ASSESSING THE POTENTIAL REMOBILIZATION OF RARE EARTH ELEMENTS FROM BLACK SHALE DURING HYDRAULIC FRACTURING OPERATIONS

The rare earth elements (REEs) are economically vital to modern society and are critical components in areas such as sustainable technologies, electronics, and high energy lasers. The rapid growth of these technologies has placed considerable economic pressure on sourcing REEs, with the current global supply dominated by mineral deposits in China. In light of this global demand for REEs, alternative sources are an area of active exploration. One potential source is ancient sedimentary black shale deposits that are currently being exploited for natural gas and oil resources. Past environmental conditions that promoted the accumulation of these black shale units were also interconnected with the cycling of sedimentary phosphorus, which is thought to be a major control on the accumulation of REEs in sediments. Drilling operations to extract natural gas from black shale units present a concomitant potential to remobilize the REEs and bring them to the surface for further extraction. However, the geochemical reactions that determine the mobility of REEs between the native black shale rock and the introduced hydraulic fracturing drilling fluids are not well understood. We present the results of our experiment as a basis for understanding these reactions between the drilling fluid and the targeted rock formations. We reacted black shale samples from the Marcellus Formation in southwestern Pennsylvania with a NaCl brine and a synthesized hydraulic fracturing fluid (FF) for a duration of one week under high temperatures and pressures that approximated those from 2.5 km below the surface. Preliminary analysis of the reacted fluids shows evidence for the dissolution of carbonates with an increase in dissolved Ca (44,000-190,000 μg/L) after one week. The presence of the synthesized FF versus the NaCl brine does not appear to have a noticeable effect on the major cation solution chemistry. The REEs show little change in the fluid phase during reactions with the NaCl brine, but do show slight decreases (<8 μg/L for all REEs) during the reaction with synthesized FF. These preliminary results suggest that the net mobility of REEs during reactions between fluid and rock is minor, although this could be an artifact of the particular rock samples utilized that may not be representative of the Marcellus Formation as a whole.