83
escape from produced water stored in open tanks or impoundments on the surface
(Butkovskyi et al. 2017).
“Fugitive emissions” is a term for natural gas and other hydrocarbon vapors that
leak directly into the atmosphere from a wellhead or from surface infrastructure
equipment such as compressors, pipelines, meter runs, etc. Fugitive emissions are
distinct from “stray gas,” which is the underground leakage of gas into a groundwater aquifer or some other receptor from a wellbore. Although often confused and
sometimes used interchangeably, these are actually two separate phenomena with
different causes, and stray gas is addressed in the next chapter on fracking and
groundwater.
Fugitive emissions can be difficult to track and are often expensive to fix. It
seems obvious that production companies would have an incentive to stop fugitive
emissions, because they are losing product into the atmosphere that they could be
selling. Unfortunately, the reality is that when gas prices are extremely low, the lost
product isn’t really worth all that much money. The cost of sending a crew out to fix
leaks can actually be higher than the amount of product they save. There is of course
a social responsibility to keep gas systems leak-tight, but many companies are more
concerned about the bottom line. A Canadian study that surveyed fugitive emissions
on production sites and informed operators of the location of gas leaks found in a
follow-up survey taken a year later that only about a quarter of these leaks had been
repaired (Ravikumar et al. 2019).
Methane gas occurs naturally at low levels in the atmosphere of around 10 ppm,
and it can be difficult to measure small leaks against this background. A number of
researchers have been monitoring the air near shale gas and tight oil drill sites in an
attempt to quantify methane emissions, and also near established, conventional
O&G fields (Pétron et al. 2014; Soeder and Kent 2018). It is not clear from these
data if methane emissions from fracked shale wells are any more significant than
those from older, conventional wells. There are, in fact, some concerns that older,
conventional wells may have suffered deterioration of the cement and casing over
time, leading to a loss in wellbore integrity and increasing the potential for gas leakage (Watson and Bachu 2009).
There are many sources of methane in the atmosphere, including anaerobic
microbial digestion of organic matter and natural seepage from shallow coal seams
or black shales. The isotopic signature of methane can be used to assess if the gas
originated from biological activity (known as biogenic gas) or from the thermal
breakdown of longer chain hydrocarbons over geological time into the simpler
methane molecule (thermogenic gas). Thermogenic and biogenic gas can often be
distinguished by their carbon isotope ratios. Stable carbon isotopes in thermogenic
gas tend to be heavier than those in biogenic gas, because microbes prefer the lighter
isotope, but other clues like the presence of noble gases are also used (Moore et al.
2018). Thermogenic gas may contain traces of longer-chain hydrocarbons like ethane and propane that are absent from biogenic gas (Claypool et al. 1980). Much of
the isotope work on gas origins is now focused on determining the temperature of
formation, which is cooler for biogenic gas and hotter for thermogenic gas (Stolper
et al. 2015).
5.2 VOCs, NOx and Fugitive Emissions
escape from produced water stored in open tanks or impoundments on the surface
(Butkovskyi et al. 2017).
“Fugitive emissions” is a term for natural gas and other hydrocarbon vapors that
leak directly into the atmosphere from a wellhead or from surface infrastructure
equipment such as compressors, pipelines, meter runs, etc. Fugitive emissions are
distinct from “stray gas,” which is the underground leakage of gas into a groundwater aquifer or some other receptor from a wellbore. Although often confused and
sometimes used interchangeably, these are actually two separate phenomena with
different causes, and stray gas is addressed in the next chapter on fracking and
groundwater.
Fugitive emissions can be difficult to track and are often expensive to fix. It
seems obvious that production companies would have an incentive to stop fugitive
emissions, because they are losing product into the atmosphere that they could be
selling. Unfortunately, the reality is that when gas prices are extremely low, the lost
product isn’t really worth all that much money. The cost of sending a crew out to fix
leaks can actually be higher than the amount of product they save. There is of course
a social responsibility to keep gas systems leak-tight, but many companies are more
concerned about the bottom line. A Canadian study that surveyed fugitive emissions
on production sites and informed operators of the location of gas leaks found in a
follow-up survey taken a year later that only about a quarter of these leaks had been
repaired (Ravikumar et al. 2019).
Methane gas occurs naturally at low levels in the atmosphere of around 10 ppm,
and it can be difficult to measure small leaks against this background. A number of
researchers have been monitoring the air near shale gas and tight oil drill sites in an
attempt to quantify methane emissions, and also near established, conventional
O&G fields (Pétron et al. 2014; Soeder and Kent 2018). It is not clear from these
data if methane emissions from fracked shale wells are any more significant than
those from older, conventional wells. There are, in fact, some concerns that older,
conventional wells may have suffered deterioration of the cement and casing over
time, leading to a loss in wellbore integrity and increasing the potential for gas leakage (Watson and Bachu 2009).
There are many sources of methane in the atmosphere, including anaerobic
microbial digestion of organic matter and natural seepage from shallow coal seams
or black shales. The isotopic signature of methane can be used to assess if the gas
originated from biological activity (known as biogenic gas) or from the thermal
breakdown of longer chain hydrocarbons over geological time into the simpler
methane molecule (thermogenic gas). Thermogenic and biogenic gas can often be
distinguished by their carbon isotope ratios. Stable carbon isotopes in thermogenic
gas tend to be heavier than those in biogenic gas, because microbes prefer the lighter
isotope, but other clues like the presence of noble gases are also used (Moore et al.
2018). Thermogenic gas may contain traces of longer-chain hydrocarbons like ethane and propane that are absent from biogenic gas (Claypool et al. 1980). Much of
the isotope work on gas origins is now focused on determining the temperature of
formation, which is cooler for biogenic gas and hotter for thermogenic gas (Stolper
et al. 2015).
5.2 VOCs, NOx and Fugitive Emissions