Rapid desorption of radium isotopes from black shale during hydraulic fracturing. 2. A model reconciling radium extraction with Marcellus wastewater production

Abstract

Radium in hydraulic fracturing wastewaters derives from two isotopically distinct end-members in the shale, labile 228Ra hosted by mineral surfaces (226Ra/228Ra atom ratio ~250) and exchangeable 226Ra hosted by organic surfaces (226Ra/228Ra ~10,000). Here we use mass balance and isotope mixing models to reconcile extraction of Ra from these phases with mechanisms of Marcellus wastewater production. Radium isotopic mass balance requires that the characteristic water-rock ratio between wastewater and shale is exceedingly low, on the order of 0.04, and that this ratio decreases with time during wastewater production. An evolving water-rock interaction drives increasing Ra concentrations (=[Ra]) and 226Ra/228Ra ratios during wastewater production, all mediated by increasing [Ca2+] that favors desorption of 226Ra from organics. Our observations and models of Ra isotope geochemistry are best reconciled with observations of water and salinity mass balance, δ18O, Na-Br-Cl, and 87Sr/86Sr if wastewater is produced by mixing of injected fluids with a limited volume of pore brine (on the order of 13% by volume), accompanied by contemporaneous extraction of excess alkaline earth elements by water-rock exchange. Validated using Ra isotope data, this model attributes the extreme salinity and [Ra] in wastewaters to the progressive, hydrologic enrichment of injected fluids during hydraulic fracturing.