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Filter cake is a deposit that forms on the surface of a filter over time as particulates become trapped because they cannot pass through the filter. Eventually, the buildup can prevent smooth operation of the filter, because only minimal amounts of fluid will be able to pass through for filtration. At this point, the filter cake needs to be flushed or removed. With a disposable filter, the technician can dispose of the filter and replace it once it is no longer functional.
These buildups can occur anywhere, but they are of particular concern in the oil and gas industry, which has dedicated considerable research to the topic to learn more about the formation and treatment of filter cake. Drilling muds and other media pass through filters on their way to and through a well, and they can leave unwanted deposits over time. These can potentially cause problems with the lines in the well, slowing or stopping production.
One issue in this industry is the formation of a sticky gel of polymers in the filter cake. These can act like glue to force the material to stick to the filter, and also create an impermeable mass. Drilling fluid cannot pass through the buildup, and the line is effectively blocked until it can be removed. Some systems may allow for flow reversal to force the filter cake off by applying pressure from the other side. In others, it needs to be manually removed with the assistance of a chemical treatment to break up the cake.
Companies that manufacture drilling muds and filters conduct testing to learn about the filter cake properties of given fluids. If a material causes a rapid buildup, it may not be suitable for drilling. They look at how quickly the deposit of particulates develops, and the level of permeability offered. If the layer of debris is highly permeable, it can allow fluids to pass through, and may not pose a significant problem at a drilling site. The company can take note of these qualities and discuss them in the technical specifications to make users aware of any issues they may experience.
Such buildups of material can appear with other types of filters as well. Since filters are often not readily visible, operators can only estimate the amount of filter cake present on the basis of changes in filtration speed and other issues. Some systems use a warning light to indicate when a filter has been in place too long, so operators know to check the filter and clean or replace it, if necessary, after a set period of time.