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### Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2017;139(6):061201-061201-7. doi:10.1115/1.4036772.

The decrease of fossil fuels availability and the consequent increase of their price have led to a rapid evolution of renewable market and policy frameworks in recent years. Renewable resources include solar radiation, which is of considerable interest as it is inexhaustible, free, and clean. In order to calculate how much work can be obtained from solar radiation, several methods have been proposed in the literature and are here reviewed. In this paper, a single exergy factor to be applied to the total radiation measured on horizontal surface in a given place is proposed. The factor is estimated from both direct and diffuse radiation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):061202-061202-9. doi:10.1115/1.4036964.

In this study, we present the results of a two-dimensional fluid-dynamic simulation of novel rotor geometry with spline function which is derivative of the traditional S-shape Savonius blade. A computational fluid dynamic (CFD) analysis is conducted using the Spalart–Allmaras turbulent model, validated using experimental data released by Sandia National Laboratory. Results are presented in terms of dimensionless torque and power coefficients, assuming a wind speed of 7 m/s and height and rotor diameter of 1 m. Furthermore, analysis of the forces acting on the rotor is conducted by evaluating frontal and side forces on each blade, and the resultant force acting on the central shaft. A qualitative representation of the vorticity around the traditional and spline rotor is shown to prove that the novel blade allows less turbulent flow through the rotor.

Commentary by Dr. Valentin Fuster

### Research Papers: Energy Conversion/Systems

J. Energy Resour. Technol. 2017;139(6):061601-061601-17. doi:10.1115/1.4037206.

A large amount of the operating costs in a building is determined by the energy requirements of its air conditioning system. The demand for more energy efficient units desired by both manufacturers and the consumers results in a dire necessity to have air conditioning units that are more energy efficient than the existing ones. In order to achieve the abovementioned features, a tool must be designed to simulate the thermal behavior of the air conditioners. In this work, a mathematical model is developed for air conditioning units and coded into a computer program to estimate the overall performance, as indicated by the unit energy efficiency ratio (EER). The main objective is to maximize the unit EER by proposing modifications or enhancements in the existing unit and to study the economics of these modifications based on the measured terms such as the energy savings and the operating cost. Finally, the effect of the proposed design modifications on the economy and environment at the national level in Saudi Arabia is estimated and presented as an example.

Commentary by Dr. Valentin Fuster

### Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2017;139(6):062001-062001-5. doi:10.1115/1.4037205.

Currently occupying only a small portion of the energy sector, nuclear power is increasingly becoming a promising contender for energy resources of the future. With growing concern of climate change and excessive carbon emissions from fossil fuels, nuclear is widely being pursued as an alternative energy resource that does not produce carbon dioxide. Nuclear power has been the source of environmentally hazardous byproducts of its own, however, and issues with radioactive waste have in many ways halted progress in nuclear power development and implementation. New advances now attempt to solve the many issues of the past, associated mostly with nuclear fission. Some of these developments, including the promising use of nuclear fusion, are evaluated as a means of solving the energy crises as well as the radioactive waste issues.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062002-062002-6. doi:10.1115/1.4037157.

Evaporation of droplets of liquid mixture is a subject of interest in combustion studies, e.g., combustion of bioethanol blends. In this paper, experimental investigation, using rainbow refractometry, on the variations of droplet diameter and composition during the evaporation of water–ethanol droplet in quiescent atmosphere is studied. The droplet is suspended on the tip of 125 μm-diameter fiberglass rod. The initial diameter is around 1000–1100 μm, and the initial composition is varied from 0% to 100% of ethanol by volume. The scattered rainbow signal from the evaporating droplet is fitted to the Airy theory to extract information on the diameter and refractive index of the liquid droplet against evolution time. To determine the accuracy of droplet diameter measurements using this technique, the diameter is also measured from the shadow image of droplet simultaneously. At 0–60% of ethanol by volume, the diameter and volume fraction accuracies are within ±30 μm and 10%, respectively, even though the temperature and composition gradients inside a droplet are neglected. The results show that the water–ethanol mixture evaporates faster at the beginning due to the higher amount of the volatile component, i.e., ethanol. The D2–t curve appears as a series of two straight lines of different slopes: a steep one initially and a moderate one at later stage. The slope at the initial or the transition stage increases with the ethanol composition, while the slope at later stage (steady stage) is equivalent to that of pure water. Likewise, the refractive index decreases rapidly at the beginning and becomes steady reaching a final value of 1.333, which is close to the refractive index of pure water.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062003-062003-8. doi:10.1115/1.4037371.

