Automation Improves Shale Shaker Performance

One of the limitations of current shale shakers is that their performance can be adversely affected by changes in the feed rate. Standard shakers operate with vibratory motors running at a constant speed, yielding a continual output force. This leads to a shaker having its peak g-force during non-loaded conditions. As soon as drilling fluid is in addition to the system (more mass), the acceleration (g-force) decreases and in turn, the shaker performance decreases as well.

A significant improvement for shale shakers has been made with the introduction of the second generation of the controlled acceleration technique. The improved technology allows the shaker to continuously adjust the shaker basket’s g-force to the proper pre programmed set point. This improvement offers significant advantages over the first generation of controlled acceleration in terms of performance, cost, safety and downtime.

Field tests have shown that a shaker equipped with the second generation of controlled acceleration is still capable of screening 2 API classes finer or better as is the case with the first generation. Use of this technology allows for improved solids removal efficiency and the potential to reduce rig non-productive time (NPT) and improve rate of penetration (ROP) by helping to yield a cleaner wellbore.

Presented will be the differences between the first and second generations of controlled acceleration along with field and theoretical data showing performance curves. [source]

Statement of Theory and Definitions

The technology has evolved since 2007. It was originally intended to be a one g-force set point controller for only one line of shale shakers. After extensive pilot plant and field tests, and input from operators and contractors, the technology evolved to a three set point controller; creating the first generation of controlled acceleration technology. In late 2010, the technology evolved into the second generation controller with multiple set points changed automatically during drilling depending on the flow conditions.

The controlled acceleration technology has also evolved into a mainstay control system on multiple shaker products (4-5 different derivatives) whether it is a new build or an upgrade kit in the field.

Original testing has shown that controlled acceleration technology improves capacity and conveyance while not affecting cuttings dryness or screen life. Appendix 1 explains how the controlled acceleration is an application of Newton’s second law of motion.

The second generation of the controlled acceleration technology revolves around two major concepts. First, not running the shaker at a higher g-force than necessary; second, not flooding the shaker during high flow conditions causing the loss of whole drilling fluid. The controller is programmed with 3 set points, High, Low and Default that automatically change depending on the drilling conditions.

The system starts out running at the first preset g-force (Low), the system will continuously run at this preset level until the load (thus mass) on the basket increases to a threshold point (T1), and the system will automatically adjust its g-force setting to High. As the flow rate decreases on the shaker basket to threshold point (T2), the shaker will automatically go back to Low and the cycle will continue unless the shaker is stopped or there is a sensor malfunction.

Note that T2 must always be less than T1. The difference between T1 and T2 is varied depending on the shaker. If T2 is set to equal T1, the system might continue to infinitely ramp up and down which may damage the motors prematurely. This insures the system is running in High mode for an extended period of time to avoid the risk of whole fluid loss. Figure 1 shows the relationship between set points T1 and T2. Notice that the dotted curve represents the frequency on the left y-axis while the non-dotted curve represents the g-force on the right y-axis. Notice on the non-dotted curve that the g-force starts to decrease as the mass goes up. This takes place because the frequency of the motors is limited to a certain maximum, the flat dotted line. The motors cannot exceed certain frequencies to protect the bearings from overheating and premature damage. The maximum frequency limit also assures that the motors are operated in compliance with the National Electrical Code and other similar standards.

The control system is equipped with a Default mode that is only enabled if the sensor is damaged or removed from the shaker basket. The controller runs the shaker motors at 60Hz until the sensor is replaced or restored in its proper place. The default mode was developed with the first generation of the control system such that the PID* loop doesn’t run the motors at maximum speed risking premature motor bearing failure.

Figure 2 shows the difference between shale shakers running in controlled acceleration mode vs. shakers running in nominal mode. Notice that the top line which represents the second generation of the controlled acceleration technology can handle up to 600 lbs. of slurry without dropping the g-force below 7.3 G’s while its counterpart, the bottom line on the graph will operate close to 4 g’s at the same weight point. 600 lbs. is approximately equivalent to 12 ppg slurry used to drill a 17.5” hole at 100 ft. /hr. Table 1 shows different weight/g-force scenarios using a model built internally to anticipate the load on the shakers under different drilling conditions assuming that there is only one shaker handling all the flow coming back from the well to be processed.

