Automatic Shale Shaker Control

Introduction

A shale shaker is designed to quickly and efficiently remove solids from mud systems in the presence of changing drilling conditions. Optimal operation typically requires manual shaker adjustments to position the fluid endpoint at ¾ of the screening area, which leaves ¼ of the screening area to dry the cuttings. Sub-optimal operation can lead to lost whole mud (in high-flow conditions) and decreased screen life (in lowflow conditions). Traditional shale shakers rely on the operator to manually adjust the shaker according to the mud flow conditions by tilting the basket, adjusting the gates and valves, or turning shakers on and off to operate near optimum1. This can lead to potential HSE issues.

These manual adjustments beckon the safety and efficiency of the automated shale shaker. This work describes the automatic shaker control that is necessary to maintain near optimal operation even under changing drilling conditions. The main features include automatically controlling the fluid end point by adjusting the basket angle, increasing the basket acceleration when necessary to prevent flooding, and preparing for the next flow. Sensors are placed in the shaker to detect the level of mud on the screens. The shaker basket angle is then adjusted to maintain the desired screen mud level, which in this configuration maintains the desired fluid endpoint. This results in increased screen life due to better screen lubrication. The automated shale shaker also detects conditions leading to flooding and will temporarily increase the g-force to process the higher flow rates2. Bailey (1998) used manually adjustable acceleration to temporarily increase flow rates3. When the shaker detects that the input flow has
stopped, it tilts the basket to a maximum uphill position in anticipation of the next flow. At any time, the automatic
features of this shaker may be bypassed by turning a switch, and the shaker will operate like a traditional manual shaker.

Description

Design changes were made to a traditional manual shale shaker to facilitate automation (Fig. 1). First, the pivot point
of the basket tilt was moved near the desired fluid endpoint (near the end of the third screen of the four screen panel
shaker). This resulted in being able to control the fluid endpoint by only measuring the fluid level on the first screen.
A “U” shaped rocker arm supports the basket via the suspension springs. The single piece rocker arm passes behind and under the backtank. The rocker arm rests on a flexible bellows style actuator (Fig. 2). Solenoids selectively open and close to control the flow of pressurized fluid into the actuator. The fluid is pressurized with 60-100 psi compressed air supplied by the rig. This air-over-hydraulic system allows for minimal deflection or bounce after the actuator has moved. For instance, an air only system would be much more compressible than the air-over-hydraulic system.

The solenoids are controlled by the single-board-computer (SBC) system. The SBC system is composed of an SBC and an input/output (I/O) module with cards to read and write analog and digital signals. The I/O module and the SBC communicate with a Modbus over Ethernet protocol. The SBC system reads data from sensors that measure the fluid level on the screens, the basket angle, and the fluid flow into the shaker backtank. Control decisions are based on the sensor data.

The three sensors are calibrated upon installation. These settings determine the fluid level set point, fully uphill basket angle position, and zero input flow. Additional calibration may be done in the field. For instance, the operator may want to adjust the set point for the fluid end position based on personal preferences or additional knowledge about the mud. Calibrations are performed while the shaker is running by adjusting the shaker and pressing the calibration buttons on the enclosure.

For maintenance and diagnostic purposes, a web interface has been developed for the SBC system. The web interface
allows control and visibility to all program parameters and values. For instance, sensor errors can be viewed, sensor data integrity checked, set points adjusted, and both solenoid open/close and variable-frequency-drive (VFD) boost
commands can be sent. The web interface makes interfacing to a laptop or other PC possible via standard network
connections or optional wireless radio. With cooperation from the rig, interface data can be monitored offsite as needed for diagnostic purposes.

Control Logic.

The control logic for the automatic shaker is intended to adjust the machine appropriately while guarding against whole mud loss. If the fluid level on the screens is above the set point, then the SBC system will move the actuator down by a discrete amount, which will adjust the basket to a more uphill position. If the fluid level is too low, then the SBC system will move the basket by a discrete amount to a less uphill position. If the fluid level is within a small range or dead band around the set point, then the actuator will not move for that cycle. This helps prevent limit cycles or repeated movements around the set point.