In recent years, a large number of renewable energy sources (RESs) have been added into modern distribution systems because of their clean and renewable property. Nevertheless, the high penetration of RESs and intermittent nature of some resources such as wind power and photovoltaic (PV) cause the variable generation and uncertainty of power system. In this condition, one idea to solve problems due to the variable output of these resources is to aggregate them together. A collection of distributed generations (DGs) such as wind turbine (WT), PV panel, fuel cell (FC), and any other sources of power, energy storage systems, and controllable loads that are aggregated together and are managed by an energy management system (EMS) are called a virtual power plant (VPP). The objective of the VPP in this paper is to minimize the total operating cost for a 24-h period. To solve the problem, a metaheuristic optimization algorithm, teaching–learning based optimization (TLBO), is proposed to determine optimal management of RESs, storage battery, and load control in a real case study.

Commentary by Dr. Valentin Fuster

### Research Papers: Environmental Aspect of Energy SourcesÂ

J. Energy Resour. Technol. 2017;139(6):062101-062101-8. doi:10.1115/1.4036962.

In the present study, a new carbon capture device that can be carried on-board vehicles has been developed and tested. The developed device uses absorption and adsorption methods of postcombustion CO2 capture. Sodium hydroxide (NaOH) pellets and calcium hydroxide Ca(OH)2 have been used as solvents and sorbents in the device. The CO2 capture efficiency has been evaluated at a wide range of operating conditions. The results showed that the higher the concentration of the solvent, the higher the capture efficiency, i.e., w 100% capture efficiency, being obtained at full saturation of NaOH. In addition, the increase in the solution temperature increases the capture efficiency up to 50 °C. Design of the gas distributer in the device has also a notable effect on CO2 capture. It was found that solvent prepared with seawater can provide high capture efficiency over a wide range of operation, but in general, it has a lower capture efficiency than that prepared by tap water. Moreover, solvents prepared by NaOH have a superior CO2 capture efficiency over those prepared by Ca(OH)2. For the adsorption technique, a 50% NaOH and 50% Ca(OH) mixture by mass has provided the highest capture efficiency compared with each sorbent when used alone.

Commentary by Dr. Valentin Fuster

### Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2017;139(6):062201-062201-9. doi:10.1115/1.4036957.

Gasification is the core unit of coal-based production systems and is also the site where one of the largest exergy destruction occurs. This paper reveals the exergy destruction mechanism of carbon gasification through a combined analysis of the kinetic method and the energy utilization diagram (EUD). Instead of a lumped exergy destruction using the traditional “black-box” and other models, the role of each reaction in carbon gasification is revealed. The results show that the exergy destruction caused by chemical reactions accounts for 86.3% of the entire carbon gasification process. Furthermore, approximately 90.3% of exergy destruction of chemical reactions is caused by the exothermal carbon partial oxidation reaction (reaction 1), 6.0% is caused by the carbon dioxide gasification reaction (reaction 2), 2.4% is caused by the steam gasification reaction (reaction 3), and 1.3% is caused by other reactions under the base condition. With increasing O2 content α and decreasing steam content β, the proportion of exergy destruction from reaction 1 decreases due to the higher gasification temperature (a higher energy level of energy acceptor in EUD), while the proportions of other reactions increase. This shows that the chemical efficiency is optimal when the extent of reactions 1 and 3 is equal and the shift reaction extent approaches zero at the same time.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062202-062202-8. doi:10.1115/1.4036932.

In this study, the emissions from three passenger cars with gasoline, methanol, ethanol, and their blend were tested. The results show that the CO and HC emissions from the exhaust of the vehicles fueled with E7.5/M7.5 decrease compared with those from the vehicles fueled with the gasoline, E10 or M15, while NOx emissions increase by 7.5–25.8%. Formaldehyde and acetaldehyde are found higher for the vehicles fueled with E7.5/M7.5, whereas a series of volatile compounds become lower. Evaporative emissions of the vehicles fueled with E7.5/M7.5 were higher than those of the vehicles fueled with gasoline, by a range of 16.39–28.28%.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062203-062203-13. doi:10.1115/1.4036771.

