Mineral Spatial Distribution Research Articles

Test Method for Mineral Spatial Distribution of BIF Ore by Imaging Spectrometer

The spatial distribution characteristics of iron ore components are important when measuring the difficulty of their beneficiation. Polarized light microscopy and scanning electron microscopy are traditional methods with some shortcomings, including complicated operation and low efficiency. Most of the laboratory hyperspectral imaging techniques that have emerged in recent years have been focused on the field of mineral resource exploration. In contrast, the mineral distribution and tectonic characteristics of iron ores have been relatively poorly studied in the field of beneficiation. To address the issue, 11 experimental samples of banded iron formation (BIF)-hosted iron ores were selected and tested using an imaging spectrometer. Then, based on the differences in spectral characteristic of the three main components (quartz, hematite, and magnetite) in the samples, the identification model of the spatial distribution of the iron ore components was established using the normalized spectral amplitude index (NSAI) and spectral angle mapper (SAM). The NSAI and SAM identify minerals based on spectral amplitude features and spectral morphological features of the sample, respectively. The spatial distribution of different minerals in the samples was tested using the model, and the test results demonstrated that the spatial distribution of the three components is consistent with the banded tectonic character of the sample. Upon comparison with the chemical test results, the mean absolute errors (MAE) of the model for quartz, hematite, and magnetite in the samples were 2.03%, 1.34%, and 1.55%, respectively, and the root mean square errors (RMSE) were 2.72%, 2.08%, and 1.85%, respectively, with the exception of one martite sample that reached an MAE of 10.17%. Therefore, the model demonstrates a high degree of accuracy. The research provides a new method to test the spatial distribution of iron ore components.

Numerical investigation of interactions between hydraulic and pre-existing opening-mode fractures in crystalline rocks based on a hydro-grain-based model

Hot dry rock (HDR) is deep geothermal reservoir rock predominantly containing granite, whose physical and mechanical properties are governed by heterogeneous crystal structures. However, the fluid conductivity of intact granite with ultra-low permeability cannot be reformed using existing techniques. Complex fracture networks must be created by connecting hydraulic fractures with natural fractures developed in reservoirs. In this study, we introduce the wall barrier method into the novel hydro-grain-based model (hydro-GBM) to build a prefractured granite model at the mineral grain scale. This model enables investigation of the effects of the stress environment, mineral spatial distribution, and fluid injection rate on hydraulic fracturing characteristics. Microcracks around the inner tips of pre-existing fractures reproduce the initiation, propagation, and coalescence behaviors similar to experimental results. Mineral spatial distributions induce random changes in the propagation paths of hydraulic fractures. Increasing the fluid injection rate delays the deflection of hydraulic fractures and extends the propagation distance along the major axes of pre-existing fractures. The average activity level and proportion of large seismic events are reduced by injecting low-rate fluid. In summary, our results reveal the interactions between hydraulic and pre-existing fractures and provide a valuable reference for constructing an efficient enhanced geothermal system (EGS) for deep reservoirs.

Effect of the mineral spatial distribution heterogeneity on the tensile strength of granite: Insights from PFC3D-GBM numerical analysis

The mechanical characteristics of crystalline rocks are affected by the heterogeneity of the spatial distribution of minerals. In this paper, a novel three-dimensional (3D) grain-based model (GBM) based on particle flow code (PFC), i.e. PFC3D-GBM, is proposed. This model can accomplish the grouping of mineral grains at the 3D scale and then filling them. Then, the effect of the position distribution, geometric size, and volume composite of mineral grains on the cracking behaviour and macroscopic properties of granite are examined by conducting Brazilian splitting tests. The numerical results show that when an external load is applied to a sample, force chains will form around each contact, and the orientation distribution of the force chains is uniform, which is independent of the external load level. Furthermore, the number of high-strength force chains is proportional to the external load level, and the main orientation distribution is consistent with the external loading direction. The main orientation of the cracks is vertical to that of the high-strength force chains. The geometric size of the mineral grains controls the mechanical behaviours. As the average grain size increases, the number of transgranular contacts with higher bonding strength in the region connecting both loading points increases. The number of high-strength force chains increases, leading to an increase in the stress concentration value required for the macroscopic failure of the sample. Due to the highest bonding strength, the generation of transgranular cracks in quartz requires a higher concentrated stress value. With increasing volume composition of quartz, the number of transgranular cracks in quartz distributed in the region connecting both loading points increases, which requires many high-strength force chains. The load level rises, leading to an increase in the tensile strength of the numerical sample.

