- Traffic-related environmental impacts of fracking studied using a novel Traffic Impact Model.
- Model estimates greenhouse gas, local air quality, noise and axle loading impacts on roads.
- Single well pad creates substantial increases in local air quality pollutants during peak activity.
- Short-duration/large-magnitude events may adversely affect local ambient air quality and noise.
- Daily NOx emissions may increase by over 30% and hourly noise levels can double (+ 3.4 dBA)
Paul S. Goodman, Fabio Galatioto, Neil Thorpe, Anil K. Namdeo, Richard J. Davies, Roger N. Bird
School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, Tyne and Wear NE1 7RU, UK
Received 16 July 2015 Accepted 1 February 2016 Available online 24 February 2016
© 2016 The Authors
Hydraulic fracturing (fracking) has been used extensively in the US and Canada since the 1950s and offers the potential for significant new sources of oil and gas supply. Numerous other countries around the world (including the UK, Germany, China, South Africa, Australia and Argentina) are now giving serious consideration to sanctioning the technique to provide additional security over the future supply of domestic energy.
However, relatively high population densities in many countries and the potential negative environmental impacts that may be associated with fracking operations has stimulated controversy and significant public debate regarding if and where fracking should be permitted. Road traffic generated by fracking operations is one possible source of environmental impact whose significance has, until now, been largely neglected in the available literature.
This paper therefore presents a scoping-level environmental assessment for individual and groups of fracking sites using a newly-created Traffic Impacts Model (TIM). The model produces estimates of the traffic-related impacts of fracking on greenhouse gas emissions, local air quality emissions, noise and road pavement wear, using a range of hypothetical fracking scenarios to quantify changes in impacts against baseline levels.
Results suggest that the local impacts of a single well pad may be short duration but large magnitude. That is, whilst single digit percentile increases in emissions of CO2, NOx and PM are estimated for the period from start of construction to pad completion (potentially several months or years), excess emissions of NOx on individual days of peak activity can reach 30% over baseline. Likewise, excess noise emissions appear negligible (< 1 dBA) when normalised over the completion period, but may be considerable (+ 3.4 dBA) in particular hours, especially in night-time periods.
Larger, regional scale modelling of pad development scenarios over a multi-decade time horizon give modest CO2 emissions that vary between 2.5 and 160.4 kT, dependent on the number of wells, and individual well fracking water and flowback waste requirements. The TIM model is designed to be adaptable to any geographic area where the required input data are available (such as fleet characteristics, road type and quality), and we suggest could be deployed as a tool to help reach more informed decisions regarding where and how fracking might take place taking into account the likely scale of traffic-related environmental impacts.
Aim of the paper
The exploration and potential production of oil and gas from shale reservoirs using hydraulic fracturing (fracking) technology has raised significant public concerns about a range of possible environmental impacts as diverse as the potential for fracking to cause earthquakes (e.g. Davies et al., 2013) to the contamination of water supplies (Osborn et al., 2011, Davies et al., 2014). The environmental impacts of road traffic associated with drilling and fracking operations has been an additional concern (King, 2012), but there are few analyses of these potential impacts in the peer-reviewed literature.
Fracking an exploration or production well requires additional, primarily Heavy Duty Vehicle (HDV) traffic associated with the use and disposal of water and chemicals used in the fracking process. Since gas resource estimates, for example in the UK and across Europe, are significant (DECC, 2010, Andrews, 2013) and there is an active debate around the environmental risks, assessment of the impact of road traffic is both timely and important.
The aim of this paper is therefore to produce a scoping-level environmental assessment for the short-term local impact of an individual site, as well as a longer-term assessment of the temporal impact of a number of sites operating in a particular region, over a timeframe of several decades, using a newly-created Traffic Impacts Model (TIM). This should help inform the current debate around the impact of increased traffic associated with the deployment of this technology, specifically for the UK (on which our hypothetical modelling scenarios are based) and the rest of Europe, but with relevance to other countries where fracking is active, such as the US and Canada, or being considered (e.g. Australia, China and South Africa).
