Field-scale Model Research Articles

Identification of distinctions of immiscible CO2 huff and puff performance in Chang-7 tight sandstone oil reservoir by applying NMR, microscope and reservoir simulation

CO 2 huff and puff (HNP) is one of the most effective methods to improve tight oil recovery after the primary depletion process. The seepage mechanisms between CO 2 and crude oil are complicated in porous media during CO 2 HNP process. Therefore, in this paper, the CO 2 HNP process of Chang-7 tight oil reservoir, Ordos Basin, China, was studied by nuclear magnetic resonance (NMR) technology, microscopic observation and numerical simulation. Experimentally, using NMR technology and microscopy methods, the distribution characteristics of residual oil during CO 2 HNP process were measured intuitively. Numerically, a group of core-scale and field-scale simulations considering molecular diffusion and asphaltene precipitation were established to further verify and elongate the experimental results. The results show that at the initial state, the crude oil in the tight core was mainly distributed in nanopores, sub-micro-nanopores and sub-micropores, where the oil content exceeded at least 73% in these pores. During CO 2 HNP process, the oil recovery was more pronounced for the 1st and 2nd rounds than for 3rd to 5th rounds. Notably, even if the cores with more nano-pores were more favorable for the 4–5th CO 2 HNP rounds, the oil molecules in nanopores were still difficult to be available. Moreover, the CO 2 sweep scope could be divided into displacement affected region and diffusion affected region. CO 2 could effectively drive the crude oil in the displacement affected region. While the oil could be successfully displaced by dissolved gas flooding in the diffusion affected region only under the appropriate conditions. Meanwhile, the core-scale numerical models confirmed that it was necessary to consider molecular diffusion and asphaltene precipitation factors, which would make the simulation results in line with the experiment. In terms of the ultimate oil recovery, the field-scale model only considering the diffusion (2.456%) > the model both considering the diffusion and asphaltene (2.436%) > the model without considering the diffusion and asphaltene deposition (2.412%) > the model only considering the asphaltene deposition (2.388%). • The oil were difficult to be available in nanopores due to the nano-constraint effect. • The CO2 displacement area was associated with diffusion and asphaltene precipitation. • SP-T2 spectrum and microscopic image were applied to obtain CO2 sweep scope. • A group of simulations were established to analyse the effect of CO2 sweep scope.

The impact of a low-permeability upper layer on transient seawater intrusion in coastal aquifers

This paper provides a thorough investigation of the effect of a top low-permeability (TLK) layer on transient saltwater intrusion dynamics prompted by water table fluctuations and sea level rise. Laboratory experiments were conducted on a 2D-sandbox and numerical simulations were performed using the SEAWAT code. Four cases were investigated, including a homogeneous case and three cases, where the top layer thickness (Wtop) was equal to 0.2H, 0.33H and 0.5H, respectively, where H was the aquifer thickness. The experimental and numerical results show that the toe length decreases linearly with increasing the thickness of the TLK layer. The results also suggest that lowering the permeability of the upper part of the aquifer causes faster saltwater removal process. The sensitivity analysis shows that decreasing the top layer permeability causes further reduction of the intrusion length. Nonetheless, the results evidence that this method yields relatively little reduction of the saline water intrusion length if the upper layer thickness is inferior or equal to a fifth of the total aquifer thickness, regardless of the permeability value of the top layer. The field-scale modelling results demonstrate that the performance of the TLK layer weakens noticeably as the hydraulic gradient decreases. The results show that the TLK layer achieved a maximum saltwater wedge reduction of 31% in the case where Wtop = 0.75H, which means that lowering the permeability of three fourths of the aquifer thickness only induced a toe length reduction by nearly a third of its original length. In addition to providing a quantitative analysis of SWI dynamics in bi-layered coastal aquifers, this study questions the performance and practicality of the artificial reduction of the upper aquifer permeability as a countermeasure for seawater intrusion control.

