A study of dissolution and water-bearing characteristics of the restricted platform dolomite facies in the karst areas of Guizhou,China

The restricted platform facies dolomites are widely distributed in the northern and northeastern Guizhou, China, and they are the main lithology of karst groundwater and a primary target for locating drinking water resources in the province of Guizhou. The geological history and deposition environment of the restricted platform dolomite facies vary significantly across the province, making it more difficult to evaluate the dissolution and water-bearing capacity of karstic dolomites. This paper explores a quantitative approach to assessing the solubility of four typical geological strata using analytical hierarchy process (AHP) modeling. Properties including CaO/MgO, clast content, porosity, permeability and fracture density are investigated and used as the major criteria in the AHP analysis to render the rankings for solubility among the studied formations. Hydrogeological characteristics are also discussed. Overall the presented results demonstrate the feasibility of AHP as a modeling tool to provide a quantitative approach when a wide range of properties need to be considered. The explored approach can have strong implications for the exploitation and utilization of karst groundwater in Guizhou, China.     

Thermodynamic analysis of heat transfer in a wellbore combining compressed air energy storage

Compressed air energy storage (CAES) is a potential energy storage technology. The gas phase and short cycle period are two key factors affecting heat transfer loss in the wellbore of CAES. A semi-analytical solution was developed by using the convolution method considering gas movement in this study to describe the transient behavior of heat transfer with a short cycle period. The comparative analysis of the presented solution with two published solutions showed that the solution matched well with the previous solutions under steady state. Parametric studies were carried out to investigate the impact of injection rate, overall heat transfer coefficient and thermal diffusivity of the formation on heat loss in the wellbore. The results indicated that a low overall heat transfer coefficient and thermal diffusivity of the formation with an appropriate injection rate can efficiently reduce the heat loss. A hypothetical case study with a short cycle period of injection and production was conducted to demonstrate the applicability of the developed solution in CAES. The results suggest that the semi-analytical solution is applicable for heat transfer in the wellbore of CAES.     

Groundwater age,mixing and flow rates in the vicinity of large open pit mines,Pilbara region,northwestern Australia

Determining groundwater ages from environmental tracer concentrations measured on samples obtained from open bores or long-screened intervals is fraught with difficulty because the sampled water represents a variety of ages. A multi-tracer technique (Cl, ¹⁴C, ³H, CFC-11, CFC-12, CFC-113 and SF₆) was used to decipher the groundwater ages sampled from long-screened production bores in a regional aquifer around an open pit mine in the Pilbara region of northwest Australia. The changes in tracer concentrations due to continuous dewatering over 7 years (2008–2014) were examined, and the tracer methods were compared. Tracer concentrations suggest that groundwater samples are a mixture of young and old water; the former is inferred to represent localised recharge from an adjacent creek, and the latter to be diffuse recharge. An increase in ¹⁴C activity with time in wells closest to the creek suggests that dewatering of the open pit to achieve dry mining conditions has resulted in change in flow direction, so that localised recharge from the creek now forms a larger proportion of the pumped groundwater. The recharge rate prior to development, calculated from a steady-state Cl mass balance, is 6 mm/y, and is consistent with calculations based on the ¹⁴C activity. Changes in CFC-12 concentrations with time may be related to the change in water-table position relative to the depth of the well screen.     

A least squares approach for efficient and reliable short-term versus long-term optimization

