Analytical model for screening potential CO<Subscript>2</Subscript> repositories

Assessing potential repositories for geologic sequestration of carbon dioxide using numerical models can be complicated, costly, and time-consuming, especially when faced with the challenge of selecting a repository from a multitude of potential repositories. This paper presents a set of simple analytical equations (model), based on the work of previous researchers, that could be used to evaluate the suitability of candidate repositories for subsurface sequestration of carbon dioxide. We considered the injection of carbon dioxide at a constant rate into a confined saline aquifer via a fully perforated vertical injection well. The validity of the analytical model was assessed via comparison with the TOUGH2 numerical model. The metrics used in comparing the two models include (1) spatial variations in formation pressure and (2) vertically integrated brine saturation profile. The analytical model and TOUGH2 show excellent agreement in their results when similar input conditions and assumptions are applied in both. The analytical model neglects capillary pressure and the pressure dependence of fluid properties. However, simulations in TOUGH2 indicate that little error is introduced by these simplifications. Sensitivity studies indicate that the agreement between the analytical model and TOUGH2 depends strongly on (1) the residual brine saturation, (2) the difference in density between carbon dioxide and resident brine (buoyancy), and (3) the relationship between relative permeability and brine saturation. The results achieved suggest that the analytical model is valid when the relationship between relative permeability and brine saturation is linear or quasi-linear and when the irreducible saturation of brine is zero or very small.     

Assessment of sediment and erosion rates by using the caesium-137 technique in a Chinese polygonal karst depression

Karst depressions comprise geomorphologically important sources and sinks for sediments and associated pollutants; yet the sedimentology of many depressions is not well understood in the world. In this paper, the ¹³⁷Cs technique was employed to estimate recent sedimentation rates in a Chinese polygonal karst depression. The results indicate that the sediment deposition rates ranged from 0.91 to 1.97 mm year⁻¹ from 1963 to 2007, and the average sediment deposition rate and specific deposit yield were estimated to be 1.47 mm year⁻¹ and 20 t km⁻² year⁻¹, respectively. These results are consistent with the local monitoring data of runoff fields, which confirms the validity of the overall approach. This shows that the soil loss rate is very low in some karst areas of Southwest China. Above all, the approach appears to offer valuable potential to study surface erosion by estimating sediment deposition rates of karst depressions, rather than the assessment of complicated soil erosion in stony soils of carbonate rock slopes. In addition, the space distribution of surface soil and ¹³⁷Cs inventories are affected remarkably by the inhomogeneous dissolution of limestone under the soil. It may be an important phenomenon, which exists widely in karst areas, and it is significantly different from other places.     

Crystal bending,subgrain boundary development,and recrystallization in orthopyroxene during granulite-facies deformation

A prominent feature of a granulite-facies shear zone from the Hidaka Main Zone (Japan) is the folding of orthopyroxene (opx) porphyroclasts. Dislocation density estimated by transmission electron microscope (TEM) and chemical etching in homogeneously folded domains is too low to account for the amplitude of crystallographic bending, leading us to propose a model similar to “flexural slip” folding, where folded layers are micrometer-wide opx layers between thin planar clinopyroxene (cpx) exsolutions. Extension (compression) in the extrados (intrados) of the folded layer is accommodated by dislocations at the cpx–opx interfaces. Alternatively to distributed deformation, crystal bending also localizes in grain boundaries (GBs), mostly oriented close to the (001) plane and with various misorientation angles but misorientation axes consistently close to the b-axis. For misorientation up to a few degrees, GBs were imaged as tilt walls composed of regularly spaced (100)[001] dislocations. For misorientation angles of 7°, individual dislocations are no longer visible, but high-resolution TEM (HRTEM) observation showed the partial continuity of opx tetrahedral chains through the boundary. For 21° misorientation, the two adjacent crystals are completely separated by an incoherent boundary. In spite of these atomic-scale variations, all GBs share orientation and rotation axis, suggesting a continuous process of misorientation by symmetric incorporation of (100)[001] dislocations. In addition to the dominant GBs perpendicular to the (100) plane, boundaries at low angle with (100) planes are also present, incorporating dislocations with a component of Burgers vector along the a-axis. The two kinds of boundaries combine to delimit subgrains, which progressively rotate with respect to host grains around the b-axis, eventually leading to recrystallization of large porphyroclasts.     

