Today’s shale shakers must be capable of maintaining optimum fluid properties to maximize drilling efficiency.Joe Bacho
Define Shale Shaker
Shale shakers are components of the solids control systems. Shale shaker mainly used in the oil and gas drilling industry. Shale shaker is the first phase of a solids control system on a drilling rig, and are used to remove large solids (cuttings) from the drilling fluid (“mud”).
Shaker screen selection has the largest impact on the overall performance of the shale shaker. It is therefore important to understand the factors which may impact screen performance and how to properly select screens. Shaker screen performance is measured by:
- Separation Performance – the size of the solids removed.
- Liquid Throughput Performance – the capability of the screen to transmit fluid.
- Service life.
Catagory of Shaker Depends on Motion
Linear motion shale shakers use two motors vibrate in-phase for separation. The structure of this type of shaker is simple, easy to maintain, and repair. Linear motion provides superior cuttings conveyance and is able to operate at an uphill slope to provide improved liquid retention.
The abrupt changes in acceleration during the vibration cycle tends to cause screens to wear more quickly unless close attention is paid to tensioning and screen support techniques. Perforated metal backing plates and pre-tensioned screen panels have been specifically developed to address this problem.
Linear motion shakers usually operate at less than 4.0 G’s (normal to the screen) to balance screen life with processing capacity. Regardless, the finer screens normally run on linear motion shakers cannot be expected to outlast the coarser screens used in the past. For screens finer than 100 mesh, expect an average service life in excess of 100 hours.
A balanced elliptical motion shale shaker can effectively remove solids and eliminate larger particles in drilling fluids. The relatively moderate vibration strength of balanced elliptical shale shakers can reduce screen consumption. It performs perfectly while drilling top-hole sections where heavy, high-volume solids are present.
On a balanced elliptical shale shaker, the difference in power between the two vibrating motors results in an unbalanced force. The structure of this type of shaker is more complex than a linear motion shaker. The price is higher.
The smooth change in acceleration with respect to direction translates into longer screen life compared to other vibration patterns. However, many circular motion shakers were designed before the advent of fine mesh screens and may provide less support for the screens. This will tend to negate much of the screen life benefit associated with circular
The definition of “acceptable” screen life must be judged within the context of the total solids removal system economics. Besides screen replacement cost, consideration must be given to the costs of drilling mud dilution and waste disposal costs when determining whether longer screen life is warranted at the expense of solids removal efficiency. In
weighted mud applications, the economic benefits of improved solids removal efficiency usually outweigh the additional screen costs.
Effect of Screen Composition
Only very general correlations may be made between screen composition and service life. Unfortunately, features that lead to improved life are usually detrimental to flow capacity. Using heavier wires with greater tensile strength or adding supporting layers of cloth can both reduce conductance.
Increasing support through an additional bonding area (smaller plate openings) eliminates the usable screening area. Also, support techniques and screen tension can have a major effect on on-screen life. As a result, screen panels are typically designed to balance flow capacity performance with screen life.
Screen life is heavily dependent upon flow line conditions. Solids loading rate drilled cuttings abrasiveness, and shaker dynamics can easily outweigh composition effects.
The separation performance of a shale shaker screen (or any other solids control device) is commonly represented by its percent separated, or grade efficiency, curve. This curve is generated from full-scale experimental measurements and depicts the percent solids
removed as a function of particle size.
It reports the screen’s probability of separating any specific particle size with a given shaker under conditions specific to the test. Grade efficiency is the preferred measure of separation performance because it is independent of feed particle size distribution.
An example of a percent-separated curve is shown in Fig. 1. In this example, the median size separated by the screen was 145 microns. This means that 50% of the solids with a diameter of 145 microns were removed. A rough estimate of the median cut point (d50) can be made in the field by the wet sieve procedure (see Field Procedure to Estimate
Shaker Screen Separation Potential
APR has developed a method to characterize the relative separation efficiency potential of shaker screens without the expense and time required for full-scale testing. The technique links the relative separation performance of screens to a volume-equivalent distribution of their opening sizes.
The screen’s openings are measured using PC-based image analysis technology. Each opening in the screen is then represented by a spherical diameter corresponding to an ellipsoidal volume calculated from the image analysis data.
The cumulative volume of these ellipsoids, when plotted as a function of spherical diameter, yields a curve that correlates well with the standard grade efficiency curve. This curve represents the “separation potential” of the screen. The word “potential” is used because the screen’s separation performance is not measured directly, but implied by the size of the screen’s apertures.
Note: Grade separation efficiencies as measured on the shaker are subject to specific shaker and flowline conditions. They may not always agree with separation potential values. For example, the separation potential value for a screen with rectangular openings may be
pessimistic when drilling clean sand sections producing predominantly spherical sand grains. The image analysis method assumes solids of all shapes and sizes are available to the screen. However, on average, the separation potential values have been shown to adequately represent the screen’s separation performance.
Liquid Throughput Performance
The liquid throughput capacity of a screen panel is primarily a function of screen conductance and usable area. Conductance describes the ease with which fluid can flow through a unit area of screen cloth. In simplistic terms, it is analogous to permeability with the length in the direction of flow (screen thickness) taken into account. Higher conductances will result in higher flow rates through the screen.
Conductance is calculated from the mesh count and wire diameters of the screen cloth by the equations given in Appendix B, Conductance Calculation. Multilayer screens can also be handled by the conductance equation. The inverse of conductance for each screen layer is summed to equal the inverse of the net overall conductance:
This is valid provided that the screen layers used in the composition are designed to remain in contact.
Oilfield screens are typically bonded to a perforated metal panel or plastic grid to provide extra strength and improve service life. This practice eliminates some of the usable areas through which fluid may pass. Some metal backing plate designs may reduce the effective
screening area by as much as 40 percent. Because conductance describes screen flow capacity per unit area, the usable unblocked area available for screening must also be considered when comparing the mud processing capacity of shaker screen panels.