Areal Heterogeneity Research Articles

Study on The Areal Sweep Efficiency and Water Breakthrough Time Prediction of Water Flooding in Carbonate Reservoirs

Abstract This study concentrates on the evaluation of areal sweep efficiency and the prediction of water breakthrough time in a representative medium-porosity, medium-high permeability thin carbonate reservoir, using the H oilfield in the Middle East as an example. Areal geological models, accounting for variations in high permeability streaks and regional heterogeneity, were established to study the impact of areal heterogeneity on the sweep coefficient and morphology of the water flooding front. The results revealed that areal heterogeneity significantly affects both the areal sweep efficiency and the morphology of the water flooding front. In models with high permeability streaks, a greater permeability ratio led to more uneven displacement in all directions, consequently, this resulted in a poorer overall development effect. Furthermore, regional heterogeneity was found to significantly impact sweep efficiency, the larger the area of relatively low permeability zones, the more severe the reduction in sweep efficiency and the overall injection production effect. A theoretical method for accurately predicting water breakthrough time was established based on the stream tube method. This method was applied to the H oilfield, which reduced the discrepancy between the predicted and actual breakthrough times, thereby confirming its feasibility. The novelty of this study lies in overcoming the limitations of the stream tube method and utilizing numerical simulation to calculate the areal sweep coefficient at the time of water breakthrough. This approach quantitatively characterized the affected streamline area, thereby enhancing the accuracy of water breakthrough time predictions. This research provides significant theoretical insights and practical references for understanding the areal sweep efficiency of thin carbonate reservoirs, predicting water breakthrough times, and optimizing injection production strategies.

Technology Focus: EOR Operations (November 2022)

Among the different enhanced-oil-recover techniques, waterflooding is still commonly practiced in part because of its economic potential. During the past 2 years, many papers presented at SPE meetings have dealt with different aspects of waterflooding design, optimization, monitoring, and low-salinity applications. Understanding the injected-water preferential paths is a key aspect of waterflood optimization in reservoirs characterized by strong vertical and areal heterogeneities. Devising specific work flows for such applications is important. These can be easy but powerful tools for visualizing the complex dynamic connections between injectors/producers and aquifer influence areas. They can enable improving the business-time decision-making cycle, resulting in increased operational performance and lower waterflood operating costs by consolidating end-to-end optimization work flows in one platform. Using artificial intelligence (AI) and machine-learning (ML) approaches with reduced-physics models can reduce the time required for this task. Also, ML allows for fast screening of waterflood performance at diverse levels (e.g., reservoir, sector, pattern, and well), enabling prompt identification of opportunities for immediate uptake into an opportunity-management process and for evaluation in AI-driven production forecasting or in a reservoir simulator. As part of waterflood optimization, changing the chemistry of the injected water has been under investigation since early 2000. This process has been named differently in different laboratories (e.g., smart water and low-salinity waterflooding, or LoSal). The new studies are related to its field application. It is interesting that new hybrid applications such as low-salinity water with surfactant flooding and carbonated smart water injection are showing up in the literature. As we move forward, it is expected that the waterflooding recovery factor will increase with additional technology advancements. Recommended additional reading at OnePetro: www.onepetro.org SPE 209426 - Impact of Brine Chemistry on Waterflood Oil Recovery: Experimental Evaluation and Recovery Mechanisms by Behdad Aminzadeh, Chevron, et al. SPE 205426 - Pump Up the Volume—Massive Water-Injection Increase Through Open-Water Stimulations by Alistair Roy, BP, et al. SPE 210154 - Middle East Giant Carbonate Field: Integrated Water-Injection-Optimization Work Flow by Stefano Del Fraro, Eni, et al.

