This is where it gets interesting. Some pretty sharp engineers have spent a lot of time on this subject and can do amazing things. Most of the technology comes from the oilfield. In the oilfield, solids control is not a luxury — it is an absolute necessity. Without proper mud chemistry and solids control, we would have drilled up all the cheap, easily available oil years ago. The fact that we now can access oil that was previously undrillable is thanks to sharp thinkers and having plenty of money available to solve the problems. Kinda like an old geezer with a bad bladder, the “trickle-down” theory finally worked for water well drillers. We now have access to the latest and best equipment available to make more hole faster, cheaper and better than ever before.
One of the simplest and most basic pieces of solids-control equipment is the desander. A desander is based on the principle of the hydrocyclone. The basic idea is: Pump mud into the outer edge of the top of a cone shape. The mud whirls around faster and faster as it travels down the wall of the cone. The solids are forced by centrifugal force to the outside, near the walls of the cone. The clean mud moves to the center of the cone. The solids eventually make it to the bottom of the cone and are forced out of an orifice by what is known as underflow. The clean mud, minus the solids, travels upward through the center of the cone and is discharged out the top, back to the suction pit.
Hydrocyclones come in a variety of sizes. The size designation is based on the inside diameter of the top of the cone. The smaller the cone, the finer the cuttings it will remove. This is because a smaller diameter produces a higher G force, which force smaller cuttings to the outside. The downside is what is called throughput. The smaller cones only will process a small amount of mud, and on large systems, a large number of cones are necessary. Large cones easily process large amounts of mud, but don’t remove all the fines.
Oilfield desander cones usually are 10 inches or 12 inches in diameter. They will process 400 gpm to 600 gpm of flow, but only the coarse cuttings will be removed. Typically, in the oilfield, 10-inch cones are followed by 4-inch cones, called desilters, to remove the smaller fines. This requires an additional pump and power. The water well industry has come up with the 5-inch cone. It removes coarser materials, but still makes a fine enough cut to satisfy most conditions. A 5-inch cone is between a typical desander and a desilter cone. It is commonly called a desander. A 5-inch desander will remove cuttings down to 20 microns.
“How big is a micron?” — you might ask. Hell, I’m a well driller, not a science geek, but I know it’s pretty dad-gummed small. Consider this: The suction filter on your hydraulic pump probably is 25 microns, and that’s clean enough for a close-tolerance hydraulic pump! Modern water well desanders typically run 5-inch cones, which allow 80 gpm per cone of throughput. A 4-inch cone only allows 40 gpm of throughput, so it takes twice as many cones to do the job. The mud will be cleaner, but only marginally. It also will plug more easily with large cuttings. Not enough difference to justify the extra expense.
Even more basic is the shale shaker. A shaker is a vibrating screen that “scalps” the larger cuttings before they are cleaned by the desander. The theory is: Lose the cuttings, and save the mud. The motion of the screen itself has a lot to do with how much mud can be run over a screen. There are several theories about which ones work best.
The simplest method is what they call orbital motion. Orbital motion shakers have been around for years; they are the staples of the shale shaker industry. A weight is rotated rapidly, imparting an orbital motion to the shaker basket. The can be driven by any power source — hydraulic, electric motor or gasoline engine. They are simple, cost effective and field-serviceable.
The newest method is what is called linear motion. This type of shaker slides back and forth in one axis, instead of two. The advantage is higher throughput. This means that a smaller shaker can process more mud. The downside is, until recently, all linear shakers required three-phase electric power — not a common item on a water well rig! Plus, they are incredibly expensive. Hydraulic power is the most economical, easiest to service and most practical for the water well industry. Some shakers are independently powered by a small gasoline engine, making them cheap and stand-alone, but who needs one more engine to fuel and maintain?
A big consideration in shale shakers is screen-size selection. Shaker screens come in a bewildering array of sizes and even opening shapes. Obviously, smaller size openings in the screen remove more cuttings before they get to the desander. With high flow rates and thick mud, they also overload, dumping cuttings and mud on the ground. Larger screen mesh sizes allow more throughput, but put more load on the desander to remove cuttings that passed the shaker screen.
Screen mesh size is highly variable, and different drilling conditions require different sizes. In the oilfield, it is not uncommon to change screen-mesh size in the middle of the hole! A common problem is screen blinding. This occurs when drilling a “balling” clay. The clay will tend to “ball up” on the screen, blinding it. This allows cuttings and clean mud to flow on the ground. The traditional solution has been to increase the mesh size. This works well until the desander is overloaded. The newest solution is to use rectangular mesh — that is, mesh that is larger in one dimension than the other. It will keep the mud clean without blinding.
The latest efforts in solids control have been directed toward the environmental market and all the spin-offs from it. In the environmental drilling business, drillers often are required to dispose of cuttings off-site. The more liquids mud entrained in the cuttings, the more they have to haul off and pay to dispose of. They have required mud systems that produce dry cuttings to lessen the amount of material they have to haul off-site.
This trend has benefited the water well driller. When dry cuttings are produced, even if you don’t have to haul them off, less mess is made, and less site disturbance happens. Production of dry cuttings has been made possible by drying out the desander underflow. Desanders typically require 10 percent to 15 percent of throughput to go to underflow to carry the discarded cuttings. This underflow usually is good mud. The newest designs allow the desander underflow to be discharged over a very fine (180 mesh) shaker screen to recover the good mud and discharge the fines. This allows a very dry discharge, which is either easily hauled off or discharged on-site with minimal disturbance.
As you can see, there are many facets to proper solids control. Good design and operation can add to your bottom line. Less rig-up time, faster hole, better hole, less site disturbance, less clean up and many other factors contribute to the reasons for modern solids control equipment.
Look around at the next trade show you go to; you probably will see some pretty amazing units!
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