There are various sources that could be used. Why not have a look at my chapter in the book Natural Gas?

Arian,

CO2 emission factor for Natural gas is: 0.056 kg CO2eq/MJ energy equivalent.

Since the higher heating value of Natural Gas is: 38 - 39 MJ/m3, CO2 emission factor would be about 2.2 kg CO2eq per m3 of natural gas.

Note that this emission factor doesn’t include emissions associated with the extraction and delivery of natural gas.  Those emissions are region specific and you may need to check in a local data source.

By performing this calculation with SimaPro and openLCA for 'Natural Gas, consumption mix, at consumer, from onshore and offshore production incl. pipeline and LNG transport, desulphurised - EU-27', I get a result 0.3635 kg CO2eq/m3 which seems very small than the one I expect. Why is there so much difference?

Arian,

For comparison, primary energy consumed per 1 m3 of Natural Gas (NG) delivered to consumers in Ontario (a province of Canada) is about 4.38 MJ/m3 of NG.  This means: upstream CO2 emissions associated with the extraction and delivery of NG to consumers in Ontario is about 0.245 kg CO2 eq./m3.  Compared with this value your calculated value (0.363 kg CO2eq/m3) is not too small.

It is important to compare gas emissions with alternative fuels. I think that for unit of energy (let say, ton of oil equivalent), natural gas gives  much less emissions  than coal (maybe, 50% or less). Here are exact numbers but unfortunately not in metric units: https://www.epa.gov/energy/ghg-equivalencies-calculator-calculations-and-references . The average carbon coefficient of natural gas is 14.46 kg carbon per mmbtu (EPA 2013). The fraction oxidized to CO2 is 100 percent (IPCC 2006). 1 MMBtu = 28.263682 m3 of natural gas (see http://www.indexmundi.com/commodities/glossary/mmbtu ) So 1 cub.m of gas should emit 14.46/28.26 = 0.512 kg of carbon. It is closer to Arian's value and well below Susantha's estimate.

@Yuri, Thanks for posting the links for EPA calculations.  I was getting the numbers from 'Natural Resources Canada' data (basically very close to the values given in EPA site).  Please see below:

0.512 kg C is about 1.89 kg CO2 equivalents.

0.512 x (44/12)

Interesting discussion and very helpful, too. Thanks to everyone on the chat, and to Google who guided me here :)

According to the BP report US gas consumption increased by 78 bcm in 2018. and ı wonder 78 bcm natural gas consumption's carbon emmisions? thank you

What will be a converstion factor for natural Gas in GJ/Nm3 if you have mmbtu value for natural Gas ?