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Initially shaker screens were easy to describe. They were square mesh made with market grade wire. Mesh is defined as the number of openings per square inch. The size of solids moving through the screen depends on mesh and on the diameter of the wire. The openings (and solids that pass through) are larger if the wire has a small diameter.
Thus, it is possible for two identical meshes to possess different flow capacities and also return different amounts of solids to the drilling fluid. In addition, wet meshes exhibit significantly different behavior than dry meshes. Specifying "mesh" does not identify the ability of a shaker screen to remove solids. The concept was generally accepted as the screen descriptor. However, double- and triple-layer screens are used today. They seem to improve flow capacity while removing large quantities of solids, but make the mesh designation concept impractical.
The task group started by trying to identify the real problem in providing a meaningful screen description. The word "mesh" was no longer appropriate because screen openings varied so much. Different wire diameters, different rectangular weaves mounted in different orientations, and different size openings prevent a simple designation to be applicable to all screens. In addition, the openings must be described in metric (SI) units when the recommended practice becomes an international standard.
The word "mesh" was also applied to screens with greatly different appearances. Images of four such screens were taken through a microscope with roughly the same magnification (Figure 1). The largest number appearing on the screen box is shown on each image, and rig workers might assume that these were mesh numbers. One, in fact, had the word "mesh" printed after the number. Clearly, the screens are quite different.
Many different methods to describe screens were considered during the past 5 years. Methods included developing a dedicated test shaker, wet sieving, different grits, mathematical models and optical methods. The new procedure finally developed is a physical test that compares the screen to be tested with standard American Society of Testing and Materials (ASTM) screens.
The task group decided a physical test would be the fairest way to make measurements to describe current screens and also any future designs. ASTM standards for screens specify openings widths, tolerances, and wire diameters for screens coarser and finer than those used in the oil field.
Solids can be sieved and sorted through a stack of standard ASTM screens. These solids, when presented to a test screen, will be sorted by the test screens into sizes equivalent to some ASTM designation. The ASTM alternative designation will be used as the API number. This will describe the effective screen openings, compared with a recognized standard and is not intended to describe performance of the screen on any particular shaker.
Shaker screens are considered go/no-go separators. Initially, with square screens, some particle size would be the maximum size that could go through the screen. This is the D100 separation point. The task group decided to return to this concept with the new screen designation. Presumably, the largest particle that could possibly reenter the drilling fluid system will be the opening size determined by the new test. Smaller particles may be rejected as they are discarded with the drilling fluid associated with the separated solids. A liquid film wetting the wire increases the apparent thickness of the wire. The effective opening size is smaller than indicated by the D100 separation point. For example, when a screen becomes water-wet, non-aqueous fluid (NAF) may not easily pass through an API 200 screen. Even without solids, the NAF could find the openings too small to permit passage.
The quickest method to determine where a test screen will fit relative to an ASTM standard screen is called the "finder's method."
Aluminum oxide grit is available commercially in specific size ranges, or individual size ranges can be created by sieving the material through a stack of standard ASTM screens mounted on a vibrating table like a RO-TAP.
To measure the API number of a screen, small amounts of different grit sizes are placed on a test screen and shaken on a RO-TAP for 10 minutes. The weight of the grit sizes remaining on the screen can be used to determine the size of grit that could not go through the screen. This would define "D100" or the separation potential of the screen to remove 100% of the particles larger than the openings. This technique is called the "finder" method and is used initially by laboratory technicians to determine roughly what size grits should be placed in a RO-TAP stack to efficiently measure the test screen's equivalent opening size. The technique allows accurate initial grit size selection for the first API test procedure.
ASTM standard screens are shown in Table 1, in the left-hand column and represent square screen openings in microns. Shale shaker screens will use the alternate designations seen in column 2. Thus, an API 200 screen would have square 75 µ openings, an API 80 screen would have square 180 µ openings, and so on. A simple test will confirm where the test screen would go into the stack for the API test.
A test mix consisting of 5 g of aluminum oxide in six different sizes is created. This would be 5 g of material from an API 70 screen; 5 g from an API 80 screen; 5 g from an API 100 screen; 5 g from an API 120 screen; 5 g from an API 140 screen; and 5 g from an API 170 screen. This 30 g test mix is placed on a test screen and shaken for 10 minutes using a RO-TAP. The weight of the captured grit indicates the equivalent size of the openings of the screen tested (see Table 2).
For example, if 14 g of aluminum oxide grit was captured on the test screen, in the above grit distribution chart, this would indicate that the test screen has opening sizes somewhere between API 80 and API 100 screens.
