Drainage Experiments Research Articles

Experimental study of three-dimensional CO2-water drainage and fracture-matrix interactions in fractured porous media

Estimation of CO2 storage capacity in fractured porous reservoirs requires a better understanding of the CO2-water flow fundamentals in fracture-matrix systems. There are few core-flood experiments on three-dimensional CO2-water drainage in a fracture-matrix system, yet none have examined the impacts of the capillary continuity of fractures and fracture-matrix interactions. In this study, twelve drainage experiments were conducted in four fracture-matrix columns, each comprising a vertical stack of a cylindrical rock core, a filter paper serving as an analogue to a horizontal fracture, and a ceramic plate. The core sample was surrounded by open space at the top and circumferential sides to model fractures, allowing for 3-D CO2-water drainage in the rock core and displaced water draining across the horizontal fracture. X-ray computed tomography was conducted to visualize the dynamic invasion/drainage processes in four rock core samples showing contrasts in anisotropy, permeability, and heterogeneity. Experimental results show (1) the equilibrium CO2 saturations in the rock matrix vary from 0.10 to 0.60 at controlled capillary pressures up to 200 kPa, (2) the CO2 saturations in the matrix increaze with water saturation in the horizontal fracture resulting in better capillary continuity for water to drain across the fracture, and (3) the fracture water saturation can be enhanced by the non-uniform fracture capillary pressure and countercurrent flow of CO2 and water across the fracture-matrix interface. In the case of high fracture water saturation, matrix CO2 saturation largely depends on matrix anisotropy and heterogeneity. The core-scale experimental results contribute to understand the fracture-matrix interactions and CO2 storage efficiency in fractured porous media.

In-situ capillary pressure and wettability in natural porous media: Multi-scale experimentation and automated characterization using X-ray images

HypothesisGeometrical analyses of pore-scale fluid-fluid-rock interfaces have recently been used for in-situ characterization of capillary pressure and wettability in natural porous media. Nevertheless, more robust techniques and multi-scale, well-characterized experimental data are needed to rigorously validate these techniques and enhance their efficacy when applied to saturated porous media. Experiments and Image analysisWe present two new techniques for automated measurements of in-situ capillary pressure and contact angle, which offer several advancements over previous methodologies. These approaches are methodically validated using synthetic data and X-ray images of capillary rise experiments, and subsequently, applied on pore-scale fluid occupancy maps of a miniature Berea sandstone sample obtained during steady-state drainage and imbibition flow experiments. FindingsThe results show encouraging agreement between the image-based capillary pressure-saturation function and its macroscopic counterpart obtained from a porous membrane experiment. However, unlike the macroscopic behavior, the micro-scale measurements demonstrate a nonmonotonic increase with saturation due to the intermittency of the pore-scale displacement events controlling the overall flow behavior. This is further explained using the pertinent micro-scale mechanisms such as Haines jumps. The new methods also enable one to generate in-situ contact angle distributions and distinguish between the advancing and receding values while automatically excluding invalid measurements.

A general capillary equilibrium model to describe drainage experiments in heterogeneous laboratory rock cores

Macroscopic observations of two-phase flow in porous rocks are largely affected by the heterogeneity in continuum properties at length scales smaller than a typical laboratory sample. The ability to discriminate among the rock properties at the origin of the heterogeneity is key to the development of numerical models to be used for prediction. Here, we present a capillary equilibrium model that represents spatial heterogeneity in dual-porosity porous media in terms of the capillary entry pressure, 1/α, and the irreducible wetting phase saturation, Sir. Both parameters are used to scale local capillary pressure curves by using three-dimensional imagery acquired during multi-rate gas/liquid drainage displacements. We verify the proposed approach by considering the case study of a dual-porosity limestone core and use the spatial variation in Sir as proxy for microporosity heterogeneity. The latter places potentially next-to-leading order controls on the observed fluid saturation distribution, which is strongly correlated to the distribution of 1/α. While microporosity is by and large uniform at the observation scale on the order of 0.1 cm3, the spatial correlation of 1/α is on the order of 1 cm and is therefore not statistically represented in the volume of typical laboratory core samples.