Micro- to nano-scale areal heterogeneity in pore structure and mineral compositions of a sub-decimeter-sized Eagle Ford Shale

Mineral and organic matter compositions & pore structures of fine-grained shale influence reservoir properties. To improve the understanding of spatial heterogeneity in core-sized samples, methods of microscale X-ray fluorescence (μXRF) mapping, (ultra-) small-angle x-ray scattering [(U)SAXS] and wide-angle X-ray scattering (WAXS) have been used to determine elemental, pore-structure variations at scales up to ~10 cm on two samples prepared at circular and rectangular orientations from a piece of Eagle Ford Shale outcrop in South Texas, USA. In addition, thin section petrography and field emission-scanning electron microscopy observations, X-ray diffraction (XRD), total organic carbon, and pyrolysis were utilized to investigate the potential spatial heterogeneity of pore types, mineral and organic matter compositions for cm-sized samples at both orientations. Overall, the siliceous‑carbonate mineral contents in these two samples (8 cm × 8 cm × 0.08 cm and 5 cm × 8 cm × 0.08 cm, in terms of width×length×thickness) of carbonate-rich Eagle Ford Shale vary between laminations at mm scales. For the circular sample, porosity and specific surface area (SSA) variations range from 0.82 to 3.04% and 1.51 to 14.1 m2/g, respectively. For the rectangular sample, values for porosity and SSA vary from 0.93 to 2.50% and 3.95 to 10.8 m2/g. By analyzing six selected sub-samples on each of two samples with (U)SAXS and XRD techniques, nm-sized pores are mainly interparticle ones in the higher calcite regions, where the porosity is also relatively lower, while the lower calcite regions consist of both interparticle and intraparticle pore types with higher porosity. Finally, the μXRF and (U)SAXS mappings are combined to generate porosity distribution maps to provide more insights about sample heterogeneity related to the laminations and fractures at our observational scales.

Multi-fracture nonuniform initiation and vertical propagation behavior in thin interbedded tight sandstone: An experimental study

Thin interbedded tight sandstone of Shanxi Formation in Ordos basin, China, contains mudstone interlayers and laminas universally, and exhibits ultra-low matrix permeability, low brittleness, and areal heterogeneity. The stimulation effectiveness of this formation depends largely on whether multiple hydraulic fractures (HFs) can initiate and penetrate into the multiple thin sandstone layers upward and downward. To clarify the multi-fracture growth behavior of such formation, multi-cluster fracturing experiment in the horizontal well was performed on specimens prepared from the sandstone-mudstone outcrops of Shanxi Formation based on a true triaxial fracturing simulation system. The influences of interlayer stress difference, mudstone thickness, fluid viscosity, pumping rate and the number of clusters were mainly analyzed through post-fracturing specimen splitting and pressure curve analysis. Results show that the difference of rock mechanical parameters between sandstone and mudstone is relatively large. When interlayer stress difference is less than 3 MPa, the mudstone layer cannot function as a barrier to restrain the vertical growth of HFs, and the essential mechanism causing HF height containment is mainly the opening of laminas or/and interfaces. By contrast, when the interlayer stress difference is equal to or greater than 3 MPa and the dimensionless thickness of mudstone is more than 0.4, the HF height tends to be restrained. Increasing the fluid viscosity or/and pumping rate can reduce filtration, and facilitates HFs penetrating through the laminas or interfaces and consequently propagating into adjacent layers. It has been observed that when fluid viscosity reaches 100 mPa s (cross-linked gel) and pumping rate is 100 mL/min, HFs can pass through the overlaying mudstones. However, uniform initiation of all clusters is difficult to achieve due to the low brittleness of sandstone and heterogeneity along the horizontal wellbore. The experimental results prove that temporary plugging within the wellbore using the plugging agent of appropriate particle size is necessary to create multiple HFs in such formation.

Micro- to Nano-Scale Areal Heterogeneity in Pore Structure and Mineral Compositions of a Sub-Decimeter-Sized Eagle Ford Shale

Micro- to Nano-Scale Areal Heterogeneity in Pore Structure and Mineral Compositions of a Sub-Decimeter-Sized Eagle Ford Shale

Crustal evolution of the Paleoproterozoic Ubendian Belt (SW Tanzania) western margin: A Central African Shield amalgamation tale

The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8 Ga, forming the Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U–Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85 Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89–1.85 Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0 Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570 Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate–Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.

