudhoe Bay, Alaska.
Laboratory studies, which correlate with field experience, are also presented. The laboratory studies show that produced waters containing relatively high concentrations of oil-in-water can be re-injected as long as the oil droplet size is less than or equal to the average pore throat diameter of the reservoir. Furthermore, these lab studies show that it is important, when developing new produced water specifications, that core flood experiments be conducted on cores at residual oil saturation rather than brine or water saturated cores.
In June 1992 a significant change occurred in the produced water quality at the Flow Station #2 (FS2) production facility at Prudhoe Bay, Alaska. Prior to this time, the oil content of the produced water averaged 50 mg/L. Since then the daily average has been closer to 200 mg/L with intermittent spikes of 500 mg/L and higher. In review, it was found the increase in oil content was coincident to a change in surface line corrosion inhibitors.