Strategies for Solids Control
Suspended solids vary in size, shape and density. In general terms, suspended solids are measured by using a 0.45-micron filter, then drying and weighing the solids. The filter size may be different from lab to lab, so you may want to verify what micron they are using and keep that size consistent. Suspended solids less than the filter size become categorized as dissolved solids.
In some cases, turbidity can be used as a measurement of suspended solids. Turbidity is measured by using light scattering. However, it is prone to interference. If you are going to use turbidity, you must do some comparisons between total suspended solids (TSS) values and turbidity to determine if there is a correlation. If there is no correlation, then turbidity should not be used to determine TSS.
TSS can be categorized into three main groups, inorganic, organic and micro-organisms. Inorganic suspended solids are typically from rock and soil. Organic solids are mainly from the degradation of plant and animal remains. And micro-organisms include bacteria, viruses and pathogens.
The main source of suspended solids is inorganic. Which can be expected when you consider drilling through rock, drilling mud consisting of clays, proppant (i.e. sand) and the occasional West Texas dust storm. Inorganic solids are further categorized into sand, silt and clay. Sand has larger particles, with silt being smaller and clay being the smallest.
Suspended solids typically have a negative surface charge and repel each other. The smaller the particle, the greater the surface charge. Although most suspended solids vary in size and density, they tend to follow Pareto’s Principle or what people commonly call the 80/20 rule. What this means for suspended solids is that the majority of solids are smaller particles, but the larger solids contribute the majority of the weight.
So, when you have a TSS result in ppm, the larger particles contribute the most weight, but the smaller particles greatly outnumber the larger ones. Larger particles move slower, and smaller particles much faster. The random nature of the movement of suspended solids is called Brownian motion.
As you might assume, heavy solids fall to the bottom at a faster rate, and smaller solids take longer to settle. But some small particles never settle and stay in suspension. These particles are so light their weight combined with gravity cannot overcome the viscosity of the fluid. Think of it this way, I can drop a bowling ball through a tub of pudding, but a ping-pong ball stays suspended. This is part of the reason that there are more smaller solids left in suspension, the heavier solids, which are typically larger, have fallen out or settled faster.
Solids control occurs throughout the produced-water cycle, starting at the wellhead, then in tank batteries, before the gun barrel separators and ultimately right before injection in a disposal well. When considering produced-water reuse, there can be additional solids control before your storage vessel or pit and finally during reuse. Solids-control devices include:
• Centrifugal separators—desanders, desilters
• Settling devices—tanks, tank batteries, gun barrels, storage tanks, weir tanks and pits
• Conventional filtration—bag filters, cartridges, disk filters
• Water treatment systems—clarifiers, dissolved air flotation (DAF)
Suspended solids typically have a negative surface charge and repel each other. The smaller the particle, the greater the surface charge.
Why Control Solids?
In a produced-water system, solids can cause:
• Pump wear
• Formation of deposits, sludges, tank bottoms
• Plugging and fouling
• Filling of vessels affecting efficacy
• Oil carryover
• Formation damage
• Injection or disposal well plugging
Most solids are settled in tanks, which can take the burden off control systems, but may cause other problems.
Although solids are typically negatively charged and repel each other, they can be coated with oil, which will negate the charge. Corrosion and scale inhibitors or emulsion breakers are attracted to solids, which can attract oil, paraffins, asphaltenes and bacteria. This growing solid is sticky and will finally settle. This is sometimes referred to as schmoo. This combination of scale, solids, bacteria, paraffins and asphaltenes causes plugging and fouling.
Another concern with suspended solids is formation damage. Solids accumulate, forming cakes or filter cakes on the face of the formation matrix, in the perforations or even in the proppant packs. You will see injectivity problems in injection wells or production problems in gas or oil wells. Sometimes this damage is irreversible. In a 20,000 BPD system, just 10 ppm could produce almost 400 pounds of solids per day.
Because formations are so different, it is hard to predict what target TSS concentration is best. There are areas where just 3 to 5 ppm causes well half-lives in three to six months. The half-life is the time for a 50-percent decline to occur. In other areas, 2,000 ppm can be injected with negligible impact to injectivity. In the Marcellus, for example, produced-water reuse has little to no solids control to avoid accumulation of naturally occurring radioactive materials (NORM). Most operators will tell you that there is little to no effect on gas production.
As we discussed earlier, there are existing solids-control devices in place throughout the produced-water systems from the wellhead, tank batteries, gun barrels and reuse system. One common strategy is to optimize these systems.
The operator might add coagulants to help settle or cause settling in tanks designed to collect solids. Coagulants act to neutralize the charge causing particles to clump. As clumps agglomerate, they become larger and grow from micro flocs. Coagulation requires good mixing. Too much mixing doesn’t have a negative effect, but if there’s not enough, the flocs won’t form.
This growing solid is sticky and will finally settle. This is sometimes referred to as schmoo.
The next step is flocculation where you want passive mixing to allow the flocs to contact each other and form pin flocs and ultimately, macro flocs, which become heavy enough to settle. You may need to add coagulant to develop larger flocs. Smaller flocs float to the surface, especially in oil/water separation systems where the floc sticks to an oil droplet and rises to the surface. This effect can be enhanced with air, which is principally what a DAF does. Much of this type of settling is accomplished in simple tank systems, or frac or weir tanks.
Filtration systems are commonly used, but the cost of maintaining them are causing them to fall out of favor. Newer automated backwashable filtration systems are starting to make their way into the oil field. The first of these systems had microns 25 and larger. Now, micron sizes are getting smaller, and there are a few 5-micron systems available.
Five microns is an important milestone for these systems because you need about 5 microns to capture iron. Iron can act as a coagulant, and allowing iron to make its way through your system will cause coagulation in the formation and formation damage.
Iron may also negatively affect your frac fluid. Filtration systems are found just prior to injection at injection and disposal wells, or just before pits to prevent buildup of solids in the pit, or after the pit, before reuse to capture solids and iron. But as mentioned, filtration systems are falling out of favor due to the cost of manpower requirements to maintain them. Automated backwashable systems may reduce this concern.
Another strategy is using the pit system itself to settle solids. Operators are building primary and secondary pits, where the primary collects solids and the secondary is for reuse. Water flows from the primary and overflows into the secondary pit, similar to a two-stage clarifier. Adding coagulants optimizes the process. Sumps have been installed in these settling pits to periodically remove solids.
Managing suspended solids is not as straightforward as you would think. Defining your goals is important, but these goals should take into consideration injectivity for injection and disposal wells, proppant conductivity and core flow for reuse. Another consideration is the proppant itself.
Iron can act as a coagulant, and allowing iron to make its way through your system can cause coagulation in the formation and formation damage.
Proppant has fines, and these fines can represent 10 percent of the proppant volume. This is a significant source of suspended solids. Although proppant fines should have a charge associated with them and not bridge, the introduction of a coagulant or iron can create formation damage and plugging. Upstream activities using coagulants need to be monitored. Iron, because it can act as a coagulant, also needs to be monitored.
Iron control and solids control go hand in hand.