We won’t run out. I worked in the NG industry for 40 years. In the 70’s we rationed NG, sure that we were going to run out, but technology unleashed new supplies. That tech never sleeps and we are constantly looking at new formations. Right now the Marcellus shale produces the most gas, and the reserves there have been written up to almost triple what was originally forecast. Just beneath it is the Utica shale, which has proven to be even more prolific based on test well results. 84 years is based on what we know today. 50 years ago we thought we only had 10 years worth of future supplies, but we found more. That cycle will continue. I think we have unlimited supplies available just waiting to be discovered. Remember we only extract less than 30% from existing reservoirs with today’s technology. In future years we will learn how to extract much higher levels.
Natural gas like all other fossil fuels are a finite commodity and at some point we will run out. Hopefully we will run out after we've found a renewable cost effect replace fuel. Other wise the planets' trees are going to pay hell.