Loaded shakers can automatically increase G rating to boost capacity by 35%, enable finer fluid screening
By Federico Mezzatesta, National Oilwell Varco
For the first time in the industry, shakers have the ability to automatically increase acceleration (G) under loaded conditions because of Constant-G Control (CGC). The technology was developed to maintain an optimal G rating on National Oilwell Varco (NOV) FluidControl Brandt shaker products during varying liquids and solids loading conditions.
CGC, developed by NOV’s Fluid Control Group, maximizes shaker screen performance, solids conveyance and throughput while enhancing screen life. A combination of field and pilot plant tests have shown an ability to increase shale shaker capacity by up to 35% or filter drilling fluid at 2 to 3 API screen classes finer than a conventional shaker.
How does CGC work?
On a conventional shaker, G drops as weight on the basket increases because the motor rpm and force are constant. The result is a unit that is less efficient while loaded and has a reduced ability to process drilling fluids. The immediate reduction in acceleration due to loading on a conventional shaker is displayed by the KC 2.5 HP Standard line in Figure 1. As loading increases, G drops because motor force is constant and does not compensate for the additional weight.
To overcome this problem, the company added an accelerometer on the basket tied to the variable frequency drive (VFD) operating the motors. By doing so, the VFD is able to constantly monitor the basket G and adjust the motor speed to compensate for the fluids and solids loading the basket. The basket runs at a lower G and reduces wear on screens and components while operating with little solids loading and automatically ramps up to a higher G when loading becomes more extreme and higher Gs are needed to process the drilling fluid.
This is shown by the VSM Multi-Sizer (VSM-MS) line in Figure 1. The unit operates at 5.3 G with up to 100 lbs of fluid and solids loading on the basket, 6.3 G when loading is between 100-300 lbs and 7.3 G when loading exceeds 300 lbs and will maintain that G with up to 500 lbs of loading. Acceleration decreases beyond this point as motor speed cannot be increased any further.
Why it matters
Shale shakers are the first and best defense against drilled solids. By increasing the shale shaker’s processing capacity, CGC enables better control of drilled solids, which has the potential to reduce rig nonproductive time (NPT), improve rate of penetration (ROP) and reduce drilling fluid costs. In addition, the ability of CGC to selectively increase or decrease G depending on loading conditions reduces wear and tear on screens and shale shaker components.
In pilot plant testing, CGC allowed for up to 35% higher fluid-processing capacity. Field tests have shown that CGC would enable the use of screens 2 to 3 API classes finer than similar shakers not equipped with CGC.
Pilot plant test results
Pilot plant and field tests have been conducted since December 2007 to measure the value added by CGC. Capacity, conveyance, cuttings dryness, screen life and motion comparison were tested to determine their relationship to changes in the G.
Flow, screen selection, basket angle and all mud properties were controlled in pilot plant testing. Results from the pilot plant showed the various relationships between the change in G and the tested variables.
Capacity, conveyance and cuttings dryness
In pilot plant tests, testing revealed that a higher G increased the shaker capacity. However, the relationship is not linear. The law of diminishing returns applies to capacity as the G increases. As the G increases, the rate of increase in capacity decreases to eventually a flat line when the G reaches a threshold point. Pilot plant testing confirmed that CGC increases shaker capacity up to 35% without any appreciable difference in screen life.
To measure capacity in the pilot plant, a constant fluid end point was selected for all Gs, and the flow was measured to determine the capacity at the selected G. The flow was measured after the fluid end point, and the G was stable for 10 min.
Field testing confirmed the results of the pilot plant. The rate of change in the fluid end point decreases as the G increases, but the screen life is not affected by the increase of G. Since controlling and measuring the flow in the field is difficult, measuring the fluid end point is a reliable measure of change in capacity assuming there is no major change in the flow. Most data collection was done in a short time frame to ensure consistency in the flow.
Testing for conveyance was measured by dropping a Ping-Pong ball (or similar object) at the fluid end point and measuring the time it took to travel to the end of the shaker. By knowing the distances (in.) and time (sec) it took the object to travel, the conveyance was calculated in in./sec. Testing was conducted over linear and elliptical motion in the pilot plant and linear motion in field tests.
Testing showed conveyance is dependent on both the G value and the square of the G value. The relationship generates a gentle, upward sloping curve, which the company was able to obtain with an R2 value, or accuracy, of 97%. Testing also showed very similar conveyance results for both linear and elliptical motions. For simplicity purposes, if the G2 value is ignored, a linear regression equation can be applied to the pilot plant results, which yields the following formulas:
Conveyance = 0.7075 * (G) – 1.1485 for linear motion with R2 of 83% and
Conveyance = 0.7108 * (G) – 0.9551 for elliptical motion with R2of 85%.