The current state of the art in waste heat recovery (WHR) from internal combustion engines (ICEs) is limited in part by the low temperature of the engine coolant. In the present study, the effects of operating a diesel engine at elevated coolant temperatures to improve utilization of engine coolant waste heat are investigated. An energy balance was performed on a modified three-cylinder diesel engine at six different coolant temperatures (90 °C, 100 °C, 125 °C, 150 °C, 175 °C, and 200 °C) and 15 different engine loads to determine the impact on waste heat as the coolant temperature increased. The relative brake efficiency of the engine alone decreased between 4.5% and 7.3% as the coolant temperature was increased from 90 °C to 150 °C. However, the engine coolant exergy increased between 20% and 40% over the same interval. The exhaust exergy also increased between 14% and 28% for a total waste heat exergy increase between 19% and 25%. The engine condition was evaluated after testing and problem areas were identified such as overexpansion of pistons, oil breakdown at the piston rings, and head gasket seal failure.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062204-062204-8. doi:10.1115/1.4036958.

Combustion processes of two fuels, pulverized coal and biomass, in furnaces take place at steady state. Combustion of condensed fuels involves one-way interfacial flux due to phenomena in the condensed phase (evaporation or pyrolysis) and reciprocal ones (heterogeneous combustion and gasification). Many of the species injected in the gas phase are later involved in gas phase combustion. This paper presents results of combustion process of two-phase charge contained coal and wetted biomass, where the carrier was the air with given flow rate. The furnace has three inlets with assumed inlet flow rate of coal, biomass, and air, and combustion process takes place in the furnace fluidized space. The simulation of such combustion process was carried out by numerical code of open source computational fluid dynamics (CFD) program code_saturne. For both fuels, the moist biomass with following mass contents: C = 53%, H = 5.8%, O = 37.62%, ash = 3.6, and mean diameter of molecules equal to 0.0008 m and pulverized coal with following mass contents: C = 76.65%, H = 5.16%, O = 9.9%, ash = 6.21%, and mean molecule diameter 0.000025 m were used. Devolatilization process with kinetic reactions was taken into account. Distribution of the main combustion product in furnace space is presented with disappearance of the molecules of fuels. This paper presents theoretical description of the two-phase charge, specification of the thermodynamic state of the charge in inlet boundaries and furnace space, and thermal parameters of solid fuel molecules obtained from the open source postprocessor paraview.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062205-062205-9. doi:10.1115/1.4036959.

More precise measurements of the fuel injection process can enable better combustion control and more accurate predictions resulting in a reduction of fuel consumption and toxic emissions. Many of the current methods researchers are using to investigate the transient liquid fuel sprays are limited by cross-sensitivity when studying regions with both liquid and vapor phases present (i.e., upstream of the liquid length). The quantitative rainbow schlieren technique has been demonstrated in the past for gaseous fuel jets and is being developed here to enable study of the spray near the injector. In this work, an optically accessible constant pressure flow rig (CPFR) and a modern common rail diesel injector are used to obtain high-speed images of vaporizing fuel sprays at elevated ambient temperatures and pressures. Quantitative results of full-field equivalence ratio measurements are presented as well as more traditional measurements such as vapor penetration and angle for a single condition (13 bar, 180 °C normal air) using n-heptane injected through a single hole (0.1 mm diameter) common rail fuel injector at 1000 bar fuel injection pressure. This work serves as a proof of concept for the rainbow schlieren technique being applied to vaporizing fuel sprays, and full details of the image-processing routine are provided. The ability of the imaging technique combined with the constant pressure flow rig make this approach ideal for generating large data sets in short periods of time for a wide range of operating conditions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062206-062206-8. doi:10.1115/1.4037190.