Evaluation of four supervised learning methods for groundwater spring potential mapping in Khalkhal region (Iran) using GIS-based features

One important tool for water resources management in arid and semi-arid areas is groundwater potential mapping. In this study, four data-mining models including K-nearest neighbor (KNN), linear discriminant analysis (LDA), multivariate adaptive regression splines (MARS), and quadric discriminant analysis (QDA) were used for groundwater potential mapping to get better and more accurate groundwater potential maps (GPMs). For this purpose, 14 groundwater influence factors were considered, such as altitude, slope angle, slope aspect, plan curvature, profile curvature, slope length, topographic wetness index (TWI), stream power index, distance from rivers, river density, distance from faults, fault density, land use, and lithology. From 842 springs in the study area, in the Khalkhal region of Iran, 70 % (589 springs) were considered for training and 30 % (253 springs) were used as a validation dataset. Then, KNN, LDA, MARS, and QDA models were applied in the R statistical software and the results were mapped as GPMs. Finally, the receiver operating characteristics (ROC) curve was implemented to evaluate the performance of the models. According to the results, the area under the curve of ROCs were calculated as 81.4, 80.5, 79.6, and 79.2 % for MARS, QDA, KNN, and LDA, respectively. So, it can be concluded that the performances of KNN and LDA were acceptable and the performances of MARS and QDA were excellent. Also, the results depicted high contribution of altitude, TWI, slope angle, and fault density, while plan curvature and land use were seen to be the least important factors.     

Evaluation of bank filtration as a pretreatment method for the provision of hygienically safe drinking water in Norway: results from monitoring at two full-scale sites

Two case studies were carried out in central Norway in order to assess the performance of bank filtration systems in cold-climate fluvial aquifers relying on recharge from humic-rich surface waters with moderate microbial contamination. Three municipal wells and two surface-water sources at operative bank filtration systems were monitored for naturally occurring bacteriophages, fecal indicators, natural organic matter (NOM) and physico-chemical water quality parameters during a 4-month period. Aquifer passage effectively reduced the microorganism and NOM concentrations at both study sites. Bacteriophages were detected in 13 of 16 (81%) surface-water samples and in 4 of 24 (17%) well-water samples, and underwent 3 ± 0.3 log₁₀ reduction after 50–80-m filtration and 20–30 days of subsurface passage. NOM reductions (color: 74–97%; dissolved organic carbon: 54–80%; very hydrophobic acids: 70%) were similar to those achieved by conventional water-treatment processes and no further treatment was needed. Both groundwater dilution and sediment filtration contributed to the hygienic water quality improvements, but sediment filtration appeared to be the most important process with regard to microbial and NOM reductions. A strengths-weaknesses-opportunities-threats analysis showed that bank filtration technology has a high potential as a pretreatment method for the provision of hygienically safe drinking water in Norway.     

Investigating the impact of the properties of pilot points on calibration of groundwater models: case study of a karst catchment in Rote Island,Indonesia

A robust configuration of pilot points in the parameterisation step of a model is crucial to accurately obtain a satisfactory model performance. However, the recommendations provided by the majority of recent researchers on pilot-point use are considered somewhat impractical. In this study, a practical approach is proposed for using pilot-point properties (i.e. number, distance and distribution method) in the calibration step of a groundwater model. For the first time, the relative distance–area ratio (d/A) and head-zonation-based (HZB) method are introduced, to assign pilot points into the model domain by incorporating a user-friendly zone ratio. This study provides some insights into the trade-off between maximising and restricting the number of pilot points, and offers a relative basis for selecting the pilot-point properties and distribution method in the development of a physically based groundwater model. The grid-based (GB) method is found to perform comparably better than the HZB method in terms of model performance and computational time. When using the GB method, this study recommends a distance–area ratio of 0.05, a distance–x-grid length ratio (d/X _grid) of 0.10, and a distance–y-grid length ratio (d/Y _grid) of 0.20.     

