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The arrival of commercial shale gas exploration in Europe, on the back of its success in the United States, has already proven controversial. There are large reserves and its successful production is likely to be welcomed in terms of its impact on energy supply and prices. In the Unites States (US) gas prices have dropped by almost an order of magnitude over recent years, and continue to drop – a very different picture to that experienced in Europe until recently.
An illustration of the significant impact of shale gas in North America is that the US was a net importer of gas until a few years ago, but now is a net exporter, and planned liquefied natural gas (LNG) import terminals are being turned into gas export facilities.
Attempts to extract gas from shale are not new. The first shale gas wells were drilled in the US almost 200 years ago, but large scale commercial extraction of natural gas from shale is a recent phenomenon made possible due to the development of two techniques: firstly, horizontal drilling allows the drilling to change directions thousands of metres below the surface, producing a horizontal well bore into the shale gas layer, and secondly, hydraulic fracking, which fractures the rock to allow the release of the gas. In simple terms the horizontal section of the well bore is perforated, and hydraulic fracking fluid, a mixture of water, proppant solids (sand or engineered materials) and chemical additives, is pumped down the well through the perforations into the rock. The water fractures the rock, releasing the gas. The proppant keeps the fractures open.
A shale gas well will produce less gas over a longer time period than a conventional gas well and typically there are a large number of wells spaced relatively closely together.
Simon Henry, Shell’s chief financial officer, recently said that the biggest challenge of extracting shale gas in Europe would be the “significant impact” on nearby communities, because shale gas exploration required “a lot of industrial activity” such as trucking, moving rigs and fluids used for fracking. The US shale gas fields have tended to be located in sparsely populated areas, locations more difficult to find in Europe. A notable exception is the Barnett Shale in Texas which runs beneath the Fort Worth urban area and where development has raised community concerns about noise and air pollution.
Environmental impact
The environmental impact of extracting gas from shale reached the UK headlines last year when two earthquakes were felt in Blackpool, north west England. Hydraulic fracking during exploration of a shale gas reservoir in the Bowland Basin was blamed, later confirmed by studies commissioned by the operator. Operations were suspended, but following an independent review, the government is likely to give permission for drilling to recommence, subject to certain restrictions.
In the United States there have been increasing concerns over the impact of shale gas exploitation on water quality, particularly potable water, and the quantity of water required for the process. The US Environmental Protection Agency (US EPA) is currently investigating the impact on drinking water, and has announced its intention to regulate wastewater from the industry. In addition, techniques for the recycling of the water are being actively investigated.
A less well publicised issue in Europe is the impact of air emissions. Workers can be exposed to the direct emissions of gas from the wells, as well as the emissions from the large number of on-site diesel powered engines required during the drilling and operation of shale gas wells. Communities close to some shale gas wells have raised concerns regarding the impact of these emissions on their health, and there is ongoing litigation in the US regarding alleged adverse exposure of nearby residents.
While there are air emissions, it is thought unlikely that the concentrations local residents are exposed to will be above risk based levels of concern, however. For many years there were few regulations addressing the fugitive emissions from the gas wells. Colorado and Wyoming were the first US states to implement control for these emissions’ sources, and in April 2012 the US EPA implemented nationwide controls. Those revisions in new source performance standards will require the capture and/or prevention of the fugitive emissions.
If shale gas exploitation becomes commercially viable in Europe, similar control will become necessary on this side of the Atlantic.
Regional ozone
During the drilling operations and hydraulic fracturing there may be tens of diesel engines running continuously emitting particles, nitrogen oxides and a range of air toxics. There are also potential sources of hydrocarbon emissions from the wellhead and equipment, as well as hydrocarbons evaporating from wastewater storage and management vessels.
These emissions combined with other natural (biogenic) and anthropogenic emissions in the region can together form ozone, other photochemical oxidants, and particles in the atmosphere. High concentrations of ozone and other oxidants in the atmosphere near the ground are of concern because of the adverse effects on human health and damage to vegetation, while particles are an important air pollutant due to their impact on human health. ENVIRON undertook a study of emissions from shale gas production areas and their impact on regional air pollution in 2010.
