As the muds became more expensive, it became more common to use metal tanks instead of earthen pits. Mud guns (high pressure jets) operated from rig main mud pumps were used to mix the mud contained in the mud tanks as shown by Figure 1. Conventional high speed electrically or air driven propeller mixers or stirrers ( mud agitators) such as those used in the chemical industry were also mounted on the mud tanks as shown by Figure 2. As hydrocyclones and centrifuges were used to supplement the simple settling of the earlier mud separation systems, the use of stirred compartments became more important. Mud powered jet assisted hoppers became the most common method of adding dim or powdered materials to the mud system.
As more mud guns and other mud operated devices were added to the systems, the use of lower pressure, high volume centrifugal pumps rather than the main rig mud pumps became the usual method of supplying mud to such devices. The application of these independent pumps provided greater flexibility and more dependability to the mud systems. Mud tanks with independent, high capacity centrifugal pumps and submerged mud gun manifolds or “roll bars” were enable. These provided a high shear environment within the mud tank and frequent passage of the fluids through the centrifugal pumps. passage of the fluids through the centrifugal pumps. The flow of the mud through the centrifugal pumps and through the jets provided significant mixing action as well. These systems proved to be very helpful when adding polymers and other hard to disperse materials to the mud system.
Mud mixing has progressed from the herd of cows run through the mud pit for the Lucas well at Spindle top to modern current systems. In general as the demands made on the systems in use were increased, improvements were developed to meet the new demands.
There were at least four serious problems encountered with mud tanks and mud mixers using conventional fixed mud gun manifolds or stirring motors.
- As the fluids moved from the region where the jets or blades were installed into other regions of the tanks, there were large changes in the velocity of the fluids. Typically, solids accumulated behind obstructions or in the square corners of the tanks as shown by Figure 3. When the mud properties were changed, these solids could be resuspended resulting in an unwanted increase in mud weight. Typically these accumulated solids also presented problems when cleaning the mud tanks.
- In addition to the problems in mixing within Each segment of the tank, there were other serious problems caused by only one suction or at best a limited number of suctions for the pump. In a simple rectangular mud tank with twenty jets and a suction on one end, the fluids flow from the segment at the suction point at a rate twenty times that experienced by the segment at the far end of the tank. This is true if all twenty of the jets are handling equal volumes of mud.
- When a conventional tank was mixed there was a tendency for the fluids in the tank to rotate about one or more axes. The materials at the center of this rotating mass near the periphery of the tank. This was particularly true with thixotropic fluids such as drilling mud where the shear forces in the center of the gel strength of the mud. In the extreme case, the material at the center would rotate as a solid mass with no mixing. See figure 4.
- The final and most serious problem with conventional mixer was that these mixer could not mix mud fast enough to control problem wells. This situation was shown on a number of wild wells or blow outs that not enough mud available to kill them. This situation created the next demand for improvements in mud mixers.
THE DEVELOPMENT OF MUD MIXER
After analyzing the performance and problem of existing mud systems, the spiral flow cylindrical mud mixer was developed. The first step in the development of mud mixer involved the use of a “U” shaped tank with series of submerged mud guns located along the bottom of the tank as shown by figure 4. The manifold for the mud guns was located on the outside of the mud tank and all other obstructions were removed from the mud tank. This arrangement provided a rotating flow without eddies around obstructions which caused the solids to settle from the mud. This provided a solution to the first problem.
This use of one suction for each mud gun solved the lack of uniform withdrawal from each segment. The use of balanced injection and withdrawal from each segment actually intensified the tendency of fluid mass to rotate around the axis of the cylinder. In a mixer 24 feet long and 8 feet in diameter filled with an 11 lb/gal mud, this amounted to approximately 100,000 lb. of mud rotating with a peripheral velocity of 25 ft/sec. these first two steps followed conventional approaches and offered only marginal improvements over existing units.
The most radical innovation in the development to the mud mixer was moving the syction from the bottom to the center of the cylinder. However, this location at the center of a strong vortex was an open invitation for problems with cavitation at the pumps.
A series of inductors were arranged along the length of the center suction to induce flow into the center suction line and reduce cavitation in the centrifugal pumps. Thus, by introducing the fluid along the periphery of the mud tank and removing all of the fluids along the axis of the cylinder, the fluids were forced to flow along a uniform path spiraling into the center of the vessel.