Well drilling and fracking
The drilling of a well requires the building of a well pad, movement of a rig and related equipment and materials to the site, for example casing, cement and the chemicals required to make up drilling mud. The fracking process is undertaken after the well has been drilled. Rather than one fracking operation, fracking is normally carried out along different sections of a horizontal or vertical well in what is called a multi-stage fracking operation. Total volumes of water used per well vary considerably, from 1500 m3 to 45,000 m3 (e.g. King, 2012).
The process requires water to be pumped down the well along with chemicals and proppant (usually ceramics or sand). After pumping has finished, the fracking fluid (including natural contaminants) returns to the surface. As with initial water demand, flowback volumes are variable. In US operations the volumes returning can be 5–50% of the injected volume (King, 2012). Over a period of a few days to a few weeks flowback decreases and gas production commences.
A production well pad could include 12 or more wells, which may be re-fracked several times, once production has declined. It subsequently becomes necessary to remove flowback water from those sites both during and after fracturing. If this transportation is done by road, as has typically been the case in the US and Canada, then considerable volumes of HDV (i.e. tanker) traffic may be generated, albeit for relatively short periods (i.e. weeks) of time.
HDVs cause visible disruption and traffic congestion, especially if routed along roads that may be considered inappropriate, such as those carrying light traffic in rural areas, or at inappropriate times of the day. Vehicle movements are associated with production of greenhouse gas (GHG) emissions, primarily in the form of Carbon Dioxide (CO2). The production of CO2 from traffic emissions due to shale gas exploitation has been previously examined by Broderick et al. (2011), with on-road emissions estimated at 38 t–59 t CO2 per well. Broderick et al. (2011) also suggested that total CO2 emissions associated with extraction of shale gas from a well were small (0.2–2.9%) compared to total emissions from combustion of the gas produced by the well.
HDVs are associated with disproportionate annoyance arising from noise emissions when compared to lighter vehicles (Sandberg, 2001), and their diesel engines are perceived as ‘dirty’ in terms of local air quality emissions of Oxides of Nitrogen (NOx) and particulate matter (PM), even though modern vehicles are fitted with a variety of exhaust treatment devices to meet emissions standards (the ‘Euro’ standards) in effect across the European Union, and adopted globally in other countries, such as China and Australia. At the time of writing, the EURO VI standard (Reg (EC) No. 595/2009) is the highest in effect for HDVs (OJEC, 2009).
Emissions of gases affecting local air quality may have been historically of less importance in the US and Canada, with much drilling taking place away from urban areas. However, this may not be the case in more densely populated areas such as in Europe, where exploration may occur relatively close (i.e. within kilometres) to sub-urban locations resulting in HDVs travelling through inhabited areas. Historically, large urban areas, and areas adjacent to major highways, especially in the UK, have had problems meeting statutory limits on concentrations of Nitrogen Dioxide (NO2 — a component of NOx) and PM10 (particulate matter with aerodynamic size under 10 μm).
Aside from physical emissions of gases and noise, concern has also been voiced about the damage caused to pavement surfaces and the underlying road structure through additional loading by HDVs. Such vehicles cause a disproportionate amount of loading when compared to lighter vehicles, and may substantially shorten the lifespan and increase deterioration of roads not designed with their presence in mind.
Other traffic-related issues that have been raised in the debate about fracking and require rigorous research are: potential increases in the number of accidents (involving direct injury and damage to property, or accidental spillage of materials or chemicals), issues with vibration, community severance, delay (to pedestrians and other drivers) and disruption of normal activity patterns, increases in traffic violations, which all feed into marginal economic effects where the cost of operation of a site may be borne by the local community, rather than the site operator, if appropriate policy mechanisms to redress imbalances are not provided.
Traffic volume and intensity
Estimation of the potential volume of traffic generated by shale gas exploitation associated with a particular drilling site, or over a given region, is compounded by a number of factors. Aside from uncertainty over total quantities and precise locations of resources to be exploited, a number of other issues and considerations may be highlighted.
First, modern techniques allow for multiple wells to be operated from a single well pad during full production. NYS DEC (2011) reports that 90% of Marcellus Shale development (Pennsylvania, US) is expected to be by ‘horizontal wells on multi-well pads’. Examination of literature, online resources and anecdotal evidence suggests 6-, 8-, 10- or 12-well pads are now standard operating practice in the US (DEC, 2008, SEAB, 2011, DrillingInfo, 2014).
