|Written by Don Keer, PE, MDC Systems® Consulting EngineerThe Marcellus Shale natural gas deposit is currently the focus of intense exploration and development that could provide energy for the entire U.S. for the next 100 years. As with any development of an energy source the level of activity is dependent on the ultimate retail price of the delivered energy. In recent months the supply of natural gas has outstripped the demand resulting in a slight slowdown in activities in Ohio, Pennsylvania, New York and West Virginia. As with other energy sources, eventually the economics will again favor aggressive development of this resource.
Despite the attractiveness of natural gas the development of the Marcellus Shale deposit is limited by the ability of developers to manage the large quantities of water required to complete the wells. The issue has such a high public profile that the EPA is aggressively conducting studies and analysis together with other federal, state and local agencies to determine the impact and options for water treatment.
Hydrofracturing (“fracking”) is a well completion process by which the surface area between the oil or gas producing geologic formation and the well is increased. With an increased surface area the oil or gas is able to enter the well bore easier thus increasing production. Fracking is normally accomplished by pumping water under extreme pressures with additives that help stress the underlying formation to the point that the rock cracks and fractures. Once a fracture occurs the completion company will typically add sand to the water to keep the fractures open. The entire process could result in three to eight million gallons of water being pumped into the well and the surrounding rock formation.
Each well is somewhat unique but all natural gas wells will return water used in the fracking process back to the surface over the next 30 to 60 days. Depending on the well 10 to 30% of the frack water can return to the surface as “flowback”. The return water contains all of the chemicals, solids and salts added during the fracking process plus any salts and chemicals present in the reservoir. Future development of the Marcellus Shale deposits depends upon the availability of sufficient quantities of water for the initial fracking process and the treatment of the flowback water.
The issue that gains all of the headlines is the treatment of the flowback. According to The Penn State University College of Agricultural Studies (Marcellus Shale Wastewater Issues in Pennsylvania – Current and Emerging Treatment and Disposal Technologies, by Charles W. Abdalla, Joy R. Drohan, Kristen Blunk, and Jessie Edson, 2011 forr Penn State Extension) Range Resources, a major developer of the Marcellus Shale formation in Pennsylvania, disclosed the average volume and chemicals used in its fracking process:
· 4.57 million pounds of sand
· 1,333 gallons of hydrochloric acid
· 1,695 gallons of friction reducer
· 2,211 gallons of an antimicrobial agent, and
· 386 gallons of scale inhibitor (which includes ethylene glycol, a component of antifreeze.
While the chemicals are a relatively small percentage of the total material used (<0.5%) the flowback includes other materials dissolved from the formation including sodium, calcium and chlorides. The Penn State group summary of flowback water quality presents a range of total dissolved solids (TDS) in the range of 38,500-238,000 parts per million (ppm) after 14 days. In comparison, sea water has a TDS of just 35,000 ppm. Traditional wastewater treatment facilities are designed to reduce solids and organics, not TDS. Therefore, without modification, existing wastewater plants eventually discharge water with more salt in it than sea water. The EPA and the PADEP became aware of the problems associated with elevated TDS levels in the Pennsylvania waterways in 2008 and 2009. In 2010 the PADEP issued new regulations to limit the TDS levels of discharges.
Due to the multistate impacts of the Marcellus Shale flowback the EPA has started evaluating various treatment options. Those options/issues include:
· Underground injection of waste disposal fluids from oil and gas wells (Class II wells)
· Discharge to treatment facilities that can meet the Clean Water Act effluent guidelines
· Stormwater management from oil and gas operations
· Use of surface pits or ponds for re-use or disposal
· Re-use or recycling of flowback to supplement the freshwater requirements.
The less appreciated issue may be the total volume of water required to complete the frack process. Three million gallons per well is the equivalent of 1” of rain over 110 acres assuming it is a 100% impervious surface. Other sources of water should be identified and captured to minimize the impact on surface water volumes.
Re-use of flowback water may be used to supplement the raw water requirement but other sources must be identified. One possibility is the use of wastewater plant effluent. Instead of releasing the water to the environment is can be easily captured and recycled for the fracking process.
The environmental pressures associated with water quality issues will continue to drive innovation in both the identification of resources, treatment technologies and management programs for successful exploitation of the Marcellus Shale. The interaction of public and private utilities, resource developers and government regulators will continue to be scrutinized by the public in order to balance environmental issues with the drive to improve energy delivery resources.