Oxycoal combustion was numerically simulated in a lab-scale cylindrical furnace (Φ200 mm × 2 m) with high-velocity oxygen jets. The mesoscopic characteristics of turbulent flame behavior such as nondimensional numbers ReT, Ka, and Da were calculated under different jet positions and jet spacing. The results show that for coflow burners, large spacing (L = 75 mm) is not favored due to poor radial mixing and the restriction of wall; except L = 75 mm, as jet spacing increases, the oxidizer flow could be internally diluted to a lower concentration and preheated to a higher temperature, at least 1000 K; for L = 60 mm conditions, the maximum temperature increase is lower than the ignition temperature (437 °C), they are, namely, oxycoal moderate or intense low oxygen dilution (MILD) combustion. For MILD conditions, the mesoscopic parameters of the flame front where temperature gradient is the largest locate in the distributed regime corresponding to l/lF > 1, ReT > 1, Kaδ > 1, and Da < 1, the global regime is depicted as 1 < l/lF < 4, 60 < ReT < 150, 50 < Ka < 500, and Da < 1; for flaming conditions, the regime is depicted as 1 < l/lF < 6, 40 < ReT < 110, 10 < Ka < 800, and Da < 1.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062207-062207-8. doi:10.1115/1.4036961.
FREE TO VIEW

Thermal stability is an important characteristic of alternative fuels that must be evaluated before they can be used in aviation engines. Thermal stability refers to the degree to which a fuel breaks down when it is heated prior to combustion. This characteristic is of great importance to the effectiveness of the fuel as a coolant and to the engine's combustion performance. The thermal stability of Sasol iso-paraffinic kerosene (IPK), a synthetic alternative to Jet-A, with varying levels of naphthalene has been studied on aluminum and stainless steel substrates at 300–400 °C. This was conducted using a spectroscopic ellipsometer to measure the thickness of deposits left on the heated substrates. Ellipsometry is an optical technique that measures the changes in a light beam's polarization and intensity after it reflects from a thin film to determine the film's physical and optical properties. It was observed that, as would be expected, increasing the temperature minimally increased the deposit thickness for a constant concentration of naphthalene on both substrates. The repeatability of these measurements was verified using multiple trials at identical test conditions. Finally, the effect of increasing the naphthalene concentration at a constant temperature was found to also minimally increase the deposit thickness.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062208-062208-7. doi:10.1115/1.4037481.

In order to eliminate pollution from ultra low calorific value gas (ULCVG) of methane and achieve energy recovery simultaneously, a novel reactor with the function of regenerator and catalytic combustor named rotary regenerative type catalytic combustion reactor is studied. The reactor walls which store and reject heat alternatively can preheat incoming ULCVG to the ignition temperature of methane, and catalytic combustion occurs rapidly. According to the features of the reactor such as rotation and catalytic combustion, considering the conjugate heat exchange, the characteristics of this reactor were calculated and analyzed with the help of computational fluid dynamics (CFD). The results show that the ULCVG can be oxidized as a primary fuel, with the methane conversion above 91%, and the feasibility of this reactor is proved in theory. The reactor can continuously generate high-temperature gas (1035 K–1200 K) which can be used by the heat consumption unit (HCU) such as turbines, boilers, and solid oxide fuel cell services. Besides, the outlet gas and exhaust gas temperature vary roughly linearly with time, and this rule is useful to estimate the outlet temperature. Periodical rotation not only provides high-temperature zone which is beneficial to catalytic combustion, but also avoids further heat accumulation.

Commentary by Dr. Valentin Fuster

### Research Papers: Oil/Gas Reservoirs

J. Energy Resour. Technol. 2017;139(6):062801-062801-11. doi:10.1115/1.4037812.

Critical condensate saturation, Scc, is a key parameter for the evaluation of well deliverability in gas condensate reservoirs. We propose a new method to determine Scc by performing three-phase flow simulations with three-dimensional (3D) pore network model. First, we establish a network model with random fractal methodology. Second, based on the condensation model in the literature of Li and Firoozabadi, we develop a modified condensation model to describe the condensation phenomenon of gas with connate water in the porous medium. The numerical model is verified by experimental measurements in the literature. Then, we investigate the influence of different factors on the critical condensate saturation, including micro pore structure (pore radius and fractal dimension), condensate gas/oil interfacial tension (IFT), and flow rate at different irreducible water saturation, Swi. The simulation results show that Scc decreases with increasing of average pore radius, but increases with increasing of fractal dimension. In the case of the same gas/oil interfacial tension, the higher the connate water saturation, the higher the critical condensate saturation. There is a critical gas/oil interfacial tension, below the critical value, the critical condensate saturation increases drastically with increasing of interfacial tension while it keeps almost unchanged when the interfacial tension is above the critical value. The critical condensate saturation decreases with increasing in the gas flow rate. High capillary number results in low critical condensate saturation. Reasonable increase in producing pressure drop can effectively improve the flow capacity of condensate oil.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2017;139(6):062901-062901-7. doi:10.1115/1.4037026.