Each cycle of reading the sensors, filtering the data, and moving the actuator takes approximately 13 sec. Testing has
determined this cycle time to be a good compromise between fast response and consistent operation. The cycle begins by reading all sensor data, typically 50 times. The data is then filtered to reduce noise and produce a single value for each sensor. The fluid level value is then compared to the set point and a decision is made on how to move the actuator. Actuator movements take a few seconds. Then, some time is allowed after moving the actuator to allow the fluid to settle on the screens. This minimizes fluid tides caused from rapid movement that affect the next sensor measurements.

The fluid flow sensor is used to determine when flow is entering the shaker. When input flow is present, the actuator
control is based on the fluid level on the screens. When the input flow is absent or zero, the actuator is adjusted to bring the basket to the fully uphill position. This uphill position is the most capable of handling high flow rates when feed returns to the shaker. Thus, this is intended to prevent a sudden high input flow rate from flooding the shaker and whole mud being lost. If the input flow is not high, then the basket will be adjusted to a less uphill position until the fluid level is at the set point.

If the fluid level is high (above the set point), the first control option is to tilt the basket to a more uphill position. If
the basket is already in the fully uphill position and the fluid level is still high, then the SBC system will automatically
increase the shaker acceleration (g-force). This is accomplished by sending a signal to the VFD on the motors to
increase the speed. In this “boost” mode, the acceleration increases by 21%. Test results have confirmed that this boost increases the flow capacity of the shaker. The automatic boost lasts a fixed amount of time and is typically preset to two minutes. Nevertheless, this and all variables in the SBC system have been parameterized and can be adjusted to suit individual rig needs. After the boost mode is completed, the fluid level is automatically measured and compared to the set point. If the fluid level is low, then the basket will be automatically adjusted to a less uphill position as needed. If the fluid level remains high, then the shaker will return to the boost state. This process will repeat as often as needed. The boost mode is only used when needed, rather than continuously in order to reduce the likelihood of degrading the solids in the mud and to increase screen life.

There is some fault tolerance built into the system. For example, if there is a sensor error on the fluid level sensor,
then the basket will tilt to a fully uphill position. If the actuator breaks, then the basket will also move fully uphill. If
a solenoid breaks, then the basket can be moved to a fully uphill position by opening a manual bypass valve in the
actuator system. At any time, the automatic features of this shaker may be bypassed by turning a switch, which will
allows the shaker to operate like a traditional manual shaker. Finally, since the SBC system is independent of the VFD motor system, a catastrophic failure of the SBC system will not result in a shaker shutdown. In this case, if the SBC system failed, then the shaker could be put into manual mode. This allows the operator to manually control the basket angle whether the vibratory motors are running or not.

High_G_Shale_shaker_M4
High G Shale Shaker

Test Procedure

Over the course of seven months, the automatic shaker was tested on two land rigs for two customers. During this time, the shaker was operated in automatic mode with the SBC controlling the shaker. Each rig used water-based drilling mud.

The shaker was also tested in a laboratory to determine the effect of the boost mode. The laboratory tests involved
measuring the flow rate of the shaker in normal and boost modes with 215 mesh rectangular opening screens. Normal mode acceleration for the shaker was nominally 6.07 g-force, while boost mode was nominally 7.34 g-force. The shaker basket was manually tilted to a fully uphill position after being put in normal mode acceleration. A knife valve in the flow line was used to adjust the flow to the shaker. The flow was set to position the fluid end point 1in. back from the end of the third screen of the four screen panel shaker. The flow was allowed to reach a steady-state level, and then if necessary, the valve was re-adjusted to maintain the fluid end point at the desired location of 1in. back from the end of the third screen. The flow was then maintained for five minutes before measurements were taken in order to ensure steady-state operation. The input flow rate was measured with a turbine style flow meter in the flow line. This test was repeated with the fluid pool moved out to the end of the fourth and final screen. This simulated an upset condition with extremely high flow rates.