Permeability-controlled migration of induced seismicity to deeper depths near Venus in North Texas

Migration of seismic events to deeper depths along basement faults over time has been observed in the wastewater injection sites, which can be correlated spatially and temporally to the propagation or retardation of pressure fronts and corresponding poroelastic response to given operation history. The seismicity rate model has been suggested as a physical indicator for the potential of earthquake nucleation along faults by quantifying poroelastic response to multiple well operations. Our field-scale model indicates that migrating patterns of 2015–2018 seismicity observed near Venus, TX are likely attributed to spatio-temporal evolution of Coulomb stressing rate constrained by the fault permeability. Even after reducing injection volumes since 2015, pore pressure continues to diffuse and steady transfer of elastic energy to the deep fault zone increases stressing rate consistently that can induce more frequent earthquakes at large distance scales. Sensitivity tests with variation in fault permeability show that (1) slow diffusion along a low-permeability fault limits earthquake nucleation near the injection interval or (2) rapid relaxation of pressure buildup within a high-permeability fault, caused by reducing injection volumes, may mitigate the seismic potential promptly.

Influence of top water on SAGD steam chamber growth in heavy oil reservoirs: An experimental study

Steam-assisted gravity drainage (SAGD) is one efficient and mature technology for recovering heavy oil and bitumen resources. The key underlying mechanism is the growth of the steam chamber after injecting steam. However, due to the complex geological environment, the thief zones exist and have a prejudicial effect on the development of the steam chamber, thus impacting the ultimate heavy-oil recovery. In this work, our objective is to investigate the effect of a top-water thief zone (i.e., water zone overlies the oil sand) on SAGD performance and further to understand the crucial mechanisms that control the heat loss during steam injection. A large-scale three-dimensional experimental apparatus is used to carry out the SAGD process with a top aquifer. Based on the similarity criterion, the field-scale model is transformed into a laboratory elemental model. To evaluate the SAGD performance quantitatively, the dynamic growth of the steam chamber is measured using the thermal detectors and the production data is recorded. The results show that the steam chamber exhibits three distinguished stages, that is, upward spread, lateral extension, and downward development in the presence of top-water zone. The bottom-water zone has less impact on the steam-chamber growth. The existence of a confined top-water zone, however, significantly affects SAGD performance, especially the lateral expansion of the steam chamber. The lateral propagation of the steam front is hindered by the top thief zone due to the heat exchange with the top water. Once the steam chamber reaches the boundary, the accumulation of energy in the water thief zone, in turn, can reduce the remaining oil saturation along the topwater–oil interface. This study provides us some key insights into the development of heavy oil resources with top thief zones when implementing SAGD technology.

Effect of proppant treatment on heat extraction performance in enhanced geothermal system

Geothermal energy, a clean renewable energy, is widely used for electricity generation. Hydraulic fracturing treatment is a potential way to effectively develop the geothermal energy in deep formation. The design of the proppant pumping schedule is a key factor, which affects the permeability and the consequent heat extraction in the hot dry rock formation. In this paper, we proposed a comprehensive model to investigate the effect of proppant pumping schedule on the heat extraction. The model was validated by an analytical solution. After that, we presented a basic case to investigate the proppant distribution and heat extraction in the reservoir. Meanwhile, we designed a series of cases to investigate the effects of physical parameters in proppant pumping design on the heat extraction. Based on case studies, some useful insights into hydraulic fracturing design for the efficient development of geothermal energy in the deep formation were obtained and summarized. This paper provides a comprehensive model to simulate the proppant transport and heat extraction in the geothermal reservoir in a field scale. Meanwhile, the case studies can give clear insights about the effect of proppant pumping schedule on the heat extraction. • A field-scale model was proposed to investigate the effect of proppant pumping schedule on the heat extraction. • Comparison of heat extraction among matrix with propped-fractures, non-propped fractures, and no fractures was performed. • Effects of physical parameters in proppant pumping design on the heat extraction were investigated. • Useful insights into hydraulic fracturing design for the effective development of geothermal energy were summarized.

