Currently, most domestic oil fields have entered the high water cut period, where water flooding remains the primary method to increase crude oil production. As the water output increases, a significant amount of oil-containing wastewater is present in the produced water returned to the surface of the oil field. To achieve the goals of ecological environmental protection and resource recycling, how to efficiently treat oil-containing wastewater with minimal investment is a challenging issue for many oil workers. In recent years, with the gradual maturity of related separation membrane technology, some experts and scholars have conducted research on the use of this technology for treating oil field wastewater. Compared to traditional oil separation methods, this technology offers simpler配套设施, easier operation, high separation efficiency, and low energy consumption, making it an important research direction for treating oil field wastewater.

Challenges in the Application of Membrane Separation Technology in Oilfield Produced Water Treatment

During oilfield extraction, the oil content in the produced water on the surface ranges from 1.2 to 100 milligrams per liter, with the total dissolved solids at a ratio of 1000 to 1500 milligrams per liter of wastewater, and organic pollutants at 20 to 12,250 milligrams per liter. Addressing these pollutants, treatment using membrane separation technology presents significant challenges, especially for organic contamination. For instance, when the oil content is at 200 milligrams per liter, the organic pollutants can exceed 5,000 milligrams per liter, and the total suspended solids can surpass 4,000 milligrams per liter. Under these conditions, using membrane separation to remove impurities may likely clog the pores of the filter membrane, reducing the lifespan of the filter. To achieve greater results in practical applications, the oilfield produced water treatment technology with membrane separation should be combined with other wastewater treatment processes.

2. Influencing Factors in the Process of Oil-Contaminated Wastewater Treatment Using Membrane Separation Technology

2.1 Materials Used in Separation Membranes and Pore Size

When treating oil-containing wastewater from oil fields, the material of the separation membrane should be determined based on the chemical characteristics of the wastewater. If the main components of the original oil in the wastewater are dispersed oil droplets and floating oil, the pore size of the filter membrane should be set between 10-100 micrometers for microfiltration membranes. However, if the oil in the wastewater consists of stable emulsified oil and dissolved oil, ultrafiltration membranes with hydrophobic properties should be selected.

2.2 Operating Temperature and Pressure Difference

The effectiveness of separator membranes in wastewater treatment is influenced by the temperature conditions they are exposed to. Typically, the treatment temperature for wastewater is around 30-50 degrees Celsius. When using filtration membranes to treat oily wastewater, an operating pressure difference at the critical state should be applied across the membrane. If the pressure difference does not exceed the critical value, the permeate flux will increase as the pressure difference gradually increases. If the applied pressure difference is less than the critical value, the permeate flux will decrease as the pressure difference increases.

2.3 Liquid Concentration and Flow Conditions

When treating oil-containing wastewater with a low concentration of feedstock, the flux of the filtration membrane is directly proportional to the pressure difference applied. However, once the feedstock concentration exceeds a certain threshold, the amount that can permeate is no longer directly related to the pressure conditions, but rather to the flow rate across the membrane surface. Since altering the flow conditions of the feedstock can enhance the efficiency of the membrane separation process for wastewater, it is advisable to select a scientifically and reasonably optimized material flow state based on the actual conditions of the feedstock in the filtration membrane separation system. This will further improve the efficiency of the membrane in treating oil-containing wastewater.

2.4 Film Contamination

The term "contamination to the filter membrane" refers to the accumulation of various substances in the oil-contaminated wastewater, forming deposits on the membrane surface through physical, chemical, or mechanical interactions. The issue of contamination in filter membranes is a constraint factor that hinders the widespread application of membrane separation technology. Therefore, when treating oil-contaminated wastewater using filter membrane separation technology, it is essential to choose a more reasonable filter membrane and adopt appropriate operational methods.

3. Design of membrane separation technology for treating oil-contaminated wastewater

In the oilfield water injection process, oil-contaminated wastewater is isolated using filtration membrane technology. Organic matter and oil components in the wastewater are pre-treated with primary buffering disinfection and catalytic oxidation. Larger particle substances in the wastewater are separated through secondary treatment using filters with different pore diameter grades. The goal is to make the treated oil-contaminated wastewater suitable for reinjection into the formation.