Predicting the Effect of Relaxation on Interfacial Area Development in Multiphase Flow

AbstractA variety of two‐phase flow experiments, currently available in the literature, are compared to study the effect of fluid relaxation on interfacial area generation. Interfacial area is an important parameter that controls mass transfer in many engineered multiphase systems, so it is important to develop accurate predictive tools describing multiphase flow to engineer efficient processes. An empirical predictive relationship was developed describing a specific interfacial area‐wetting saturation relationship that depends on the number of quasi‐equilibrium relaxation points obtained during a drainage or imbibition experiment. The empirical expression was tested and verified using a number of existing datasets. We found that different relationships were needed depending on the fluid properties as well as the porous medium. However, clear trends were observed that can, once a predictive relationship is established for the system, allow us to design multiphase flow systems to produce a desired amount of interfacial area tailored to a particular application.

Analytical Solutions of Vertical Airflow in an Unconfined Aquifer with Rising or Falling Water Table

The rise and fall of the groundwater level can drive air flow in the vadose zone. In turn, the air flow can interact with the water flow. When the unconfined aquifer is covered by a low-permeability media, the coupling of the water flow and the air flow is more obvious. In this study, a conceptual model is established for coupling of air flow and water flow in the vadose zone in response to rapid fluctuations of the water table. Water injection and drainage experiments are conducted in a double-layered sand column with a thick layer (80.5 cm) of coarse sand and a thin layer of fine sand as a low-permeability cap. Different cap thickness (2 cm, 5 cm, 7.5 cm) and different thickness of the vadose zone (30 cm, 40 cm) are set for the experiments. Negative pressure (NP)/positive pressure (PP) of the air in the vadose zone is observed in the drainage/injection experiments, with higher pressure in experiments of thicker cap layer. In each experiment, NP or PP increases rapidly to reach a maximum in the early stage, and gradually becomes zero in the late stage. Analytical solutions on three subdivided stages indicate the permeability and thickness of the cap layer, as well as permeability and porosity of the media in the vadose zone are the key controlling factors on the process of coupling of air flow and water flow. The solutions also reveal the formation mechanism of air pressure in the vadose zone with a low-permeability cap. This study has both theoretical significance and engineering applications.

Quantitative experimental investigation of multiple P-T effects on primary drainage process during scCO2 storage in deep saline aquifers

Deep saline aquifer is the preferred reservoir for CO2 geological storage. However, during the primary drainage process of injecting scCO2 into deep saline aquifers, residual water will form and cannot be drained, which will have adverse effects on the capacity and safety of CO2 storage. This is a new study based on the preliminary exploration, 9 groups of scCO2 drainage experiments in a natural rock core are carried out under multiple pressure (8, 10 and 12 MPa) and temperature (40, 50 and 60 °C) conditions to investigate the formation mechanism of residual water during primary drainage process. A quantitative power function relationship between residual water saturation and drainage time is proposed; a quantitative exponential function relationship between irreducible water saturation and P-T conditions is also presented. The coefficients a, b, c and d of two fitting functions are discussed in detail. In addition, the irreducible water saturation increases with the increase of temperature and decreases with the increase of pressure. Through deep analysis on the interfacial tension, viscosity ratio and logCa-logM stability phase diagram, the interfacial tension of CO2-H2O system, which determines the capillary entry pressure, is regarded as the guiding factor influencing the experimental results, rather than the viscosity ratio. It is worth mentioning that under the given P-T conditions in this study, the influence of temperature on the experimental results is greater than that of pressure.

