The rates of mineral dissolution reactions in porous media are difficult to predict, in part because of a lack of understanding of mineral reactive surface area in natural porous media. Common estimates of mineral reactive surface area used in reactive transport models for porous media are typically ad hoc and often based on average grain size, increased to account for surface roughness or decreased by several orders of magnitude to account for reduced surface reactivity of field as opposed to laboratory samples. In this study, accessible mineral surface areas are determined for a sample from the reservoir formation at the Nagaoka pilot CO2 injection site (Japan) using a multi-scale image analysis based on synchrotron Xray microCT, SEMQEMSCAN, XRD, SANS, and FIB-SEM. This analysis not only accounts for accessibility of mineral surfaces to macro-pores, but also accessibility through connected micro-pores in smectite, the most abundant clay mineral in this sample. While the imaging analysis reveals that most of the micro-and macro-pores are well connected, some pore regions are unconnected and thus inaccessible to fluid flow and diffusion. To evaluate whether mineral accessible surface area accurately reflects reactive surface area a flow-through core experiment is performed and modeled at the continuum scale. The core experiment is performed under conditions replicating the pilot site and the evolution of effluent solutes in the aqueous phase is tracked.Various reactive surface area models are evaluated for their ability to capture the observed effluent chemistry, beginning with parameter values determined as a best fit to a disaggregated sediment experiment (Beckingham et al., 2016) described previously.Simulations that assume that all mineral surfaces are accessible (as in the disaggregated sediment experiment) over-predict the observed mineral reaction rates, suggesting that a reduction of RSA by a factor of 10-20 is required to match the core flood experimental data. While the fit of the effluent chemistry (and inferred mineral dissolution rates) greatly improve when the pore-accessible mineral surface areas are used, it was also necessary to include highly reactive glass phases to match the experimental observations, in agreement with conclusions from the disaggregated sediment experiment. It is hypothesized here that the 10-20 reduction in reactive surface areas based on the limited pore accessibility of reactive phases in core flood experiment may be reasonable for poorly sorted and cemented sediments like those at the Nagaoka site, although this reflects pore rather than larger scale heterogeneity.

SignificanceMicroorganisms persisting in hydraulically fractured shales must maintain osmotic balance in hypersaline fluids, gain energy in the absence of electron acceptors, and acquire carbon and nitrogen to synthesize cell building blocks. We provide evidence that that cofermentation of amino acids (Stickland reaction) meets all of these organismal needs, thus functioning as a keystone metabolism in enriched and natural microbial communities from hydraulically fractured shales. This amino acid-based metabolic network can be rationally designed to optimize biogenic methane yields and minimize undesirable chemistries in this engineered ecosystem. Our proposed ecological framework extends to the human gut and other protein-rich ecosystems, where the role of Stickland fermentations and their derived syntrophies play unrecognized roles in carbon and nitrogen turnover.

Our limited understanding of mineral reactive surface area contributes to significant uncertainties in quantitative simulations of reactive chemical transport in subsurface processes. Continuum formulations for reactive transport typically use a number of different approximations for reactive surface area, including geometric, specific, and effective surface area. In this study, reactive surface area estimates are developed and evaluated for their ability to predict dissolution rates in a well-stirred flow-through reactor experiment using disaggregated samples from the Nagaoka pilot CO 2 injection site (Japan). The disaggregated samples are reacted with CO 2 acidified synthetic brine under conditions approximating the field conditions and the evolution of solute concentrations in the reactor effluent is tracked over time. The experiments, carried out in fluid-dominated conditions at a pH of 3.2 for 650 hours, resulted in substantial dissolution of the sample and release of a disproportionately large fraction of the divalent cations. Traditional reactive surface area estimation methods, including an adjusted geometric surface area and a BET-based surface area, are compared to a newly developed image-based method. Continuum reactive transport modeling is used to determine which of the reactive surface area models provides the best match with the effluent chemistry from the well-stirred reactor. The modeling incorporates laboratory derived mineral dissolution rates reported in the literature and the initial modal mineralogy of the Nagaoka sediment was determined from scanning electron microscopy (SEM) characterization. The closest match with the observed steady-state effluent concentrations was obtained using specific surface area estimates from the image-based approach supplemented by literature-derived BET measurements. To capture the evolving effluent chemistry, particularly over the first 300 hours of the experiment, it was also necessary to account for the grain size distribution in the sediment and the presence of a highly reactive volcanic glass phase that shows preferential cation leaching.