Achieving Near-Uniform Fluid and Proppant Placement in Multistage Fractured Horizontal Wells: A Computational Fluid Dynamics Modeling Approach

SummaryMultistage plug-and-perforate fracturing of horizontal wells has proved to be an effective method to develop unconventional reservoirs. Various studies have shown uneven fluid and proppant distributions across all perforation clusters. It is commonly believed that both fracturing fluid and proppant contribute to unconventional well performance. Achieving uniform fluid and proppant placement in all perforation clusters is an important step toward optimal stimulation. This paper discusses how to achieve such uniform placement in each fracturing stage by means of a computational fluid dynamics (CFD) modeling approach.A laboratory-scale CFD model was built and calibrated using experimental data of proppant transport through horizontal pipes available from several laboratory configurations. A field-scale model was then built and validated using perforation erosion data from downhole camera observations. With the field-scale model validated, CFD simulations were performed to evaluate the impact of key parameters on fluid and proppant placement in individual perforations and clusters. Some key parameters investigated in this study included perforation variables (orientation, size, and number), cluster variables (count and spacing), fluid properties, proppant properties, pumping rates, and stress shadow effects.Both laboratory and CFD results show that bottom-side perforations receive significantly more proppant than top-side perforations because of gravitational effects. Laboratory and CFD results also show that proppant distribution is increasingly toe-biased at higher rates. Proppant concentration along the wellbore from heel to toe varies significantly. Gravity, momentum, viscous drag, and turbulent dispersion are key factors affecting proppant transport in horizontal wellbores. This study demonstrates that near-uniform fluid and proppant placement across all clusters in each stage is achievable by optimizing perforation/cluster variables and other treatment design factors. CFD modeling plays an important role in this design-optimization process.

Multi-scale modelling of gas transport and production evaluation in shale reservoir considering crisscrossing fractures

Advancements in recent shale gas development technologies like creating hydraulic fracturing have greatly improved the production of unconventional shale gas reservoirs (USGR). However, the production of USGR is still low mainly due to the inefficient gas transport in tight shale formation. Towards this end, the investigations about the mechanism of gas transport through the fractures and the effects of both hydraulic and natural fractures on the overall gas production are urgently needed. In this study, a multi-scale modelling is established to analyze the gas transport and production performance in shale formation with crisscrossing natural fractures using finite element method (FEM). For the base case with fracture-scale model, the gas transport behavior and pressure distribution within the matrix and crisscrossing fractures are revealed, demonstrating that the crisscrossing fractures serve as the fast paths to enhance the gas transport from shale matrix to wells. Then, the field-scale model is developed by considering the complex fracture networks consisting of the hydraulically fractured horizontal wells and crisscrossing natural fractures, which is well verified against field data. On this basis, the comprehensive analysis for the influence of various parameters, including the properties of matrix, hydraulic fractures and natural fractures, on the production performance are conducted to optimize the exploitation strategy of shale gas reservoir. It is believed that the multi-scale simulation framework and the optimized exploitation strategies presented in this study are useful for the enhanced gas recovery (EGR) of shale reservoir.

Soil Greenhouse Gas Responses to Biomass Removal in the Annual and Perennial Cropping Phases of an Integrated Crop Livestock System

Diversifying agronomic production systems by combining crops and livestock (i.e., Integrated Crop Livestock systems; ICL) may help mitigate the environmental impacts of intensive single-commodity production. In addition, harvesting row-crop residues and/or perennial biomass could increase the multi-functionality of ICL systems as a potential source for second-generation bioenergy feedstock. Here, we evaluated non-CO2 soil greenhouse gas (GHG) emissions from both row-crop and perennial grass phases of a field-scale model ICL system established on marginally productive, poorly drained cropland in the western US Corn Belt. Soil emissions of nitrous oxide (N2O) and methane (CH4) were measured during the 2017–2019 growing seasons under continuous corn (Zea mays L.) and perennial grass treatments consisting of a common pasture species, ‘Newell’ smooth bromegrass (Bromus inermis L.), and two cultivars of switchgrass (Panicum virgatum L.), ‘Liberty’ and ‘Shawnee.’ In the continuous corn system, we evaluated the impact of stover removal by mechanical baling vs. livestock grazing for systems with and without winter cover crop, triticale (x Triticosecale neoblaringhemii A. Camus; hexaploid AABBRR). In perennial grasslands, we evaluated the effect of livestock grazing vs. no grazing. We found that (1) soil N2O emissions are generally higher in continuous corn systems than perennial grasslands due to synthetic N fertilizer use; (2) winter cover crop use had no effect on total soil GHG emissions regardless of stover management treatment; (3) stover baling decreased total soil GHG emissions, though grazing stover significantly increased emissions in one year; (4) grazing perennial grasslands tended to increase GHG emissions in pastures selected for forage quality, but were highly variable from year to year; (5) ICL systems that incorporate perennial grasses will provide the most effective GHG mitigation outcomes.

