EXPERIMENTAL MEASUREMENT OF ALPHA RECOIL FLUX OF RADIONUCLIDES AND ITS APPLICATION IN HYDRAULIC FRACTURING

Hydraulic fracturing alters the pore size distribution and increases the effective surface area of gas-bearing earth materials, which may enhance transfer of short-lived radionuclides into porewater-wastewater through alpha recoil (radionuclide decay). However, evaluating the rate and extent of this process, and its impact on porewater radioactivity, remains uncertain; numerous factors contribute to this uncertainty, including the spatial distribution of parent radionuclides (e.g. U²³⁸ and Th²³²) within native materials, differences in nuclide recoil length and the physical structure of the rock strata (pore size distribution and porosity). Here, we experimentally measure radionuclide activities within porewater contributed through alpha recoil, and analyze its variations as a function of pore structure and parent nuclide distribution within host matrices, including Marcellus shale rock and Serrie-Copper Pegmatite. The shale cores originate from the Marcellus formation in Mckean, Pennsylvania collected at depths ranging from 1000-7000 feet, and the U-Th-rich Pegmatite is obtained from South Platte District, Colorado. Columns are packed with granulated rock of varying surface area (30,000-60,000 cm2/g) and subjected to low salinity sodium chloride solution in a close loop configuration. The activity of Radium isotopes (Ra²²⁶, Ra²²⁴, and Ra²²⁸) in the saline fluid is measured over time to determine recoil supply rates. Mineralogical and trace element data for rock specimens are characterized using XRD and XRF, and detailed geochemical profiles are constructed through total dissolution and analysis using ICP-MS and ICP-OES. Naturally occurring Radium nuclides are quantified using a low-energy Germanium detector. The parent nuclide (U²³⁸ and Th²³²) distribution in the host rock is studied using X-Ray Absorption Spectroscopy (XAS). Our study elucidates the contribution of alpha recoil on the appearance and distribution of short-lived radium isotopes within porewater of representative gas-bearing rock subjected to hydraulic fracturing. Further, we illustrate the effects of chemical and physical heterogeneity on the rate of this process, which may inform models predicting the fate and transport of radionuclides in subsurface environments.