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observedC chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values ofC nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

Molecular motion of hydrocarbons under confinement exhibits several peculiarities and has important implications in industries like gas recovery. A quasielastic neutron scattering (QENS) study of the dynamics of propane in nanoporous silica aerogel was carried out to quantify its molecular mobility. The dynamical properties of propane were studied as a function of temperature, pressure and presence of CO2. The effects of pressure, i.e., fluid density and composition, are found to be more pronounced than the effects of temperature. At low pressures of propane, many propane molecules are adsorbed onto the pore surfaces and are thus immobile. As the pressure of propane loading is increased, more molecules become available to take part in the diffusional dynamics and thus enhance the diffusivity. At low pressure the propane molecules take part in a continuous diffusion, while at higher pressures, the diffusion of propane molecules within the aerogel occurs via the mechanism of jumps. Presence of CO2 enhances the jump rate of propane molecules, thereby increasing the diffusion coefficient. This study aims to aid in understanding the complex processes involved in hydrocarbon migration in porous quartz-rich rocks and enhanced hydrocarbon recovery.

Gallium arsenide (GaAs) encapsulated at 450 °C with thin films of amorphous silicon has been annealed at temperatures up to 1050 °C and the resulting polysilicon (poly-Si)/GaAs interfaces investigated with secondary ion mass spectroscopy, Rutherford backscattering, and transmission electron microscopy. Little or no interdiffusion is detected at undoped Si/GaAs interfaces whereas Si diffuses from P- or As-doped Si to depths as great as 550 nm in the GaAs after 10 s at 1050 °C. The flux of Si into the GaAs is correlated with the flux of Ga and As into the Si and both increase with increases in the dopant concentration of the Si. The diffusion of other Si dopants into the GaAs, including P and In, is also detected. This enhanced diffusivity of Si, P, and In in GaAs results from the diffusion of point defects into the GaAs created by the diffusion of the Ga and As into the encapsulant. Numerical simulations using position-dependent impurity diffusion coefficients predict that Si, P, and In diffusivities in GaAs at doped poly-Si interfaces are enhanced by factors of 104 above their respective intrinsic bulk equilibrium diffusivities, where known.

Hydraulic fracturing is the prevailing method for enhancing recovery of hydrocarbon resources from unconventional shale formations, yet little is understood regarding the microbial impact on biogeochemical cycling in natural-gas wells. Although the metabolisms of certain fermentative bacteria and methanogenic archaea that dominate in later produced fluids have been well studied, few details have been reported on microorganisms prevelant during the early flowback period, when oxygen and other surface-derived oxyanions and nutrients become depleted. Here, we report the isolation, genomic and phenotypic characterization of Marinobacter and Arcobacter bacterial species from natural-gas wells in the Utica-Point Pleasant and Marcellus Formations coupled to supporting geochemical and metagenomic analyses of produced fluid samples. These unconventional hydrocarbon system-derived Marinobacter sp. are capable of utilizing a diversity of organic carbon sources including aliphatic and aromatic hydrocarbons, amino acids, and carboxylic acids. Marinobacter and Arcobacter can metabolize organic nitrogen sources and have the capacity for denitrification and dissimilatory nitrate reduction to ammonia (DNRA) respectively; with DNRA and ammonification processes partially explaining high concentrations of ammonia measured in produced fluids. Arcobacter is capable of chemosynthetic sulfur oxidation, which could fuel metabolic processes for other heterotrophic, fermentative, or sulfate-reducing community members. Our analysis revealed mechanisms for growth of these taxa across a broad range of salinities (up to 15% salt), which explains their enrichment during early natural-gas production. These results demonstrate the prevalence of Marinobacter and Arcobacter during a key maturation phase of hydraulically fractured natural-gas wells, and highlight the significant role these genera play in biogeochemical cycling for this economically important energy system.

No abstract