Numerical Simulation Study of Brittle Rock Materials from Micro to Macro Scales Using Digital Image Processing and Parallel Computing

The multi-scale, high-resolution and accurate structural modeling of rocks is a powerful means to reveal the complex failure mechanisms of rocks and evaluate rock engineering safety. Due to the non-uniformity and opacity of rocks, describing their internal microstructure, mesostructure and macro joints accurately, and how to model their progressive fracture process, is a significant challenge. This paper aims to build a numerical method that can take into account real spatial structures of rocks and be applied to the study of crack propagation and failure in different scales of rocks. By combining the failure process analysis (RFPA) simulator with digital image processing technology, large-scale finite element models of multi-scale rocks, considering microstructure, mesostructure, and macro joints, were created to study mechanical and fracture behaviors on a cloud computing platform. The Windows-Linux interactive method was used for digital image processing and parallel computing. The simulation results show that the combination of a parallel RFPA solver and digital image modeling technology can achieve high-resolution structural modeling and high-efficiency calculation. In microscopic simulations, the influence of shale fractures and mineral spatial distribution on the fracture formation process can be revealed. In the mesostructure simulation, it can be seen that the spatial distribution of minerals has an impact on the splitting mode of the Brazilian splitting model. In the simulation of a joined rock mass, the progressive failure process can be effectively simulated. According to the results, it seems that the finite element parallel computing simulation method based on digital images can simulate the multi-scale failure process of brittle materials from micro to macro scales. Primarily, efficient parallel computing based on a cloud platform allows for the multi-scale, high-resolution and realistic modeling and analysis of rock materials.

Pore‐Scale Dissolution by CO2 Saturated Brine in a Multimineral Carbonate at Reservoir Conditions: Impact of Physical and Chemical Heterogeneity

AbstractWe study the impact of physical and chemical heterogeneity on reaction rates in multimineral porous media. We selected two pairs of carbonate samples of different physical heterogeneity in accordance with their initial computed velocity distributions and then injected CO2 saturated brine at reservoir conditions at two flow rates. We periodically imaged the samples using X‐ray microtomography. The mineralogical composition was similar (a ratio of dolomite to calcite of 8:1), but the intrinsic reaction rates and mineral spatial distribution were profoundly different. Visualizations of velocity fields and reacted mineral distributions revealed that a dominant flow channel formed in all cases. The more physically homogeneous samples had a narrower velocity distribution and more preexisting fast channels, which promoted dominant channel formation in their proximity. In contrast, the heterogeneous samples exhibit a broader distribution of velocities and fewer fast channels, which accentuated nonuniform calcite distribution and favored calcite dissolution away from the initially fast pathways. We quantify the impact of physical and chemical heterogeneity by computing the proximity of reacted minerals to the fast flow pathways. The average reaction rates were an order of magnitude lower than the intrinsic ones due to mass transfer limitations. The effective reaction rate of calcite decreased by an order of magnitude, in both fast channels and slow regions. After channel formation calcite was shielded by dolomite whose effective rate in slow regions could even increase. Overall, the preferential channeling effect, as opposed to uniform dissolution, was promoted by a higher degree of physical and/or chemical heterogeneity.

Hyperspectral Alteration Information Extraction of Tangchi Geothermal Area in Eastern Liaoning, China

To assist in the exploration of geothermal energy, Using remote sensing technology in eastern Liaoning Tang Chi geothermal mineral alteration information extraction, based on Tang Chi region of Hyperion satellite hyperspectral remote sensing image data, combined with hot spring Tang Chi region ground spectral information of the measured mineral samples alteration minerals, verification Tang Chi region alteration mineral spatial distribution and the relationship between the geothermal energy. The results show that most of the mineral alteration information extracted is the exploration of geothermal energy services.

Spatial zonation limits magnesite dissolution in porous media

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/Keq>0.1. Two important surface areas are defined to understand the observed rates: the effective surface area (Ae) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/Keq<0.1), while the interface surface area (AI) 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 Ae and AI. Under the conditions examined in this work, the value of Ae varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates RMgCO3,B (in units of mol/s) vary linearly with Ae and AI. Using Ae to normalize rates, we calculate a rate constant (10−9.56mol/m2/s) that is very close to the value of 10−10.0mol/m2/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 Ae 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.

Magnesite dissolution rates at different spatial scales: The role of mineral spatial distribution and flow velocity