Second, there is a large amount of variability reported in the amount of water and proppants required at each well, which depends on the underlying geology, and the number of potential stages of fracking. The single well to date in the UK at Preese Hall (Lancashire) required 8400 m3 of water (Cuadrilla, 2014a). A summary of US literature, produced for the European Parliament (EP DGIP, 2011) suggests volumes per well (including initial drilling) in the range of 1500 to 45,000 m3.
Jiang et al. (2013) reported using a normal distribution with mean 15,000 m3 and overall range from 3500 to 26,000 m3 to model demand for the Marcellus Shale deposits. Broderick et al. (2011) suggest that each stage of a fracking operation for a single well will require between 1100 and 2200 m3 of water, leading to a total demand of 9000 to 29000 m3 per well, or 54,000 to 174,000 m3 for a six-well pad.
If a base requirement of 20,000 m3 of water and 5% by volume proppant is assumed, this equates to a transportation need for delivery of 1000 m3 of sand, or 1700 t of sand (assuming dry sand with density 1700 kg/m3), though fracturing additive requirements in some countries may be lower than typical US values, based on exploratory data (Broderick et al., 2011).
Third, there is large variability reported in the amount of flowback material produced. The US Environmental Protection Agency (US EPA, 2010) suggests that the rate recovery of injected fluids from hydraulic fracturing is variable — ranging between 15 and 80%. NYC DEP (2009) suggests use of a ‘worst case’ option of 100% for the calculation of tanker demand, whilst NYS DEC (2011) reports 9% to 35% returned water for Marcellus Shale wells in Pennsylvania, and Cuadrilla report values of 20 to 40% (Cuadrilla, 2014b). For modelling purposes Broderick et al. (2011) assumed 50%.
Fourth, during the operational lifetime of the well, which may be a period of 5 to 20 years, there may be the need for further stimulation via periodic re-fracturing (or re-fracking). Each re-fracturing event may entail similar, if not higher, water and waste demands to the initial process (Sumi, 2008). Broderick et al. (2011) assumed a single re-fracturing of 50% of wells for UK-wide shale gas scenarios.
Finally, aside from the actual fracking and waste disposal transport demands, there will also be vehicle movements associated with: construction of access roads and site facilities, excavation and concrete pouring for the pad, transportation of drilling equipment, well casings, water tanks and pump equipment to site, excavation equipment used to dig waste pits, completion and capping material transport, and general movement of workers to and from a site.
However, the vast majority of movements (typically 70% or greater) are associated with the fracking and flowback processes. Table 1 presents data from NYC DEP (2009), cited by Broderick et al. (2011), illustrating total truck movements associated with a fracking operation requiring between approximately 12,500 m3 (low) and 18,750 m3 (high) of water and proppants per well, with approximately 50% flowback of fluid waste.
Table 1. Truck visits over the lifetime of a six-well pad.
Re-fracturing activities could add a further 2000–3000 HDV trips to the estimates in Table 1 (Broderick et al., 2011). Limited tanker movements (2–3 per well per annum) to remove produced water during the operational lifetime of a well may also be necessary (NYS DEC, 2011).
The duration of activities at a site also determines the intensity of demand for transportation over a given period. NYC DEP (2009) suggests that total operations for the six well pad outlined in Table 1 would span between 500 and 1500 days. However, the peak of HDV demands occur in the delivery of water and proppant between 30–60 days prior to commencement of fracking, with flowback fluid return occurring for 42–56 days after.
Transport demands may be complicated by the overlapping of phases between individual wells on a pad (e.g. water deliveries occurring during horizontal drilling), or by operation of multiple well activities in parallel (though actual drilling of more than two wells simultaneously is considered unlikely). Demand intensity may also be mitigated by the provision of water storage facilities and lined waste pits on-site, acting as buffers to demand.
It cannot be ruled out that water transportation to the well pad during exploration, development or after production has commenced could be via pipeline, as was the case for the UK’s first fracked shale exploration well at Preese Hall, Lancashire (Mair, 2012).