Assessment of mechanical dispersion effects on mixing zone under extreme saltwater intrusion

The previous studies primarily identify the two types of saltwater encroachment processes like active SI (seawater intrusion) and passive SI (submarine groundwater discharge). Under passive SI, the hydraulic gradient inclines towards the sea, making density and fresh groundwater flow work in opposite directions. Under active SI, the hydraulic gradient inclines landward, and density and fresh groundwater flow work in the same direction, causing more aggressive salinization and wider mixing zones. Mechanical dispersion's effects in controlling the mixing zones in coastal aquifers have become the subject of ongoing debate in SI studies. In this study, numerical experimentations of scaled-tank and a field-scale model were used to explore the influence of longitudinal and transversal dispersivities, separately, on the width of the mixing zone under passive and active SI, in response to inland freshwater head decline. The results show that under steady SI conditions, increasing the longitudinal dispersivity value widened the lower part of the mixing zone while increasing the transversal dispersivity value increased the mixing zone's width both at the lower and the upper part of the saltwater wedge. The same phenomenon as steady-state SI results was observed for passive SI conditions even though with larger effects. While under active SI, both an increase of longitudinal and transversal dispersivities widened the width of the mixing zones but their quantitative impacts are larger compared to those in passive SI conditions. As an instance, when the transverse dispersivity is increased 50 times, the width of the mixing zone at the middle of the saltwater wedge is almost doubled compared to it in the passive case. Ultimately, it is found that under active circumstances, the shear effect of transverse dispersivity pushes the toe position back.

Field Scale Modelling of Explosion-Generated Crack Densities in Granitic Rocks Using Dual-Support Smoothed Particle Hydrodynamics (DS-SPH)

Field Scale Modelling of Explosion-Generated Crack Densities in Granitic Rocks Using Dual-Support Smoothed Particle Hydrodynamics (DS-SPH)

A Simulation Study of Steamflood Process with Co-Injection of Steam-Carbon Dioxide for Oil Recovery

Steamflood as one of thermal EOR method is effective to reduce the amount of oil left in the reservoir by mechanisms such as reducing the viscosity of the oil, oil evaporation, and displacement by steam. In this method, fluid is injected continuously into some injection wells to displace oil and obtain production from other wells. However, there are problems with steamflood process that are still not optimally solved. There is an early channelling of steam under the cap rock to the production well which leads to investigating a solution by co-injecting gases to steamflooding processes. Carbon dioxide is co-injected with steam to form gas-drive so it is expected to be able to add sweep in reservoir and also increase oil in place as gas dissolves in oil. Thermal simulation was carried out in this study using CMG STARS reservoir simulator by constructing two kind of injection schemes; steamflood only and steam-CO2 to study the performance of each method. The analysis in this research investigates the effects of steam-gas simultaneous injection in a heavy-oil recovery process specifically to the microscopic and macroscopic efficiencies. The sensitivity of some parameters in the simulation including injection rate, steam quality and slug size of gases injected are also evaluated. The result of simulation shows the highest efficiency from field-scale model is obtained from steam-CO2 method. From sensitivity analysis, it is shown that the increase of injection rate and steam quality of gas injected gives the higher recovery, but the oil recovery decreases as the slug size of gas increasing.

Numerical Simulation of Nonequilibrium Foamy Oil for Cyclic Solvent Injection in Reservoirs after Cold Heavy Oil Production with Sand