The variable mass flow in perforated horizontal wells is very complex. One reason is that the perforation can increase the roughness of the pipe wall which will increase the frictional pressure drop. The other is the fluid boundary layer and velocity profile of axial flow will be changed due to the “mixing” of the inflow with the axial flow. The influences of the perforation parameters and flux rate on the pressure drawdown in horizontal wellbore are investigated. The perforation parameters include perforation phasing, perforation diameter, and perforation density. According to the experiment results, some modes such as friction factor calculation model (the accuracy of the model is 4%), “mixing” pressure drop calculation model (the accuracy of the model is 3%), and total pressure drop calculation model (the accuracy of the model is 2%) are developed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062902-062902-11. doi:10.1115/1.4036960.

A novel and pragmatic technique has been proposed to quantify the nonequilibrium phase behavior together with physical properties of foamy oil under reservoir conditions. Experimentally, constant-composition expansion (CCE) experiments at various constant pressure decline rates are conducted to examine the nonequilibrium phase behavior of solvent–CO2–heavy oil systems. Theoretically, the amount of evolved gas is first formulated as a function of time, and then incorporated into the real gas equation to quantify the nonequilibrium phase behavior of the aforementioned systems. Meanwhile, theoretical models have been developed to determine the time-dependent compressibility and density of foamy oil. Good agreements between the calculated volume–pressure profiles and experimentally measured ones have been achieved, while both amounts of evolved gas and entrained gas as well as compressibility and density of foamy oil were determined. The time-dependent effects of entrained gas on physical properties of oleic phase were quantitatively analyzed and evaluated. A larger pressure decline rate and a lower temperature are found to result in a lower pseudo-bubblepoint pressure and a higher expansion rate of the evolved gas volume in the solvent–CO2–heavy oil systems. Apparent critical supersaturation pressure increases with either an increase in pressure decline rate or a decrease in system temperature. Physical properties of the oleic phase under nonequilibrium conditions follow the same trends as those of conventionally undersaturated oil under equilibrium conditions when pressure is higher than the pseudo-bubblepoint pressure. However, there is an abrupt increase of compressibility and decrease of density associated with pseudo-bubblepoint pressure instead of bubblepoint pressure due to the initialization of gas bubble growth. The amount of dispersed gas in the oleic phase is found to impose a dominant impact on physical properties of the foamy oil. Compared with CCE experiment at constant volume expansion rate, a rebound pressure and its corresponding effects on physical properties cannot be observed in the CCE experiments at constant pressure decline rate.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062903-062903-8. doi:10.1115/1.4036963.

One of the serious challenges encountered in cementing oil and gas wells is the failure of the cement sheaths and its debonding from casing or formation rock. Shrinkage of the cement during setting is identified as one of the driving factors behind these issues. Some expansive cement systems have been developed in the oil and gas industry to compensate for the shrinkage effect. All the expansive additives which have been developed so far have chemical reactions with the cement itself that would significantly impact the mechanical strength of the cement. In this paper, we present a new class of polymer-based expandable cement additive particles which are made of shape memory polymers (SMP). This class of polymers is designed to expand to the required extent when exposed to temperatures above 50–100 °C (122–212 °F) which is below the temperature of the cementing zone. It is notable that expansion occurs after placement of the cement but before its setting. The API RP 10 B-2 and 5 have been followed as standard test methods to evaluate expansion and strength of the cement slurry after utilizing the new additive. The proposed additive does not react with the water or cement content of the slurry. Mechanical evaluation tests confirm the potential benefit of this additive without any deteriorative effect on mechanical properties or setting time of the cement paste and significant impact on its mechanical properties. Hence, this additive would provide a reliable way to prevent cement channeling, debonding, and fluid migration to upper formations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062904-062904-9. doi:10.1115/1.4037215.