Investigating low salinity waterflooding via glass micromodels with triangular pore-throat architectures

Glass micromodels have been extensively used to simulate and investigate crude oil, brine, and surface interactions due to their homogeneous wettability, rigidity, and ability to precisely capture a reservoir’s areal heterogeneity. Most micromodels are fabricated via two-dimensional patterning, implying that feature depths are constant despite varying width, which sub-optimally describes a three-dimensional porous architecture. We have successfully fabricated micromodels with arbitrary triangular cross sections via femtosecond pulsed laser direct writing resulting in depth-dependent channel width. As such, we have achieved arbitrary geometric control over device fabrication and thus a more accurate recapitulation of a geological porous media. With this fabrication technique, we are now able to directly observe pore-level, depth-dependent multiphase flow phenomena. This platform was used to study the low salinity effect (LSE) by simulating waterflooding processes using various brine solutions that differ in cation type and salinity. Patterned pore-throat structures were created to investigate displacement behavior during waterflooding. Real-time monitoring of the displacement processes, combined with a comparison of the brine chemistry before and after waterflooding provides an insight into realistic interactions occurring between crude oil and brine. The results indicate that produced emulsions were prone to coalesce in the presence of lower salinity brine. Combined with previous work, the LSE was interpreted as favored coalescence and resisted breakup that resulting in a more continuous aqueous phase during waterflooding therefore improving the displacement efficiency.

Research on integrative multi-media modelling and simulation method for tight formation horizontal well development

Tight formations usually have large area of coverage. There is large areal heterogeneity for reservoir petrophysics, rock facies, formation thickness, physical properties and oil saturations, in different reservoir blocks, in area between different wells, and also in different segments of a single well. Tight reservoirs have different scales of natural fracture development. Hydraulic fracturing also forms different scales of complex hydraulic fracture network, which has large macroscopic heterogeneity with prominent multi-scale features. Unconventional tight reservoirs consist of different scales of porous media from nano-sized to nano-micron to micron-sized. There is large difference for compositions and quantity distributions in pore spaces with different scales. Different scales of pores present discrete discontinuous distribution in space, with large heterogeneity in spatial distribution. There is large difference for geometric parameters and physical properties in different scales of porous media. Different scales of pores present large microscopic heterogeneity and multiple porous media features. All of these features make conventional reservoir modelling and numerical simulation theory and technology not applicable in this scenario. In this paper, a new multimedia modelling and simulation method for volume fracturing of horizontal wells in tight reservoirs is introduced in detail by using the unconventional reservoir modelling software Untog, taking a single well as an example.

Understanding the Conductive Carbon Additive on Electrode/Electrolyte Interface Formation in Lithium-Ion Batteries via in situ Scanning Electrochemical Microscopy.

The role of conductive carbon additive on the electrode/electrolyte interface formation mechanism was examined in the low-potential (3.0–0 V) and high-potential (3.0–4.7 V) regions. Here the most commonly used conductive carbon Super P was used to prepared electrode with polyvinylidene fluoride binder without any active material. The dynamic process of interface formation was observed with in situ Scanning Electrochemical Microscopy. The electronically insulating electrode/electrolyte passivation layer with areal heterogeneity was formed after cycles in both potential regions. The low-potential interface layer is mainly composed of inorganic compounds covering the conductive carbon surface; While the electrode after high-potential sweep tends to lose its original carbon structure and has more organic species formed on its surface.

Mechanisms and prevention & control countermeasures of water breakthrough in horizontal wells in multi-layer unconsolidated sandstone gas reservoirs: A case study of the Tainan Gas Field in the Qaidam Basin

