Marcellus Shale Play – Water Treatment Options Worth Considering
Centered in western Pennsylvania, the Marcellus stretches over 650 miles of the Appalachian Basin from West Virginia to the state of New York. Marcellus has been estimated to contain anywhere from 58 trillion to 500 trillion cubic feet of gas. A member of the Devonian black shales, Marcellus is categorized as a dual porosity reservoir, wherein fractures can be drained rapidly while the shale matrix is drained more slowly.
Gas exploration today not only requires better reservoir knowledge and superior drilling methods, but also highly targeted completion technologies. Marcellus Shale Play developers are looking at ways to improve their use of precious water assets to support not only multi-stage fracturing, but also well completion efficiencies and improved water conservation.
A gallon of water involved in fraccing has an interesting journey. First it receives a mixture of chemical additives: a friction reducer (a polymer to reduce the viscosity of the water and improve its flowability so it’s easier to pump down the well), a special grade of light sand, and a proprietary gel that helps to carry the sand down into the well. This fraccing fluid is injected into a gas hole at a high flow rate and pressure to break up the formation, increasing the permeability of the rock and helping the gas flow toward the surface. As the water cracks the rock formation, it deposits the sand. As the fractures try to close, the sand keeps them propped open. Fraccing typically occurs once when a well is newly drilled, and again after a couple of years when the rate of gas flow begins to decline.
Underground, the fraccing fluid picks up other contaminants present in the rock formation, including barium, calcium bicarbonate, iron, magnesium sulfate, sodium chloride, and strontium.
Each gas well in the Marcellus Shale uses two- to four- million gallons of water for drilling and fracturing and even more if the well must be re-fractured.
As little as 25 percent and as much as 100 percent of the fracture water returns to the surface. This water, often referred to as flow-back water, contains hydrocarbons, salts, dissolved solids, etc. The first flow-back water has a salt content of only between 1,500 and 2,000 parts per million, but the longer the water remains in the Marcellus Shale, the saltier it becomes. By the end of the first week, the salt content can reach 45,000 parts per million. Sea water averages between 10,000 and 35,000 parts per million. The high salt content makes the water highly corrosive to metals and harmful to land, vegetation, and other living organisms.
Some water continues to flow out of gas wells once they are in production. This water is referred to as produced water. Some wells may indeed have more produced water than flowback water.
A typical Marcellus well development will involve approximately 30,000 barrels (1.26 MG) of flowback. The present philosophy is to send this flowback, as well as produced water to off-site treatment and disposal that may be many, many miles away, requiring a fleet of some 300 vehicles to haul the flowback water to an acceptable treatment facility. Furthermore, the current off-site treatment and disposal capacity in Western Pennsylvania is severely lacking. Furthermore, in May of 2008, the PA DEP sent formal letters to all of the publically owned treatment works (POTWs) explicitly outlining that they were prohibited from taking this waste stream without a rigorous permitting step, if at all.
Under current off-site disposal philosophy, this flowback water is not recycled. Millions of gallons of natural water resources are lost. For every well, the estimated 300 transport trucks will also release an estimated 40 tons of CO2 emissions into the environment. These transport trucks make hundreds of round trips a day, congesting traffic flow in the surrounding community and deteriorating roads and highways. In addition, at the current price of diesel, significant cost impact is realized via this current philosophy. Off-site treatment is estimated to cost between $0.03 and $0.05 per gallon.
The solution to off-site disposal is on-site treatment and reuse. An estimated 50% — 60% total savings can be realized by treating and reusing flowback and produced water on-site. There are a number of on-site treatment options worth considering including evaporation/crystallization, advanced oxidation, membrane filtration, etc. A few of these technologies are discussed in more detail below.
At Venture, we are working with our clients to determine the most economically viable treatment option for each site. Site factors such as available power and final water quality are often the determinant in treatment selection. After determining the most economical on-site treatment solution, Venture designs (using 3D AutoCAD Inventor) mobile skids that can be deployed directly to the well-site to improve water recovery, reduce environmental impact, eliminate hauling costs, and drastically reduce off-site treatment costs. Further, because the treatment and recycle plant is mobile, it can be moved and reused at multiple well sites, and requires far less time to permit and install (since it is not a permanent facility).
On-site treatment technologies have proven capable of recovering between 70% — 80% of the initial water to potable water standards and made immediately available for reuse in the frac process. The remaining 20% — 25% is very brackish and considered brine water. In addition produced water has even higher salt concentrations than flowback. Some technologies can further treat the brine water, increasing recoverability by as much as 24%. In most cases, this recovered brine water cannot achieve potable water standards, but can sufficiently be cleaned for other on-site or off-site uses as ‘process water’. In other cases, the brine water is simply sent for off-site treatment and disposal. The economics here are primarily distance from a treatment facility with available capacity to undertake your development.
On-Site Flowback/Produced Water Treatment Alternatives
Advanced Oxidation Process
The advanced oxidation process (AOP) is successfully used to decompose many hazardous chemical compounds to acceptable levels, without producing additional hazardous by-products or sludge which require further handling. The term advanced oxidation processes refers specifically to processes in which oxidation of organic contaminants occurs primarily through reactions with hydroxyl radicals. AOPs usually refer to a specific subset of processes that involve O3, H2O2, and/or UV light. The most widely applied advanced oxidation processes (AOP) have been:
- Peroxide/ultraviolet light (H2O2/UV)
- Ozone/ultraviolet light (O3/UV)
- Hydrogen peroxide/ozone (H2O2/O3)
- Hydrogen peroxide/ozone/ultraviolet (H2O2/O3/UV) processes
- Ozone/Ultrasonic cavitation
Advantages of Advanced Oxidation Processes
- Rapid reaction rates
- Small foot print
- Potential to reduce toxicity and possibly complete mineralization of organics treated
- Does not concentrate waste for further treatment with methods such as membranes
- Does not produce materials that require further treatment such as “spent carbon” from activated carbon adsorption
- Non selective pathway allows for the treatment of multiple organics at once
Disadvantages of Advanced Oxidation Processes
- Capital Intensive
- Complex chemistry must be tailored to specific application
- For some applications quenching of excess peroxide is required
Mechanical Vapor Recompressor Evaporation/Crystallization
A vapor-compression evaporator, like most evaporators can make reasonably clean water from any water source. In a salt crystallizer, for example, a typical analysis of the resulting condensate shows a typical content of residual salt not higher than 50 ppm or, in a different concept, not higher than 10