Helium (4He) measurements during vacuum crushing of rock

This work extends research directed towards developing the concept of noble gas release as a deformation signal. Geogenic noble gases are probably contained in all crustal rocks at inter- and intracrystalline sites. These gases represent a unique signal source, their release can be related to the deformation state of rock; sensing this signal will inform us of natural and man-made deformation. We have developed a lab-based system to measure helium (4He) during deformation by modifying a vacuum die system to crush small (~2 to 10 gm) samples of room-dry and water-wet rock and simultaneously measure the amount of 4He released during and after the crushing. For room dry measurements a sample is placed in the die and the vacuum is pumped down to 10-4 mbar. A 4He flow rate of 10-11 cc/s attained and maintained as background conditions for 4 to 24 hours. During crushing, axial force (maximum of 100kN) was slowly applied and maintained for 4 to 24 hours, while measuring the 4He flow rate and system pressure. The helium flow rate is measured using a Leybold Phoenix Vario helium leak detector. During loading and hold-at-load the sample is pulverized to varying degrees. After this time, the load was reduced to zero, and later the helium measurements ceased. In Westerly granite samples, the amount of geogenic helium detected is on the order of 1 to 2.2 (x10-7) cc/g rock. The cc/g of helium increases with decreasing sample size, implying more efficient comminution as sample size decreases. Release of helium from dry samples is relatively short lived, background levels attained in about an hour after deformation is stopped. The timing for release/detection of saturated samples is at least an order of magnitude longer and depends on dewatering of the sample (a water-freeze trap is used) for complete detection. The results (1) provide a means to quantify noble gas generation during comminution, and they imply (2) increases in fracture surface area will increase the amount of geogenic gases during fracturing events and (3) the presence of subsurface water may muddle noble gas flow detection. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525; SAND2021-9153 A.