A Traffic Impacts Model (TIM) has been developed to produce a broad, scoping-level environmental assessment for a small number of well pads operating in a region, over a timeframe of several years. As outlined in the introduction, the primary input drivers of the model are the water and proppant demands of fracking at well pads, whilst the primary outputs are local air quality, GHG and noise emissions, plus estimates of axle loadings in the region.
The TIM consists of four main components:
- a traffic demand model, which utilises information about the scale and duration of activities in a region where fracking is being undertaken, in order to produce the pattern of traffic associated with those activities;
- a traffic assignment model, which uses the traffic demand data to calculate temporal and spatial patterns in vehicle flow and speeds on the roads being used to supply the region;
- an environmental model, which uses the patterns of assigned traffic to calculate impacts for a number of assessment criteria; and
- a post-processing module that collates and statistically summarises information, to compare the impact of operations with regards to the baseline traffic.
Each component is discussed in more detail in the following sections. The traffic demand, assignment and post-processing components are novel in this work, whilst the environmental model forms part of the pre-existing PITHEM (Platform for Integrated Traffic, Health and Environmental Modelling) tool (Namdeo and Goodman, 2012).
The purpose of the traffic demand model is to calculate the temporal pattern of vehicles within the study region associated with fracking activities. The model is implemented using a hierarchical series of C++ programming language classes that reflect real-world components (both physical and procedural) in a fracking operation. Class names are initially shown in single quotes (‘…’) in the following text.
At the top of the hierarchy is the ‘region’ class, which represents in abstract form an area of a country being studied. The region contains a number of ‘well pads’ where fracking activities take place. Each well pad may contain a number of ‘wells’. Activities at the well pads and wells, outlined below, drive the generation of traffic demand. Additionally, a region contains a transport ‘network’, comprised of road ‘links’ that carry both baseline (i.e. without fracking) and fracking ‘traffic’. The amalgamation of fracking demand, plus the baseline activity, give the overall level of spatiotemporal demand. The physical and spatial concepts in the TIM model are shown in Fig. 1.
Fig. 1. Physical and spatial concepts in the TIM.
As experience with fracking operations has developed, the number of possible wells per pad has increased (Section 1.4). At present the upper limit in the model is set to twelve wells per pad, though this is adjustable. It is presently assumed that pads in a given region are spaced at approximately 1.5 km distant, with a 15 km × 15 km (225 km2) region therefore having around 100 well pads, though not all would be active at any given time (AEA, 2012).
Both well pads and wells are associated with a number of ‘phases’ of operation. Each phase possesses a number of attributes and classes:
- a defined start and end time;
- the total volume of materials needing to be serviced over the duration of the phase. Note that this volume may be either an input requirement to the phase (e.g. required building materials), or a waste product produced by the phase (e.g. flowback water);
- links to other phases where required, defined by the volume of materials passed to subsequent phases;
- a list of ‘vehicles’ that may be used to service the needs of the phase; and
- the ‘routes’ used by the vehicles to and from the well pad.
In the default TIM model, the well pad has two key phases defined:
- commissioning and construction — encompassing initial survey requirements, the development of access roads to the pad location, pad ground- and earthworks, pad concrete pouring, set-up of shared drilling equipment for wells, excavation of shared waste pits used by wells and delivery of ancillary and site equipment (e.g. site offices); and
- decommissioning and landscaping — representing the removal of equipment from site and making good via any required landscaping at the end of the pad’s operational lifespan. The decommissioning phase may be linked to the commissioning phase so that a certain volume of materials used in construction need to be removed from site.
For each well the following phases are assumed:
- construction and drilling — representing the arrival of drilling equipment, drill casings and drilling water at the well pad, and the removal of bored material and drilling water from site;
- hydraulic fracturing — encompassing the delivery of water, proppant materials and chemicals to the well;
- flowback treatment — representing the removal of waste water from the well. The flowback phase is linked directly to the fracturing phase, so that a certain percentage of waste water needs to be removed from the well; and
- miscellaneous — encompassing all other operations and activities associated with the well, such as routine movements of site staff.
