Absolute filter is a kind of filter rated to remove particles or organisms larger than a specific size from a fluid stream. Absolute filters are rated in micrometers (microns).
An absolute filter rating of 20 microns means that the filter has the capacity to remove particles or organisms with diameter larger than 20 microns from the fluid stream.
Sometimes, the term absolute filter is used to refer to the filter itself rather than the filter rating. An absolute filter is supposed to have 100% efficiency such that any particle or organism greater than the rated size of the opening will be completely prevented from passing through the filter. However, this is not always the case.
Limitations of Absolute Filter
The rating of an absolute filter is done in laboratories using spherical glass particles under low pressure conditions. This laboratory method of rating an absolute filter fails when the filter is used on the field.
For instance, some of the particles we are trying to keep off may not be spherical at all; they may be cylindrical in shape. Cylindrical particles with a thickness smaller than the size of the filter opening will pass through the filter even though the filter was designed or rated to remove them.
Another limitation is in the pressure exerted on the filter. Low pressure is used when rating the filter to remove particles of a specific size in the laboratory.
On the field however, pressure from the fluid stream could be high with pressure surges at some other times. The high pressure and flow rate may force larger particles through the filter openings.
Field Application of Absolute Filters
In well workover and stimulation with a treatment fluid, absolute filters can be placed adjacent the producing formation to keep off particles or organisms of certain micron sizes from getting into the producing formation through the treatment fluid.
This tactic helps reduce any form of damage that could arise when such particles or organisms (like microbes) get into the producing formation that is being treated. Microbes, for example, can reduce the oil and form undesirable emulsions.