These environmentally safe sands are the future of hydraulic fracturing.
by Phon Malone, Preferred Sands

Today, the oil boom in North America would not be possible without hydraulic fracturing, which makes recovering oil and gas locked within rock formations economically viable. The hydraulic fracturing process has received tremendous publicity during the past few years, particularly highlighting potential negative environmental effects. For hydraulic fracturing to be a sustainable method of building domestic energy, companies must address these environmental issues proactively by investing in research and development of environmental technology. One fracturing sand producer is doing its part with its first environmentally safe resin-coated sand (resin).

The Importance of Proppants

Service companies use proppants such as raw sand, resin and ceramics during hydraulic fracturing. Proppants are critical because they help keep fractures open and provide permeable conductivity channels back to the well bore. The price varies with each type of proppant. Sand is the cheapest, with resin priced higher and ceramic being the highest priced.

The industry segregates wells into shallow depth, intermediate depth and deep depth. Sand is typically used in shallow- and intermediate-depth wells. Resin is used in all three depths and ceramic is typically used in intermediate- and deep-depth wells. Some companies may use whatever proppants are available when there is a supply shortage. Unfortunately, some companies may use inferior proppants as a cost-cutting strategy, which can be counter-productive.

Fracturing sands are unique because they are strong and crush-resistant. The sand particles’ shape is round, and logistically, it must be mined in locations that are economical for the transportation of the sand to the drilling site. The largest cost associated with sand is transportation from the mine to the drilling site. This portion can be approximately 70 percent of the total cost.

The most common gradations for raw sand are 16/30 mesh, 20/40 mesh, 30/50 mesh, 40/70 mesh and 50/140 mesh (or commonly known as 100 mesh). The gradation is noted in the mesh size of the sand particles. For example, 20/40 mesh refers to a product that has 90 percent of the sand particle between a 20 and 40 mesh sieve screen. The most popular grade is 20/40 mesh. Typically the price is higher for coarser grade of sand. Sand is the most-used proppant.

Resin-Coated Sand Benefits

Resin-coated sand technology has been available for several decades. Two main types of resin-coated sand are used in the industry—curable and pre-cured resin. Curable resin has the ability to react and bond to other resin particles, while pre-cured resin does not have this ability. Pre-cured resin has higher crush resistance and interacts less with the fracturing fluid because the resin is fully cured. Curable resin is not fully cured and will result in greater interaction with the fracturing fluids, but this characteristic allows it to cure to other resin particles. Curable resin is preferred.

Putting a resin coating on raw sand increases the strength of the proppant. The coating helps retain the small particles that are generated when enough pressure is applied to crush the sand particles. Fine particles will reduce the conductivity and permeability of the proppant because they will migrate and plug up openings in the proppant bed.

When companies buy proppant, they essentially buy the space between the proppant. Research has shown that 5 percent of fines generated could reduce conductivity by 60 percent. One of the greatest properties provided by curable resin is its ability to bond to other resin sand particles. This phenomenon eliminates a problem called flow back, where proppants that are pumped into the well come back to the surface. Too much proppant coming back to the surface can create damage to collection equipment.

Phenolic-Resin-Coated Sand

Most resin-coated sands used in hydraulic fracturing are based on phenolic resin. This type of chemistry has been used for decades because phenolic resin can provide the characteristics and properties needed for resin-coated sand. The environment of the well is probably the harshest and few chemistries and technologies can hold up to an environment that is hot, wet and high pressure.

Unfortunately, phenolic-resin-coated sand could leach harmful chemicals that could potentially reach the water aquifer and contaminate water relied upon by so many people if the protective casing is damaged. Phenolic resin leaches formaldehyde and phenols. Many industries have already banned formaldehyde because this is known carcinogen. Phenol is a known mutagen, which can cause birth defects. Many industries are required to test products for these harmful compounds to ensure the safety of people.

Unfortunately, the hydraulic fracturing industry does not yet require full disclosure of all chemicals and potential hazards from all chemicals and resin used in the process. This will eventually be required as more pressure is applied to the industry.

Polyurethane Chemistry

The new resin technology is based on polyurethane chemistry—the same kind of coating the people eat at their kitchen tables. For the industry to accept this new technology, it must meet or exceed the performance of phenolic resin currently in use. The polyurethane resin has exceptional crush strength, comparable conductivity and is more effective at stopping flow back than its counterpart. Just as an example, one 40/70 polyurethane resin is an 18K proppant, while competitive material is a 14K proppant. The new technology produces equivalent conductivity and permeability as phenolic resins. Its coating technology is suitable for intermediate and deep wells.

Low-Temperature Resin

A new resin technology was recently launched that will impact the industry. The temperatures of shallow wells are typically low, which creates problems for resins that are intended to cure at low temperatures. It is a challenge to create a product that cures at a temperature as low as 90 F in well condition, but is stable when the product is stored in silos and transported in hot weather in rail cars.

The new low-temperature cure resin has flow back control capability in temperatures as low as 90 F, and the resin does not require an activator. Most low-temperature resins do not have flow back capability at low temperatures even using excessive amounts of activator. The new coating chemistry is safe enough to pass the tap water drinking limit. Testing has illustrated that if this resin is boiled in water, the water is just as safe as the tap water drinking limit. This is not the case for most phenolic resins because they leach formaldehyde and phenols.

The Safer Alternative

Before the polyurethane coating technology, no safer alternative was available. This new alternative resin has the same or better performance as its phenolic counterpart. At the same time, it is a product that is safer for the environment. Hydraulic fracturing is vital to sustain energy production in North America. Companies operating within this industry are socially responsible for creating a product that is safe to use to protect the environment that will be inherited by our children.