The uncertainties related to long-term forecasts of oil prices impose significant financial risk on ventures of oil production. To minimize risk, oil companies are inclined to maximize profit over short-term horizons ranging from months to a few years. In contrast, conventional production optimization maximizes long-term profits over horizons that span more than a decade. To address this challenge, the oil literature has introduced short-term versus long-term optimization. Ideally, this problem is solved by a posteriori multi-objective optimization methods that generate an approximation to the Pareto front of optimal short-term and long-term trade-offs. However, such methods rely on a large number of reservoir simulations and scale poorly with the number of objectives subject to optimization. Consequently, the large-scale nature of production optimization severely limits applications to real-life scenarios. More practical alternatives include ad hoc hierarchical switching schemes. As a drawback, such methods lack robustness due to unclear convergence properties and do not naturally generalize to cases of more than two objectives. Also, as this paper shows, the hierarchical formulation may skew the balance between the objectives, leaving an unfulfilled potential to increase profits. To promote efficient and reliable short-term versus long-term optimization, this paper introduces a natural way to characterize desirable Pareto points and proposes a novel least squares (LS) method. Unlike hierarchical approaches, the method is guaranteed to converge to a Pareto optimal point. Also, the LS method is designed to properly balance multiple objectives, independently of Pareto front’s shape. As such, the method poses a practical alternative to a posteriori methods in situations where the frontier is intractable to generate.     

Pore to pore validation of pore network modelling against micromodel experiment results

Pore network modelling (PNM) has been widely used to study the multiphase flow and transport in porous media. Although a number of recent papers discussed the PNM validation on core-scale parameters such as permeability, relative permeability and capillary pressure; quantitative predictive potential of PNM on pore by pore basis has rarely been studied. The aim of this paper is to present a direct comparison between PNM simulations and corresponding micro-model experiments at the same scale and the same geometry. A number of well-defined and constrained two-phase flow in porous medium experimental scenarios were utilized to validate the physics solving part in PNM (filling rules, capillary and viscous pressure). This work validates that a dynamic pore network flow solver can predict two-phase flow displacements for these experiments for drainage situations at both pore and plug scales. A glass-etched micro-model is used to quantify the accuracy of a dynamic PNM solver on pore and core levels. Two-phase drainage micro fluidic experiments at different flow conditions are performed on micro-models. PNM simulations are performed on the same pattern and flow conditions as used in micro-model experiments. The two-phase distribution extracted from experiment images is registered onto rsults of PNM simulations for direct pore to pore comparison. Pore-scale matching level is found at around 75 % for all three test cases. The matching level of core-scale parameters such as S _{w c} and oil-phase permeability varies from case to case; the relative error to micro-model experiment measurements varies from 15 to 60 %. Possible reasons leading to discrepancies on core-scale parameters are discussed: missing considerations during validation of the combination of uncertainty in both simulator input parameters and experiments are seen as the principal factors.     

Hydraulic Conductivity of Fly Ash-Amended Mine Tailings

The objective of this study was to evaluate the effect of fly ash addition on hydraulic conductivity (k) of mine tailings. Mine tailings used in this study were categorized as synthetic tailings and natural tailings; two synthetic tailings were developed via blending commercially-available soils and natural tailings were collected from a garnet mine located in the U.S. Two fly ashes were used that had sufficient calcium oxide (CaO) content (17 and 18.9 %) to generate pozzolanic activity. Hydraulic conductivity was measured on pure tailings and fly ash-amended tailings in flexible-wall permeameters. Fly ash was added to mine tailings to constitute 10 % dry mass of the mixture, and specimens were cured for 7 and 28 days. The influence of fly ash-amendment on k of mine tailings was attributed to (1) molding water content and (2) plasticity of the mine tailings. Tailings that classified as low-plasticity silts with clay contents less than 15 % exhibited a decrease in k when amended with fly ash and prepared wet of optimum water content (w _opt ). Tailings that classified as low-plasticity clay exhibited a one-order magnitude increase in k with addition of fly ash for materials prepared dry or near w _opt . The decrease in k for silty tailings was attributed to formation of cementitious bonds that obstructed flow paths, whereas the increase in k for clayey tailings was attributed to agglomeration of clay particles and an overall increase in average pore size. The results also indicated that the effect of curing time on k is more pronounced during the early stages of curing (≤7 days), as there was negligible difference between k for 7 and 28-days cured specimens.     

