★◆Structural features and application ranges of various filters◆★

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Although the filters used in oil fields come in a wide variety, analyzing their structural characteristics based on the filtration mechanism mainly involves the filter layer thickness, pore size, and pore structure. Whether the filter operates primarily through screening or adsorption depends on the thickness of the filter layer. Filters with relatively thin filter layers primarily serve a screening function ； Filters with thicker filter layers rely primarily on adsorption. The precision of a filter depends on the size of the pores in its filter media. However, when the pore sizes are the same, the filtration precision achieved through adsorption is much higher than that achieved through screening. Therefore, deep-bed filters that rely primarily on adsorption are widely used in oil fields. However, backwashing filters that rely primarily on adsorption is difficult; desorption is the key to backwashing such filters, and it depends both on the adsorption strength of the filter media for suspended particles and on the size of the pores in the filter layer. If a filter whose pore structure remains unchanged during filtration and backwashing is called a fixed-pore filter, while one whose pore structure can change during backwashing is called a non-fixed-pore filter, then backwashing a fixed-pore filter is much more difficult than that of a non-fixed-pore filter. Since the oil present in water has a high adsorption strength for most filtering media, desorption is very difficult in fixed-porosity filters; therefore, fixed-porosity filters are generally not suitable for filtering oil-containing produced water. The following is an analysis of several typical filters used in oil fields: 1. Quartz sand filter – A quartz sand filter is a typical deep-bed filter; its structural feature is a thick filter layer, the high density of the quartz sand as the filtering medium, and a relatively stable filter bed. The working mechanism of a quartz sand filter is primarily adsorption, with screening playing a secondary role. Since the filter bed remains fixed during backwashing and is a type of fixed-porosity filter, it is difficult to dislodge the tiny particles that are adsorbed in the filter layer; as a result, backwashing has limited effectiveness in restoring the filtering capacity. After being in use for a while, the filtering performance declines significantly, often requiring the replacement of the filter media. This type of filter is generally used for filtering clean water where the requirements regarding water quality are relatively low. 2. Light-weight media filters: The light-weight media filters used in oil fields are mainly walnut shell filters. The basic structure and filtration principle of these filters are the same as those of quartz sand filters; the difference lies in the fact that walnut shells, which serve as the filtering medium, have a lower density, typically around 1.2 g/cm³. Due to the light weight of the filter media, during backwashing the filter layer becomes a bubbling bed under the action of water flow; the micropores formed in the gaps between the filter media are eliminated, allowing the adsorbed suspended solids to be desorbed. Therefore, this type of filter belongs to the category of non-fixed-pore filters; it has strong backwashing and regeneration capabilities, stable filtration performance, and is suitable for filtering produced water from formations with medium to high permeability. 3. Microporous ceramic filter: The filtering element of this type of filter is a porous ceramic tube that has been sintered; its body serves both as the filtration layer and as the support layer. This type of filtering medium is characterized by uniform and stable pores; it filters out larger suspended particles while adsorbing smaller ones. Backwashing involves flushing the filtering medium with reverse and lateral flows, which is effective at removing particulates, but has little effect on removing adsorbed substances; as a result, it can easily lead to blockages. This filter is suitable for filtering clean water from formations with moderate permeability. When used for filtering oil-containing produced water, it is difficult to backwash and regenerate the filter elements; cleaning agents must be added and methods such as air blowing are required, making the backwashing process very complex. 4. Membrane filter: The core component of this type of filter is the membrane, which is a thin film covered with even smaller pores and fabricated on a microporous support layer (substrate). There are many materials used to make filter membranes, which are divided into organic membranes (such as polysulfone hollow fiber membranes) and inorganic membranes (such as ceramic membranes). As a filtering element, the filter membrane is characterized by an extremely thin filter layer; therefore, its filtration mechanism is primarily based on screening, with little adsorption effect. Therefore, membrane filters have high filtration precision, stable particle size control, and their performance can be easily restored through backwashing. However, if there is oil in the water, it can easily get clogged and is difficult to backwash. Many researchers, both domestically and internationally, have hoped to use ceramic membranes to treat produced water based on the hydrophilic properties of ceramic materials; however, after research, it is generally believed that the problem of membrane fouling remains difficult to resolve. Membrane fouling is a very complex issue, but one thing is certain: the presence of large amounts of organic matter among the pollutants is undeniable. However, attention is usually focused on the membrane itself, with less consideration given to the support layer. . . By analyzing based on the adsorption mechanism and taking into account the pore characteristics of the membrane layer and the support layer, it is possible to explain why membrane filtration is less prone to clogging in clean water but highly susceptible to clogging in oily wastewater. For general solid suspensions, particles larger than the membrane pores are retained (screened out) on the surface of the membrane, while particles smaller than the membrane pores pass through them. Since the membrane serving as the filter layer is very thin, it is unable to adsorb these particles. Although the support layer is relatively thick, its pores are larger compared to those of the membrane, so it is not sufficient to enable adsorption. In this way, there is only a screening effect, so it is easy to backwash and remove. As for oil in water, due to the instability of oil droplets, those larger than the membrane pores are not necessarily filtered out, as they may split into droplets smaller than the membrane pores and pass through them. Although they can penetrate the membrane itself, they can aggregate into larger droplets in the support layer, where they are trapped by the micropores of that layer and cannot be released. Therefore, oily wastewater can clog the membrane filter, and backwashing becomes difficult. Therefore, this type of filter is suitable for filtering clean water with the quality requirements of low-permeability formations, but not for filtering oil-containing produced water. 5. Fiber media filter: This is also a type of deep-bed filter, with the filtering medium made of fiber materials, usually synthetic fibers; common types include fiber balls and fiber bundles. The filtration mechanism of this filter is that the fibrous medium is compressed under external forces (hydraulic or mechanical forces), resulting in the formation of tiny pores; adsorption then occurs, allowing the suspension particles in water to be filtered out. During backwashing, the compressive force is removed to loosen the fiber filter media, causing the adsorbed suspended solids to desorb and be removed by the flow of water used for backwashing. Since the fiber material is very fine, the pores formed after compression are also very small; as a result, the filtration accuracy is extremely high, making it an ideal fine filter. However, if there is oil in the water, it will be very troublesome. This is because synthetic fibers are generally oil-loving and have a strong affinity for oil; once oil is adsorbed onto the fiber filter material, it acts as an adhesive that binds the fibers together, making it difficult to separate them and thus complicating backwashing. This filter is suitable for filtering the water quality used in water injection into low-permeability formations with fresh water as the water source. It has been applied with relatively good success so far. In addition, some domestic research institutions (such as the Jianghan Machinery Research Institute) are studying oil-repelling fibers as well as fiber modification techniques, with the aim of using such filters for the filtration of oil-containing produced water; significant progress has already been made in this area. If the oil adsorption capacity of the fiber material itself can be reduced to a sufficient level, fiber media filters, especially fiber bundle filters, will become ideal devices for filtering the water recovered from low-permeability oil fields prior to its reinjection.