Source partitioning of anthropogenic groundwater nitrogen in a mixed-use landscape,Tutuila, American Samoa

This study presents a modeling framework for quantifying human impacts and for partitioning the sources of contamination related to water quality in the mixed-use landscape of a small tropical volcanic island. On Tutuila, the main island of American Samoa, production wells in the most populated region (the Tafuna-Leone Plain) produce most of the island’s drinking water. However, much of this water has been deemed unsafe to drink since 2009. Tutuila has three predominant anthropogenic non-point-groundwater-pollution sources of concern: on-site disposal systems (OSDS), agricultural chemicals, and pig manure. These sources are broadly distributed throughout the landscape and are located near many drinking-water wells. Water quality analyses show a link between elevated levels of total dissolved groundwater nitrogen (TN) and areas with high non-point-source pollution density, suggesting that TN can be used as a tracer of groundwater contamination from these sources. The modeling framework used in this study integrates land-use information, hydrological data, and water quality analyses with nitrogen loading and transport models. The approach utilizes a numerical groundwater flow model, a nitrogen-loading model, and a multi-species contaminant transport model. Nitrogen from each source is modeled as an independent component in order to trace the impact from individual land-use activities. Model results are calibrated and validated with dissolved groundwater TN concentrations and inorganic δ¹⁵N values, respectively. Results indicate that OSDS contribute significantly more TN to Tutuila’s aquifers than other sources, and thus should be prioritized in future water-quality management efforts.     

Probabilistic domain decomposition for the solution of the two-dimensional magnetotelluric problem

Probabilistic domain decomposition is proposed as a novel method for solving the two-dimensional Maxwell’s equations as used in the magnetotelluric method. The domain is split into non-overlapping sub-domains and the solution on the sub-domain boundaries is obtained by evaluating the stochastic form of the exact solution of Maxwell’s equations by a Monte-Carlo approach. These sub-domains can be naturally chosen by splitting the sub-surface domain into regions of constant (or at least continuous) conductivity. The solution over each sub-domain is obtained by solving Maxwell’s equations in the strong form. The sub-domain solver used for this purpose is a meshless method resting on radial basis function-based finite differences. The method is demonstrated by solving a number of classical magnetotelluric problems, including the quarter-space problem, the block-in-half-space problem and the triangle-in-half-space problem.     

Modeling core-scale permeability anisotropy in highly bioturbated “tight oil” reservoir rocks

A high-resolution simulation model of a heterogeneous low-permeability rock sample is used to investigate the effects of physical and biogenic sedimentary structures on scaling and anisotropy of absolute permeability at the core scale. Several simulation sub-samples with random locations and volumes were also selected for evaluation of the effects of scale and lithological composition on the calculated permeability. Vertical and horizontal permeability values (from whole core simulation) are in good agreement with routine core analysis (RCA) measurements from offsetting cores. Despite relatively good reservoir quality associated with geobodies of biogenic and relic bedding structures, results from the full diameter core simulation demonstrate that their limited volumetric abundance and restricted connectivity prevent these features from controlling fluid flow in these rocks. In fact, permeability seems to be dominated by the tighter encasing matrix, which exhibits average permeability values very close to those reported from RCA. Geometric averaging offers a better representation for the upscaling of horizontal permeability datasets; whereas, both geometric and harmonic averaging work similarly well for the vertical measurements. The methodology used in this work is particularly applicable to the detailed characterization of reservoir rocks with a high degree of heterogeneity caused by biological reworking and diagenesis.     

Effects of Microparameters on Macroparameters of Flat-Jointed Bonded-Particle Materials and Suggestions on Trial-and-Error Method

The aims of this paper are to analyze the effects of microparameters on macroparameters of flat-jointed bonded-particle materials of particle flow code in two dimensions (PFC2D) and improve the efficiency of trial-and-error method in calibration of microparameters. The trial-and-error method is always used to calibrate the microparameters for a PFC2D model, but it is time-consuming and empirical. To improve the efficiency of the trial-and-error method, the effects of microparameters on macroparameters need to be analyzed. By using the trial-and-error method, a desirable set of microparameters for a uniaxial compression PFC2D model (with a flat-joint contact model as the contact model) of limestone were calibrated to match the macroscopic properties obtained from laboratory tests. Based on the calibration set of microparameters, effects of microparameters on macroparameters of the PFC2D model were studied by the orthogonal design method. On this basis, a flowchart of calibrating microparameters via trial-and-error method was proposed. This flowchart was used to calibrate the microparameters of granite. The macroparameters and failure characteristics determined from numerical simulation were very similar to those of laboratory tests, confirming the effectiveness of the proposed flowchart.