A detailed inventory of all sources of air emissions from the shale gas development in the Haynesville Shale formation, beneath the border of north east Texas and north west Louisiana, were estimated for each year between 2009 and 2020 under three development scenarios. Exploration and production of the Haynesville Shale began in 2008. The inventory predicted increasing emissions over time, as the number of wells, and production, increases. To elucidate data on future activity levels the leaseholders and producers were surveyed, but little data was available at the time of the study, approximately one year after production commenced, to predict future production.
The three growth scenarios were developed using data from Texas and Louisiana to define the possible future production. For each scenario the number of new wells (and spuds, the initial penetration of the ground or seafloor), and production profiles over time, were used to build up estimates of the number of formation-wide spuds and wells, and gas production.
The low growth scenario assumed that the March 2009 drill rig count of 95 remained constant each year until 2020. The high growth scenario assumed that initially the number of rigs grew from 95 to 200 at the same rates as in the Barnett Shale near Dallas-Fort Worth between 2001 and 2008, estimated to reach 200 in 2014. The moderate scenario assumed half the growth rate of the high scenario, and 200 was reached in 2018.
The number of rigs was capped at 200 as this is ten percent of the total number of rigs in the US in 2009. It was assumed that each well took 63 days to drill including the time to move the rig to a new site, mobilise it, drill the well and demobilise it for transport for the next well, allowing each rig to drill 5.8 wells per year. The success rate of the wells was initially assumed to be 55 percent, increasing to 100 percent by 2018 as understanding of the shale gas formation increases.
Production profiles over the life of a typical well, or decline curves, were derived from the eight longest running Haynesville wells, in combination with data on drilling and production from the Louisiana Department of Environmental Quality (LDEQ). These were compared to data available from two leaseholders. The average of the latter data produced cumulative well production almost three times higher than the derived figures. The production per well estimates were combined with the number of active wells to produce the Haynesville Shale production estimates per year. The lower estimates were used in the low and moderate scenarios and the higher estimate in the high scenario.
The gas production rates were used together with known emission factors for natural gas production in the region to produce an emission inventory of ozone precursors. The impacts of new emission standards (e.g. for off road mobile sources) implemented up to 2020 was taken into account.
Emission sources
The projected importance of different emission sources evolves over time as the field matures. For nitrogen oxides (NOx) drill rigs are the largest source in the early years, but midstream central compressor stations and natural gas processing facilities begin to dominate in the later years. A wider variety of sources contribute to emissions of volatile organic compounds (VOCs) adding fugitive sources and completion venting to the sources already named.
The emission inventory is being updated now that more data is available and it is anticipated that other important sources may be identified such as wellhead compressors for NOx, and liquid storage tanks for VOCs. Total NOx and VOC emissions were predicted to increase by 124 percent and 271 percent respectively between 2009 and 2020 in the moderate scenario. By 2020 the emissions were predicted to be more than 120 and 35 tons per day respectively.
Regional photochemical modelling
The Comprehensive Air-quality Model with extensions (CAMx) was used to model ozone across the eastern half of the US with a focus on the Haynesville Shale region. CAMx is a three dimensional, chemical transport grid model used to predict concentrations of tropospheric ozone, aerosols, air toxics, and related air pollutants.
It is used for air quality planning in both Texas and Louisiana. The model was used here to estimate the near term ozone impacts of the exploitation of the Haynesville Shale in 2012. An historic ozone episode from May 20 to June 30, 2005, was used to evaluate the model performance. It was able to reproduce measured ozone data with good accuracy within the Texas-Louisiana-Arkansas-Oklahoma region. The Texas Commission on Environmental Quality (TCEQ) anthropogenic emission inventory for 2005 was projected forward to 2012, and the model was rerun using the same input data (e.g. boundary condition and meteorological data), except the 2012 emission inventory replaced the 2005 inventory.
The model was then rerun three times with the addition of the Haynesville Shale emissions – once for each scenario. This enabled the impact of each of the scenarios to be assessed against the baseline (i.e. no Haynesville Shale gas exploitation) ozone concentrations.
