Migration of high-pressure air during gas well drilling in the appalachian basin

We present the details of a numerical model simulating the migration of pressurized air used for pneumatic drilling of a well in an aquifer. We used an incident that occurred in West Virginia during June 2012 as a basis for making the simulations realistic. We developed a 3D conceptual model using the multipurpose model TOUGH2 to simulate the events during this incident. Input parameters for the model were obtained from field measurements, and a number of reasonable assumptions were made for other parameters. Our results showed that compressed air from a drilling well is capable of creating a high pressure gradient in groundwater at hundreds of meters from the drill hole, even if the air leakage from the drilling well occurs in a confined aquifer, and even if the leakage duration is only 2 h. Therefore, one way to prevent the pressure buildup in the surrounding aquifers is through emplacement of observation wells before drilling, which would alert the drillers to any unusual pressure buildup inside the confined aquifer. However, air leakage in unconfined aquifers seems to have a much smaller spatial extent (less than tens of meters). Sensitivity analysis revealed that air pressure, fracture permeability, and injection time are critical parameters for the propagation of air.