Table 1

Mud Density	Hole Size	ROP(ft/hr)	GPM	Load Range(lbs)	G-force Range
12	6.5	25	450	256 – 504	7.48 – 8.10
12	8.5	50	525	269 – 513	7.46 – 8.06
12	12.25	75	650	300 – 535	7.40 – 7.98
12	17.5	100	700	378 – 592	7.28 – 7.78

The solids control model used to calculate Table 1 assumes the following:

1. 68% of the shaker screens are covered with slurry. Slurry is defined as the combination of drilling fluid and cuttings generated during drilling. This is equivalent to 2.75 out of 4 total screens.
2. The solid/liquid ratio coming out of the well doesn’t change on the shaker basket.
3. The model assumes that 100% of the solids coming out of the well add mass to the shaker.
4. Since not all liquid is touching the shaker screen, it is difficult to predict the exact mass of the liquid part of the slurry, so the model shows a range of 50% of the liquid portion is in contact (thus adding mass) with the shaker and 100% of the liquid portion is in contact with the shaker.

The control enclosure is equipped with a pilot light indicating if the sensor is properly functioning or needs attention. The pilot light blinks if the system is in Default mode and is solid green when running in automatic mode. A blinking light indicates that the VFD* controlling the motors is not receiving the feed signal from the accelerometer mounted on the side of the shaker.

Description and Application of Equipment and Process

Field testing on a land rig in Oklahoma included three shale shakers. The first shaker was a standard linear motion equipped with 2.5 hp motors without controlled acceleration controllers. This shaker runs at 6.1g’s nominal and when equipped with screens and loaded with fluid the g-force decreases to 5.7 g’s. The second shaker was a standard linear motion equipped with 2.5 hp motors with controlled acceleration control. This shaker with controlled acceleration controls operates at a continuous 6.3 g’s with up to 300 lbs. of slurry loading at maximum motor rpm as indicated in figure 2. The third shaker was a linear motion equipped with 3.5 hp motors with the second generation of controlled acceleration technology. Figure 2 shows the relationship between the weight on the shaker and the g-force each shaker can maintain. The three shakers described are represented in Figure 2 with the bottom line representing the first shaker, the middle line representing the second shaker and the top line representing the third shaker.

All three shaker controls were adjusted such that they all ran at 6.1 g’s nominal at the beginning of the test. Setting the shakers to perform equally was executed by removing the sensor from shakers 2 and 3 triggering the default mode that was programmed to run at 6.1 g’s nominal. Figure 3 shows the fluid end point of all three shakers as well as the drilling conditions at the beginning of the field test. All shakers were equipped with identical new screens. These steps were executed in order to insure that all three shakers have the same flow rates before enabling the controlled acceleration controls on shakers 2 and 3. The total flow on the three shakers was 980 gpm, split equally at approximately 330 gpm per shaker.

The field test was then for each shaker to process 100% of drilling fluid returns at a rate of 980 gpm using API 100 screens while drilling a 17 ½-inch section at an average rate of 100 ft./hour with water-based mud (WBM).

Data and Results

On the land rig test, the first shaker (shaker 1) without controlled acceleration processed 85% (830 gpm) of the returns; the second and third shakers with controlled acceleration processed 100% of the flow at 980gpm, 18% more than the first shaker.

In addition, the second shaker (Shaker 2) processed 980 gpm with the fluid endpoint at the end of the 3rd screen while the third shaker (shaker 3) processed 980 gpm with the fluid endpoint at the end of the 2nd screen. This increase in screen beach length represents a 25% and 50% increase in unused screen area as compared to the shaker without controlled acceleration controls. Figure 4 shows graphically the difference in performance between the 3 shakers.

The unused screen area provides the basis to allow the use of finer API screens and still process the same flow rate.

Conclusion

The second generation of controlled acceleration technology automatically adjusts the shakers motor rpm depending on the flow condition. This eliminates operator intervention and optimizes shakers performance. The controlled acceleration technology can be installed on new build shakers or as an upgrade kit to shakers currently operating in the field.

Shakers equipped with controlled acceleration operate at maximum g in loaded conditions and nominal g when the shaker is not loaded. This provides increased solids conveyance, higher flow capacities and the capability to screen finer in optimum g shaker operation, while enhancing motor, bearing and screen life when programmed to run at nominal g in unloaded conditions.

The second generation of controlled acceleration has been installed and shipped on over 60% of the newly manufactured shakers and has become standard equipment for multiple major drilling contractors.

Appendix 1

Using Newton’ second law of F=ma where F represents the force, m represents the mass and a represents the acceleration, the g force on the shaker basket represents the acceleration (a) such that 1 g = 9.8 m/s². The motors generate the force (F) and the basket and the slurry represent the mass (m).

Rewriting the equation the equation in terms of the shaker’s variables:

f(Motor RPM) = (basket + slurry mass) X ( basket acceleration)

Solving for the basket g-force the formula can be re-arranged to the following:

basket acceleration = f(Motor RPM) /  (basket + slurry mass);

By changing the motor RPM (i.e. Force), we can control the basket acceleration (g-force) depending on the basket load (slurry + basket mass).