Phases themselves may overlap in time, both between individual well pads and wells, as well as internally, between activities at a specific pad or well. For example, hydraulic fracturing may be taking place at one well, whilst another well is being drilled, and flowback water is being removed from a third. Likewise flowback water removal may begin whilst a well is still in the process of being fractured. Default durations of phases have been derived from Broderick et al. (2011).
The vehicle class contains information on utilisation rate and capacity, which are used to determine the overall number of vehicles required by the phase. The vehicle class also contains the description of the vehicle, required by the environmental model (Section 2.4), and the passenger-car unit (PCU) scaling factor required by the traffic assignment model (Section 2.3). The vehicle class also contains flow ‘profile’ information.
The profile determines when vehicles servicing the phase are considered active on the road network, and holds data for each day of the week and hour of the day. As an example, a profile may be used to apply all traffic for the phase outside of peak traffic hours, as a form of traffic management policy. Finally, the route class defines a simple static list of network links that vehicles associated with a profile will use to service the site. Fig. 2 summarises the phases and classes associated with activities at a well pad.
Fig. 2. Temporal phases and objects associated with well pad activities.
When run, the traffic demand model produces a temporally-ordered series of time-sliced profiles linked to their parent vehicle classes and route information for its parent region.
Whilst the initial, six phase model (two phases for the well pad and four for the well) is considered an abstract, ‘high-level’ model, it is considered sufficiently representative of pad activities. Further disaggregate phases, in order to model at a higher level of detail (e.g. for separate treatment of input water from one location, proppant from another, chemicals from a third etc.), or to incorporate additional technological scenarios (e.g. water is brought to the pad site via pipeline, but waste-water is still removed by tanker) are possible.
Re-fracturing of wells (Section 1.4) in order to extend their operational gas production is not explicitly covered, though may feasibly be modelled through the combination of individual runs at differing epochs, using appropriate assumptions for demand and flowback.
The traffic assignment model combines information on the baseline status of a transport network in a region, with the temporal profiles, plus vehicle and routing information, output from the traffic demand model, to produce vehicle kilometres travelled (VKT) and average speed information for individual roads, which forms input to the environmental model.
The ‘network’ class contains a collection of ‘links’. Each link contains an identifying name, the type and length of the road, and baseline ‘traffic’ information. This baseline traffic information consists of a total Annual Average Daily Total (AADT) flow in vehicles per day, coupled with a diurnal ‘profile’, which is applied to the AADT flow to give hourly variation throughout a week.
Open information from the UK Department for Transport (DfT) ‘Road Traffic Statistics’ datasets (DfT, 2013) was used to produce the baseline diurnal profiles for our hypothetical scenarios. Each type of road has an associated ‘speed-flow’ curve, which alters the calculated speed on the link, based on the flow in each hour. The road type and calculation year, are used to give the traffic fleet composition on the link. Fleet composition tables are inherited from the data in the parent PITHEM model, and are discussed further in Section 2.4.
Appropriate capacity values for the speed-flow curves are defined for types of link, with reference to UK Design Manual for Roads and Bridges (DMRB) guidelines (Highways Agency, 1999, Highways Agency, 2002). At present there are parameter values for 30 road types defined in the model, broadly covering categories of rural, sub-urban, central-urban and town and village roads. It is recognised that speed-flow curves are an abstract representation of the real-world behaviour of traffic.
A congested situation will be represented as a high-flow, low-average-speed situation, rather than a more realistic low-flow, static or slow-moving queue of vehicles. This has potential implications for the accurate modelling of emissions in congested conditions in that emissions may be under-estimated. Additionally, the parameters for rural links exclude the effects of delay at junctions, which should be separately modelled (Highways Agency, 2002), but are not represented in the current approach.
Assessment criteria and fleet composition
As mentioned in Section 2.1, the PITHEM software (Namdeo and Goodman, 2012) has been used to provide the assessment of key environmental criteria based on the output from the traffic assignment model. The environmental criteria used are split into four broad categories:
- greenhouse gas emissions (tailpipe ‘ultimate CO2’ or ‘uCO2’ emissions);
- local air quality emissions (tailpipe NOx, HC and primary NO2 emissions, PM10, PM2.5 including brake and tyre wear components);
- noise level at the roadside; and
- standardised axle loading applied to the road.
