Field Evaluation of D.E. and Cartridge Filters for Completion/Workover Fluid Filtration
Authors: Cantu L.A. 
Published: December 2007

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Abstract
Proper filtration of "clean" brines is an important step in well workover or completion operations. The use of properly filtered brines has resulted in increased oil production in several Conoco wells in the Gulf Coast region. This is a direct result Of reduced formation damage. The results of a 3 ear field evaluation of D.E. and cartridge filter systems are reported and compared to findings of laboratory core flow tests to deter-mine needed brine "clarity" for minimum formation damage. The evaluations show that even highly contaminated completion/workover fluids can be satisfactorily filtered to contain less than 50 mg/i of solids with either diatomaceous earth (D.E.) and/or cartridge filter systems. Both systems removed as much as 96 to 99 percent of influent solids. The D.E. filters, in general, handled a much wider range of solids concentration more efficiently than the "nominal rating" cartridge filters. Laboratory core flow tests have shown that completion/workover fluids with approximately 5 mg/i of solids do not cause formation damage with limited pore volume injection.

Introduction
Formation damage by particle invasion and/or filter cake buildup was recognized by the oil industry as early as 1896. Rike states that as low as 50 ppm of solids in a completion fluid can plug up to 16 perforations if 50 bbl of brine is lost to the formation during a Job. Consequently, the school of thought among many oil producers is to use clean brine to control formation damage rather than stimulate later. Once the damage in permeability has incurred, it cannot be 100 percent removed by any means. The loss in permeability translates to a decrease in anticipated production rates. The use of clean completion fluids has resulted in more production at faster and more efficient rates. Pasztor and Snover state that these "solids-free" fluids have caused increases in well productivity of as much as 850 percent. McLeod and Crawford calculate a twelvefold improvement ratio by the use of ultraclean brine. Thus the use of clean brine for completion operations translates into higher profit and lower operating cost for oil producers. As a result, a proper filtration of brine becomes an important step in well workover or completion operations.

Background
Filtration is a process used to remove suspended materials from liquids (as discussed in this paper). There are various filter systems available as Nall describes. In the late 1970s, it was recognized that high-rate completions could be achieved in Gulf of Mexico wells if good completion practices were followed. One such completion practice is the use of clean completion fluids. Fluids, at that time, were being filtered with nominal cartridge filters or bag filters, if they were filtered at all. Quality of the filtration was being evaluated by centrifuge, or by reading a newspaper through a sample container.
In completion/workover fluids, the suspended solids which need to be removed vary. As Allen indicates, the contaminating solids may be weighting materials, clays, viscosity builders, fluid-loss control materials, perforating-charge debris, lost-circulation materials, drill solids, cement particles, rust and mill scale, pipe dope, undissolved salt, gravel-pack or frac-sand fines, precipitated scales, paraffin, or asphaltenes. Because these various contaminants are widely different in composition, size, and shape, filtration of completion and workover fluids is not a simple task. Other parameters affecting the filtration process, such as fluid density, and viscosity, are mentioned by Glaze and Echols.
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