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Suspended solids in produced water
Solids are almost always present in an oil, gas, and water-producing stream. Unfortunately, the solids are usually ignored until the problems caused by the solids become so onerous that action is required. This article discusses the impact of suspended solids in produced water.
In some cases, the reservoir sands are known to be unconsolidated, and sand control is part of the project development. However, even if sand control is successful, fine solids will still be produced and end up in the produced water system. If the volume of water handled is small, the solids issues may never be important. When a lot of water is present, problems begin to appear, such as:
- Pump wear
- Formation of deposits
- Injection well plugging
- Filling vessels
- Oil carryover
Water intended to be injected is often specified to meet certain levels of particulates with a maximum size.
The solids can be produced from the reservoir rock (e.g., clays, quartz) or from hydraulic fractures (proppant flowback), or it can precipitate from the produced water (e.g., iron sulfide). Particle sizes cover a wide range. In the hydraulic fracture case, the proppant size may be 1 mm or larger, while iron sulfide precipitate can be < 0.1 μm.
Formation of schmoo
Because the solids are denser than either the oil or the produced water, they tend to sink to the bottom of the pipes, vessels, or tanks. Systems with low flow rates usually build stagnant deposits of the solids. A rule of thumb that can be used is < 3 ft/sec for lines that will build solids deposits. The solids often are coated with oil and become neutrally buoyant in the water or water/oil interface, so they remain suspended and can travel great distances. Treating chemicals such as corrosion or scale inhibitors or emulsion breakers are surface active, are strongly attracted to the surfaces of the solids, and act to attract oil, paraffins, asphaltenes, and bacteria, so that the once-dense particle is now much larger and less dense. They also are sticky and agglomerate easily, eventually forming what has been termed "schmoo."  The resulting schmoo is an organic/inorganic scale that effectively coats the surfaces exposed to the produced water: piping, vessels, meters, tubing, and injector-well perforations. This heterogeneous coating has been observed in produced-water piping more than 1 to 3 in. thick around the full pipe circumference.
Schmoo deposits harbor bacteria implicated in corrosion of the produced water system. Also, schmoo can plug injector wells, primarily in the perforations and formation face, although occasionally large quantities slough off the tubing and fill the wellbore. Many times, the produced water in the injector well tubing will build a thick enough layer of schmoo that wireline tools cannot be run. Oiled tubing-fill cleanouts are effective at removing the schmoo and restoring injectivity. Soaking the system with particular surfactant formulations has been effective in removing the schmoo deposits, preventing corrosion, and restoring injectivity.
- Hsi, C.D., Dudzik, D.S., Lane, R.H. et al. 1994. Formation Injectivity Damage Due to Produced Water Reinjection. Presented at the SPE Formation Damage Control Symposium, Lafayette, Louisiana, USA, 7–10 February. SPE-27395-MS. http://dx.doi.org/10.2118/27395-MS_
- Ly, K.T., Chan, A., Bohon, W.M. et al. 1998. Novel Chemical Dispersant for Removal of Organic/Inorganic "Schmoo" Scale in Produced Water Injection Systems. Presented at the 22-27 March 1998. NACE-98073.
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