SummaryNonequilibrium foamy oil behavior and solvent transport are two important recovery mechanisms for cyclic solvent injection (CSI) processes in post-cold heavy oil production with sand (CHOPS) reservoirs. The nonequilibrium solvent exsolution and gas bubbles generated during the pressure depletion stage have the typical characteristics of foamy oil flow. In this paper, a field-scale post-CHOPS model is constructed and upscaled from a core model, which was calibrated against detailed experimental data involving various propane (C3H8)-based and carbon dioxide (CO2)-based solvent mixtures. The field model is upscaled from the core model to analyze the impacts of simulation scales, heterogeneous wormholes, and the operating schedules on foamy oil behavior of different solvent systems.Reaction kinetics are implemented to represent the nonequilibrium gas dissolution and exsolution for foamy oil flow. A fractal wormhole network is modeled. To analyze the impacts of pressure depletion strategies, single-stage pressure depletion involving three oil solvent systems, as well as two cycles of CSI production processes, are examined. Detailed sensitivity analyses involving different solvent compositions are discussed.The results illustrate that both C3H8-based and CO2-based solvents exhibit significant nonequilibrium foamy oil characteristics, enabling the oil viscosity to remain close to its value with dissolved solvent during the pressure depletion process. However, the amount of nonequilibrium foamy oil flow is strongly dependent on the pressure depletion rate: A faster depletion rate is beneficial for higher oil recovery. The core model results are more sensitive to the solvent types, whereas the field-scale simulations show comparable recovery performance for both C3H8-based and CO2-based solvents. This observation highlights the significance of domain size, time scale, and wormhole heterogeneities on the ensuing foamy oil behavior.Although several post-CHOPS models were developed in the past, detailed field-scale models that simulate nonequilibrium foamy oil kinetics in a realistic wormhole network are lacking. The simulation model developed here has been calibrated against detailed experimental measurements and upscaled from a core-scale model. Improving our understanding of solvent dissolution/exsolution would aid in the design of operating strategies (e.g., pressure depletion and solvent injection schemes) for enhanced solvent/oil mixing and transport.

Characterizing fluid flow paths in the Hellisheidi geothermal field using detailed fault mapping and stress-dependent permeability

The Husmuli zone of the SW-Iceland Hellisheidi geothermal field is currently being used for re-injection of geothermal fluids and geothermal CO2 for its permanent storage in the form of carbonate minerals. A fully coupled hydro-thermo-mechanical numerical model was employed to investigate the coupled impacts of these complex processes on the calibration of fluid flow paths, which can have significant implications for the long-term performance of this subsurface reservoir.Employing a combination of high-resolution fault mapping with laboratory measurements of stress dependent permeability coupled into a dual porosity field-scale model, the flow paths were calibrated using results of tracer tests performed at the site using stress-dependent permeability tensors. Although vertically extended faults are the primary fluid flow paths, fractures connecting the faults can play an important role in fluid transport. As the upward flow streamlines manifest, deep geological layers can also deviate the fluid flow towards the shallower layers provoking the vertical flow of geothermal fluids. This highlights the sweet spot for sustainable flow and heat extraction in vicinity of faults intercepting the geological layers at depth of 1100 m.It was also shown that the inclusion of the geomechanical calculations in the history matching of the tracer test could lead to changes in arrival time and peak of the tracer profiles. Results of an independent tracer test were used to validate the model and to demonstrate the predictive capability of the calibrated model. This verifies the consistency of our methodology to incorporate the stress-dependent permeability. The results of this comprehensive modelling study provide insight into the likely fluid flow paths, which can have profound impact on the evaluation of various processes such as CO2 mineralisation taking place in Hellisheidi geothermal reservoir.

Modelling the impact of Alkaline-surfactant and Alkaline-surfactant-polymer flooding processes on scale precipitation and management

Abstract Mineral scaling issues have been reported in many alkaline and Alkaline-Surfactant-Polymer (ASP) projects. The role of the in situ mineral reactions on the produced scaling ions and pH has been little reported in the literature. The objective of this study is to investigate the impact of the in situ chemical and geochemical interactions on the scale precipitation risk when the fluids reach the wellbore, and their inhibition during Alkaline-Surfactant (AS) and ASP flooding processes. Reservoir simulation is used to model the geochemical interactions and chemical flood flow behaviour using 2D areal and vertical homogeneous and heterogeneous models. Data from the literature is used to model oil, water and rock interactions (interfacial tension, reaction rate parameters, relative permeability, chemical adsorption and polymer viscosity) for surfactant, and sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) alkalis, and HPAM polymer. At the wellbore, squeeze modelling is used to investigate the volume, concentration and cost of calcite scale inhibitor for three different AS and ASP flooding options. Results show that the in situ rock dissolution, mineral precipitation and brine mixing reduce the produced ion concentrations (Ca2+, Mg2+, HCO3-) and pH compared to the initial concentration and the injected pH value. The calcite scaling risk can be high during Na2CO3 injection while silica and Mg(OH)2 scales are potential minerals that will precipitate in the production system during NaOH injection. Uncertainty in the mineral reaction rate parameters, especially mineral surface area, is important and must be captured, as this may impact the scaling risk in the producer. Among the studied flooding options, ASP with pre and post polymer slugs shortens the calcite scaling period, reduces the scaling ion concentrations and the produced water rates. This case, then, requires the least number of squeeze treatments, the lowest scale inhibitor volume, and delivers the highest incremental oil recovery. This paper gives a workflow for assessing the scaling risks for AS and ASP flooding, with crucial role played by reservoir complexity. It is therefore recommended that scaling assessment calculations following our workflow be carried out for specific AS and ASP field cases.