Screen life was determined by analyzing the number of screens used to drill several wells during the field tests. The
number of screens used on the automatic shaker was then compared to the number used on a similar style manual shaker processing mud on a rig in a similar geographic region. The well depths ranged from 11,100 to 12,500 ft. deep and spanned 22-38 days for completion.

A wireless Ethernet radio was placed inside the explosion proof enclosure on the field test shaker to determine how far away the operator could maintain an adequate wireless network connection with the SBC. This was tested with the wireless radio and 5 dB gain whip antenna completely inside the explosion proof enclosure on the shaker and also tested with the antenna on the outside of the enclosure. When the antenna was on the outside of the enclosure, it was connected to the wireless radio with an explosion proof connector that penetrated the enclosure. Two wireless radios were used for this test, an Asus WL-300g consumer style radio (approx. $80) and a Locus OS2400-HSE industrial radio (approx. $1550). The standard Windows XP wireless network interface was used to determine the signal strength between the wireless radio and the laptop. The laptop was then moved from the shaker until the signal strength dropped below the “good” level. The wireless range was determined to be the maximum
distance on a particular side of the shaker that the “good” signal level was maintained. Communications were
determined to be adequate between the SBC and the laptop even below the “good” signal level.

Results

These field tests confirmed that the automatic shaker is capable of functioning in an oilfield environment. When
operating in water-based drilling mud, the faces of the ultrasonic transducers over the screen (measuring fluid level)
and in the backtank (detecting fluid flow) required a light daily cleaning. Cleaning involved rinsing the face with a
water hose or wiping with a hand or cloth. The sensors performed well with a light coating of mud, as long as the mud did not dry or become too thick. The suggested cleaning procedure is to rinse the fluid flow sensor face each time the screens are rinsed. This requires minimal additional operator time. If the screens are not rinsed, a daily wipe of the hand over the sensor face is usually adequate.

Conditions where more frequent cleaning is necessary include high flow rates of sticky solids. In this case, the
screens would also typically require more frequent cleaning. Thus, the total additional cleaning time for the sensors is very low. The sensor over the rocker arm that measures the basket angle never required cleaning. Typically, this sensor would only require cleaning if something were spilled on it or if debris collected beneath it. The sensor in the backtank that detects fluid flow was originally positioned such that solids could buildup beneath it and cause inaccurate measurements. This was corrected by relocating the sensor in front of the weir gate. This new position prevents solids from building up beneath the sensor. When flow to the shaker stops, the sensor measures a consistent distance to the smooth fluid pool below.

The flow tests showed that the boost mode increased the shaker flow capacity. As shown in Figure 3, the flow capacity at the optimal fluid endpoint increased 13% (measured with a turbine style flow meter). In extreme conditions where the fluid endpoint is at the very end of the last shaker screen, the boost mode increased the shaker flow capacity by 20%. The water-based mud for this test had a density of 9.6 lb/gal, a funnel viscosity of approximately 70-80 sec., and contained 4% low-gravity solids.

Mud Flow Rate vs. Acceleration
Fig 3. Mud Flow Rate vs. Acceleration

Figures 4 and 5 show improved shaker screen life when using the automatic shaker. Figure 4 shows that the average number of days an individual screen was used increased by more than double for the automatic shaker. The screens on the manual shaker on Rig 1 lasted an average of 5.18 days. This is very close to the standard expected screen life of 5 days with these screens in this geographic region. Figure 5 shows that the number of shaker screens consumed per 10,000 ft. of drilling decreased by at least 42%.

