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As the use of water in multizone fracturing operations continues to increase, water availability presents a challenge. Each operation uses as much as 19 million liters (5 million gallons) of water, depending on the well depth and length. Although this consumption brings up concerns for the Environmental Protection Agency (EPA) and other environmental organizations, the oil and gas industry in the U.S. uses less than 1% of all the water consumed by other industries.
Challenges related to water usage
Drilling and fracturing operations often take place in remote locations that have inadequate supplies of freshwater. As a result, operators must truck in water and bear the expense of major logistical costs. After each job, the operators must also transport the flowback and produced water from the site to treat and pump it into a wastewater disposal well, which results in additional operational and maintenance costs.
Furthermore, the heavy-duty trucks that transport the water can damage the road infrastructure in these remote locations. The costs associated with making the water available represent a considerable percentage of the overall operational costs because one truck can carry just 19,000 l (5,000 gal).
Fracturing fluid overview
The freshwater historically used for fracturing operations contains many naturally occurring compounds, to which operators add chemicals such as scale inhibitors, surfactants, biocides, friction reducers, gelling and breaking products, and sand proppant. The resulting mixture contains 82.5% water, 16.4% sand proppant and 1.1% chemicals on average. Although an extremely small percentage of the mixture, the chemical content frequently incites much discussion and misinformation.
Flowback, produced water
An unconventional well flows back about 20% to 40% of the total fluid pumped within the first year of production. In the first 30 days of flowback, the fluid generally contains production chemicals and multivalent and divalent ions. In addition to flowback water, the well produces naturally occurring water with high salt content and similar ions throughout the life of the well.
Traditionally, operators have not had the ability to directly use this water in subsequent fracturing operations for various reasons. However, the ability to reuse this water reduces freshwater consumption and presents other major advantages. For example, 2005 oil production resulted in 25 Bbbl of water (an American Petroleum Institute statistic). This amount of water has the potential to supply irrigation water to 8,333 ponds provided that filtration returns the total dissolved solids (TDS) to acceptable levels. As an added advantage for operators, reusing produced and flowback water also significantly reduces overall operational costs.
Advanced bipolymer fluids are compatible in fields with a higher concentration of TDS than traditional fracturing fluids, which enables them to be used with unfiltered water. (Source: Weatherford)
Treating produced, flowback water for fracturing operations
A common approach to reduce freshwater use involves onsite or centralized filtration that returns produced and flowback water to a usable base fluid and even to freshwater in some cases. This freshwater can then be used to create conventional fracturing fluids, depending on the fracturing chemistry used. However, many frack chemistries use high-TDS produced or flowback waters that require increased filtration methods. These methods, such as reverse osmosis, create not only increased solid waste but also a major associated cost burden.
Disposal costs can be substantial. In some cases, operators must transport wastewater for disposal out of the state where the producing well is located. The requirements depend on county and state laws.
Effective treatment that reduces water usage
Removing bacteria is vital to prepare water for reuse. Tests conducted with chlorine dioxide (ClO2) biocide verify its effectiveness for killing bacteria in produced water. A powerful bacteria oxidizer solution, ClO2 has been used in the U.S. for more than 74 years with a focus on water treatment facilities and municipal applications.
The EPA accepted ClO2 as a safe yet powerful disinfectant in the 1974 Clean Water Drinking Act (EPA). When used at the required levels to control the bacteria, this biocide does not adversely affect the fluid pH in the well or react with most organics essential for the fracturing operation.
Traditional methods of treating produced and flowback water require ultrafiltration combined with clarification to mitigate solids and bacteria in the source water. These methods commonly require 16 days to filter 200 Mbbl of water at 12 Mbbl/d (assuming that filtration does not generate a significant amount of organic waste). ClO2 treatment is an effective way to avoid these slower filtration methods and results in long-term bacteria mitigation and reduction of real-time suspended solids.
Advancements in fluid design such as the use of advanced biopolymers to create linear and crosslinked fluids with the addition of robust friction reducer additives enable making optimal fracturing fluids from 100% produced water. Recent advances have increased the compatibility of both crosslinked and slickwater fluids with produced water. When combined with ClO2 technology to effectively deal with the bacteria challenge, this fluid can form an efficient solution in fracturing applications using 100% produced water.
Although filtration is not necessary for a crosslinked fluid to remain optimal, it is recommended that operators mitigate suspended solids to a level of less than 400 μ to avoid damage to permeability or conductivity (400 μ in reference to the canister filtration pod).
New robust fracturing fluid systems have been developed and tested. When added to the produced water, the result is a clean, low-residue stimulation fluid. The fluid imitates the properties of freshwater-based systems, thus precluding any formation or conductivity damage, and it can handle salt concentrations of more than 300,000 parts per million (ppm), divalent ions of up to 30,000 ppm, boron of more than 400 ppm and chlorides of 185,000 ppm. This water performs well in temperatures from 38 C to 149 C (100 F to 300 F) and maintains high viscosity levels that can carry proppant for long periods at high bottomhole pressures.
The recycled fluid reduces the amount of disposal required at the time of the flowback and production phase, which results in a major positive impact on costs and environmental concerns. On an annual basis, fracturing operations use about one-eighth of the total water used in golf course irrigation. Five jobs are created for every 19 million liters of water used in hydraulic fracturing, compared to less than one for nuclear power and agriculture. Operators can filter only to the level required by the frack chemistry. Filtration quality increases the amount of waste and escalates costs. Recycled produced water can be filtered to low enough levels for agricultural purposes.
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