Determining the controlling mechanisms of hydrate dissociation front using optimized characteristic time

The advance of the hydrate dissociation front is crucial to both the energy recovery process and the geological hazards prevention. In this work, the role of the scale of gas hydrate reservoirs in the controlling mechanisms is investigated numerically with the lab-scale and field-scale models. It is suggested that the scale of hydrate reservoir is not reliable in discriminating the three controlling mechanisms, i.e., fluid-flow (FF), heat-transfer (HT), and hydrate-dissociation-kinetics (HDK). For instance, the controlling mechanism of both the lab- and the field-scale hydrate reservoirs both might be FF as revealed in this work. Therefore, an approach of characteristic time is proposed to discriminate controlling mechanisms by employing dimensionless analyses. Also, mathematical models of the velocity of hydrate dissociation front conditioned to the single controlling mechanisms have been established, which are used to optimize the characteristic times. According to the optimized results, single characteristic times are sufficient for the HT and HDK controlling mechanisms. For instance, the time consumed to transfer the heat in the hydrate reservoirs by heat conduction through the gas phase may serve as the characteristic time of the HT controlling mechanism. As for the FF controlling mechanism, two different characteristic times should be used, i.e., a gas-phase based characteristic time for the initial stage and a water-phase based one for the later stage. The optimized characteristic times are capable of capturing the variation of hydrate reservoir controlling mechanism with geological and production parameters.

Modeling Approaches to Assess Soil Erosion by Water at the Field Scale with Special Emphasis on Heterogeneity of Soils and Crops

Information on soil erosion and related sedimentation processes are very important for natural resource management and sustainable farming. Plenty of models are available for studying soil erosion but only a few are suitable for dynamic soil erosion assessments at the field-scale. To date, there are no field-scale dynamic models available considering complex agricultural systems for the simulation of soil erosion. We conducted a review of 51 different models evaluated based on their representation of the processes of soil erosion by water. Secondly, we consider their suitability for assessing soil erosion for more complex field designs, such as patch cropping, strip cropping and agroforestry (alley-cropping systems) and other land management practices. Several models allow daily soil erosion assessments at the sub-field scale, such as EPIC, PERFECT, GUEST, EPM, TCRP, SLEMSA, APSIM, RillGrow, WaNuLCAS, SCUAF, and CREAMS. However, further model development is needed with respect to the interaction of components, i.e., rainfall intensity, overland flow, crop cover, and their scaling limitations. A particular shortcoming of most of the existing field scale models is their one-dimensional nature. We further suggest that platforms with modular structure, such as SIMPLACE and APSIM, offer the possibility to integrate soil erosion as a separate module/component and link to GIS capabilities, and are more flexible to simulate fluxes of matter in the 2D/3D dimensions. Since models operating at daily scales often do not consider a horizontal transfer of matter, such modeling platforms can link erosion components with other environmental components to provide robust estimations of the three-dimensional fluxes and sedimentation processes occurring during soil erosion events.

