Study Of Filters Used For Filtering Workover Fluids
Authors: Sparlin Derry , Guidry J.P. 
Published: December 2007

Abstract
Improved productivities can be obtained from wells if clean fluids are used for workovers and completions. All clays, silts, or sands that are suspended in wellbore fluids can be deposited in the producing formation and the perforation channels where they will reduce a well's production rate.
Filters are often used to clean up fluids. Although two micron cotton filters are commonly used for this purpose, there are other types of filters and filtration systems available that could do a more efficient job for less cost.
A study has been made of the efficiency and life of various filter elements. Some of the factors evaluated were: filter materials of cotton and polypropylene; micron size ratings of 2 microns to polypropylene; micron size ratings of 2 microns to 75 microns; and flow rates of 1 to 3 barrels per minute.
Due to the extremely large number of variables, this study is continuing, but some important conclusions have been reached. These conclusions are:

Conclusions
Filter life is longest at low rates. When more than one barrel per minute rate is required, the flow should be split into 2 or more filter pots in parallel. Series filtration through 50 micron and 2 micron rated filters provides good filtration and filter life at flow rates to 3 BP per pot. per pot. Sock type polypropylene filters showed best overall efficiency of the three types of filters tested. Cotton sock type filters have best efficiency at low flow rates, but fail at higher flow rates. Polypropylene filters should be changed when the pressure differential reaches 35 psi.
A better understanding of filters and filtration technology can provide cleaner fluids and reduce the high costs and wastes of ineffective filter systems.

Introduction
Formation damage by solids in completion and workover fluids can result in considerable loss of production from oil and gas wells. Solids injected production from oil and gas wells. Solids injected into the formation may be trapped by the matrix or in the perforation tunnels and reduce productivity. This problem frequently occurs in operations such as acidizing, washing perforations, killing a well, injecting chemicals into a formation and in sand control jobs.
It is very difficult to remove these solids once they have been placed into the formation, especially if they invade some distance into the formation matrix. The pore sizes of typical productive miocene sands in the Gulf Coast are between productive miocene sands in the Gulf Coast are between 0.2 and 7.5 micron diameters. Thus, solids of these sizes can invade into the formation sand where they may be trapped.
To estimate the volume of this size range of solids required to significantly damage a formation sand, one may calculate the amount of solids that will completely fill a pore volume of the formation sand l' radially around the well bore. For instance, one foot radial volume around a 7" well bore would require only one quarter of a cubic foot of solids to completely fill the pore spaces of a one foot interval assuming twenty percent porosity. Using this calculation, 7.14 barrels of water containing only one thousand parts per million (0.1%) of solids, would completely fill the pores if all of these solids were trapped in the first radial foot of the formation.