Pre-production emissions before the well is producing gas are almost certain to be higher from unconventional wells than conventional wells due to extra energy required for horizontal drilling and hydraulic fracturing. Pre-production emissions from shale gas exploitation are likely to be in the range of 656 up to 1790 tCO2e/well (Jiang et al. 2011; Santoro et al. 2011; NYSEDC, 2011), which is up to 369 t CO2e/well larger than emissions from the pre-production of a conventional well (Broderick et al. 2011). However, of central concern for understanding the greenhouse gas emissions from shale gas exploitation are the methane emissions from well completion and flowback fluids, just as combustion emissions have been the central concern for electricity production, which are 420-520 gCO2e/kWh for natural gas compared to that of 840-1150 gCO2e/kWh for coal (Mackay & Stone, 2013). In the USA, the shale gas revolution outpaced scientific research and policy regulation, so emissions were not strictly regulated from 2000-2010. From 2002-2014, US methane emissions increased by 30%, which has been suggested to be caused by shale gas exploitation among other sources (Tuner et al. 2016; Hausmann et al. 2016). However, the EPA (2014) improved regulation by requiring that from 2015 all new shale gas wells were fitted with Reduced Emission Completion (REC) technology to capture methane emissions which would otherwise have been vented or flared.
From a UK perspective, it is important to ensure methane emissions do not increase, and this should involve monitoring at onshore shale gas well pads to assist in super-emitter mitigation. By increasing the precision of methane measurements, it allows for better-informed policy decisions on how to minimise methane and to what extent from other sources if there are surplus methane emissions from shale gas exploitation. Although Howarth et al. (2011) state that shale gas has a larger greenhouse gas impact than coal, this declarative conclusion is supported by assumptions of widespread worst practice, which includes venting during well completion flowback and no REC implementation. Under appropriate regulation, shale gas would have a life cycle greenhouse gas impact of at most 60% of that of coal and as low as 75% of that of LNG (Mackay & Stone, 2013). At an estimated rate of 100 shale gas wells coming online per year in the UK, even under worst case scenarios, methane leakage from flowback and drill out could not exceed 20% of
annual enteric fermentation methane emissions from the UK dairy cow, sheep and cattle population. Overall therefore, shale gas will have an emission factor equivalent to, or slightly higher than conventional gas depending on pre-production emission control and well productivity (EUR). Methane emissions under surplus ‘minimum necessary regulation’ required by the Committee on Climate Change (2016) could not nullify the shift from coal to natural gas.
Over the short to medium term, shale gas can be a ‘bridge fuel’ in the UK if domestic production is used to replace natural gas imports and displace coal use. Over the longer term, net UK carbon emissions will have to decrease in line with the fifth carbon budget to reduce emissions by 80% from 1990 levels by 2050 (DECC, 2015). Meeting such a target will either involve the decreased used of fossil fuels or the implementation of CCS technology, with the latter being described by Oxburgh (2016) as the central focus for reducing annual emissions. At the very least, the next generation of efficient CCGS plants should be ‘capture ready’ if and when the appropriate CCS infrastructure is implemented by the government.
A global adoption of shale gas could reap huge climate benefits in nations that are largely dependent on coal, such as China, which generates over 70% of its electricity from coal (IEA, 2015). The US has seen a 9% reduction in CO2 emissions from 2002- 2012 of which a large reduction proportion has resulted from the switch to increased gas use, produced domestically from shale (EPA, 2016). It would be environmentally undesirable for shale gas to contribute surplus energy generation on top of current fossil fuel use or replace renewable energy production as this would increase the carbon burden and increase the risk of 2 °C warming.
It must be duly noted that even with efforts from OECD nations to decrease net emissions, global net emissions are projected to increase from current rates of 50 Gt CO2e/year to 80 Gt CO2e/year by 2050, with 85% of the growth from emerging economies (BP World Energy Report, 2016). This ‘business as usual’ emission growth is undesirable and will only contribute to the enhanced accumulation of CO2 in the atmosphere. The UK currently emits ~1.5% of global greenhouse gas emissions, and even under the strictest possible climate regime, little contribution to global net emissions reduction would result (DECC, 2015). Therefore global initiatives such as the Paris Climate Agreement of 2016 are of great importance, and the UK should develop technology such as CCS to minimise emissions, and the technology can then be traded with developing nations who will be less likely to implement strict environmental controls. Lackner (2016) believes that CCS is
inadequate and that negative emissions technologies, such as direct air capture must be implemented because at current trajectories of CO2 accumulation of 2.2ppmv annually, the 2 °C warming target will be passed in 2038, well before CCS could be globally effective.
To ensure shale gas does not exceed carbon budget limits, pre-production emissions must be minimised, shale gas must displace coal and replace gas imports, and any methane emissions from shale gas exploitation must be addressed by reduction from other methane sources, which Howarth et al (2011) describe as the ‘low hanging fruit’. In terms of CO2 emissions, the greatest potential for de- carbonisation is from the power sector, but the power sector only represents 13% of UK energy consumption and in terms of total energy usage, renewables are inadequate to meet national demand in the short and medium term without an energy revolution. Therefore, CO2 and CH4 emissions must be nullified and CCS is essential to ensure UK climate targets, with a wider variety of emission mitigating techniques being required over longer terms to prevent global warming above 2 °C regardless of shale gas exploitation.