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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 November 2017, Vol.217, pp.421-440
    Description: Shale formations account for 25% of the land surface globally and contribute a large proportion of the natural gas used in the United States. One of the most productive shale-gas formations is the Marcellus, a black shale that is rich in organic matter and pyrite. As a first step toward understanding how Marcellus shale interacts with water in the surface or deep subsurface, we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions, constrained by soil and water chemistry data. The simulation was carried out for 10,000 years since deglaciation, assuming bedrock weathering and soil genesis began after the last glacial maximum. Results indicate weathering was initiated by pyrite dissolution for the first 1000 years, leading to low pH and enhanced dissolution of chlorite and precipitation of iron hydroxides. After pyrite depletion, chlorite dissolved slowly, primarily facilitated by the presence of CO and organic acids, forming vermiculite as a secondary mineral. A sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases (CO and O ), specific surface area, and flow velocity of infiltrating meteoric water. The soil chemistry and mineralogy data could not be reproduced without including the reactive gases. For example, pyrite remained in the soil even after 10,000 years if O was not continuously present in the soil column; likewise, chlorite remained abundant and porosity remained small if CO was not present in the soil gas. The field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1–3 orders of magnitude smaller than surface area values measured for powdered minerals. Small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings. In addition, some mineral surface is likely interacting only with equilibrated pore fluids. An increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far-from-equilibrium conditions. We conclude from these observations that availability of reactive surface area and transport of H O and gases are the most important factors affecting rates of Marcellus shale weathering of the in the shallow subsurface. This weathering study documents the utility of reactive transport modeling for complex subsurface processes. Such modelling could be extended to understand interactions between injected fluids and Marcellus shale gas reservoirs at higher temperature, pressure, and salinity conditions.
    Keywords: Chemical Weathering ; Reactive Transport Modeling ; Critical Zone ; Marcellus Shale ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, Feb 1, 2014, Vol.126, p.555(19)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.10.051 Byline: Li Li, Fatemeh Salehikhoo, Susan L. Brantley, Peyman Heidari Abstract: We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a "Mixed column" containing uniformly distributed magnesite and a "One-zone column" containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6-2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/K.sub.eq 0.1. Two important surface areas are defined to understand the observed rates: the effective surface area (A.sub.e) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/K.sub.eq 〈0.1), while the interface surface area (A.sub.I) reflects the effective magnesite surface that lies along the quartz-magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of A.sub.e and A.sub.I . Under the conditions examined in this work, the value of A.sub.e varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates R.sub.MgCO.sub.3,B (in units of mol/s) vary linearly with A.sub.e and A.sub.I . Using A.sub.e to normalize rates, we calculate a rate constant (10.sup.-9.56 mol/m.sup.2/s) that is very close to the value of 10.sup.-10.0 mol/m.sup.2/s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60-70% of the A.sub.e is at the magnesite-quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution. Article History: Received 24 April 2013; Accepted 30 October 2013 Article Note: (miscellaneous) Associate editor: Daniel E. Giammar
    Keywords: Carbonate Minerals -- Analysis ; Magnesium Compounds -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 February 2014, Vol.126, pp.555-573
    Description: We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a “Mixed column” containing uniformly distributed magnesite and a “One-zone column” containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36 m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6–2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1 cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/ 〉 0.1. Two important surface areas are defined to understand the observed rates: the effective surface area ( ) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/ 〈 0.1), while the interface surface area ( ) reflects the effective magnesite surface that lies along the quartz–magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of and . Under the conditions examined in this work, the value of varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates (in units of mol/s) vary linearly with and . Using to normalize rates, we calculate a rate constant (10 mol/m /s) that is very close to the value of 10 mol/m /s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60–70% of the is at the magnesite–quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 4
