Oil and gas producers in the US are required by law to seal and cap their wells once they’re finished producing. But a new survey of wells along the Gulf of Mexico coast indicates that there are 14,000 wells that aren’t producing, are unlikely to be brought back into service, and are uncapped.

The bad news is that the estimated cost of capping them all would run into the area of $30 billion dollars. The good news is that, in most cases, one of the major oil companies will be responsible for these costs.

Put a cork in it

The basic risk of uncapped wells is that material doesn’t necessarily stop coming out of them when the equipment the well was connected to is switched off and removed. One obvious potential problem is continued seepage of hydrocarbons. Light material like methane and simple hydrocarbons typically ends up being digested by microbial life, which converts it to carbon dioxide that will typically find its way to the atmosphere. More complicated molecules will be insoluble and remain behind as contamination.