Estimating water content of unsaturated sandy soils by GPR during a drainage experiment

This study shows how to estimate water content of unsaturated sandy soils from ground-penetrating radar (GPR) data acquired during water drainage experiment. Using the Topp equation, water content can be converted to dielectric permittivity. Afterwards, based on 1D forward modeling method a GPR synthetic trace is generated. Finally, a trial-and-error match of modeled and observed GPR traces was performed to estimate the optimum set of parameters characterizing the materials composing the soil. According to the comparison of the modeling signal and the real signal, we go back to update the water content. Repeating the procedure until the modeling signal fits the real signal very well. We used this procedure to obtain the evolution of water content of unsaturated sandy soils. The method is presented on drainage experiment carried out on unsaturated sandy soil of a large-controlled experimental site. In order to follow the change of the water level, we measured GPR profiles at different times and picked up four GPR profiles, then the same trace (No. 700) were picked up on each GPR profile and estimated the water content from each signal.

Characterizing Preferential Flow Paths in Texturally Similar Soils under Different Land Uses by Combining Drainage and Dye-Staining Methods

Preferential flow paths have been widely characterized by many visualization methods. However, the differences in preferential flow paths under various land uses and their relationships to hydraulic properties remain uncertain. The objectives of this study are to (1) characterize preferential flow paths under various land uses (forest and orchard) by combining drainage and dye-staining methods and to (2) build a connection between preferential flow paths and hydraulic-related parameters and extract the proportion of preferential flow paths from the compounding effects of matrix flow and preferential flow. The dye-staining experiments were conducted in five sandy soils and one sandy clay loam in situ, including four soils from forest and two soils from orchards. A total of 47 soil cores, 4 cm in height and 9 cm in diameter, were collected in each layer of the dye-stained soils for drainage experiments in the laboratory. Dye coverage and hydraulically equivalent macropore parameters (macroporosity, pore size distribution, and number of macropores) and their relationships were analyzed. The results show that the volume of preferential flow is partly affected by the total macropore volume. The effect of macropores on preferential flow varies by macropore size distribution. Dye coverage exhibited a significant (P < 0.01) correlation with macroporosity (correlation coefficient 0.83). Based on the value of macroporosity or steady effluent rates, the part of the dye coverage that was due to preferential flow on the surface dye-stained soil (resulting from both matrix and preferential flow) could be identified in this study. Compared with orchards, forestland has more preferential flow paths in both surface soil and subsoil. Further studies are needed to quantify the 3-D preferential flow paths and build a connection between preferential flow paths and hydraulic properties.

The Effect of Salt on Stability of Aqueous Foams

The properties of foams are often affected by environmental variables such as salt contamination. The objective of this study is to investigate the impact of salt on the drainage behavior of aqueous foams. To accomplish this objective, drainage experiments were conducted on aqueous foams. Test variables were foam quality (40–65%), and salt content (0% to 18%), and type. To investigate drainage, the foam was generated in a flow loop and trapped in a vertical test section. Then, the pressure profile in the foam column was measured using ten pressure sensors. Foam drainage is determined as a function of time using measured pressure profiles. The results show that the drainage of NaCl-containing foams decreased with foam quality, whereas the CaCl2-containing foams did not exhibit a clear trend with foam quality. The effect of salt content on foam rheology was minimal.

Role of Pore‐Scale Disorder in Fluid Displacement: Experiments and Theoretical Model

AbstractThe flow of multiple immiscible fluids in disordered porous media is important in many natural processes and subsurface applications. The pore‐scale disorder affects the fluid invasion pathways significantly and induces the transitions of displacement patterns in porous media. Extensive studies focus on pattern transitions affected by disorder under quasistatic or dynamic conditions, but how the disorder controls the pattern transitions from capillary‐dominated regime to viscous‐dominated regime is not well understood. Here, we combine microfluidic experiments and theoretical analysis to investigate the role of disorder in fluid displacement. We perform drainage experiments with four different disorders under six flow rate conditions and show that increasing disorder destabilizes displacement fronts for all flow rates considered. Based on the scaling analysis of pore‐filling events, we propose a theoretical model that describes the pattern transitions from compact displacement to capillary to viscous fingering as functions of disorder and capillary number. The effects of disorder on both capillary and viscous forces are quantified within the theoretical model. The phase diagram predicted by this model agrees well with our experimental results. We further elucidate the role of disorder in fluid displacement via energy conversion and dissipation. We find that increasing disorder enhances the capillary instabilities and induces more energy dissipated in a capillary‐dominated regime, with the dissipation ratio increasing from 28.3% to 56.7%. Our work extends the classic phase diagram to consider the effect of disorder and provides a better understanding of the impact of the disorder on flow behaviors by energy dissipation.