    In: Water Resources Research, October 2014, Vol.50(10), pp.8240-8264
    Description: This work examines how heterogeneity structure, in particular correlation length, controls flow and solute transport. We used two‐dimensional (2D) sandboxes (21.9 cm × 20.6 cm) and four modeling approaches, including 2D Advection‐Dispersion Equation (ADE) with explicit heterogeneity structure, 1D ADE with average properties, and nonlocal Continuous Time Random Walk (CTRW) and fractional ADE (fADE). The goal is to answer two questions: (1) how and to what extent does correlation length control effective permeability and breakthrough curves (BTC)? (2) Which model can best reproduce data under what conditions? Sandboxes were packed with the same 20% (v/v) fine and 80% (v/v) coarse sands in three patterns that differ in correlation length. The Mixed cases contain uniformly distributed fine and coarse grains. The Four‐zone and One‐zone cases have four and one square fine zones, respectively. A total of seven experiments were carried out with permeability variance of 0.10 (LC), 0.22 (MC), and 0.43 (HC). Experimental data show that the BTC curves depend strongly on correlation length, especially in the HC cases. The HC One‐zone (HCO) case shows distinct breakthrough steps arising from fast advection in the coarse zone, slow advection in the fine zone, and slow diffusion, while the LCO and MCO BTCs do not exhibit such behavior. With explicit representation of heterogeneity structure, 2D ADE reproduces BTCs well in all cases. CTRW reproduces temporal moments with smaller deviation from data than fADE in all cases except HCO, where fADE has the lowest deviation. Large correlation length leads to distinct stages of solute breakthrough Two‐dimensional ADE reproduces the BTC in HCO with lowest deviation from data Correlation length potentially has stronger control than permeability variance
    Keywords: Solute Transport ; Scaling ; Spatial Heterogeneity ; Non‐Fickian Transport ; Correlation Length ; Permeability Variance
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 5
    Language: English
    In: Applied Geochemistry, April, 2013, Vol.31, p.119(14)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.apgeochem.2012.12.014 Byline: Seyed Mehran Heidari (a), Majid Ghaderi (a), Peyman Afzal (b) Abstract: a* Multifractal modeling for delineation of mineralization phases in a deposit. a* Determination of four mineralization phases in the deposit for Au, Ag, As and Cu. a* Recognition of high and extreme gold mineralization in an epithermal deposit. a* Correlation of mineralization phases with geological phases based on field geology. Author Affiliation: (a) Department of Economic Geology, Tarbiat Modares University, Tehran, Iran (b) Department of Mining Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran Article History: Received 2 July 2011; Accepted 20 December 2012 Article Note: (miscellaneous) Editorial handling by A. Danielsson
    ISSN: 0883-2927
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: Journal of Petroleum Science and Engineering, March 2019, Vol.174, pp.291-305
    Description: Clays in sandstone are thought to be a key factor in low salinity (LS) water flooding. This study investigates the effects of quartz, kaolinite and illite in LS water flooding in synthetic sand columns as a function of temperature. Four chromatography columns containing different amounts of pure quartz sand, illite, and kaolinite (100% quartz sand; 95/5% sand/illite; 95/5 sand/kaolinite; and 95/2.5/2.5% sand/illite/kaolinite). The use of these synthetic columns gives full control over mineralogy. These columns were saturated with high salinity (HS) formation water with 0.01 M (M) sodium acetate and aged for a week at 70 °C. They were then flooded with waters (yes, the use of water to displace water) with various salinities at four different temperatures (25, 70, 90 and 120 °C). Effluent concentrations of Ca and acetate (CH COO ) and pH were measured. This is a novel experimental design, where formation water containing sodium acetate is flooded with waters of varying salinity to gain insight into LS flood performance and possible recovery mechanisms. The hypothesis pursued here is that the behavior of the acetate ion (attached to matrix minerals during the aging process and then released due to ionic exchange) in an LS flood would provide a valuable analog that would simulate the bonding of carboxylic acids in crude oils with reservoir matrix minerals (and subsequent desorption during LS flooding). Key results include the following: (a) Our hypothesis was correct—the use of Na acetate behaved analogously to carboxylic acid, giving useful insights; (b) The quartz-only column showed strong evidence (elevated pH, presence of Ca and acetate in effluent) of ionic exchange due to LS flooding, showing that pure quartz is responsive to LS flooding and that clays are not absolutely needed; (c) Quartz plus kaolinite plus illite gave a slightly higher response than pure quartz so clays can play a role. Oil displacement results were also done for a pure quartz sand column, and this showed a strong response to the LS flood, confirming that clays are not absolutely needed.
