All pits and ponds used for storing produced water, even temporarily, should be lined. This is because produced water is particularly hazardous to the environment and hard to remediate once it has escaped. Preventing spills and leaks can save companies thousands of dollars per year, which quickly adds up to offset the cost of liner installation. Some soils may have the perfect qualities to hold back seepage naturally, but few are located near where hydraulic fracturing is also widely used. Installing a synthetic liner, such as a geomembrane from BTL Liners, is the best way to ensure the same control over produced water loss no matter the location or soil conditions.
Requirements for a Reliable Liner
There are a few basic requirements that all pit liners must share to handle the challenges of produced water storage. The first is chemical compatibility with the hydrocarbon rich mixture that will be stored in it. Without chemical compatibility, cracks and leaks can develop quickly. The second requirement is the durability and strength to withstand large scale installation over relatively steep sides. Produced water storage pits tend to be dug rapidly and scaled to hold hundreds of thousands of gallons. Sheets of geomembrane must resist stretching and tearing under great force when used to line such large ponds. Finally, the liner must have a very low permeability to keep the water from slowly seeping out over time.
Why Clay and Compacted Soil Don’t Work
The final point above is the biggest reason why it’s a poor idea to simply smooth and compact the soil of the waste holding pit. Even the soils with the highest ratios of clay, still achieve a relatively poor impermeability rating. Highly permeable soil allows a lot of water and other contaminants to escape, increasing the risk of groundwater contamination and other environmental issues as time goes by. No amount of compaction, or spreading thick layers of bentonite and similar products, will result in a reliable level of impermeability. It’s easier to test and ensure the impermeability of a geomembrane since it’s manufactured in a controlled setting, unlike clay layers that are applied in the field.
Concrete as a Frac Pit Liner
Concrete can be used to reinforce and protect the banks of a produced water pit, but it’s not ideal as a primary liner material. First, concrete varies in its resistance to hydrocarbon reactivity depending on the exact cement formulation used as the binder. Second, additives in the concrete can often react separately and cause surface damage that weakens the material. Even without these issues specific to produced water ponds, concrete isn’t as impermeable as most geomembranes. Tiny openings in the surface may be invisible to the eye, but they’re large enough to let water seep out over time.
Geomembranes for Flexibility
Geomembranes are ideal for lining produced water pits and ponds because the material is flexible, chemical resistant, and highly impermeable. Of course, only some geomembranes offer all of the necessary features and specifications. Other materials or manufacturing processes may create a liner that can’t handle the hydrocarbons mixed into the water or that break down under direct sun exposure. It’s important to select a geomembrane specifically designed for petroleum industry use. Most mining and industrial liners will work, but some still feature weaknesses against hydrocarbons. Checking chemical compatibility should be the first step in selecting a geomembrane liner for any produced water pit.
Impermeability
Not all flexible liners and polymer materials offer the same level of impermeability. Woven materials, in particular, tend to lack the impermeability needed for produced water storage or treatment. Permeability is the material’s ability to allow water to flow through it, so an impermeable material is relatively good at containing water and other liquids. Look for the highest impermeability rating when comparing materials since produced water is both a valuable resource and a potential environmental hazard. Losses to seepage, either way, are a waste when impermeable materials are available from suppliers like BTL Liners.
Pits and Ponds vs Tanks
With so many considerations going into the design and installation of an in-ground pond, many hydraulic fracturing planners turn to tanks instead. However, tanks don’t automatically solve any of the issues that can come up with produced water ponds. Tanks are much more limited in their capacity and buying either larger tanks or smaller ones in great numbers both come with high price tags. These structures can still leak as they age, requiring secondary containment in the form of a surrounding basin lined with a geomembrane. If you’re already planning a containment area for tanks, you might as well combine primary containment features into it by making it the main pit or pond. Tanks are a good fit for holding small amounts of highly concentrated wastewater in the midst of processing, but they’re not generally a replacement for larger holding pits for produced water.
Material Options for Flexible Liners
The exact material used for manufacturing the geomembrane or flexible liner will determine how well it works for containing produced water. Narrowing down a general category of material is the best way to start searching for a product with the appropriate chemical and mechanical damage resistance as well.
RPE
Reinforced polyethylene (RPE) is the best option for most produced water applications, including pits, ponds, and tank lining. This material combines layers of both HDPE and LDPE to get the best of their features while canceling out the limitations of using either alone. Polyethylene in general is one of the most widely used polymers in the world because it’s highly chemical resistant and can be easily made UV resistant. When installing liners in a produced water pit, both features are needed in the same material. Many geomembranes, made of only one material, can either handle the exposed installation or high levels of reactive compounds, but not both. The reinforced design also increases the liner’s resistance to mechanical damage, both during the initial installation and as the liner ages.
HDPE
High density polyethylene (HDPE) is commonly used as a liner in challenging water storage situations because it’s a strong and tear-resistant material. However, it’s also stiffer than other polyethylene and PVC products due to its density. HDPE is also particularly weak against hydrocarbon penetration, creating leaks and seepage opportunities within a few years of installation. Most homogeneous geosynthetic membrane products, in general, are weak against hydrocarbons, therefore making it important to look for products combining multiple polymers. RPE achieves a higher level of chemical resistance while still offering plenty of mechanical strength due to its reinforcement.
LDPE
Low density polyethylene (LDPE), including very low-density polyethylene (VLDPE), are also sometimes used in short-term holding pits and tanks for produced water. LDPE is somewhat more chemical resistant than HDPE, but it’s still weak against hydrocarbons when used alone in a mono-polymer design. The lower density of the polymer allows for a flexible and tight fit against tanks and other existing structures, making it a popular repair material. However, it won’t last as long as RPE, which is nearly as flexible.
CSPE
Chlorosulfonated polyethylene (CSPE), is a unique option, commonly used for lining potable water ponds and tanks. It’s very chemical and UV resistant, but it’s also much more expensive than other materials used for lining in-ground ponds. This is because it’s resistant to leaching as well and is safe for storing drinking water long-term with lifespans of up to 30 years. It’s usually too expensive to use on large scale pits like those used for storing produced water and is limited to either smaller or higher value operations. This material is also not recommended for any kind of use with hydrocarbons because of its reaction and subsequent breakdown. While it may work for freshwater ponds for hydraulic fracturing, it’s a poor choice for lining produced water pits.
PVC
Polyvinyl chloride (PVC) is commonly used in other types of pond lining applications, including some wastewater processing plants. It’s better than some polymers at handling hydrocarbons, but it still tends to break down in the presence of oil or grease. The material is flexible like LDPE but tends to be far weaker than RPE. PVC can be used for short-term lining, but it’s not a long-term lining option for produced water.
Reinforced Design
Stick with reinforced liners when building produced water ponds and pits. Reinforced liners are much less susceptible to damage during installation, even if the soil is less than ideal after grading. The reinforcement mesh at the center of the liner is the key to increasing tear and rip resistance. This protection carries on during the years of use as the weight of millions of gallons of produced water presses down on the material. Stretching is minimal, reducing the chances of a tear developing because of weight or a folded liner.
The right liner will protect a produced water pit for thirty years or more with minimal maintenance. Make sure to check the warranty terms and recommendations for installation on all geomembrane products before choosing one. You may find that a product is only warrantied for buried installation and not direct exposure to the sun or chemicals in the water. If you plan to use the exposed installation style, favored for keeping produced water pits easy to empty and clean, you’ll need a liner designed for that purpose. Check out our selection of exposed installation liners here at BTL Liners that can handle the challenges of the oil and gas industries.
