Shale Oil Production Wastewater: Sources, Characteristics, and Treatment Challenges

Source of Oil-contaminated Wastewater in Shale Oil Production

The oil-contaminated wastewater in shale oil production primarily comes from the following stages:

· Distillation Unit Drainage: This is the main source of oil-contaminated wastewater, including moisture from the shale, water carried by the main air, water vapor from the furnace basin, and a small amount of water produced by chemical reactions. These waters participate in the distillation process and form oil-contaminated wastewater.

· Distillation Product Washing Water: In the process of washing distillation products, gas purification, and shale oil recovery, a large amount of oil-contaminated wastewater is generated.

· Cooling Water System Drainage: Oil-contaminated wastewater is also produced during the cooling cycle of gas and pump operations.

· Tank Area Dehydration: The shale oil in the measuring and finished product tanks generates a certain amount of oil-contaminated wastewater during dehydration.

Characteristics of Oil-contaminated Wastewater

This type of wastewater has the following characteristics:

· High Organic Content: The wastewater is rich in volatile phenols, petroleum compounds, and other organic pollutants.

· High Ammonia Nitrogen Concentration: Nitrogen compounds, especially ammonia nitrogen, are present in high concentrations.

· Presence of Multiple Pollutants: The wastewater contains other pollutants, including sulfides, cyanides, etc.

· Poor Biodegradability: The aromatic and oxygen-containing compounds in the wastewater inhibit microbial activity, making it difficult to degrade the pollutants effectively through biological methods.

Treatment Methods for Oil-contaminated Wastewater

The common treatment methods for this type of wastewater include:

· Oil Separation: Oil-water separators or oil separation tanks are used to remove floating oils. The oil-water separator uses the density difference between oil and water to allow the oil to float and be removed. The oil-water separator physically separates floating oil and suspended solids.

· Demulsification and Dissolved Air Flotation Treatment: After the oil is separated, emulsified oil is further treated in a demulsification flotation tank. Demulsifiers and flotation agents are added to break the emulsion and allow the oil to float and separate.

· Biological Treatment: Microbial metabolic processes are used to convert organic pollutants, with common methods like activated sludge and biofilm methods. However, due to the presence of microbial inhibitors in the wastewater, the effectiveness of biological treatment may be limited.

· Physicochemical Treatment: Adsorption, oxidation, reduction, and other methods are employed to remove pollutants. This method is efficient and stable but requires careful selection of reagents and control of conditions.

· Advanced Treatment: If the water quality does not meet the required standards after the above methods, advanced processes like advanced oxidation and membrane separation can be used. Advanced oxidation uses strong oxidants to degrade organic substances, while membrane separation utilizes selective separation mechanisms to remove pollutants.

Problems with Traditional Treatment Methods

· Poor Oil-Water Separation: Insufficient chemical dosing and improper settling temperature reduce separation efficiency; delayed cleaning of inclined plate tanks and improper clear water control lead to increased oil and mechanical impurities in the effluent, raising the subsequent treatment load.

· Corrosion and Blockage of Equipment: The high content of sulfides and bacteria in the wastewater accelerates equipment corrosion, shortening the service life. Calcium carbonate precipitation easily forms scale, clogging pipes and reducing water flow capacity, affecting system operation.

· Insufficient Treatment Capacity: As the amount of produced water increases, the system cannot keep up with the treatment load, requiring capacity expansion, which increases investment and operational costs. Oil separation tanks that do not collect oil in a timely manner may clog filter pipes, affecting separation. Filtration tank filter media compaction can reduce filtration efficiency, affecting the effluent quality.

· Environmental Risks: The treatment process generates a large amount of oily sludge, and the existing sludge reinjection capacity is insufficient, posing environmental risks. If the effluent does not meet standards and is discharged directly, it will pollute the soil and water environment.

· Technological Limitations: Heavy oil wastewater has high viscosity and is difficult to treat. High-pressure steam extraction wastewater contains high levels of oil, making purification challenging. Low-permeability oil production wastewater can cause formation plugging, requiring adjustments to membrane coefficients, making the process complex and costly. Polymer flooding wastewater has increased viscosity due to polymer additives, making oil-water separation even more difficult, thus further complicating treatment.