This is background from the Environmental Protection Agency explaining why infrared imagery can be useful in spotting leaks or stray emissions of methane from natural gas wells, oil tanks or other infrastructure. Related Dot Earth posts here: http://j.mp/dotgasleaks EPA background here: http://www.epa.gov/gasstar/tools/recommended.html

1. This is an EPA background sheet, provided for the Dot Earth blog, on imaging of methane
and other emissions from oil and gas facilities:
Summary
- The IR camera detects methane and other light hydrocarbons that include butane, propane
and pentane.
- Gas evolving from stock tanks and compressor station condensate tanks is a mixture of
methane and other light hydrocarbons.
- We agree with the statement that methane would be “boiling at ambient conditions if any
significant amount of methane were still in the liquid”, and this is in fact the major source
of the vented methane emissions. Crude is transferred from containment (i.e. pipeline) at
a high pressure to containment (i.e. storage tank) at low pressure. Crude at high pressure
is highly saturated with methane, the saturated methane flashes out of the crude as soon
as it is transferred into storage tanks that are at ambient conditions.
- Based on the last bullet, the composition of the gas mixture evolving out of stock tanks
and compressor station condensate tanks is usually dominated (by volume) by methane.
Detailed response
The three major sources for losses of light hydrocarbons are flash losses, working losses
and standing losses. Emissions from a tank vary based on numerous factors including the
frequency of cycling, duration of time within tank, temperature, pressure and the API gravity.
The composition of the emitted vapor varies; however; the largest component (by volume) is
methane (between 40 and 60 percent)1. “Other components include more complex hydrocarbon
compounds such as propane, butane, and ethane; natural inert gases such as nitrogen and carbon
dioxide; and HAP like benzene, toluene, ethyl-benzene, and xylene (collectively these four HAP
are referred to as BTEX)”Error: Reference source not found. During storage, these components
“flash out” of the crude and collect in the space between the liquid and the fixed roof. This vapor
is oftentimes vented into the atmosphere.
A variety of components including methane, ethane, butane, propane, pentane as well as
other hydrocarbons chains, benzene, toluene, methanol and various other components can be
detected by an IR camera. While it is true that the IR camera can detect components other than
methane, the emission composition exiting from production stock tanks and compressor
condensate tanks are primarily methane. These tanks typically have a high pressure inlet
gas/condensate which flashes bringing the inlet stream to tank conditions. When the stream
flashes, a significant amount of methane is emitted. Conversely, tanks that house crude oil which
was previously at atmospheric conditions will not have significant quantities of methane in their
emissions. Tanks which receive crude at high pressure (i.e. crude experiences high pressure
drops) will have more methane emissions.
All tank emissions will include a range of vapors from methane to pentane plus. In a
couple of simulations done using geographic data and the simulation software E&P tank, noted
in the results listed below. It is clear that at standard industry operating conditions, the majority
of emissions are methane by mole percent.
1
http://www.epa.gov/gasstar/documents/ll_final_vap.pdf

2. Table 1: Emissions from tank calculated using Table 2: Emissions from tank calculated using
E&P Tank program while assuming an API E&P Tank program while assuming an API
crude oil gravity of 20. crude oil gravity of 45.
Composition Composition
Inlet to Tank Emissions Inlet to Tank Emissions
Component mol% wt% mol% wt% Component mol% wt% mol% wt%
C1 0.2 0.0 51.9 25.8 C1 1.8 0.2 62.1 35.0
C2 0.2 0.0 13.7 12.8 C2 0.6 0.1 11.8 12.5
C3 0.8 0.2 14.0 19.1 C3 1.3 0.4 10.8 16.7
C4 2.1 0.8 11.9 21.4 C4 2.5 1.1 7.6 15.4
C5 3.4 1.7 5.9 13.2 C5 3.7 2.1 3.4 8.6