Experimental and Modelling Study of Gravity Drainage in a Three-Block System

Gravity drainage is known as the controlling mechanism of oil recovery in naturally fractured reservoirs. The efficiency of this mechanism is controlled by block-to-block interactions through capillary continuity and/or reinfiltration processes. In this study, at first, several free-fall gravity drainage experiments were conducted on a well-designed three-block apparatus and the role of tilt angle, spacers’ permeability, wettability and effective contact area (representing a different status of the block-to-block interactions between matrix blocks) on the recovery efficiency were investigated. Then, an experimental-based numerical model of free-fall gravity drainage process was developed, validated and used for monitoring the saturation profiles along with the matrix blocks. Results showed that gas wetting condition of horizontal fracture weakens the capillary continuity and in consequence decreases the recovery factor in comparison with the original liquid wetting condition. Moreover, higher spacers’ permeability increases oil recovery at early times, while it decreases the ultimate recovery factor. Tilt angle from the vertical axis decreases recovery factor, due to greater connectivity of matrix blocks to vertical fracture and consequent channelling. Decreasing horizontal fracture aperture decreases recovery at early times but increases the ultimate recovery due to a greater extent of capillary continuity between the adjacent blocks. Well match observed between the numerical model results and the experimental data of oil recovery makes the COMSOL multiphysics model attractive for application in multi-blocks fractured systems considering block-to-block interactions. The findings of this research improve our understanding of the role of different fracture properties on the block-to-block interactions and how they change the ultimate recovery of a multi-block system.

Investigation on water phase trapping mechanisms in tight gas reservoirs: Pore-scale visualization observation and core-scale flooding analysis

Water phase trapping (WPT) is one of severe formation damage issues for the safe production in tight gas reservoirs. In this work, both pore-scale and core-scale investigation of the formation process of WPT were performed. Real-time observation of the water invasion and formation of trapped gas was achieved using visualized micromodel. Meanwhile, core flooding experiments including water imbibition and drainage experiments were conducted to analyze the responses of gas permeability and water saturation changes to the formation of WPT. The micromodel visualization experiments presented an intuitive gas-water two-phase flow scenarios which revealed the microscopic mechanisms of the WPT vividly. It pointed out that snap-off and circumfluous flow are the two major mechanisms to break the bulk gas flow and generate the trapped gas during WPT formation. The results of core flooding provided a causality link between the changes of gas permeability and water saturation, which showed a macroscopic consequence to the gas flow ability caused by WPT damage. These findings will benefit a better understanding of the formation process and damage mechanisms of WPT.

Experimental and simulation investigations of carbon storage associated with CO2 EOR in low-permeability reservoir

CO2 based water-alternating-gas (WAG) injection and surfactant-alternating-gas (SAG) injection have shown to be two effective and practical methods to enhance oil recovery, which, also achieves carbon sequestration as well. In this study, a combination of experiment and numerical simulation was used to evaluate the performances of WAG and SAG flooding in low-permeability reservoir. Core drainage experiments were conducted, and NMR equipment was used to reveal the microscopic fluid distribution and explain the macroscale drainage results for each of the enhancing oil recovery (EOR) technique. In addition, the two EOR techniques were further evaluated in a realistic low-permeability reservoir, Yushulin oil field from Songliao Basin, China. The results show that, gas phase can lower gas-oil interfacial tension (IFT) in WAG process which contributes the oil displacement from smaller pores. Besides that, the surfactant in SAG process can also improve the oil displacement efficiency in larger pores due to the generation of foams. The optimal WAG and SAG ratios (1.25–1.5) in the realistic reservoir are all higher than the typical ratio (equal to 1) from the core experimental results and that in the most conventional oilfields due to the higher heterogeneity in low-permeability reservoir with easier gas breakthrough. Furthermore, the CO2 utilization efficiency for SAG is higher than WAG.