Designing smart water (SW) by optimizing the chemical composition of injected brine is a promising low-cost technique that has been developed for both sandstone and carbonate reservoirs for several decades. In this study, the impact of SW flooding during tertiary oil recovery phase was investigated by core flooding analysis of pure limestone carbonate rocks. Increasing the sulfate ion concentration by using CaSO4 and MgSO4 of NaCl concentration and finally reducing the total salinity were the main manipulations performed to optimize SW. The main objective of this research is to compare active cations including Ca2+ and Mg2+ in the presence of sulfate ions $(SO42−)$ with regard to their efficiency in the enhancement of oil production during SW flooding of carbonate cores. The results revealed a 14.5% increase in the recovery factor by CaSO4 proving its greater effectiveness compared to MgSO4, which led to an 11.5% production enhancement. It was also realized that low-salinity water flooding (LSWF) did not lead to a significant positive effect as it contributed less than 2% in the tertiary stage.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062905-062905-7. doi:10.1115/1.4037156.

Formation damage from aqueous phase trapping in low-permeability sandstones can be removed using mutual solvents, blends of alcohols and mutual solvents, and surfactants. These treatments modify the interfacial tension of the trapped fluids or the wettability of the formation. However, treatments intended to remove a certain type of damage may cause other types of formation damage due to incompatibility with the rock and formation fluids. High-frequency acoustic waves have been used in industrial applications to clean up and remove contaminants. Important studies have been conducted to extend the use of acoustic waves for wellbore stimulation. This technical paper presents a laboratory investigation to determine the effects of ultrasonic (UT) treatment on interfacial tension and wettability alteration during invasion of fracturing fluids treated with surfactants in low-permeability sandstones. An experimental program consisting of a series of spontaneous imbibition experiments was conducted to measure the spontaneous imbibition potential of sandstone rock cores treated with surfactants in the presence of UT energy. Spontaneous imbibition tests were conducted in two steps. In the first, spontaneous imbibition tests were conducted on untreated low-permeability sandstone core samples in the presence of UT radiation. In the second step, spontaneous imbibition tests were conducted on cores flooded with surfactant while exposing the core to UT radiation from an acoustic horn. In each series experiments, the power output was changed to monitor the effect of acoustic power on wettability alteration. Results obtained from the experiments showed that acoustic stimulation improves imbibition of water in both water wet and intermediate wet cores. Wettability alteration is attributed to enhancement of capillary forces in water wet cores. For cores treated with water repelling surfactants, improvement in imbibition is attributed to detachment of surfactant molecules from the pore walls due to acoustic streaming of sonic waves. This research was originally intended to investigate removal of trapped water from fluid injection of low-permeability sandstones by acoustic stimulation. However, the obtained results show that it is possible to improve the recovery of trapped hydrocarbon in low-permeability sandstones under the influence of acoustic stimulation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(6):062906-062906-12. doi:10.1115/1.4037811.

Ensemble Kalman filter (EnKF) uses recursive updates for data assimilation and provides dependable uncertainty quantification. However, it requires high computing cost. On the contrary, ensemble smoother (ES) assimilates all available data simultaneously. It is simple and fast, but prone to showing two key limitations: overshooting and filter divergence. Since channel fields have non-Gaussian distributions, it is challenging to characterize them with conventional ensemble based history matching methods. In many cases, a large number of models should be employed to characterize channel fields, even if it is quite inefficient. This paper presents two novel schemes for characterizing various channel reservoirs. One is a new ensemble ranking method named initial ensemble selection scheme (IESS), which selects ensemble members based on relative errors of well oil production rates (WOPR). The other is covariance localization in ES, which uses drainage area as a localization function. The proposed method integrates these two schemes. IESS sorts initial models for ES and these selected are also utilized to calculate a localization function of ES for fast and reliable channel characterization. For comparison, four different channel fields are analyzed. A standard EnKF even using 400 models shows too large uncertainties and updated permeability fields lose channel continuity. However, the proposed method, ES with covariance localization assisted by IESS, characterizes channel fields reliably by utilizing good 50 models selected. It provides suitable uncertainty ranges with correct channel trends. In addition, the simulation time of the proposed method is only about 19% of the time required for the standard EnKF.

Commentary by Dr. Valentin Fuster