3.1 Analysis of Oily Wastewater Characteristics

The produced oilfield wastewater, formed by waterflooding, is a type of particularly special industrial wastewater with a complex composition and high mineralization. It contains a variety of organic substances. These characteristics provide a breeding ground for the presence and abundant growth of microorganisms. The performance indicator for the chemical oxygen demand required to treat organic substances is in an unstable state. The chemical oxygen demand is a key parameter in the oilfield wastewater treatment process, and the amount of it becomes a focal point in the filtration membrane separation technology for treating oilfield wastewater.

3.2 Selection of Film Material and Pore Size Parameters

Filter membrane composition is a major factor affecting the treatment of oil-containing wastewater. It is essential to conduct a thorough study and analysis of the components in oil-containing wastewater, determining their chemical properties. For instance, when using microfiltration membranes for wastewater treatment, hollow fiber materials should be chosen for the filter membranes, primarily composed of inorganic ceramics. On the other hand, for ultrafiltration and nanofiltration membrane filtration technologies, filter membranes made of high molecular materials should be selected. During the filtration process of oil-containing wastewater, the detection indices of the effluent include solid suspended matter, pH level, organic matter content, sulfur compounds, and the concentration of petroleum substances. When treating the wastewater, the size of the filter membrane's pore diameter should be determined based on the particle size of the substances, ensuring the treated water meets the requirements for discharge or injection into the formation.

3.3 Procedures for the Treatment of Various Harmful Substances in Oily Wastewater

The main procedures for filtering and eliminating various substances from oily wastewater are as follows:

1) The treatment of oily wastewater involves a buffering disinfection process, utilizing an ozone generator to produce ozone gas without secondary contamination. This ozone is then introduced into the oily wastewater, where it eliminates microorganisms and treats organic matter, while also preventing flammable gases from entering subsequent treatment equipment. Oil droplets in suspension rise along with the generated ozone gas, forming an oil film or layer, with droplet particle sizes not less than 100 nanometers. When the wastewater treatment is at rest, the dispersed oil droplets remain suspended in the wastewater. After a period of settling, they aggregate into larger oil lumps that float on the surface, with droplet particles reaching 10-100 micrometers.

2) The microfiltration membrane achieves a one-time separation of wastewater, featuring high treatment efficiency, minimal adsorption on the membrane surface, and resistance to media shedding, making it highly valuable for promotion and application. The microfiltration membrane can filter out organic biological matter and smaller ions with sizes between 0.02-10 micrometers from wastewater. Emulsified oil droplets can also be isolated during this treatment process due to its high flux, which allows its application in the pretreatment processes before ultrafiltration and nanofiltration membranes. Ultrafiltration membranes can allow substances with diameters of approximately 2-20 nanometers to pass through, effectively isolating colloids, proteins, and viruses with larger molecular weights.

3) The secondary treatment of oil-contaminated wastewater utilizes nanoscale filtration membranes to concentrate and then remove salt from the wastewater. This process can only be employed if the wastewater discharged prior to this stage does not meet the required water quality standards. It acts as a supplementary process to the preceding treatment techniques, especially in conditions where the oil-contaminated wastewater has severe quality issues and a high presence of impurities. The nanoporous membrane has a pore size of 1-2 nanometers, providing filtration and separation capabilities between ultrafiltration and reverse osmosis. It effectively treats substances like low molecular weight organic matter, antibiotics, and inorganic salts.

4. Closing Remarks

Utilizing membrane separation technology to treat oily wastewater generated during the oil field water injection process, the tube bundle condenser achieves the goals of green ecological environmental protection and energy conservation. Moreover, the treated wastewater can be reinjected into the formation, presenting promising application prospects. However, to achieve optimal results, the membrane separation technology should be efficiently combined with various water treatment techniques.