We examined the role of mineral spatial distribution and flow velocity in determining magnesite dissolution rates at different spatial scales. One scale is the column scale of a few to tens of centimeters where dissolution rates are measured. Another is the “local” in situ scale defined as approximately 0.1mm. The experiments used two columns with the same bulk concentration but different spatial distributions of magnesite. In the “Mixed” column, magnesite was evenly distributed spatially within a quartz sand matrix across the whole column, while in the “One-zone” column, magnesite was distributed in one zone in the middle of the column. The two columns were flushed with the same inlet acidic solution (pH 4.0) under flow velocities varying from 0.18 to 36m/d. Columns of different lengths (22, 10, and 5cm) were run to understand the role of length scales. Reactive transport modeling was used to infer local-scale and column-scale dissolution rates.Under the acidic-solution flushing conditions used in this study, local in situ dissolution rates vary by orders of magnitude over a length scale of a few to tens of centimeters. Column-scale rates under different conditions vary between 6.40×10−12 and 1.02×10−9mol/m2/s. The distribution of local-scale rates, which collectively determine the column-scale rates, depend on flow velocity, column length scale, and mineral distribution. A two orders of magnitude difference in flow velocity results in more than two orders of magnitude difference in the column-scale rates. Under the same conditions of flow velocity and mineral distribution, column-scale rates are higher in short columns and are lower in long columns. Mineral spatial distribution made a maximum difference of 14% in the medium-flow velocity regime where the reaction kinetics of the system operates under mixed-control conditions. Under such mixed-control conditions, the larger difference between the two columns in their spatial variation of pH and saturation state lead to a larger difference in the spatial distribution of local dissolution rates and therefore column-scale rates. In contrast, under slow-flow velocity conditions, the system is mostly at equilibrium without much spatial variation, i.e., the regime of local equilibrium. Under fast-flow velocity conditions, the system is kinetically controlled, the local aqueous geochemistry is everywhere similar to the inlet condition, and is also relatively uniform. Under these two conditions, there is almost no difference between the two columns. Column-scale rates were best understood in terms of the Damkohler number (DaI) that quantifies the relative dominance of advection and dissolution processes. The observations in this study lead us to surmise that rates of weathering and other natural processes may be similarly affected by chemical heterogeneity in natural systems under conditions where reaction rate and flow rate are comparable.

Effects of mineral spatial distribution on reaction rates in porous media

This study examined the effects of variations in mineral spatial distributions on reaction rates in porous media. Pore‐scale network models were constructed to represent sandstones with anorthite and kaolinite present in various amounts and in different spatial patterns. Simulation conditions corresponded to geological sequestration of carbon dioxide. To examine scaling effects arising from pore‐scale heterogeneities, reaction rates from network models were compared to rates from continuum models that use uniform concentrations. With small percentages of reactive minerals the scaling effects are large, and the effect of spatial distribution is significant. Scaling effects are largest when reactive minerals are closely clustered and oriented parallel to flow. Conversely, spatial distributions that enhance mass transport to and from reactive minerals, such as small mineral clusters or elongated clusters oriented transverse to flow, result in reaction rates that are well represented by the continuum model.

Digital image-based numerical modeling method for prediction of inhomogeneous rock failure

Abstract This paper presents a two-dimensional digital image-based numerical modeling method for prediction of inhomogeneous rock failure behavior under loadings. Actual inhomogeneities of granitic rocks are extracted from color images of the granite cross-sections. They are represented as the internal spatial distribution of three main granite minerals (quartz, feldspar and biotite). The actual mineral spatial distribution on granite cross-section is then incorporated into conventional numerical software packages to examine the rock failures under loading. Some digital image processing algorithms are presented to isolate and identify the main internal minerals and their distribution from color digital images. A simple method is proposed to transform the actual image data into vector data for generation of finite meshes or grids. The vector data are used directly as uniform square element meshes or grids that can be inputted into the existing software packages. The finite difference software package FLAC is used as an example for the present investigation. The conventional Mohr–Coulomb and tensile stress failure criteria are used to examine the failure behavior of a circular granite cross-section under the conventional Brazilian indirect tensile test loading conditions. The numerical results indicate that the vertical tensile crack initiates in a biotite located near the geometrical center of the granite cross-section and the actual spatial distribution of the three minerals plays an important role in modifying the propagation pattern of the tensile crack from its theoretical position at the central vertical diameter of a homogeneous circular cross-section. The numerically predicted failure load and tensile strength values for inhomogeneous granite are much lower than the expected values.

Dynamic Mechanism of Migmatization in the Dabie Complex, Northeastern Hubei, China1

Abstract On the basis of the detailed field work, compositions and contents of plagioclase and K‐feldspar, determination of ordering degree, statistical analysis of plagioclase elongation index, mass‐balance calculation and mineral spatial distribution and geochemistry, it is concluded that the migmatites in the Dabie complex are characterized by the presence of thermocenters. There are regular changes in mineral character in the migmatites from the centers outwards. The dominant genetic mechanism is anatexis and metasomatism, whose intensities decrease from the centers outwards. Finally, according to the simulated experiment on Liesegang' s rings and non‐linear dynamics (dissipative structure theory), the dynamic mechanism of migmatization is profoundly expouded as consisting of the early‐stage metasomatism induced by the thermal anomaly, the cardinal‐stage anatexis induced by the early‐stage matasomatism and finally the last‐stage post‐anatexis metasomatism.