Options for statistical analysis of unreplicated paired design drainage experiments

Field scale experiments are seldom replicated due to size constraints and limitations of other resources. This presents a difficulty in performing statistical analysis and interpretation of the results collected. In this paper we will evaluate water and nutrient effluent data from two fields, one managed under controlled subsurface drainage (CSSD) compared with an adjacent field under conventional free subsurface drainage (FSSD). The purpose of this paper is to compare approaches for statistical analysis of unreplicated paired design drainage experiments. The methods tested include the paired watershed design, a before-after control-impact paired (BACIP) design, time-series, and analysis using a beta and a gamma distribution. The assumptions used in the paired watershed and BACIP designs require the data to follow a normal distribution with identically and independently distributed errors, while the beta and gamma distributions do not require the assumption of normality. Analysis of the data set using a normal distribution was found to be inappropriate due to the nature of the data set which violates the assumptions required for analysis of variance; whereas, the analysis of the data using a beta distribution was found to produce the most appropriate results. The time series approach mimicked the physical treatments applied to FSSD and CSSD fields and led to a significant effect of CSSD compared to FSSD. Future work should focus on confirming that the beta distribution or time series are the best approaches for statistical analysis of field scale unreplicated paired design drainage experiments.

Experimental study of the temperature effect on two-phase flow properties in highly permeable porous media: Application to the remediation of dense non-aqueous phase liquids (DNAPLs) in polluted soil

The remediation of aquifers contaminated by viscous dense non-aqueous phase liquids (DNAPLs) is a challenging problem. Coal tars are the most abundant persistent DNAPLs due to their high viscosity and complexity. Pumping processes leave considerable volume fractions of DNAPLs in the soil and demand high operational costs to reach cleaning objectives. Thermally enhanced recovery focuses on decreasing DNAPL viscosity to reduce residual saturation. The oil industry has previously applied this technique with great success for enhanced oil recovery applications. However, in soil remediation, high porous media permeabilities and product densities may invalidate those techniques. Additionally, the impacts of temperature on coal tar's physical properties have not been thoroughly discussed in available literature. Here, we investigated how coal tar's physical properties, the capillary pressure-saturation curve and the relative permeability of two-phase flow in porous media depend on the temperature and flow rate experimentally. Drainage and imbibition experiments under quasi-static (steady-state) and dynamic (unsteady-state) conditions have been carried out at 293.15 K and 323.15 K in a 1D small cell filled with 1 mm homogeneous glass beads. Two different pairs of immiscible fluids have been investigated, coal tar-water and canola oil-ethanol. Results demonstrated similar trends for temperature effect and values of fluid properties for both liquid pairs, which backs up the use of canola oil-ethanol to model coal tar-water flow. It was found that there is no temperature effect on drainage-imbibition curves or residual saturation under quasi-static conditions. In dynamic conditions, the DNAPL residual saturation decreased by 16 % when the temperature changed from 293.15 K to 323.15 K. This drop was mainly linked to decreasing viscous fingering, as well as the appearance of wetting phase films around the glass beads. Both phenomena have been observed only in dynamic experiments. A high enough pumping flow rate is needed to generate dynamic effects in the porous medium. Ethanol and oil's relative permeabilities also increase with temperature under dynamic measurement conditions. Our findings indicate that flow rate is an important parameter to consider in thermal enhanced recovery processes. These effects are not taken into account in the classically used generalized Darcy's law for modeling two-phase flow in porous media with temperature variation.

