Water management is an environmental and logistical challenge for many operators across the U.S. Because the typical unconventional horizontal well with a standard stimulation design requires as much as 8 million gallons of water, there is an increasing need for an alternative to freshwater. In areas such as the northeast U.S., legislation regarding water restrictions and disposal-well limitations presents additional challenges.
A common approach to mitigating freshwater use during fracturing operations is water reclamation, which is the process of returning produced and flowback water to a usable base fluid using on-site or centralized filtration. This practice is frequently applied in other industries, such as electrical power generation and agriculture, to address environmental concerns. Filtration can reduce the amount of total dissolvable solids (TDS) to less than 1,000 parts per million (ppm) and reduce the amount of interfering ions found in production water.
In addition to reducing reliance on freshwater, appropriately filtered produced water can increase base-fluid consistency. Increased base-fluid consistency enables operators to detect common incompatibilities such as a high concentration of divalent ions, which arrests gel hydration performance. Another common incompatibility is a boron concentration higher than 12 ppm, which increases the likelihood of over-crosslinking the fracturing fluid and results in substandard fluid stability.
Because of its ability to conserve freshwater and improve fluid quality, filtration is an established way to lower completion costs. However, a major disadvantage of filtering produced water is the amount of waste produced during filtration. When filtering quality increases, so does the amount of waste captured in each filtering module. At the same time that filtering out TDS and boron ions improves the quality of the produced water and eliminates wastewater disposal costs, it introduces the need to dispose of these particles, which increases filtration time and per-barrel costs. Operators must therefore weigh the advantages and disadvantages of filtration to determine whether it will be cost effective for each application.
The variable cost of wastewater disposal is another challenge in determining whether filtration will produce cost savings. On average, wastewater disposal costs can range from 50 cents to $2.25 per barrel. Adding associated transportation costs can bring the total disposal cost from 63 cents to as much as $16 per barrel. In contrast, filtration can cost $1.20 per barrel for suspended solid removal. More elaborate filtration treatments provide additional environmental benefits but also create more waste that requires disposal. When applying reverse osmosis or dewatering methods, costs can exceed $10 per barrel, and total water costs can reach more than $16 per barrel.
In response to this dilemma, researchers pursued an alternative to traditional filtration. One of the most vital objectives of water treatment is the removal of bacteria. Chlorine dioxide is a powerful bacteria oxidizer solution that has been used in the U.S. for more than 74 years, primarily by water treatment facilities and for municipal applications. In 1974, chlorine dioxide was accepted by the U.S. Environmental Protection Agency (EPA) as a safe yet powerful disinfectant as part of the Safe Drinking Water Act. The first well was treated with chlorine dioxide in 1987, though the method was not commonly used until recently.
As the need for economical use of produced water has grown, so has the acceptance of this environmentally friendly biocide that does not adversely affect the fluid pH in the well or react with most organics. Chlorine dioxide helps to address suspended solids and bacteria, which enables more effective filtration and results in more effective produced water utilization. Once suspended solids and bacteria are addressed, chemical methods of negating the effect of interfering ions, salinity, boron, calcium and chlorides are tested.
Increased acceptance of oxidizing bactericides such as chlorine dioxide, which quickly penetrates encapsulated bacteria, has lowered completion costs by reducing the need for filtration and addressing another hurdle to the use of produced water: the presence of free-floating iron content. By enabling oxidation at a controlled rate, bactericides flocculate out iron sulfide (FeS), solid-soluble particles, suspended solids and organic materials. This allows impurities to settle out of solution and enables more effective removal of flocculants with on-the-fly canister filtration. Oxidization transforms FeS and iron into ferric iron and results in accelerated settling of flocculated particles by final composition of ferric hydroxide.
Another practical method for lowering completion costs is using a slickwater fracturing fluid additive that is robust enough to withstand high-salinity divalent environments and boron content in applications with crosslinked fluids.
Advancements in crosslinked and slickwater fluids, in combination with the increased use of chlorine dioxide, make it possible to reduce the amount of suspended solids present in the produced water while mitigating bacteria in the source water. As a result, two new robust fracturing fluid systems have been developed and tested that, when added to the produced water, result in a clean, low-residue stimulation fluid. These materials are environmentally friendly gelling agents and crosslinkers unaffected by the remaining water content and capable of treating water that is high in salinity, TDS, multivalent ions and boron, while providing excellent proppant transport and conductivity.
The resulting water imitates the properties of freshwater, thus precluding formation damage, and can handle salt concentrations greater than 300,000 ppm, up to 30,000 ppm of divalent ions, and greater than 400 ppm boron and 185,000 ppm chlorides. This water performs well in temperatures from 100 F to 300 F (38 C to 149 C) and maintains high, closely controlled viscosity levels that can carry proppant for long periods at high bottomhole pressures.
As more economical completion methods are considered, advancements in fluid technology will allow for increased efficiencies and for reduction in per-barrel cost, filtration needs and pretreatment requirements. With savings per well ranging from $55,000 to more than $600,000, chlorine dioxide and bipolymer fluid technology offer tremendous potential for increasing produced water use throughout the industry.