Reading  
Read the following text and correct mistakes in the following state-ments. 
1. Leakage and migration can cause a hydraulic fracturing, although in many cases the method of fracturing is such that gas is not able to mi-grate through the well casing. 
2. Often, sources of methane are also sources of oil, allowing methane emissions to be derived based on ethane emissions and ethane/methane ratios in the atmosphere. 
3. Approximately 40 % of ethane emissions in Canada occur within Al-berta, according to the National Inventory Report. 
4. Broken or leaky well casings are often the result of geophysically un-stable or brittle cement. 
5. Of the anthropogenic methane emissions in Alberta, 71 % are avoided by the oil and gas sector. 
6. It is estimated that approximately 0,6–7,7 % of methane emissions produced during the lifetime of a fossil fuel well occur during activi-ties that take place either at the well site or before processing. 
7. Sources can include broken or leaky well casings (either at abandoned wells or unused, but not properly abandoned, wells) or lateral migra-tion through the geological formations in the subsurface after being emitted to groundwater or atmosphere. 
 
Fugitive gas emissions 
Fugitive gas emissions can arise as a result of operations in hydrocarbon exploration, such as for natural gas or petroleum. 
Often, sources of methane are also sources of ethane, allowing methane emissions to be derived based on ethane emissions and ethane/methane ratios in the atmosphere. This method has given an estimate of increased methane emis-sion from 20 Tg per year in 2008 to 35 Tg per year in 2014. A large portion of methane emissions can be contributed by only a few «uper-emitters». The an-nual ethane emission increase rate in North America between 2009 and 2014 was 3–5 %. It has been suggested that 62 % of atmospheric ethane originates from leaks associated with natural gas production and transportation opera-tions. It has also been suggested that ethane emissions measured in Europe are affected by hydraulic fracturing and shale gas production operations in North America. Some researchers postulate that leakage problems are more likely to happen in unconventional wells, which are hydraulically fractured, than in con-ventional wells. 
Approximately 40 % of methane emissions in Canada occur within Alberta, according to the National Inventory Report. Of the anthropogenic methane emissions in Alberta, 71 % are generated by the oil and gas sector. It is estimat-ed that 5 % of the wells in Alberta are associated with natural gas leaking or venting. It is also estimated that 11 % of all wells drilled in British Columbia, or 2739 wells out of 24599, have reported leakage problems. Some studies have estimated that 6–30 % of all wells suffer gas leakage.  
Sources can include broken or leaky well casings (either at abandoned wells or unused, but not properly abandoned, wells) or lateral migration through the geological formations in the subsurface before being emitted to groundwater or atmosphere. Broken or leaky well casings are often the result of geochemi-cally unstable or brittle cement. One researcher proposes 7 main paths for gas migration and surface casing vent flow: (1) between the cement and adjacent rock formation, (2) between the casing and encompassing cement, (3) between the casing and the cement plug, (4) directly through the cement plug, (5) through the cement between casing and adjacent rock formation, (6) through the cement between linking cavities from the casing side of the cement to the annu-lus side of the cement, and (7) through shears in the casing or well bore. 
Leakage and migration can be caused by hydraulic fracturing, although in many cases the method of fracturing is such that gas is not able to migrate through the well casing. Some studies observe that hydraulic fracturing of hori-zontal wells does not affect the likelihood of the well suffering from gas migra-tion. It is estimated that approximately 0,6–7,7 % of methane emissions pro-duced during the lifetime of a fossil fuel well occur during activities that take place either at the well site or during processing.