Next, a stack of screens will be mounted on the RO-TAP such that an API 120 and an API 100 is above the test screen and an API 80 and an API 70 screen is positioned below the test screen. A new mix of aluminum oxide grit is formed with 5 g to 8 g from each API screen size. By placing the test screen between the API 80 AND API 100 ASTM screens, the quantity of aluminum oxide captured will reveal the effective screen opening size (Table 3).
In this test, 1.16 g of aluminum oxide was captured on the test screen. The rest of the sample between API 100 and API 80 (150 µ and 180 µ) was captured on the API 80 (180 µ) screen. The new API RP 13C recommends that this procedure be repeated twice more for accurate laboratory results.
The API number for this screen could be determined by reading a graph (the sample is close to the API 100 screen) or the actual value can be determined by calculating the equation of the line describing the distribution of grit between API 80 and API 100. The API designation is API 100 (155 µ).
The API number is the ASTM screen size closest to the value measured with the aluminum grit test. The test does not address, nor is it intended to address, the performance of a screen on any particular shaker. This test only compares a shaker screen with a square mesh standard screen.
The second major change made in the API procedure is the manner in which conductance is measured. In the original document, a mathematical procedure published by Armour and Cannon but with an experimentally derived constant was used to calculate conductance. In the Appendix of API RP 13E, a "Darcy's Law" experimental procedure required measuring the pressure loss across a single screen layer while flowing a 35 cp glycerin/water mixture to calculate screen permeability.
The new procedure uses a larger screen sample and larger quantities of fluid (motor oil) to determine conductance.
Procedures were changed so that the screen used to measure conductance can also be used to determine screen description. Screens must be stretched - in the same manner they are mounted on shakers in order to make meaningful measurements. A screen without tension will appear to be a much coarser screen.
Motor oil was selected as the Newtonian fluid to flow through the screen because the viscosity was much larger than the viscosity of glycerin/water solution. This allows a larger pressure head to be applied with a reasonable flow rate.
Test screens are mounted between two short pieces of 6-in. Schedule 80 PVC pipe. The screens are stretched before mounting to allow the shape of the openings to resemble the shape the openings will have when mounted on a shale shaker. A 6-in. plastic pipe was selected instead of mounting the screen in an 8-in. metal ring (like the standard ASTM screens used in the RO-TAP) because the flow volume through the screen is more manageable. The screens are stretched and fixed on the pipe with epoxy to create a sample that could be used for both conductance and screen designation.
A large container (around 50 gal) of motor oil is located above the test screen. Motor oil flow rate is adjusted so that it flows through the screen at a constant rate. A chart of oil viscosity and density as a function of temperature is developed before the test. Oil overflowing the screen is directed away from the fluid flowing through the screen. The flow through the screen is captured in a container on a scale connected to a recorder. When the weight increase is constant, the flow rate through the screen is determined from change in weight measurement and the density of the oil. The height of the fluid above the screen determines the head (or pressure) causing flow through the screen.
[Where: Q = flow rate; ∆P = pressure differential; A = screen area; L = screen thickness; K = conductance; and µ = oil viscosity].
Solve for conductance:
Measure Q, ∆P, A, and µ [The thickness L does not have to be measured.]
The procedure is written so that there will be flexibility in making the measurements. The equipment shown here is only one possible way to make the measurements.
A large volume of motor oil is needed to allow equilibrium and to prevent large temperature changes.
Motor oil was selected because it will oil-wet the screen and has a high Newtonian viscosity. If the screen is to be used subsequently to determine the screen description, the screen must be washed with a detergent to remove the oil. The screen is then dried for use in the RO-TAP.
The flow velocity through the screen is maintained less than 2 in./sec to make certain that the flow is laminar. The equations apply to only laminar flow and not turbulent flow. Fluid flowing through a screen opening at a large velocity will create eddies and turbulence as it emerges from the screen (Fluid flowing at 400 gpm through a 4-ft by 5-ft [1.2-m by 1.5-m] screen with 35% open area would have an average velocity of 1.5 in./sec).
The goal of the mounting procedure was to develop a technique that would also allow screens to be shipped in from a drilling rig for confirmation testing. The same screen could be used for both the conductance and the screen description. Conductance testing will normally be completed after the screen designation is completed.
The conductances for the screens shown in Figure 1 are presented in the table below. Three different heights of motor oil were used to make three determinations of conductance. Care was taken to make certain that the flow was laminar by limiting the velocity through the screens to less than 1 in./sec.