Geochemical Modeling and Analysis of the Frac Water Used in the Hydraulic Fracturing of the Marcellus Formation, Pennsylvania

The hydraulic fracturing of the Marcellus Formation creates a byproduct known as frac water. Five frac water samples were collected in Bradford County, PA. Inorganic chemical analysis, field parameters analysis, alkalinity titrations, total dissolved solids(TDS), total suspended solids (TSS), biological oxygen demand (BOD), and chemical oxygen demand (COD) were conducted on each sample to characterize frac water. A database of frac water chemistry results from across the state of Pennsylvania from multiple sources was compiled in order to provide the public and research communitywith an accurate characterization of frac water. Four geochemical models were created to model the reactions between frac water and the Marcellus Formation, Purcell Limestone, and the oil field brines presumed present in the formations. The average concentrations of chloride and TDS in the five frac water samples were 1.1 �± 0.5 x 105 mg/L (5.5X average seawater) and 140,000 mg/L (4X average seawater). BOD values for frac water immediately upon flow back were over 10X greater than the BOD of typical wastewater, but decreased into the range of typical wastewater after a short period of time. The COD of frac water decreases dramatically with an increase in elapsed time from flow back, but remain considerably higher than typicalwastewater. Different alkalinity calculation methods produced a range of alkalinity values for frac water: this result is most likely due to high concentrations of aliphatic acid anions present in the samples. Laboratory analyses indicate that the frac watercomposition is quite variable depending on the companies from which the water was collected, the geology of the local area, and number of fracturing jobs in which the frac water was used, but will require more treatment than typical wastewater regardless of theprecise composition of each sample. The geochemical models created suggest that the presence of organic complexes in an oil field brine and Marcellus Formation aid in the dissolution of ions such as bariumand strontium into the solution. Although equilibration reactions between the Marcellus Formation and the slickwater account for some of the final frac water composition, the predominant control of frac water composition appears to be the ratio of the mixture between the oil field brine and slickwater. The high concentration of barium in the frac water is likely due to the abundance of barite nodules in the Purcell Limestone, and the lack of sulfate in the frac water samples is due to the reducing, anoxic conditions in the earth's subsurface that allow for the degassing of H2S(g).

The molybdenum isotopic composition in river water: Constraints from small catchments

We report molybdenum isotope compositions and concentrations in water samples from a variety of river catchment profiles in order to investigate the influence of anthropogenic contamination, catchment geology, within-river precipitation, and seasonal river flow variations on riverine molybdenum. Our results show that the observed variations in δ98/95Mo from 0‰ to 1.9‰ are primarily controlled by catchment lithology, particularly by weathering of sulfates and sulfides. Erosion in catchments dominated by wet-based glaciers leads to very high dissolved molybdenum concentrations. In contrast, anthropogenic inputs affect neither the concentration nor the isotopic composition of dissolved molybdenum in the rivers studied here. Seasonal variations are also quite muted. The finding that catchment geology exerts the primary control on the delivery of molybdenum to seawater indicates that the flux and isotope composition of molybdenum to seawater has likely varied in the geologic past.

Molybdenum isotope fractionation in pelagic euxinia: Evidence from the modern Black and Baltic Seas

Here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the Black Sea and two deeps of the Baltic Sea. Dissolved molybdenum in all water samples is depleted in salinity-normalized concentration and enriched in the heavy isotope (δ98Mo values up to + 2.9‰) compared to previously published isotope data of sedimentary molybdenum from the same range of water depths. Furthermore, δ98Mo values of all water samples from the Black Sea and anoxic deeps of the Baltic Sea are heavier than open ocean water. The observed isotope fractionation between sediments and the anoxic water column of the Black Sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions. An extrapolation to a theoretical pure MoS42− solution indicates a fractionation constant between MoS42− and authigenic solid Mo of 0.5 ± 0.3‰. Measured waters with all thiomolybdates coexisting in various proportions show larger but non-linear fractionation. The best explanation for our field observations is Mo scavenging by the thiomolybdates, dominantly — but not exclusively — present in the form of MoS42−. The Mo isotopic compositions of samples from the sediments and anoxic water column of the Baltic Sea are in overall agreement with those of the Black Sea at intermediate depth and corresponding sulphide concentrations. The more dynamic changes of redox conditions in the Baltic deeps complicate the Black Sea-derived relationship between thiomolybdates and Mo isotopic composition. In particular, the occasional flushing/mixing, of the deep waters, affects the corresponding water column and sedimentary data. δ98Mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations. The results can be explained by non-conservative behaviour of Mo under suboxic to anoxic conditions in the shallow bottom parts of the basin, most pronounced on the NW shelf of the Black Sea.

Biogeochemical processes in a clay formation in situ experiment: Part F - Reactive transport modelling

Reactive transport modelling was used to simulate solute transport, thermodynamic reactions, ion exchange and biodegradation in the Porewater Chemistry (PC) experiment at the Mont Terri Rock Laboratory. Simulations show that the most important chemical processes controlling the fluid composition within the borehole and the surrounding formation during the experiment are ion exchange, biodegradation and dissolution/precipitation reactions involving pyrite and carbonate minerals. In contrast, thermodynamic mineral dissolution/precipitation reactions involving alumo-silicate minerals have little impact on the fluid composition on the time-scale of the experiment. With the accurate description of the initial chemical condition in the formation in combination with kinetic formulations describing the different stages of bacterial activities, it has been possible to reproduce the evolution of important system parameters, such as the pH, redox potential, total organic C. dissolved inorganic C and SO(4) concentration. Leaching of glycerol from the pH-electrode may be the primary source of organic material that initiated bacterial growth, which caused the chemical perturbation in the borehole. Results from these simulations are consistent with data from the over-coring and demonstrate that the Opalinus Clay has a high buffering capacity in terms of chemical perturbations caused by bacterial activity. This buffering capacity can be attributed to the carbonate system as well as to the reactivity of clay surfaces.