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Abstract: It analyzes the screening and adsorption functions of filters, as well as the working principle of backwashing operations. It outlines the structural features and application ranges of quartz sand filters, lightweight media (walnut shell) filters, microporous ceramic filters, membrane filters, and fiber media filters used in oil fields. It is pointed out that fixed-pore filters are not suitable for filtering oil-containing produced water ； By studying non-lipophilic fibers and fiber modification techniques to reduce the oil absorption of the fibers to an adequate level, fiber bundle filters will become an ideal device for filtering the water recovered from low-permeability oil fields prior to its reinjection. To meet the requirements of various reservoir conditions regarding injection water quality, a variety of filters have been developed and applied in different oil fields in recent years. However, due to a insufficient understanding of the principles and performance of filters, especially regarding the role of adsorption during filtration and backwashing, many engineering and technical personnel face certain difficulties when designing and using water injection filters. Based on the characteristics of oil field produced water, the author conducts an analysis and study of filtration principles in order to achieve a proper understanding of the applicability of various filtration methods and equipment, thereby providing assistance in the development, selection, and management of oil field water injection filters. Quality characteristics of produced water: The components of produced water include suspended solids (SS), crude oil, total dissolved solids (TDS), and dissolved gases. Affected by geological conditions and treatment processes, the composition and properties of produced water vary greatly among different oil fields or even different blocks. From the perspective of water injection, there are still some common characteristics, among which oil content is a common feature of produced water. The requirement of the reservoir for the quality of the injected water is that it must not cause blockage of the percolation pores in the injection layer. China’s industry standard for petroleum and natural gas, SY/T 5329–94 \"Recommended parameters and analysis methods for water quality used in water injection in clastic rock reservoirs\", specifies the requirements for water quality conditions for water injection in different permeable layers. The main cause of formation blockage when injected into water is suspended solids. The sources of suspended solids in produced water can be diverse; they include solid particles from the formation, such as clay, as well as microorganisms like bacteria. They may also arise from the interaction between dissolved solids and gases, resulting in insoluble substances such as calcium carbonate, magnesium carbonate, barium sulfate, iron oxide, and iron sulfide. Additionally, related to blockages are the tiny oil droplets that are suspended in water in a dispersed state. These oil droplets are unstable particles that may merge into larger oil droplets or split into smaller ones. Acting as binders, they combine with fine solid particles in water, causing severe formation blockages and also contaminating filtration equipment. Therefore, the key step in purifying the produced water for reinjection is to remove suspended solids and oil through filtration. The main working principle of a filter: Filtering is a process of separating two phases, achieved by using a filtering medium (filter media) to trap the suspended solids, which are in the dispersed phase, while allowing the water, which is in the continuous phase, to pass through. The structure of a typical filtration device includes a filter layer and a support layer; Figure 1 shows an abstract schematic of the filtration principle. The retention of suspended solids by filter media can be divided into screening and adsorption. The screening effect is aimed at larger suspended particles, which are retained on the surface of the filter layer since they cannot pass through it ； Smaller suspended particles, although able to enter the filter layer, come into contact with the filtering medium as they pass through it and are adsorbed in the filter layer, thus being removed – this is the adsorption process. For deep-bed filters such as sand filters, the particles that can be filtered out are much smaller than the pores in the filter media, indicating that their working mechanism is primarily adsorption rather than screening. How does adsorption occur? Simply put, adsorption takes place when the attraction exerted by the filtering medium on the suspended particles is greater than the drag force exerted by the water flow on those particles. The suction force of the filtering medium on suspended particles mainly depends on two factors: the material properties and the structure of the filtering medium. The material property factor is the surface attraction resulting from its chemical properties ； Structural factors are due to the fact that the porous structure enhances adsorption. When the material is the same, the greater the contact area between the particles and the surface of the medium, the stronger the suction force. The micropores formed in the filtering medium, on the one hand, force the small suspended particles to come into contact with the filtering medium, and on the other hand, they increase the contact area exponentially, as shown in Figure 2. In this way, the suspended particles within the pores of the filtering medium are subjected to a sufficient suction force. At the same time, the water flow within the pores is generally in a laminar state, with a low flow velocity; as a result, the drag force on the suspended particles is also low, which leads to a strong adsorption effect. Therefore, the adsorption effect of the filter layer pores has a significant impact on filtration performance, and creating more and smaller filter layer pores can effectively improve filtration accuracy. After the filter has been in operation for a certain period of time, as the amount of debris accumulated reaches a certain level, its performance will decline, primarily manifested in a decrease in filtration speed and a reduction in filtration accuracy. At this point, backwashing is required to remove the trapped particles and restore the filtering capacity. The mechanism of backwashing is actually the reverse process of filtration. For the residues resulting from the screening process, they can be removed simply by reversing the flow of the filtered water ； However, the case of the retained substances due to adsorption is not so simple, because even with high backwashing intensity, the reverse flow remains in a laminar state within the pores; the magnitude of the drag force is limited, and it cannot completely change the adsorption state. Therefore, the removal of adsorbed contaminants must employ appropriate desorption methods tailored to the adsorption characteristics of the specific filtration device. Generally speaking, the material factors of the filtering medium that cause adsorption are difficult to change; desorption can only be achieved by altering the structural factors of the filtering medium, usually by breaking down the micropores it forms. Therefore, whether the pores of the filter layer can be opened is the key to determining the backwashing effect. The oil contained in oil well production water can be regarded as suspended particles during filtration operations, but compared to solid particles, they are unstable particles. Dispersed oil droplets tend to split into smaller ones, as well as merge to form larger ones; moreover, their shape can also change easily. Therefore, during the filtration process, oil droplets cannot truly be screened out; they can only be adsorbed, either onto the solid particles that are retained by screening or onto the filtering medium itself. Compared to solid particles, oil droplets have a stronger adsorption effect, making desorption more difficult.