Water breakthrough in horizontal wells is now the main factor restricting the stable production of the Tainan Gas Field in the Qaidam Basin. In view of this problem, the distribution characteristics of irreducible water saturation were investigated by using the nuclear magnetic resonance logging interpretation technology. And combined with the production situations of the horizontal wells in Tainan Gas Field in the initial stage of their production, the characteristic parameters of the reservoir which produced the intrastratal water as soon as it was put into production were determined. Then, the factors influencing the production of irreducible water were studied by means of gas drive water core experiments, and the factors influencing the sealing ability of the interbeds were researched by conducting mudstone breakdown tests. What's more, the effects on the bottomhole pressure by the length of horizontal section at different daily gas productions were investigated through numerical simulation. Finally, the prevention & control countermeasures for water breakthrough in horizontal wells were proposed. And the following research results were obtained. First, the horizontal well which drills into the reservoir with mobile water saturation higher than 7.2% and gas saturation lower than 63.5% produces formation water in its initial stage of production. Second, the lower the shale content is and the greater the production pressure difference is, the more favorable it is for the production of irreducible water. The production of irreducible water in the reservoirs with stronger areal heterogeneity is a continuous process. Third, the sealing capacity of the interbed increases with the decrease of its vertical permeability and water saturation and with the increase of its shale content and thickness. Fourth, the breakthrough pressure of type I mudstone (shale content > 90%) is about 4 MPa, that of type II mudstone (80% < shale content < 90%) is about 2 MPa, and that of type III mudstone (60% < shale content < 80%) is about 1.5 MPa. In conclusion, the edge water encroachment can be delayed and the water-free gas production period of gas well can be extended by increasing the length of horizontal section and controlling the production pressure difference. It is recommended to carry out balanced gas production in horizontal wells in the initial stage of production, enhance dynamic monitoring in the process of production, and drain the water in time in the case of water breakthrough so as to increase its cumulative gas production.

Nitrogen gas channeling characteristics in fracture-vuggy carbonate reservoirs

Tahe Oilfield, located in Tarim Basin, Northwest China, is a typical carbonate fracture-vuggy reservoir. Nitrogen gas flooding is a vital technology for improving oil recovery by enlarging sweep volume in fracture-vuggy reservoirs. However, gas channeling restricts further application of nitrogen gas flooding. This paper studied the characteristics of gas channeling in fracture-vuggy carbonate reservoirs using physical simulation. Visual physical models and macroscopic physical models were designed and fabricated based on similarity criterion. Nitrogen gas flooding experiments were conducted in both types of models. Experimental results demonstrated that once the dominant channel of gas is formed, the remaining oil cannot be effectively swept. The greater the areal heterogeneity of fracture-vuggy carbonate reservoir is, the easier the formation of gas channeling is. Gas channeling was accompanied by a large amount of oil and water production. The initial gas channeling was followed by several additional occurrences of gas channeling. In addition, PIR risk assessment method was proposed to predict gas channeling by assigning values to each coefficient of P, I and R. This method verified by oilfield data can be used to prevent and evaluate the risk of gas channeling in fracture-vuggy carbonate reservoirs.

AGROECOLOGICAL LAND TYPING AS A NECESSARY STEP IN DESIGNING LANDSCAPE FARMING SYSTEMS

The paper highlights the data on agroecological assessment and land typing on the example of farm “Kremlin” located in Kochenevo district of Novosibirsk region. This method aims at solving two problems. The first problem deals with typing and it is carried out on the basis of the associated analysis of triunity which implies environmental conditions of the area, agricultural crops, and agricultural technology; the second problem deals with topology: the boundaries of spatial plots of land types coincide with the boundaries of geochemical positions, which are characterized by external features of the relief elements and soil contours. Agroecological land typing includes grouping of lands and their mapping. The criterion for land grouping is seen in similarity of factors that restrict certain crop yields and application of certain technological methods. The authors suggest to use morphometric parameters of the soil mantle for identifying land types: average number of permeable intervals in the section of soil areal, contrast ratio, complexity coefficient and soil cover heterogeneity coefficient. A quantitative parameter of agroecological conditions similarity within the plot is lower contrast ratio. The efficiency of this method is confirmed by economic parameters. Digital mapping and quantitative analysis of morphometric soil mantle parameters was carried out by means of MapInfo Professional software.