Corridor development in Gauteng,South Africa

The development corridor concept has been regarded as an important development instrument in spatial planning and geography for many decades. Expanding literature on the theme indicates that development centres or nodes play an important role in the establishment of such corridors. Flows of goods and information between such centres are essential in creating conditions that are potentially favourable for further urban development along the communication axes connecting such centres. Combined, the various nodes form a unique flexible exchange environment allowing for dynamic synergies of interactive growth to achieve scope economies aided by fast and reliable corridors of transport and communication infrastructure. In the South African development sphere, there is strong belief that functional relationships between nodes can play a decisive role in the establishment of development corridors. The research showed that the degree to which economic activities are concentrated in the greater Gauteng region; the resulting current or emerging multinodal structural composition; and the flows of economic activities between the various nodes result in the creation of development corridors that channel and focus economic growth between networks of cities. This confirms the importance of corridors as spatial and economic development instruments. The research concluded that the core cities of Johannesburg and Pretoria are the central driving force in corridor development in Gauteng. They feature as the most dominant nodes, exerting great forces of attraction on the distribution of development and economic growth in the region. The research also suggests a degree of polycentrism whereby economic growth is channelled between networks of cities, however development potential is most favourable in areas in close proximity to primary centres and tends to be functionality driven.     

The contingency of medicare physician spending on population densities and sizes

The exploration of the spatial association between Medicare physician spending and population densities and sizes could possibly facilitate the investigation of the causal mechanisms beneath the variation in medical care. We acquired the U.S. Medicare physician expenditures and regional demographic and geographic data in 2006 from the Dartmouth Atlas of Health Care. Six geographic units—states, counties, Hospital Referral Regions, Hospital Service Areas, Metropolitan Statistical Areas, and state non-Metropolitan Statistical Areas—were used as units of study. Pearson correlation analysis, multivariable regression, and partial correlation analysis were employed. Among six geographic units, Pearson correlation coefficients between Medicare physician expenditures and logarithmic population densities ranged from 0.42 to 0.63 (p < 0.05 for all), and between the expenditures and logarithmic population sizes from 0.31 to 0.65 (p < 0.05 for all). When population health, differential demand, market structure, and data reporting bias were controlled, population densities and sizes were positively associated with Medicare physician expenditures in most models. Population densities and sizes could explain considerable amounts of regional variation in Medicare physician spending. We concluded that Medicare physician spending was contingent on population densities and sizes. Because population densities and sizes are produced by more fundamental qualities such as natural environments and resources and thus are not easily manipulated, they are suggestive in health policy studies. Further research might investigate population distribution associated properties such as geographic distribution of health care resources, spatial dynamics of medical technology distribution, and cultural and psychological factors.     

Seasonal residence and counterurbanization: the role of second homes in population redistribution in Finland

Since the late twentieth century, many developed countries have experienced population deconcentration, labelled as counterurbanization. There has been an academic discussion on the meaning, validity and universality of this concept, drivers of counterurbanization and its impacts on rural areas. To date, research on counterurbanization mostly apply static and discrete definitions of residence, migration and population, which is an increasingly simplistic view in the contemporary reality of a growing multitude of forms of mobility, often related to dual residence. Particularly large-scale quantitative studies on counterurbanization are confined by existing statistical practices. This paper attempts to overcome this obstacle and describe the transformation of the settlement system in Finland acknowledging the role of second home mobility. To achieve this goal, it introduces two alternative measures of population, seasonal and average population, and analyses their spatial dynamics between the years 1990 and 2010 based on georeferenced grid statistical data. The study finds that although registered population has been concentrating during the period in analysis, seasonal population has been increasingly dispersed due to the growing number of second homes. It shows that the counterurbanization process, though not noticed by conventional statistics, does occur in Finland, manifested by seasonal rather than permanent moves. The article concludes that various forms of mobility should be taken into account when analysing the urban–rural population dynamics and transformations of settlement systems as well as in rural development planning.