Field-scale soil moisture bridges the spatial-scale gap between drought monitoring and agricultural yields

Abstract. Soil moisture is highly variable in space and time, and deficits (i.e., droughts) play an important role in modulating crop yields. Limited hydroclimate and yield data, however, hamper drought impact monitoring and assessment at the farm field scale. This study demonstrates the potential of using field-scale soil moisture simulations to support high-resolution agricultural yield prediction and drought monitoring at the smallholder farm field scale. We present a multiscale modeling approach that combines HydroBlocks – a physically based hyper-resolution land surface model (LSM) – with machine learning. We used HydroBlocks to simulate root zone soil moisture and soil temperature in Zambia at 3 h 30 m resolution. These simulations, along with remotely sensed vegetation indices, meteorological data, and descriptors of the physical landscape (related to topography, land cover, and soils) were combined with district-level maize data to train a random forest (RF) model to predict maize yields at district and field scales (250 m). Our model predicted yields with an average testing coefficient of determination (R2) of 0.57 and mean absolute error (MAE) of 310 kg ha−1 using year-based cross-validation. Our predicted maize losses due to the 2015–2016 El Niño drought agreed well with losses reported by the Food and Agriculture Organization (FAO). Our results reveal that soil moisture is the strongest and most reliable predictor of maize yield, driving its spatial and temporal variability. Soil moisture was also a more effective indicator of drought impacts on crops than precipitation, soil and air temperatures, and remotely sensed normalized difference vegetation index (NDVI)-based drought indices. This study demonstrates how field-scale modeling can help bridge the spatial-scale gap between drought monitoring and agricultural impacts.

Evaluations of the feasibility of oil storage in depleted petroleum reservoirs through experimental modelling studies

The global storage space for crude oil is now facing intense challenges due to excess supply. In this situation, increasing oil storage capacity is urgently required. The currently available oil storage facilities, such as tanks and underground caverns, are of limited storage capacity, which cannot be expanded within a short time. Depleted petroleum reservoirs seem to be an ideal alternative since they are geographically ubiquitous and abundant, structurally safe, and cost-effective for storing large amounts of crude oil. Hence, this work aims to investigate the feasibility and approve the concept of oil storage in depleted petroleum reservoirs using both laboratory and modeling approaches. A core flooding experiment was conducted to physically model oil storage and withdrawal in a sandstone core. The results show that around 77% of the stored oil in the core is withdrawn. A core-scale model is then developed to match experimental data and collect inputs for the following studies. A sensitivity analysis is performed to determine the effect of porosity and permeability of the rock and bottom hole pressure (BHP) of the oil injectors on the storage capacity and withdrawal efficiency. The results demonstrate that the storage capacity increases with each of the parameters, but the withdrawal efficiency nearly holds constant. In the Field-scale modeling study, the reservoir stores 7.7 MMbbl oil, and the withdrawal efficiency are 67% and 74% for dead and live oil storage. It indicates oil storage in depleted reservoirs has a great potential to enlarge the global oil storage capacity.

A study on seismic response of buildings on coir mat reinforced sand bed

The present study examines the efficacy of coir mat as a seismic isolation material for reducing the earthquake energy transferred and, so to reduce the dynamic response of the building. A three-dimensional finite element simulation of field-scale models of G+2 and G+3 buildings resting on raft foundation in a sand bed, with and without the seismic isolation mechanism has been carried out by time history analysis with El Centro (1940) input motion. Seismic responses such as roof acceleration, roof deflection, bending moment and shear force of G+2 and G+3 buildings are studied. Variation in the base shear ratio which is defined as the ratio of seismic base shear to the total weight of the building is also analyzed by incorporating the three-dimensional soil-structure interaction effect. The results from the analysis inferred that the reinforcement of the coir mat as a seismic isolation medium limits the transmission of earthquake energy to the superstructure.

Numerical modelling of the resistance of the coarse sand barrier against backward erosion piping

The coarse sand barrier (CSB) is a novel remediation measure to reinforce embankments against the internal erosion mechanism of backward erosion piping (BEP). The feasibility of the CSB is investigated in a research programme consisting of experiments at different scales, numerical analysis and application at a pilot site in the Netherlands. Laboratory experiments showed that the CSB increased the critical head drop for BEP by one order of magnitude. In this paper, the strength of the CSB is quantified using numerical modelling of the experiments. A local, scale-independent, strength criterion for the CSB is derived based on medium-scale experiments. The application of this local criterion is illustrated for a field-scale model, in which the critical head drop that can be retained increases by a factor two to three with the CSB.