Terra-mare comparison of small young craters on the Moon

Several characteristics of very small impact craters of lunar surface have recently been improved by high resolution datasets. Lunar impact craters with diameter below 100 m and especially below 10 m were surveyed and compared at mare and terra surfaces in this work, altogether at 0.6 km2 area with 4866 craters there. The surface density values (of all craters/km2) at the mare regions were 1.5–2.0 times larger than at the terra regions, and this difference came mainly from <10 m diameter craters. Although the crater retention and stability might have been a reason for such a difference causing shorter crater lifetime on steeper terra units, similar spatial density was found at sloping and nearly horizontal parts of the terra area, and no signature of partly destroyed craters was observed there, leaving the resolution of the difference open. The difference in the properties of target material is a relevant candidate reason for it, however based on shadow length measurements no depth difference was identified between terra and mare craters.There is an ongoing debate on the reliability and usage of small craters for age estimation (Xiao and Strom, 2012), this work provides some further information to evaluate this aspect. No correlation was found between the areal density values of craters and ages determined from size-frequency distributions, confirming (using examples of smaller craters than analysed in most earlier works), that large sized craters dominate the age estimation despite the larger number of <10 m craters. Characteristic age values for <10 m diameter crater size category ranged between 65 and 100 Ma, while for <100 m craters the estimated ages were 76–250 Ma. Substantial fluctuations in age within terra and mare regions were observed in the absence of nearby secondary craters, suggesting that the role of nearby secondaries might be minor in the production of the observed areal heterogeneity. Although the role of distant secondaries could not be excluded, the results suggest natural random fluctuations should be more carefully considered in the future. Analysis of the depth of regolith layer that different size groups of craters influence, roughly the top 1–2 meter layer could be mixed during 30–60 Ma age, and this age range is close to recent estimations by other authors.

Combining preformed particle gel and low salinity waterflooding to improve conformance control in fractured reservoirs

The recovery from fractured reservoirs is usually low. The areal heterogeneity is one result of the fractured reservoir. Low salinity waterflooding (LSWF) and preformed particle gel (PPG) have recently drawn great interest from the oil industry. LSWF can only increase displacement efficiency, and it has little or no effect on sweep efficiency whereas PPG can plug fractures and improve sweep efficiency, but they have little effect on displacement efficiency. The coupled method bypasses the limitations of each method when used individually and improves both displacement and sweep efficiency.The main objective of this study was to determine whether the combining technologies can improve conformance control in fractured sandstone reservoirs. Before the study was conducted, the effects of low salinity waterflooding, number of fractures, and PPG strength were studied. The PPG was injected into the fracture at a flow rate 2.0 ml/min. Brine was injected at a different flow rate after PPG placement to test the effect of flow rate on the PPG’s plugging efficiency. Laboratory experiments showed that the oil recovery factor and the Frrw increased when the concentration of injected brine changed from conventional salinity to low salinity and the areal sweep efficiency was improved. However, the PPG extruded pressure decreased when the PPG swelled in a low-brine concentration. At a high-flow rate, there was no significant effect on the Frrw. Combining two different EOR technologies can improve displacement and sweep efficiency and, in turn, enhance conformance control.

Performance improvement of CO2 flooding using production controls in 3D areal heterogeneous models: Experimental and numerical simulations

CO2 flooding is a promising technology used to enhance oil recovery as it increases oil production and sequesters CO2 in low permeability oil reservoirs. However, the areal heterogeneity of a reservoir can seriously affect CO2 flooding efficiency. The importance of production control treatments (sweeping area regulation, differential production pressure control, and real-time producing regulation) are thus proposed to improve the performance of CO2 flooding in areal heterogeneous reservoirs. Three dimensional (3D) physical models and core-scale numerical models with areal heterogeneity and 5-spot patterns are designed using dimensionless scaling groups. Conventional CO2 flooding and CO2 production control flooding are conducted successively in the 3D models, and flooding efficiency is evaluated through oil recovery increments and changes in the displacement pressure drop curves. Mechanisms of CO2 performance improvement using production control treatments are also discussed through the analytical analysis of gas distribution and gas/oil mobility ratio.3D experimental and numerical simulation results show that CO2 is displaced unevenly in the areal heterogeneous reservoir, leaving plenty of oil in the area of relatively low permeability. However, oil recovery of CO2 flooding in the areal heterogeneous reservoir can be doubled using production control treatments (sweeping area regulation, differential pressure control, or real-time producing regulation). The oil remaining in the area of relatively low permeability can be effectively displaced using sweeping area regulation, and both a larger sweeping area and a better CO2 flooding profile can be obtained using differential pressure control and real-time producing regulation. Through the analytical analysis of gas distribution and gas/oil mobility ratio, it is found that gas/oil mobility ratio is more sensitive to the areal heterogeneity, and real-time producing regulation has a powerful mobility control ability for CO2-EOR. Improvements in the performance of CO2 flooding using production control treatments provide a feasible technical strategy to treat with the areal heterogeneity and enhance the oil recovery for 5-spot well patterns in the areal heterogeneous reservoir.

