In the North American onshore oil & gas industry, on average nearly eight barrels of water are brought to the surface for every barrel of oil. This produced water is often highly saline and contaminated by hydrocarbons and even radioactive elements: it is a hazardous waste that requires treatment, disposal, and increasingly, recycling.
Increased resuse of produced water is driving the implementation of physical, chemical, biological, and thermal treatment methods. Physical, chemical, and biological treatment methods can reduce the concentrations of certain pollutants, but the volume and salinity of the produced water is unchanged. Thermal desalination of produced water is a proven method to completely separate the salts from the water, so both can potentially be beneficially reused.
Veolia has applied proven process designs on HPD® Evaporation and Crystallization technologies used in the salt, fertilizer, and chemical industries to develop a simple and robust process to separate the flowback and produced water from hydraulic fracturing into clean water and a stable, non-hazardous solid for disposal and/or reuse.
Produced Water Chemistry
Some portion of the frac fluid injected into a well will return to the surface during the first few days to weeks. This is referred to as flowback water. Over a much longer period of time, additional water that is naturally present in the shale formation (produced water) continues to flow from the well.
Both flowback and produced water can contain very high levels of TDS, composed mainly of dissolved chloride salts of sodium, calcium and magnesium. Significant quantities of barium and strontium salts may also be present as well as some heavy metals and naturally occurring radioactive material (NORM). The produced water is also contaminated with a range of hydrocarbons.
Limitations of Conventional Methods
Conventional thermal processes for desalination of produced water require complete softening of the produced water using lime, soda ash, caustic, and other chemicals to replace the clacium and magnesium ions in the produced water with sodium ions in order to produce a crystalline solid.
In some cases, a final drying step is necessary to produce a stable solid suitable for disposal. Softening pretreatment equipment includes chemical feed/storage facilities, solids settling or filtration equipment, and sludge dewatering equipment. Drying equipment is capital and energy intensive. These additional facilities increase the footprint of the ZLD system as well as the capital costs and overall maintenance. The logistics of unloading, storing, and preparing chemicals, and dewatering and transporting sludge for disposal substantially increase the OPEX.
The CoLD® Process operates under vacuum at low temperature. The chemistry of many produced waters favor the formation of hydrates and double salts which precipitate at low concentrations as the temperature of the solution is lowered. When concentrating the waste stream at low temperature, dissolved solids will crystallize at relatively low concentration, without the need for chemical softening pretreatment and the resulting sludge production.
Advantages of CoLD Crystallization
The CoLD Process will completely desalinate high TDS produced water containing significant quantities of chloride salts. Substantial savings in CAPEX and OPEX are achieved by eliminating the chemical softening step and discharging the final solid product as a wet cake, which does not require any further drying in order to transport it to a disposal site.
This results in a simpler flow scheme, less equipment to operate, and a smaller footprint. It also eliminates the cost of buying, shipping, storing, and handling of bulk chemical softening reagents and the dewatering, storage, transport, and disposal of softener sludge. The CoLD Process recovers all the water contained in the feed at a quality suitable for discharge under an NPDES permit or reuse.