Results
The study focused on the 8-hour average ozone concentrations. The US national ambient air quality standard (NAAQS) of 75 ppb applies to the fourth-highest daily maximum 8-hour concentration in a year (i.e. the 99th percentile), averaged over three years. In the EU a different approach has been adopted. In recognition of the difficulty in achieving the desired ozone levels to protect public health and vegetation there is no absolute standard. Instead, a target value for the protection of public health of 120 µg/m3, as a daily maximum eight hour average not to be exceeded on more than 25 days per calendar year (97th percentile) averaged over three years, has been adopted. Comparison between the US standard and the EU target is not straightforward, but 120 µg/m3 is approximately equivalent to 60 ppb for ozone. Two indicators of the ozone impact were used in the Haynesville Shale study.
Firstly, the episode average difference in daily maximum eight hour concentrations; secondly, the episode maximum difference in daily maximum eight hour concentrations. These were calculated for each of the three emission scenarios. For the low emission scenario the biggest effect on episode average difference in daily maximum eight hour concentrations was in north west Louisiana, with a peak increase of four ppb in southern Bossier Parish. For the high emissions scenario the peak increase was seven ppb with a similar spatial pattern, but over a wider area.
The area exceeding one ppb difference extended eastward to the edge of Dallas- Fort Worth and northwards into Oklahoma and Arkansas. The impact on the episode maximum difference eight hour concentration was, unsurprisingly, much greater and for the high emission scenario was predicted to be up to 17 ppb in southern Bossier Parish. The area where concentrations were predicted to increase by more than six ppb covered a large area of north Texas and north west Louisiana. The pattern for the medium and low emission scenarios was similar but less intense.
The impact on compliance with the NAAQS was tested using the US EPA’s ‘Modeled Attainment Test Software’ (MATS). MATS combines model results and monitoring data to estimate future ozone ‘design values’ which determine whether or not the ozone NAAQS is attained. The MATS results showed ozone design value increases of two ppb in the low emission scenario, and four to five ppb in the high emission scenario in the Haynesville Shale counties of Harrison in Texas, and Bossier and Caddo in Louisiana.
For Gregg and Smith counties in Texas the design value increases range from one ppb for the low emissions up to one to two ppb for the high emission scenario. These locations currently attain the ozone NAAQS by only a few ppb.
Implications
The impact of the exploration and exploitation of the Haynesville Shale gas reservoir on ozone concentrations over a wide area provides cause for concern about future air quality, particularly in Texas and Louisiana. Even if development occurs relatively slowly, areas of north east Texas and north west Louisiana may become ozone non attainment areas. It should be noted that since this study was completed that the EPA has introduced new regulations controlling fugitive emissions from shale gas wells.
The impact of this on the ozone generated by the Haynesville Shale exploitation has not yet been assessed. The fugitive VOC emissions, however, are estimated to contribute about 20% of the total emissions in the moderate emission scenario. Control of fugitive emissions may increase NOx emissions if flaring is used to destroy the collected VOCs. The study has implications for Europe, where ozone remains a difficult pollutant to control.
If there are a large number of wells, the combination of fugitive emission from the wells themselves and the emissions from the on surface equipment could make a significant contribution to ozone levels when weather conditions are favourable to its formation in the atmosphere. This is another area where the regulators need to consider the impacts at an early stage of the commercial exploitation of shale gas in Europe.
References
1 Royal Dutch Shell eyes fledgling UK shale gas market as profits jump to $7.7bn on higher oil prices The Telegraph, 26 April 2012
2 US to Set rules for Fracking on Federal Land, Wall Street Journal, 3 May 2012, http://online.wsj.com/article/SB10001424052702303877604577382460699241978.html
3 Ozone Impacts of Natural Gas Development in the Haynesville Shale, S Kemball-Cook, Bar-Ilan A, Grant J, Parker L, Jung J, Santamaria W, Mathews J, and Yarwood G, Environ Sci, Technol, 2010 Dec 15;44(24):9357-63
4 CAMx.com
5 Overview final amendments to air regulations for the oil and natural gas industry: fact sheet’, http://www.epa.gov/airquality/oilandgas/pdfs/20120417fs.pdf
Acknowledgments
I am grateful to my colleagues Sue Kemball-Cook, Amnon Bar-Ilan and Greg Yarwood, based in ENVIRON’s Novato office, California, for their assistance in preparing this article. The study described was undertaken for Northeast Texas Air Care with support from the Texas Commission on Environmental Quality.
Published: 07th Jun 2012 in AWE International
