According to satellite data estimates, 350 million tons of natural gas were wastefully burned at the wellhead in 2012 [nature.com], about 3.5% of worldwide natural gas production. To put this into perspective, this amount of natural gas could provide electrical power to the entire continent of Africa. The CO2 emissions from natural gas flares are roughly equivalent to 10% of all CO2 emissions of the European Union.
The problem, as you might surmise, is the handling and transportation of the natural gas produced as a by-product of oil wells in areas without the infrastructure to handle gas. In addition, some gas produced as oil by-product has relatively low levels of methane. Russia flares more natural gas than any other nation, followed by Iraq, Iran, Nigeria, Venezuela, Algeria, and the United States.
The World Bank [worldbank.org] is trying to stop all routine flaring of natural gas by 2030. North Dakota [insideclimatenews.org] and New Mexico [nmpoliticalreport.com] are taking steps to reduce gas flares.