Estimating Fracture Network Damage After a Subsurface Detonation Using Geogenic Noble Gases.

Noble gas release can be used to investigate the timing, location and magnitude of fracture creation.  Here, a numerical model of gas release and transport, resulting from fracturing events, is used to estimate first-order fracture network characteristics after subsurface detonation.  Released radiogenic noble gases after detonation of three different subsurface explosions of varying source characteristics were interpreted.  A broad suite of gases was sampled from 62 discrete sampling intervals in a 3-D array surrounding the explosion location using an automated field sampling system and a capillary inlet quadrupole mass spectrometer.  Gases analyzed include: ⁴He, ³⁶,⁴⁰Ar, ²⁰Ne, N₂, O₂, NO and CO₂/N₂O.  Geogenic gas arrivals were observed in a subset of sampling locations.  All geogenic gas arrivals were observed in ports with explosive-derived gas arrivals.  Helium amount and arrival time were used to estimate fracture network damage using a numerical model which allows dynamic changes in fracture aperture, matrix porosity and permeability.  The amount of fracture damage was significantly different between the three different explosions and consistent with other observations of damage.  These results illustrate how geogenic noble gases can be used to understand damage, transport, and fracture creation in fracture networks, with implications for a variety of subsurface topics including hydraulic fracking, mine failure, earthquake and volcanic monitoring.