Investigating Field Scale Fracture Network Deformation Using Radiogenic Noble Gases

Abstract:

Recent investigations have shown that radiogenic noble gases released from rocks undergoing mechanical deformation can be measured from rocks at the laboratory scale and in the field from newly created hydraulic fractures at the ~10 m scale. Here, we report on an experiment where we observed radiogenic noble gas signature in the near-field subsurface during a large mechanical deformation event from a subsurface detonation. A conventional explosive was detonated at 59.1 m depth in a water filled borehole. We measured subsurface air yield, noble gas isotopic composition (⁴He, ²⁰Ne, ³⁶Ar, and ⁴⁰Ar) and helium flow rate in a series of multi-level sampling ports ~4 to ~8 m distance from the shot location before and after detonation. We mapped visual fractures in the shot and observation boreholes before finishing all wells. We observed significant increases in specific air yield in 13 of the 64 sampling locations, 11 of which had mapped fractures. We did not observe a significant increase in radiogenic noble gas composition for either ⁴He and ⁴⁰Ar after the shot. Helium flow rate decreased after the shot. The vast majority of locations with air yield increases have visible fractures. The minimal production of radiogenic gases and the observed decrease in the helium flow rate are consistent with an absence of newly fractured rock. Our results show that a large portion of the strain was accommodated by movement along existing fractures, with minimal newly created fractures. Noble gas release can be used to infer subsurface deformation style and type.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2020-7711 A