Fugitive gas emissions
Fugitive gas emissions are emissions of gas (typically natural gas, which contains methane) to atmosphere or groundwater[1] which result from oil and gas or coal mining activity.[2] In 2016, these emissions, when converted to their equivalent impact of carbon dioxide, accounted for 5.8% of all global greenhouse gas emissions.[2]
Most fugitive emissions are the result of loss of well integrity through poorly sealed well casings due to geochemically unstable cement.[3] This allows gas to escape through the well itself (known as surface casing vent flow) or via lateral migration along adjacent geological formations (known as gas migration).[3] Approximately 1-3% of methane leakage cases in unconventional oil and gas wells are caused by imperfect seals and deteriorating cement in wellbores.[3] Some leaks are also the result of leaks in equipment, intentional pressure release practices, or accidental releases during normal transportation, storage, and distribution activities.[4][5][6]
Emissions can be measured using either ground-based or airborne techniques.[3][4][7] In Canada, the oil and gas industry is thought to be the largest source of greenhouse gas and methane emissions,[8] and approximately 40% of Canada's emissions originate from Alberta.[5] Emissions are largely self-reported by companies. The Alberta Energy Regulator keeps a database on wells releasing fugitive gas emissions in Alberta,[9] and the British Columbia Oil and Gas Commission keeps a database of leaky wells in British Columbia. Testing wells at the time of drilling was not required in British Columbia until 2010, and since then 19% of new wells have reported leakage problems. This number may be a low estimate, as suggested by fieldwork completed by the David Suzuki Foundation.[1] Some studies have shown a range of 6-30% of wells suffer gas leakage.[7][9][10][11]
Canada and Alberta have plans for policies to reduce emissions, which may help combat climate change.[12][13] Costs related to reducing emissions are very location-dependent and can vary widely.[14] Methane has a greater global warming impact than carbon dioxide, as its radiative force is 120, 86 and 34 times that of carbon dioxide, when considering a 1, 20 and 100 year time frame (including Climate Carbon Feedback [15] [16][9] Additionally, it leads to increases in carbon dioxide concentration through its oxidation by water vapor.[17]
Detection methods[edit]
There are several methods used to detect fugitive gas emissions. Often, measurements are taken at or near the wellheads (via the use of soil gas samples, eddy covariance towers, dynamic flux chambers connected to a greenhouse gas analyzer),[3] but it is also possible to measure emissions using an aircraft with specialized instruments on board.[4][24] An aircraft survey in northeastern British Columbia indicated emissions emanating from approximately 47% of active wells in the area.[8] The same study suggests that actual methane emissions may be much higher than what is being reported by industry or estimated by government. For small-scale measurement projects, infrared camera leak inspections, well injection tracers, and soil gas sampling may be used. These are typically too labour-intensive to be useful to large oil and gas companies, and often airborne surveys are used instead.[7] Other source identification methods used by industry include carbon isotope analysis of gas samples, noise logs of the production casing, and neutron logs of the cased borehole.[25] Atmospheric measurements through both airborne or ground-based sampling are often limited in sample density due to spatial constraints or sampling duration limitations.[19]
One way of attributing methane to a particular source is taking continuous measurements of the stable carbon isotopic measurements of atmospheric methane (δ13CH4) in the plume of anthropogenic methane sources using a mobile analytical system. Since different types and maturity levels of natural gas have different δ13CH4 signatures, these measurements can be used to determine the origin of methane emissions. Activities related to natural gas emit methane plumes with a range of -41.7 to -49.7 ± 0.7‰ of δ13CH4 signatures.[5]
High rates of methane emissions measured in the atmosphere at a regional scale, often through airborne measurements, may not represent typical leakage rates from natural gas systems.[19]
Addressing and remediating fugitive gas emissions[edit]
The process of intervention in the case of leaky wells affected by surface casing vent flows and gas migrations can involve perforating the intervention area, pumping fresh water and then slurry into the well, and remedial cementing of the intervention interval using methods such as bradenhead squeeze, cement squeeze, or circulation squeeze.[25]