Water circulating during steam flooding to recover additional oil from reservoirs often becomes enriched in dissolved silica. As the silica concentration increases in the water, silicate minerals become supersaturated. At supersaturation conditions, amorphous silica and metal silicates may deposit in heat exchangers. Scale deposits not only reduce heat exchange efficiency due to fouling, but they may plug tubes resulting in tube failure. Control of these deposits in boilers and co-generation equipment is necessary to reduce down-time for mechanical cleanout.
In the present study at a steam flooded field, steam boiler and co-generation tube deposits consisted of opal-A, pectolite, aegerine, clinamphibole, serpentine-like minerals, iron oxide corrosion products, and apatite. Iron in the silicate minerals appears to derive from transporting incompletely oxygen scavenged circulating water to the boilers and co-generation unit in carbon steel pipe. Increasing the concentration of oxygen scavenger and EDTAbased scale inhibitor in the feed water has significantly reduced scale deposition.
Other methods that may be used to control silica/silicate scaling in steam flood operations include:
dilution with fresh water,
reducing the pH of the water,
treating the water with reducing, complexing and sequestering agents,
removing silica from water by lime softening,
precipitation of silica in water with metals or cationic surfactants, and
treating the water with geothermal silica scale inhibitors/dispersants.
Scale deposition and corrosion in steam flooding equipment was recognized early as a problem when this enhanced oil recovery method was initially developed and implemented. Boilers and tubes failed due to heavy scale fouling and corrosion. Depending on the water source used for steam generation, scaling was usually caused by the presence of calcium, magnesium, iron and silica. Corrosion was most often caused by dissolved oxygen in the water. Early attempts to mitigate scale and corrosion in field heaters included scavenging oxygen, softening the makeup water, treating with chelating/sequestering agents and controlling pH with caustic.1 (Bradley, 1975) More recently, scaling and corrosion has had to be addressed in once-through-steam-generators (OTSG), steam-assist-gravity-drainage operations (SAGD), tar sands oil recovery projects, and co-generation units used to supply steam for oil recovery and electricity for pumps.2–8 Methods described to control silica scaling in oilfield steam generation systems include hot-lime softening, crystallizer-clarifiers, retention ponds, filters, ion exchange softeners, pre-precipitation with aluminum ion 9, reverse osmosis, removal of metals that precipitate silica by chelating ion exchange, dilution with fresh water, and silica scale inhibitors.10