Estimating Radium Activity in Shale Gas Produced Brine

Abstract

Shale gas reservoir-produced brines may contain elevated levels of naturally occurring radioactive material, including Ra-226 and Ra-228, which come from the decay of U-238 and Th-232 in shale. While the total Ra activity in shale gas wastewaters can vary by over 3 orders of magnitude, the parent radionuclides tend to only vary by 1 order of magnitude. The extent of Ra mobilization from the shale into produced brines is thought to be largely controlled by adsorption/desorption from the shale, which is influenced by shale cation exchange capacity (CEC) and reservoir brine salinity, often reported as the total dissolved solids (TDS). To determine how these factors lead to such large variation in Ra activity of produced brines, the U content and CEC of shale samples from the Antrim and Utica-Collingwood shales in Michigan and the Marcellus shale in Pennsylvania were evaluated. Analysis of produced brine from 17 Antrim shale gas wells was then used to develop an empirical relationship between Ra-226 activity and produced water TDS for a given U content of the shale. This correlation will provide an a priori estimate of the expected Ra activity of a produced brine from a given shale gas play when the brine salinity and U content of the shale are known. Such information can serve as a guide for optimal wastewater treatment and disposal strategies prior to any drilling activity, thereby reducing risks associated with elevated Ra activity in shale gas wastewaters.

Potential Impacts of Shale Gas Development on Inorganic Groundwater Chemistry: Implications for Environmental Baseline Assessment in Shallow Aquifers

“…Shales are primarily composed of clay minerals (i.e., illite, mixed-layer illite/smectite, smectite, kaolinite, chlorite) and quartz and can contain significant proportions of other minerals such as feldspars (i.e., K-feldspars, plagioclase), micas (i.e., biotite, muscovite), and carbonate minerals (i.e., calcite, dolomite, siderite, ankerite). ,,,, Formed under reducing conditions, black shales contain abundant organic matter (>2% total organic carbon) and sulfide minerals, primarily pyrite. ,, Black shales are commonly enriched in trace elements that are primarily associated with sulfide minerals and to a lesser degree with organic matter and clay minerals. ,,, In particular, black shales are often enriched in trace metals and metalloids such as As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, U, V, and Zn ,,− and may contain significant contents of naturally occurring radioactive material (NORM). ,,, The high levels of radioactivity result from the natural abundance of U-238 and Th-232 and their decay products including isotopes of Ra, Po, Rn, and Pb in many organic-rich shale formations. , It is noteworthy that black shales can host sulfide ore deposits that usually contain very high contents of trace metals and metalloids. , As a result of water–rock interactions, elevated concentrations of trace metals/metalloids have been reported in shallow groundwater associated with organic-rich shale occurrences. ,− However, the concentrations of a number of trace metals/metalloids and NORMs were likely limited by sequestration in Fe–Mn oxyhydroxides and clay minerals in shallow oxidizing groundwater and by sulfide mineral and organic matter stability in reducing groundwater. , …”

“…Although there is no WHO guideline for Mn and Sr, concentrations as high as tens of ppm Mn and thousands of ppm of Sr may pose a risk to human health , (Figure ). Elevated concentrations of the alkaline earth elements Sr, Ba, and Ra in flowback/produced waters have been attributed to (1) the limited precipitation of sulfate minerals (e.g., barite, celestite) due to strongly reducing conditions and elevated temperatures in the reservoir and (2) the increased competition for sorption sites on clay minerals and organic matter due to high ionic strength of the fluids. ,, There is still a debate whether Ba and Ra in flowback/produced water are derived from formation brines or are released from black shales during hydraulic fracturing. − Elevated B concentrations are thought to result either from carbonate dissolution or desorption from clays. , To our knowledge, no information is currently available regarding the mobilization of F and Mn in flowback/produced waters.…”

“…28,31,36,43 The high levels of radioactivity result from the natural abundance of U-238 and Th-232 and their decay products including isotopes of Ra, Po, Rn, and Pb in many organic-rich shale formations. 43,44 It is noteworthy that black shales can host sulfide ore deposits that usually contain very high contents of trace metals and metalloids. 30,45 As a result of water−rock interactions, elevated concentrations of trace metals/metalloids have been reported in shallow groundwater associated with organic-rich shale occurrences.…”

Wastewater management strategies for sustained shale gas production

“…barium, strontium) and naturally occurring radioactive material (e.g. radium) that were leached from the shale [23,24]. Concentrations of these elements tend to increase over the lifetime of a well, indicating stricter monitoring may be necessary with increasing volumes of produced fluids as the field ages [25].…”

Reactive Transport Modeling of Shale–Fluid Interactions after Imbibition of Fracturing Fluids

“…For each production well, a concentrated acid spear head (15% HCl) is injected for cleaning purposes, after which tens of millions of gallons of hydraulic fracturing fluid (HFF) are injected into the shale reservoir to fracture the shale formation and increase reservoir permeability . These injected fluids are not in chemical equilibrium with the shale reservoir, resulting in chemical reactions in fractures and shale matrices. − These chemical reactions include mineral dissolution, which can increase porosity, as well as mineral precipitation, which can occlude porosity, both affecting fluid transport through the shale matrix . In addition, chemical reactions also alter the chemical composition of flow-back water, providing signatures that can be used to identify the source of the produced water. , Therefore, information about the spatial distribution and extents of these chemical reactions is helpful for understanding and estimating reservoir performance and flow-back water composition …”