Inferring Gas Flow and Fracture Network Damage After a Subsurface Detonation Using Explosive and Geogenic Noble Gases

We report on gas transport in disturbed fractured rock systems after detonation of a subsurface chemical explosive using derivative gases of the explosive and deformation released radiogenic noble gases. We sampled a broad suite of gases from 62 discrete sampling intervals in a 3-d array surrounding the explosive location. Gases were sampled using an automated field sampling system and dynamic analysis performed using a capillary inlet quadrupole mass spectrometer. Gases analyzed include: 4He, 36Ar, 40Ar, 20Ne, N2, O2, NO and CO2/N2O. Each sampling location was actively pumped for 20-minute intervals, just long enough to get representative gas into the mass spectrometer for the deepest sampling locations, and then the gas stream was switched to the next location. Explosive gas arrivals were in 10 of the 64 sampling locations. Geogenic gas arrivals were observed in <5 sampling locations. All geogenic gas arrivals were observed in ports with explosive gas signatures. Gas type and arrival time can used to infer fracture network damage and gas migration. Preferential gas flow along a limited number of fractures is indicated by the small spatial extent of explosive gas arrivals. Damage and creation of new fractures appears to be limited with geogenic gas observed in an even more reduced number of locations. These results show how geogenic noble gases along with explosive-derived gases can be used to understand damage, fracture creation and gas transport in fracture networks as a result of rapid pressure increase and deformation events. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525; SAND2021-9154 A.