The article “Norwegians develop new method to measures shaker screen performance” indicated correctly that the simple designation of a “mesh” size could no longer be used when describing current shale shaker screens (OGJ, Nov. 19, 2007, p. 37). An American Petroleum Institute task group working on solids control procedures also found that the optical image size distribution curve resembled a classic cut point curve but failed to match field data.
The task group first sought to develop a universal procedure similar to that described in the OGJ article to describe performance of shaker screens. This seemed to have too many variables to be universally applicable for all shaker motions and all water-based fluids and all nonaqueous fluids (NAFs). The most prominent of these screen performance modifiers is the fact that the bound liquid layer affects the effective screen opening size particularly with screens having very small openings. The most dramatic, of course, is the blinding of these screens in a NAF when the screen becomes water-wet. The bound layers of wetting fluid have different thicknesses. Some of the properties controlling this involve surface tension properties (that are usually not measured) and gel strengths depending upon the viscosifiers used in the drilling fluid. This will greatly affect the opening size for particles to pass through. The task of defining screen performance for all fluids and all shaker motions seemed too large for the API task group to develop recommended procedures (RPs) that could be universally applied.
As indicated in the article, the new API RP 13C does not attempt to describe shaker screen performance. The objective of the test procedure is simply to describe a shaker screen in a reproducible manner. Aluminum oxide particles were selected because they were the only particles which gave reproducible results in blind comparison tests.
Sand was the first choice because sand can be found in abundance worldwide. Blind comparison tests revealed that the data was not reproducible. The next choice was glass beads. Blind comparison tests by the service companies represented on the task group revealed that these data were not reproducible. Glass beads were also not a good representation of cuttings arriving at the surface in a borehole that had adequate cuttings carrying capacity. Rounded cuttings on the shaker indicate poor hole-cleaning from tumbling action in the annulus. Aluminum oxide grit provided good reproducibility, and microscopic examination revealed that the particles resembled shale cuttings that had been circulated from boreholes with good hole-cleaning.
As mentioned in the article, it was never the intention of the API task group to describe shale shaker screen performance. API RP 13C simply provides an indication of the largest openings in a shaker screen. The distribution of openings does not give a good picture of performance because the small cuttings do not seek only holes their size. Many go through the large openings. However, if a screen is labeled a 200 mesh, it should retain particles larger than 75 µm. Testing with the new API procedure revealed that many of the screens that were labeled as 200s were not retaining particles that were 150 µm in size. Needless to say, many manufacturers were very upset because screen designations had to be changed from 200 to 80.
As an International Organization for Standardization document the concept of “mesh” would make no sense. “Mesh” is defined as the number of openings per inch in each direction. Changing to metric units would mean that the “mesh” would need to be openings per millimeter. Clearly, rig hands would have trouble translating this from their previous knowledge base. This is the reason that the new screen designation is the alternative designation of an API number. An API200 screen would have the equivalent of a 200 mesh opening. Below that number, the actual measured largest particles retained on the screen would be recorded in microns. The word “mesh” is no longer being used to describe shaker screens.
While the new API RP 13C does not give any information about performance, it would seem reasonable that a screen that retains 75 µm dry particles would remove more solids than a screen which lets all of those particles pass through.
Mark Morgan, Vice-Chairman
Leon Robinson, Chairman
