Possibly nitrogen pumping operation?
That's what it looks like to me. cryotanker and pumper on location, not nearly enough equipment for a sand frac (insufficient frac tanks, no frack pumpers, no sand mixing equipment or proppant hoppers.
Also, the wellheads are in a line outside the yellow cage he was indicating in the still.
Had I suspected a large natural gas leak (and while C1-C4 are odorless), there would be almost always some sweet smell from more complex volatile organic compounds which are produced with the gas, and the very idea of some spark from a vehicle or even a drone would preclude driving through the cloud. Since the guy wasn't immolated, didn't pass out, and the neighborhood didn't blow up from a pilot light somewhere, no H2S alarms (lights) were in evidence, I would say any hazard posed was more likely from the pressure bleeding down.
Early attempts to drill underbalanced wells in the Ratcliffe interval in North Dakota on the Nesson Anticline were conducted by an operator I worked for using Nitrogen charged drilling fluid to reduce weight.
Failure of a float valve (like a check valve) required that we bleed off pressure in the drill string before making connections, and when the atmospheric conditions were right, the cloud looked similar, and it took some 30-40 minutes to bleed down the pressure (longer than it took to drill the next 31 feet). This led to shutting off the nitrogen and drilling with brine, a technique which was adopted for future wells.
Also, It doesn't take many firemen to assess flammability of the gas in the atmosphere (just one, actually), so the number of firemen and the number of pieces of firefighting apparatus is irrelevant.
Had there been a fire/explosion/health hazard, the firefighters would have responded with evacuation orders. Since H2S is detectable by humans at 1 PPM or less, well below toxic levels, intermittent whiffs of the odor of rotten eggs likely indicate very low concentrations, well below exposure levels which require air packs or evacuation.