Reserves Augmentation by Designing an Optimum Waterflood Pattern with Black Oil Simulator

In petroleum production system, reservoir pressure is considered to be main source of hydrocarbon production from reservoir to the surface. With passage of producing time; fluids can only be lifted at the economic rates from subsurface to the surface by some secondary recovery method which sweeps remaining oil from the reservoir to improve its overall recovery. Waterflooding is the dominant fluid injection technique and is frequently applied worldwide secondary recovery process, which involves water injection in the oil formation under high pressure through an injection well to enhance oil recovery of the well(s) of interest. Selection of optimum number of wells and their optimum location is a whip hand to plan and implement a successful waterflooding operation on a depleted reservoir to prevent the wastage of substantial capital investment. This involves efficacious and judicious selection of waterflooding pattern to augment the reserves. This study emphasizes on importance and effect of efficiently selecting an optimum waterflood pattern for primary production depleted reservoir “W” by simulating its performance for regular 5-spot & 9-spot patterns to acquire, best technical & economic match for subject reservoir for a particular injectivity, reservoir areal heterogeneity, direction of formation fractures, existing production wells and their spacing etc. Where different opportunities involving a particular measurement or calculation are involved, there is no substitute for thinking out the best solution to the problem. The mistakes should be made on paper where an eraser can remove them, not in the field where someone must live with it.

Impact of seasonal variations in hydrological stresses and spatial variations in geologic conditions on a TCE plume at an industrial complex in Wonju, Korea

AbstractSpatial and temporal variations in a trichloroethylene (TCE) plume at an industrial complex in Wonju, Korea, were examined based on hydrogeological data and seven rounds of groundwater quality data collected over a year. The site has considerable vertical heterogeneities; the top layer of soil is covered by impermeable paving material at several locations, followed by a series of reclaimed or residual soil layers, and with weathered rocks to the crystalline biotite granite at the bottom. Areal heterogeneity in the surface conditions plays an important role in controlling groundwater recharge. The heterogeneity structure is influenced by complex surface conditions paved with asphalt and concrete. Owing to the presence of limited recharge area and concentrated summer precipitation events, the effects of seasonal variations on groundwater hydraulics tend to diminish with distance from the recharge area. This result was established by analysing the influence of the contrasting surface recharge conditions between the near‐source zone and the far zone, and the seasonally concentrated precipitation on the transport patterns of a TCE plume. In addition, variations in the plume's downstream contaminant flux levels were also analysed along a transect line near the source zone. The results show that the general tendency of the TCE plume contaminant concentration and mass discharges were reproducible if we account for seasonal recharge variations and the associated changes in the groundwater level. During recharge events, the TCE concentration variations appear to be influenced by leaching of the residual dense non‐aqueous‐phase liquid (DNAPL) TCE trapped in the unsaturated zone. This result shows that seasonal variations in contaminant plume near the source zone is inevitable at this site, and that these variations indicate the presence of residual DNAPL at or above the water table, at least in some isolated locations. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Monte Carlo analysis of field water flow comparing uni- and bimodal effective hydraulic parameters for structured soil

Soil structure critically affects the hydrological behaviour of soils. In this paper, we examined the impact of areal heterogeneity of hydraulic properties of a structured soil on soil ensemble behaviour for various soil water flow processes with different top boundary conditions (redistribution and drainage plus evaporation and infiltration). Using a numerical solution of the Richards' equation in a stochastic framework, the ensemble characteristics and flow dynamics were studied for drying and wetting processes observed during a time interval of ten days when a series of relatively intense rainfall events occurred. The effects of using unimodal and bimodal interpretative models of hydraulic properties on the ensemble hydrological behaviour of the soil were illustrated by comparing predictions to mean water contents measured over time in several sites at field scale. Although the differences between unimodal and bimodal fitting are not significant in terms of goodness of fit, the differences in process predictions are considerable with the bimodal soil simulating water content measurements much better than unimodal soil. We also investigated the relative contribution of the soil variability of each parameter on the variance of the water contents obtained as the main output of the stochastic simulations. The variability of the structural parameter, weighting the two pore space fractions in the bimodal interpretative model, has the largest contribution to water content variance. The contribution of each parameter depends only partly on the coefficient of variation, much more on the sensitivity of the model to the parameters and on the flow process being observed. We observed that the contribution of the retention parameters to uncertainty increases during drainage processes; the opposite occurs with the hydraulic conductivity parameters.

