One of the hot new topics in the horizontal directional drilling industry today is fluids recirculation and reclamation. This is nothing new to the river crossing guys with the larger rigs. Because of the volumes of fluid they use, larger hole diameters and utilization of mud motors, they’ve been using solids removal (solids control) systems for years. They adopted the equipment from the oil field, which in turn, borrowed the original technology from the grain, flour and cream separation industries. However, smaller rigs are starting to make longer bores, pull larger reamers and use mud motors. In many cases, the use of solids control equipment and systems are becoming cost effective even for conventional blade bit technology. The problem is that we hear about shakers, desanders, desilters, centrifuges, mesh size, and micron size. These are new terms for many of us and can be confusing. Let’s take this step by step.
With solids control systems, we want to take out solids in order of size. The larger particles are taken out with a vibrating screen assembly called a shale shaker in the oil field but usually referred to as a shaker. It’s also called a “scalping shaker” because it takes the larger solids “off the top.” When the solids laden fluid is pumped over the shaker, the larger solids are retained on the screen, the fluid with smaller solids goes through the screen, and the larger solids travel off the screen into a holding tank or onto the ground.
That sounds simple enough but, like most things, it’s not. There is quite a bit of tuning that we can do to a shaker. We can change the angle of the forward tilt of the shaker. This effects the retention time of the fluid and solids on the screen. We can change the mesh size of the screen (Mesh size deals with the number of openings per square inch built into the screen.) We would like to use the finest mesh size possible to take out finder particles, but there is a limit to how small we can go. Factors governing optimum mesh size include volume (gallons per minute), solids content and particle size distribution of the solids. Using a screen with a mesh size too small can cause plugging or “blinding” of the screen, and everything can go off the end with nothing going through it. It can be a good idea to have two or three different mesh size screens for different soil situations.
Hydrocyclones, classified as desanders or desilters, are conical solids separation devices in which hydraulic energy is converted to centrifugal force. Solids laden fluid is fed by a centrifugal pump through the feed inlet into the feed chamber. The centrifugal forces that are developed multiply the settling velocity of the solids, forcing them toward the wall of the cone where they are moved downward in an accelerating spiral and are discharged at the bottom of the cone. This is called the underflow. The cleaned fluid moves inward and upward in a spiraling vortex to the overflow. The difference between desanders and desilters is cone size diameter. Desanders are cones that are 5 in. (125 mm) in diameter or larger. Desanders can process larger volumes of liquid per single cone. However, the smallest size particle that can be removed is in the 40 micron range at best with 6 in. cones. Desanders have a place in horizontal directional drilling, when volume dictates, but desilters are much more efficient.
Desilters are classified as 4-in. (100 mm) cones or smaller and are usually arranged in a battery of two or more depending on the size of the cones and the volume of the fluid to be processed. A particle size between 6 and 40 microns can be obtained–again, depending on the size of the cone. You might have guessed by now that the smaller the cone diameter, the finer the cut. Why? The smaller the cone, the higher the centrifugal force. Bigger isn’t necessarily better.
The cut obtained by hydrocyclones is determined by pump pressure and manifold design. Lower pump pressures result in coarser separation. Poor manifold design results in lower pressures, or, at best, uneven pressure distribution in a battery of cones.
Are all cones designed the same? Pretty much so. A conventional 4-in. (100-mm) cone runs at 50-55 gpm (190-209 Lpm) with 59 psi manifold pressure can obtain about a 25-micron cut. This is about all that can be expected. There is one patented cone and manifold system on the market that will process the same 50-55 gpm (190-209 Lpm) at 25 psi with a 16-micron cut. The difference between a 25 and a 16 micron cut might not sound like much, but it is to a micron.
Another popular solids removal device is the mud cleaner. A mud cleaner removes solids in a two-stage process. First, the fluid is processed by a battery of desilters. These desilters are mounted over a shaker fitted with a very fine mesh screen. A centrifuge is a solids control device that consists of a steel bowl that rotates at extremely high speeds and separates solids by centrifugal force. A centrifuge can separate solids down to 2-3 microns. They are not used in the horizontal industry for at least four basic reasons. They’re too big, they’re too heavy, and they can cost as much as a decent house. The fourth reason is that 1 and 2-in. (25 and 50-mm) desilters are available that will obtain almost the same micron cut at a fraction of the cost.
How do we know if our solids equipment is working efficiently? There is drilling fluid testing equipment available that can tell us exactly what is going on. One device is the sand content tube. This tells us the percentage of particles larger than 200 mesh or 74 microns. To obtain total solids content percentage we can use a mud balance. With this piece of equipment we can obtain the weight of the slurry or cleaned fluid in pounds per gallon. We can subtract the weight of water (8.33 pounds per gallon) from this number, multiply the difference by 8 and determine the total percent of solids.
By calculating the solids content of the slurry coming out of the hole, after each piece of solids removal equipment and at the pump suction, we can gauge the efficiency of each piece of equipment and the solids removal system as a whole.
There are some traps and pitfalls that can be connected with recirculating and cleaning fluids. The main problem is that the only piece of fluids testing equipment on many rigs is a marsh funnel for testing viscosity. The marsh funnel gives a measurement of thickness–that’s all. In many cases the bentonite in the original fluid is replaced by fine solids that build up in the fluid over time. We can have the same viscosity but lose our gel strengths and wall cake building ability. This is when other pieces of testing equipment such as a shearometer or rheometer and a filter press can be valuable. WIth this testing equipment, there’s no guessing. Everything is expressed with a number value.
Speaking of numbers, what’s a micron? A millionth of a meter.