Calculation of all of the above criteria depends on appropriate vehicle characteristics, combined with tabulated fleet composition information, to produce correctly weighted additions of contributions from both the vehicles used to describe fracking traffic demand and from the baseline fleet. For our demonstration scenarios in 3.2 Impacts of a single well pad, 3.3 Region and UK wide scenarios, fleet data are taken from the UK National Atmospheric Emissions Inventory (NAEI, 2011), supplemented with data from the Emissions Factor Toolkit (EFT), Version 5.1.3 (DEFRA, 2012).
These data provide a description of current and projected vehicle types (broken down by chassis type, weight, fuel, engine size and emissions control technology) for a range of years from 2008 to 2035. Separate fleet descriptions are given for broad categories of road: urban roads, rural roads and motorways.
Within the fleet composition tables, there exist temporally evolving splits of Euro emission control measures, driven by underlying vehicle uptake, renewal and scrappage assumptions. Sample ‘Euro class’ splits for rigid and articulated heavy goods vehicles for the years 2010, 2015 and 2020, from EFTv5.1.3 are given in Table 2 below.
Table 2. Example percentage splits of emissions control technologies (EURO classes) for rigid and articulated HGVs in select years.
As can be seen from Table 2, in 2010 the majority of goods vehicles were assumed to fall within the EURO III and IV emissions control categories, whilst by 2015 the EURO V and VI categories form the largest segments. At 2020 (and beyond) the majority of goods vehicles fall into the cleanest EURO VI control category.
Regarding the weight and capacity of vehicles, it has been assumed that a 40 t laden-weight articulated tanker has a capacity of 30,000 l (30 m3) of water, whilst a 26 t laden-weight rigid body tanker has a capacity of 15,000 l (15 m3). Further assumptions on the weights and capacities of other vehicles are required for each operational phase. These have primarily been taken from analysis of data in NYS DEC (2011), supplemented with information from the transport section Environmental Statement made for the potential Roseacre Wood, UK site (ARUP, 2014).
Greenhouse gas emissions
Greenhouse gas (GHG) emissions are represented by ultimate CO2 emissions (total mass of CO2 after all exhaust components of fuel exhaust have oxidised). The calculation of uCO2 is based on the speed-emissions curves presented in Boulter et al. (2009), which also form part of the UK Emissions Factor Toolkit, Version 5.1.3c.
The emissions function forms a ‘U-shaped’ curve, considered valid over a type-specific speed range (typically 5–120 km/h for light vehicles and 10–90 km/h for heavy vehicles). Outside of these ranges, values are clamped to prevent excessive emissions rates. Total emissions for a link are calculated from the summation of individual contributions for all fracking traffic types active in the period, for comparison to the baseline fleet contribution.
Local air quality emissions
Emissions for those pollutants considered detrimental to local air quality (LAQ) are calculated using the same methodology for uCO2, outlined above, with additional fuel quality and vehicle mileage scaling correction factors from the EFT applied (DEFRA, 2012) after calculation of the base emissions rate. For particulate matter and hydrocarbons, the emissions functions have the same form as that for uCO2 above, with coefficients from Boulter et al. (2009). For NOx a variety of functions are used, based on those found in the COPERT4 (COmputer Program for Emissions from Road Transport) version 8.1 methodology (EEA, 2007, NaeI, 2012).
In accordance with the EFT, particulate matter emission rates (PM10 and PM2.5) are scaled from an initial tailpipe emissions calculation, plus contributions from brake and tyre wear and re-suspended particles. Primary emissions of NO2 (i.e. those emissions of NO2 directly from the vehicle tailpipe, prior to the additional generation of NO2 from photochemistry) are calculated based on the initial NOx emission, with the percentage conversion factors presented in Boulter et al. (2009) applied.
At present, both the GHG and LAQ emissions functions assume that heavy vehicles are 56% laden over a round trip, based on UK DfT freight transport statistics (DfT, 2013), which is considered equivalent to a tanker arriving on site full, then departing almost empty. Ideally, separate emissions rates would be calculated for each journey leg. The same limitation exists in the standard EFT (DEFRA, 2012) emissions factors.