Runoff from heterogeneous small bare catchments during soil surface sealing

The combined effects of areal heterogeneity of the soil hydraulic properties and the surface seal formation dominate the hydrological response of arid and semiarid water catchments. Here these two phenomena were simulated to study their mutual role in runoff generation in small bare catchments. Seal formation during rainfall was simulated applying the dynamic model of Assouline and Mualem (1997). Areal heterogeneity of the soil was represented by a lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil, Ks, and by related distributions of the other soil parameters. The runoff hydrograph, at the outlet of a hypothetical bare catchment of 0.5 km2, was calculated using the cell model of Diskin et al. (1984). Two water catchments types, homogeneous and heterogeneous, with two different soil surface states, unsealed (mulched) and ongoing dynamic sealing, were considered. Four spatial organizations of the 10 cells in the heterogeneous catchment, all representing the same discrete distribution of Ks, were studied. Rainfall events of two rainfall intensities, 20 and 40 mm h−1, of different durations ranging from 10 to 120 min, were applied uniformly over the catchments to study the effect of rainfall intensity and duration on runoff characteristics under the different soils and catchments conditions. The state of the soil surface seal was found to be a dominant factor with regard to runoff generation. Relative to the runoff produced in the homogeneous unsealed catchment under a (40 mm h−1, 45 min) rainfall, the runoff was augmented by a factor of 10 during soil surface sealing and still more, by a factor of 20, when the soil surface was already sealed. On a relative basis the impact of soil sealing on runoff is much more important than that of soil heterogeneity. The effect of areal heterogeneity on runoff seems to depend on both the soil surface condition and the rainfall intensity and duration. When the soil is unsealed, the total runoff and the discharge peak are higher for the heterogeneous catchment. When the soil surface undergoes a sealing process, the total runoff and the discharge peak are higher for the heterogeneous catchment for the lower rainfall intensity of 20 mm h−1. For the 40 mm h−1 rainfall the catchment relative response was found to depend on the rainfall duration. A higher peak was obtained in the homogeneous catchment for rainfall durations above 60 min, and more runoff was produced for rainfall durations above 90 min. The spatial pattern of the cell organization in the heterogeneous catchment is an additional factor affecting the hydrological response. The hydrographs corresponding to each of four patterns representing the same areal heterogeneity displayed differences regarding concentration time (which varied between 7 and 28 min), timing of the peak runoff (varying between 77 and 146 min), and the peak discharge (varying between 0.41 and 0.57 m3 s−1).

Estimating snow depletion curves for American River basins using distributed snow modeling

We used a well evaluated temperature-based distributed energy balance model for snow accumulation and ablation with a 1 km × 1 km resolution to derive estimates of snow water equivalent (SWE) and snow cover area (SCA) for the American River basin during the water years 1960–1999. These model prognostic variables were then used to derive and analyze the properties of the snow depletion curves (SDC) of the drainage basins of the three Forks of the American River. Such curves are commonly used in spatially lumped model applications as a priori estimates that account for the areal heterogeneities of the snow pack. Assuming that SDC can be derived for a basin and the annual variability is small, a single SDC offers an adequate description of the snow pack area distribution. Unique model-derived SDC were obtained for each Fork but with large inter-annual variability. The curve variability introduces large uncertainty in the prediction of snowmelt timing and magnitude when spatially lumped models are used. Further analysis of the model derived SDC showed significant correlation between the maximum annual SWE of the basin and the SWE values for a given SCA. In addition, the maximum annual SWE of the basin is also correlated with the SWE from snow courses surveys conducted in the beginning of March. These empirical relationships provide a procedure to estimate the SDC at the beginning of the melting season and, therefore, reduce the uncertainty in snowmelt estimation associated with the use of SDC.