    Keywords: Geochemistry ; Low Salinity Water Flooding ; Mechanisms of the Low Salinity Water ; Enhanced Oil Recovery ; Engineering ; Geology
    ISSN: 0920-4105
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  • 7
    Language: English
    In: 01 January 2018
    Source: SciTech Connect (U.S. Dept. of Energy - Office of Scientific and Technical Information)
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  • 8
    Language: English
    In: SSRN Electronic Journal, 2012
    ISSN: SSRN Electronic Journal
    E-ISSN: 1556-5068
    Source: CrossRef
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  • 9
    Language: English
    In: Ore Geology Reviews, December 2017, Vol.91, pp.278-295
    Description: Correlation between map of Au mineralization phases obtained by S-A fractal model and factor analysis with their field geological evidences. This research paper aims to delineate and recognize different gold mineralization stages based on surface lithogeochemical data using factor analysis and Spectrum-Area (S-A) modeling, as well as geological data in Arabshah sedimentary rock hosted epithermal gold deposit, NW Iran. Based on the factor analysis, Au and Mn were allocated to factor 2 (F2) and then classified by the S-A fractal modeling. In addition, Au and F2 values were transformed to spectrum data, which were categorized by the S-A log-log plots. Accordingly, the main mineralization phase contains Au and F2 (Au-Mn) values greater than 800 ppb and 0.3, respectively, and is associated with the occurrence of minerals such as pyrite, arsenian pyrite, realgar, orpiment and oxidized sulfides. The first phase of gold mineralization, where Au typically ranges between 80 and 350 ppb, is associated with base metal sulfides, arsenian pyrites and F2 values between 0.1 and 0.2. The second gold mineralization phase consists of Au values from 350 to 800 ppb and F2 values between 0.2 and 0.3. Combination of the S-A modeling, factor analysis and geological data outlined three gold mineralization stages in Arabshah gold deposit. The main mineralization stage showed a strong positive correlation with the NE-SW and NW-SE trending structures, the altered intrusive rocks such as microdiorite and granodiorite, and the altered subvolcanic dacitic domes.
    Keywords: Spectrum-Area (S-A) Fractal Model ; Gold Mineralization Stage ; Factor Analysis ; Takab ; Engineering ; Geology
    ISSN: 0169-1368
    E-ISSN: 1872-7360
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  • 10
    Language: Persian
    In: بررسی‌های حسابداری و حسابرسی, 01 November 2016, Vol.23(3), pp.3533-372
    Description: In this study, the effect of audit quality on agency costs and information asymmetry has been examined. Among the accepted companies 99 ones in Tehran Stock Exchange were selected for statistical samples from 1385 to 1392 in order to attain the research purposes. To measure the audit quality, it has been used the observable variables such as the percentage of institutional investors, the type of audit opinion, audit size and the audit tenure. Also, it is used these variables like assets utilization ratio, the rate of assets revenue, Tobin's Q ratio and leverage ratio to measure agency costs. It was utilized the stock price volatility, earning forecast error, film size, growth opportunities and the percentage of non-institutional investors to quantify the amount of variable related to information asymmetry from observable variables. After having trusted the acceptable process of the research measurement and structural models, the results indicate that the increase of audit services quality leads to decreasing challenges over the separation of ownership and management including agency costs and information asymmetry. The results of the study also show that the increase of agency costs results in the increase of information asymmetry level.
    Keywords: "Agency Costs" ; " Audit Quality" ; " Iformation Asymmetry" ; " Structural Equation Modeling Approach"
    ISSN: 2645-8020
    E-ISSN: 2645-8039
    Source: Directory of Open Access Journals (DOAJ)
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