Days per Screen vs. Shaker Automation
Fig 4.Days per Screen vs. Shaker Automation

The increase in average screen life on the automatic shaker is likely due to better screen lubrication by not running the screens dry. This was accomplished by automatically keeping the fluid pool slightly back from the end of the third screen of the four screen panel shaker. An increase in screen life for these tests could have also been affected by better shaker maintenance. The automatic shaker was watched over more closely during these field tests than a typical shaker in an effort to collect data. It is possible that the shaker screens were washed more often than the screens of non-attended shakers. More frequent screen washing could lead to increased screen life.

Screen Usage per 10,000 ft. Drilling vs. Shaker Automation
Fig 5. Screen Usage per 10,000 ft. Drilling vs. Shaker Automation

The range of the wireless radio in communication with a laptop was found to be dependent on radio type and laptop
location in relation to which side of the shaker it resided. With the Locus OS2400-HSE radio and antenna inside the
explosion proof enclosure on the shaker, the useable range to the laptop was found to be approximately 20 ft. on both sides of the shaker. When the antenna was placed outside the shaker with an explosion proof antenna connector, the range increased to 80 ft. on the left and 100 ft. on the right side of the shaker. The less expensive consumer radio, Asus WL- 300g, had a range with the external antenna of 30 ft. on the left and 40 ft. on the right side of the shaker.

The difference in range on either side of the shaker is likely due to the amount of steel between the radio antenna and the laptop. The antenna was located under the steel backtank with an 8 in. vertical bypass pipe on the left side, and an aluminum explosion proof enclosure on the right side. The range could have been greatly extended if the antenna were located high on the shaker or on another structure. However, this configuration would not have been as rugged in the oilfield environment. The range of the wireless radios enabled monitoring of the automatic shaker system from a truck parked on the ground near the shaker.

Conclusions and Recommendations

The seven months of initial field testing has shown that the automatic shaker can perform in an oilfield environment. The flow capacity tests using the g-force boost indicates that this feature will be useful in reducing whole mud loss in upset conditions. The field test results also indicate a significant increase in screen life. More accurate screen life data will be determined as the automatic shale shaker is used on more rigs and in different locations. Interviews with the rig operators on the test locations have confirmed that the automatic shale shaker is a useful tool.

Fast Reading

Shale shakers have been manually operated machines. Key constituents such as basket angle and acceleration are featured as either fixed or manually adjustable components, which tend to yield limitations with a shaker’s screen life, flow capacity, and operator safety. This work proposes an automatic shaker control to adjust the basket angle, manage acceleration, and decrease operator intervention.

In order to achieve automatic shaker control, a manualstyle shale shaker design was modified with electronic sensors, an actuator, an on-board computer, and software to automatically control the fluid level on the screens. The computerized shaker system measures the fluid level on the screens, detects input flow, and measures the basket angle. The shaker then adjusts the basket angle and acceleration boost in order to control the fluid level on the screens. The shaker maintains the fluid level near a constant optimum value, which is set by the user.

In low flow conditions, this proper fluid pool coverage ofIn low flow conditions, this proper fluid pool coverage ofthe screens promotes screen lubrication and reduces friction,which increases screen life. In high flow conditions, the fluidlevel control helps prevent whole mud loss. The automatedshaker does not merely rely on basket angle control to managehigh flow conditions. The automated shaker’s arsenal againstmud loss also includes an automatic boost and anticipation ofnew flows. The automatic boost feature temporarily increasesthe shaker acceleration (g-force) applied to the basket in orderto process higher flow rates. Finally, when the input flow tothe shaker stops, the basket is automatically tilted to the fullyuphill position to prepare for the next flow. This decreases thepossibility of sudden rushes of fluid from flooding the shakerbefore it can automatically adjust to a more uphill position.These shaker modifications maintain screen lubrication, increase screen life, and assist in the prevention of whole mud loss, all while reducing operator HSE exposure.

Tests have shown that the automated shaker increases screen life, decreases operator intervention, and helps prevent whole mud loss. Automatic shaker control can operate a shaker near optimum. The benefits of this can lead to increased shaker performance and decreased expenses.

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