The Effect of Inclination on the Stability of Foam Systems in Drilling and Well Operations

SummaryTo minimize fluid loss and the associated formation damage, foam is a preferred fluid to perform cleanout operations and reestablish communication with an open completion interval. Because of their high viscosity and structure, foams are suitable cleanout fluids when underbalanced well-cleanout operations are applied. Although several studies have been conducted to better understand foam-flow behavior and hydraulics, investigations performed on foam stability are very limited. Specifically, very little is known regarding the impact of wellbore inclination on the stability of foams. Unstable foams do not possess high viscosity, and as a result, they are not effective in cleanout operations, especially in inclined wellbores. Predicting the downhole instability of foam could reduce the number of drilling problems associated with excessive liquid drainage, such as temporary overbalance, formation damage, and wellbore instability. The objectives of this study are to investigate the effects of wellbore inclination on the stability of various types of foams and develop a method to account for the effect of inclination on foam stability in inclined wells.In this study, foam-drainage experiments were performed using a flow loop that consists of a foam-drainage-measurement section and pipe viscometers. To verify proper foam generation, foam viscosity was measured using pipe viscometers and compared with previous measurements. Drainage experiments were performed with aqueous, polymer-based, and oil-based foams in concentric annulus and pipe under pressurized conditions. Tests were also conducted in vertical and inclined orientations to examine the effect of wellbore inclination on the stability of foams. The foam-bubble structure was examined and monitored in real time using a microscopic camera to study bubble coarsening. The foam quality (i.e., gas volume fraction) was varied from 40 to 80%.Results show that the drainage rates in the pipe and annular section were approximately the same, indicating a minor effect of column geometry. More importantly, the drainage rate of foam in an inclined configuration was significantly higher than that observed in a vertical orientation. The inclination exacerbated foam drainage and instability substantially. The mechanisms of foam drainage are different in an inclined configuration. In inclined wellbores, drainage occurs not only axially but also laterally. As a result, the drained liquid quickly reaches a wellbore wall before reaching the bottom of foam column. Then, a layer of liquid forms on the low side of the wellbore. The liquid layer flows downward because of gravity and reaches the bottom of the test section without facing the major hydraulic resistance of the foam network. This phenomenon aggravates the drainage process considerably.Although foam-drainage experiments have been reported in the literature, there exists only limited information on the effects of geometry and inclination on foam drainage and stability. The information provided in this paper will help to account for the effect of inclination on foam stability and subsequently improve the performance of oilfield operations involving foam as the working fluid.

Experimental Study on the Gas Flow Characteristics and Pressure Relief Gas Drainage Effect under Different Unloading Stress Paths

Coal seam gas is a critical substance because it can be a source of a large quantity of clean energy as well as a dangerous source of risk. A pressure relief gas drainage is an effective and widely used method for coal seam gas recovery and gas disaster control in coal mines. A series of pressure relief gas drainage experiments were conducted using large-scale coal samples under different unloading stress paths in this study to explore the unloading stress paths. From the experimental results, the dynamic evolutions of gas pressure, coal temperature, and gas production were analyzed. The trends of gas pressure and coal temperature during pressure relief gas drainage were similar: dropping rapidly first and then slowly with time. Correspondingly, gas production was fast in the early stage of pressure relief gas drainage and became stable thereafter. Meanwhile, gas flow characteristics were significantly affected by the unloading stress paths. Gas pressure and coal temperature had the maximum descent by unloading stress in three directions simultaneously, and the unloading stress of the Z direction had the minimal impact when only unloading in one direction of stress. However, the influence of unloading stress paths on gas production was complex and time dependent. The difference coefficient parameter was proposed to characterize the influence degree of unloading stress paths on the pressure relief gas drainage effect. Eventually, the selection of unloading stress path under different situations was discussed based on time, which is expected to provide the basis for pressure relief gas drainage.

Contrasting responses of soil exoenzymatic interactions and the dissociated carbon transformation to short- and long-term drainage in a minerotrophic peatland