When using its center suction, the mud mixer has the same basic configureation as a hydrocyclone. Particularly in the region near the center axis, the centrifugal forces tended to move heavier particles towards the wall of the cylinder. If the cylinder were positioned vertically, this separation would have been enhanced. By placing the cylinder in a horizontal position and using the “U” shaped vessel with a linear flow path across the top, the separation tendencies were overcome.
In order to conserve the momentum of the rotating fluids, the next units of the mud mixer were constructed with the round or circular shaped cross section shown in figure 6. The circular shaped mixers were evaluated under field conditions. Solids separation was not a problem. In fact high density objects such as rocks, bolts, and pocket knives have been removed from the inductors mounted in the center suction line after the units were emptied. Low density
Materials such as wood and plastics have also been removed from these inductors. Under full power, air was frequently drawn into the center suction several feet below the liquid level in the mix.
Two versions of the circular Shaped mixer were produced. Both were 8 feet in diameter. One was 24 feet long like the “U” shaped mixers with
a capacity of 214bbl. The length of the other model was reduced to 12 feet to facilitate its placement on offshore platforms and had a capacity of 107 bbl. The basic unit had a self contained centrifugal pump
usually powered by a 125 hp diesel engine. A conventional hopper with a mud jet at the bottom was provided for adding small amounts of materials to the mud system.
In most situation the mud mixer could meet all the demands placed upon it. Typically, the delays incurred in filling the mixer with water and/or diesel oil and adding the solids such as bentonite and barite, were longer than the time required to mix the materials. The later mixers were designed to accept barite blown directly from a bulk barite transport trailer.
MUD MIXER DESCRIPTION AND PERFORMANCE
The basic mud mixer unit is approximately 8 feet in diameter and 24 feet long with capacity of 214 bbl. A typical pump used with the unit will produce a flow rate of 1900 gpm with a discharge head of 150 ft. Such a pump can move all of the fluid within the mixer through the pump in less than five minutes.
Various models predict peripheral velocities inside the mixer that vary from 100 ft/sec. near the wall of the mixer. The centrifugal forces calculated using these velocities in equation 2 vary from 78g to 4.8g at the wall to as much as 312g to 38g at the inductors, which are 6 inches from the axis of the mixer.
The problems created by a single or limited number of suction points can be considered by analyzing an Ideal mixer with twenty segments and ideal mixing within each segment. (figure 7) A jet in each segment will discharge the volume of the segment per unit of time. In the 20th segment, the average molecule will spend 1 unit of time before it is displaced from the segment. In the 19th segment the average molecule will spend 0.5 units of time and so on, until the first segment ( the suction segment ), the average molecule will spend only 0.05 units of time before it is removed. Thus, the contents of the first segment will be displaced alost 72 times while the fluids from the 20th segment are moving through the mixer only once yielding a displacement factor of 72. Moving the suction to the center of the mixer of between the 10th and 11th segments will reduce this factor to 29. The mud mixer reduces this factor to 1.0 because each jet has one or more inductors located with it in each segment of mixer. The performance within each segment of the mud mixer probably has a similar ration of improvement over conventional mixer although it is not as easy to model or calculate the result for individual segments.
APPLICATION OF THE MUD MIXER
- The most spectacular application of the mud mixer is in the control of wild wells or blowouts. It was specifically designed and developed to meet the needs for large volumes of high density kill muds under field conditions. This does not mean that it should be parked on a special location alongside the water cooled crane used to position explosive, because the mud mixer is useful tool in other more normal day operations.
- It is an ideal mixer for central locations used to premix mud systems. It is particularly suited for mixing invert and other difficult to prepare muds.
- In areas with lost circulation problems, the mud mixer can save most of the time expended in mixing mud with carefully controlled properties. It enables the operator to change mud properties in the shortest possible time.
- Because mixing the barite without any settling out is difficult with rig systems, the mud mixer offers a sure solution on wells where extremely high mud weights are planned.
- On a normal well as a supplement to the regular mud tanks the mud mixer provides special benefits. The use of the mud mixer permits the operator to reduce the volume of mud in the active system while still providing substantial volumes of mud in the mud mixer for back up. This results in savings of materials and time whenever the mud properties must be altered or the active mud discarded. When the drilling engineer requires carefully controlled mud properties, the mud mixer is the fastest and may be the most efficient system to provide it.
In addition, if you have encountered equipment limitations when trying to mix and control fluid properties in other situations, the mud mixer provides an added measure of control for complex operations such as , providing carefully blended cement slurries or providing precisely regulated slurry properties for fracturing operations.