Field tests confirmed the pilot plant results, and a linear regression equation can be written as:
Conveyance = 0.3468 * (G) + 0.402 with a R2of 82%.
Differences between pilot plant and field test results can be attributed to the difference in mud properties and the screens used. In the pilot plant, API 100 screens were used, while API 120 screens were used in the field test. Pilot plant tests used water-based mud (WBM) while the field tests used oil-based mud, which shows that the generally linear relationship still holds regardless of mud type.
The cuttings dryness was measured by collecting the cuttings from the discharge end of the shaker. The cuttings collected were weighed and dried. The dried cuttings were re-weighed, and the new mass was divided by the old mass to get the percent of dried solids.
The sample of wet cuttings was placed in a drying oven at 355°F until it was dried. The dryness samples collected during the capacity tests of the fluid end point were similar for all G set points (Figure 2). The dryness was consistent regardless of G when fluid end point is maintained the same. In addition, pilot plant testing showed there was no appreciable difference in cuttings dryness between linear and elliptical motions. Field tests were not able to confirm pilot plant results as they lacked the steady-state conditions necessary to obtain accurate data.
Field test results
A side-by-side flow capacity field test of the King Cobra 2.5 hp, the King Cobra 2.5 hp with CGC and the King Cobra Venom 3.5 hp with CGC was held by a major drilling contractor in Oklahoma. Each shaker attempted to process 100% of drilling fluid returns at a rate of 980 gal/min using API 100 screens while drilling a 17 ½-in. top-hole section at a rate of 100 ft/hr with WBM.
While the King Cobra 2.5 hp without CGC managed to process 85% (830 gal/min) of the returns, both King Cobra shakers with CGC processed 100% of the flow at 980 gal/min (18% more than the non-CGC equipped shaker).
In addition, the King Cobra 2.5 hp with CGC managed to process 980 gal/min with the fluid end point on the third screen, while the King Cobra 3.5 hp with CGC managed the same with the fluid end point close to the end of the second screen. This represented a 25% and 50% increase in unused screen area, respectively, relative to the non-CGC equipped shaker (Figure 3). This increase in unused screen area would have allowed the contractor to dress the shakers with screens 2-3 API sizes finer than the non-CGC equipped unit and still process the same flow rate.
Due to performance, the contractor chose the King Cobra Venom 3.5 hp for all newbuild rigs and replacement shakers on existing rigs.
CGC testing also was conducted on the VSM Multi-Sizer separator, a triple-deck separator consisting of a scalping deck and two lower decks, the latter of which can be operated in series or parallel modes. During a multi-well test on a land rig in Zapata County, Texas, a VSM Multi-Sizer was set up to receive 100% of drilling fluid returns. With the unit operating in parallel mode and CGC engaged, it was able to handle 100% of returns through every section of the well.
In the 12 ¼-in. top-hole section, the unit processed 100% of returns at 850 gal/min using API 140 screens while drilling with a ROP of 150-300 ft/hr. This was six API screen classes finer than standard operating practice with the rig’s original single-deck linear motion shakers.
The intermediate and lower hole sections were drilled with the same ease; CGC always allowed the unit to exceed the rig’s process requirements. The intermediate 8 ¾-in. section was drilled at a rate up to 760 gal/min with a ROP of 75-350 ft/hr and API 140 screens (two sizes finer than the original shakers). In addition, the lower 6 ½-in. hole section was drilled with up to API 200 screens (three classes finer than the original shakers).
CGC also was tested offshore as part of the VSM Multi-Sizer separator in the UK sector of the North Sea. Two VSM Multi-Sizer units were set up alongside three VSM 300 shakers to evaluate the performance of the CGC-equipped separators. A single VSM Multi-Sizer was able to process 100% of returns during the 16-in. well interval, which was drilled from 3,230 to 11,050 ft. The flow rate handling capacity in the 16-in. well interval was measured at multiple pump rates ranging from 800 to 980 gal/min; 10-mesh scalping screens were used. Two 800 gal/min tests ran API 60 filtering screens with drilling rates of 130 ft/hr and 12.5-ppg fluid. Three 950 gal/min tests used API 170 screens to process 13.15-ppg fluid at rates up to 120 ft/hr, and a fourth 950 gal/min test used API 200 screens with an ROP of 60 ft/hr to process 13.15-ppg fluid. CGC enabled the VSM Multi-Sizer to never experience flooding conditions.
Shale shakers are the first and best defense against drilled solids. A study showed that CGC-equipped shakers can process fluids at rates up to 35% higher and with screens 2 to 3 API classes finer than conventional shakers. By increasing the shaker’s capacity, CGC enables better control of drilled solids. Since its introduction into the market in 2009, CGC has been adopted on more than 150 shakers onshore US as well as offshore.
Constant-G Control, Brandt, VSM, King Cobra, and King Cobra Venom are registered trademarks of NOV.