Methane and CO2 are released during natural gas production several ways, including intentional venting and flaring, and accidental leakage. Venting and flaring are more prevalent in oil wells than natural gas wells, but both still happen during the gas drilling process. Venting is the intentional release of gas from the well bore into the atmosphere. In addition to CH4
, vented gas from a Marcellus well could contain hydrogen sulfide (H2S). Flaring involves a controlled burn of gas, the byproducts of which (mostly CO2 and H2O) enter the atmosphere While flared gas is mostly CO2 and H2O, it can also contain sulfur dioxide (SO2), nitrogen dioxide (NO2), particulate matter, carbon monoxide (CO), and about 2% of the original methane (because combustion is not 100% efficient).