Between 2000 and 2011, the number of producing gas wells in the US increased by 50%, reaching 514,637. This surge in exploration and production from unconventional sources has been accompanied by public concerns about various environmental issues—including air quality, water quantity and quality, and human health impacts. Moreover, with this fast-moving industry, scientists have been struggling to obtain adequate funding and data access for research studies, and regulators have been grappling with the development of new rules and policies along with limited resources for enforcement during the surge in drilling. Decision and rule making at the state and national levels in the US have been informed in part by limited, out of date, and sometimes incomplete emission inventories and self-reported industry data.

Further confounding the ability to adequately assess the industry’s environmental impacts are a number of other factors including (1) a lack of independent field measurements to evaluate assumptions, quantify risks, and assess actual impacts; (2) contradictory scientific results; and (3) polarizing political and sociological dichotomies (i.e., jobs vs environmental stewardship).

—Moore et al.

The team, from the Desert Research Institute, University of Colorado Boulder and NOAA, and Stanford University, began by defining a consistent vocabulary, first by jettisoning the popular umbrella label of “fracking” applied to unconventional oil and natural gas development. They separated the drilling process from the term “hydraulic fracturing”, which describes the process of fracturing low permeability rocks using water mixed with sand and proprietary chemicals pumped into the borehole under high pressure. Hydraulic fracturing originated in the 1940s, but the pressures and volumes used today are much higher than in the past, they noted.

They delimited five stages of the natural gas lifecycle using terminology defined in a 2012 World Resources Institute working paper:

• preproduction;
• natural gas production;
• natural gas transmission, storage, and distribution;
• natural gas end-use; and
• well production end-of-life.

Potential species emitted to the atmosphere during specific stages of the natural gas life cycle. Credit: ACS, Moore et al. Click to enlarge.

In terms of the life cycle, unconventional natural gas differs from conventional natural gas in three main ways. First, extraction of unconventional natural gas often requires directional or horizontal drilling. Second, well-completion (hydraulic fracturing) procedures for unconventional natural gas are much more extensive than for conventional wells. Third, unconventional natural gas wells typically have a sharper production decline curve and a less well constrained total volume of natural gas recovered per well (based on both economical and practical constraints). Once out of the ground, however, unconventional natural gas is subject to the same fate (e.g., processing, transport, end-use) as conventional natural gas, and the atmospheric impacts are indistinguishable between the two forms.Moore et al.

They then reviewed the available literature on air quality impacts for each of the stages. Air emissions from the natural gas life cycle include greenhouse gases, ozone precursors (volatile organic compounds and nitrogen oxides), air toxics, and particulates.

To fill the critical knowledge gaps that they found, they recommended a number of actions:

• Air quality measurements need to be made prior to oil and gas development, including during drilling and hydraulic fracturing, to more clearly understand the direct impacts of these activities. Air monitoring during these operations can help ensure emissions management strategies are effective and exposure to air pollutants, including silica, are kept to a minimum.

• A full chemical classification of emissions, including air toxics, during all life cycle stages needs to be obtained to properly perform source apportionment modeling and to understand all potential air quality and health impacts.

• Independent scientific data on the true nationwide extent of methane leaks from the production, processing, transmission, storage, and distribution infrastructure, including measurements of flows and fluxes, should be acquired.

• An inventory of abandoned/orphaned wells should be collected so that emissions can be properly estimated.

• Measurements on the variation of air emission composition and magnitude by natural gas and oil plays need to be made.

• Collaborations between independent scientists, regula- tors, and operators need to be increased to gain access to areas where measurements should be made and to inform effective emissions detection, reduction, and monitoring strategies.

Resources

• Christopher W. Moore, Barbara Zielinska, Gabrielle Pétron, and Robert B. Jackson (2014) “Air Impacts of Increased Natural Gas Acquisition, Processing, and Use: A Critical Review,” Environmental Science & Technology doi: 10.1021/es4053472

• Branosky, E.; Stevens, A.; Forbes, S. (2012) “Defining the Shale Gas Life Cycle: A Framework for Identifying and Mitigating Environmental Impacts,” World Resources Institute