Lowering of water table owing to climate warming and drainage has threatened the large reservoir of the preserved carbon (C) in peatlands. However, the consequent effects on northern nutrient-deficient peatlands are still under-investigated. Here, we compared a short- (42 days) with a long-term (30 years) drainage experiment, and quantified the activities of the decomposition-related extracellular enzymes as well as C, nitrogen (N) and phosphorus (P) stoichiometries in a minerotrophic peatland in northeastern China. The results showed that the key hydrolytic enzymatic activities increased by 2–145% and 25–465% under short- and long-term drainage respectively, but the preserved nutrients and C in the peat did not vary considerably. The minerotrophic peatland seems to be a complex adaptive system with great resilience to tackle the short- and long-term drainage with two disparate mechanisms. Specifically, the intrinsic nature of severe nutrient-deficiency may weaken enzymatic constraints on decomposition (the ‘enzymatic latch’ mechanism) during the short-term drainage. In contrast, the increases in phenolics input and litter quality due to the shift in vegetation composition may alleviate the threat to C stores during the long-term drainage. These distinct regulatory mechanisms over the course of progressive lowering of water table may potentially provide new insights to assess the risk and benefit of drainage in peatlands.

Multiphase and multicomponent simulation of acid mine drainage in unsaturated mine waste: Modeling approach, benchmarks and application examples

Acid mine drainage (AMD), caused by weathering of sulfide-rich mine wastes, is arguably the most critical environmental issue in the mining sector. Despite the fact that individual mechanisms responsible for the generation/propagation of AMD are fairly well-known, the prediction of overall dynamics in a mine waste system is challenging due to the nonlinearity and multilevel coupling associated with the key physico-chemical processes. This work presents a new three-phase multicomponent reactive transport model for AMD simulations, taking into account variably saturated water flow, multicomponent advective-dispersive transport both in aqueous and gaseous phase, and kinetic and equilibrium chemical reactions. The model is capable to capture the species-species coupling due to solute/surface charge or gas phase pressure by solving Nernst-Planck equation for the aqueous transport, and Maxwell-Stefan equation for the gaseous transport. Furthermore, the unsaturated flow and transport code is coupled with PHREEQC, utilizing the PhreeqcRM module, to simulate a wide range of geochemical reactions. The capabilities of the code are benchmarked against other software, previously published laboratory data, and pilot-scale drainage experiments in mica schist containing waste rocks performed in this study. The results are analyzed in terms of spatial and temporal profiles. The simulations reveal that the proposed model, called AMD-PHREEQC, is capable of accurately reproducing the experimental results, and thus it can be effectively used in environmental risk assessment, long-term drainage quality predictions, and data interpretations in mine waste.

Estimation and mapping of field capacity in Brazilian soils

Modeling soil water dynamics is important to support decision in planning and management policies. Field capacity (FC) is an important parameter in this context and its proper determination allows the use of simple models to assess soil water balance components. In this study we aimed estimation and mapping of FC in Brazilian soils based on available soil global data and pedotransfer functions (PTF). Soil basic data were obtained from the SoilGrids system in a 0.05 × 0.05° spatial grid covering the Brazilian territory, with a resolution of approximately 5 km. Soil saturated hydraulic conductivity (Ks) and water retention curve (SWRC) were estimated by PTFs and used as input in the agro-hydrological model SWAP to simulate an internal drainage experiment without evapotranspiration, until the drainage at the bottom profile reached negligible flux density values (qfc) of: 0.25, 1.0 and 4.0 mm d−1, considered to correspond to FC. Simulations were performed considering three soil depths (zfc) of 30, 60 and 100 cm. FC was classified according to major soil textural and pedological classes. Uncertainty was evaluated by the sensitivity of the model to the soil hydraulic properties and the grid cell size. The highest soil matric potential at FC (hfc) was observed for loam texture while the lowest hfc was observed for sandy loam texture. Between soil pedological classes, Gleysols presented the lowest hfc. The results showed that FC estimation is sensitive to zfc and qfc. Spatial agreement between patterns of Ks, FC and soil class were observed according to the evaluated database. The qfc value of 4.0 mm d−1 presented reasonable results for estimating FC in Brazilian soils, in agreement with expected drainage time following saturation, with average values below 4 days for the evaluated depths and hfc around −30 cm. Estimation of FC with PTF showed underestimation of hfc varying from double to almost fourfolds times. The usually adopted hfc of −330 cm does not represent the FC for Brazilian soils. Furthermore, a qfc lower than 1.0 mm d−1 did not yield usable values for FC.