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### Research Papers: Energy Storage/Systems

J. Energy Resour. Technol. 2017;140(1):011901-011901-11. doi:10.1115/1.4037535.

This paper investigates the nature of entropy generation in stratified sensible thermal energy stores (SSTES) during charging using a dimensionless axisymmetric numerical model of an SSTES. Time-varying dimensionless entropy generation rates and the cumulative entropy generation in SSTES were determined from finite volume computations. The aspect ratios (AR), Peclet numbers (PeD), and Richardson numbers (Ri), for the stores considered were within the ranges $1≤AR≤4, 5×103≤PeD≤100×103$, and $10≤Ri≤104$, respectively. Using the Bejan number (Be), the total entropy generation was shown to be almost entirely due to thermal effects in the SSTES. The Be is practically unity for most of the SSTES' charging duration. The contributions of radial thermal gradients to the thermal entropy generation were further shown to be largely negligible in comparison to the contributions of axial thermal gradients, except at low Ri. Entropy generation numbers, Ns, in the SSTES were also computed and found to increase with decreasing AR and PeD and with increasing Ri. PeD was found to have the most significant influence on Ns. Based on this axisymmetric analyses of time-varying entropy generation in SSTES, estimates have been obtained of (1) the relative significance of radial effects on entropy generation within SSTES and (2) the relative significance of viscous shear entropy generation mechanisms within SSTES.

Topics: Entropy , Temperature
Commentary by Dr. Valentin Fuster

### Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2017;140(1):012001-012001-11. doi:10.1115/1.4037365.

This paper depicts the results of a detailed energy audit, analysis, and implementation of energy efficient operations and maintenance strategies in a large commercial mall in Kuwait. Initially, the cooling towers (CTs) operated only at high speed, and on a typical summer day, nearly one-fourth of the make-up water was used for self-cooling of air. The study based on measured data and analysis, for a period of one year, revealed that the use of variable frequency drive (VFD) could reduce the water wastage for self-cooling of air by as much as 75% and overall water consumption by 18.6% while keeping the cooling system performance at the design level. An optimization model was developed, suitable exclusively for arid climatic conditions. Implementation of various energy efficient operation and maintenance strategies (EEO&MS) with ventilation and air-conditioning (VAC), and lighting systems obtained an overall reduction of 9879 MWh/y, equivalent to a percentage reduction of 11.7% in the annual energy consumption and a 345 kW in peak power demand. The study estimated an economical benefit of 19,958 KD/y for the owners and to the Ministry of Electricity and Water, in addition to a considerable environmental benefit of deduction in CO2 emissions by 6990 t/y.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012002-012002-9. doi:10.1115/1.4037482.

Fuel vapor-containment system (FVS) is a kind of vapor recovery device in the hybrid electric vehicle (HEV) and has a great advantage on the fuel vapor recovery. The general refueling progress of the FVS has been studied in detail and divided into two different stages: the decompression stage and the refueling stage. Then, the two different stages' mathematical models have been developed based on the binary diffusion theory and time-variation diffusion theory, and simulated using matlab to calculate the evaporative emission with regard to time. Finally, the author has made the experiments on the decompression emissions and refueling emissions. The analysis shows that the test results are well coincided with the evaluation of mathematical model.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012003-012003-8. doi:10.1115/1.4037352.

Controlling the charging power system in an electrical vehicle, presents a serious challenge for the engineer in order to find the best solution that guarantee the system effectiveness and performance. Related to this objective, this paper is presented to offer an intelligent power management algorithm, which guarantees the best process of power extraction and injection, respectively, from an electrical generator (EG) linked to an internal combustion engine (ICE) to a system of batteries via a direct current to alternative current power converter. This intelligent process was based on the fuzzy technology and the system tuning is made after a various test. Obtaining the necessary power in the exact moment and in the specific condition, that presents the goal of the presented algorithm. For obtaining the best instruction from the present intelligent process, the state of charge (SOC) of the battery, the measured output voltage from the battery and the acceleration decision of the user, are used as a real's input parameters for having a real statue of the electrical vehicle. This new process will be an asset to the highway electrical vehicle for optimizing the power consumption. To evaluate the algorithm performance matlab/simulink is used and a simulation results are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012004-012004-6. doi:10.1115/1.4037375.

The motion of a fluid in a defined domain is called thermodynamically admissible if it satisfies the global system of the principles of balance of continuum mechanics and the principle of entropy or its equivalent differential system, consisting of differential equations and jump conditions. In an earlier publication, we have shown that the motion of a three-dimensional rigid body in an irrotational viscous and heat-conducting fluid violates the entropy jump condition, referred to as the Clausius–Duhem jump condition. Such a motion is thermodynamically inadmissible and could not persist. In a more recent publication, we have demonstrated that if the fluid–solid interface is isentropic, boundary conditions at a material interface, such as the no-slip condition and the continuity of the temperature, follow directly from the Clausius–Duhem jump condition. It is the purpose of this analysis to extend this methodology for the derivation of boundary conditions at isentropic material interfaces to nonisentropic material interfaces. We show that if the boundary conditions at the fluid–solid interface are a priori selected to satisfy the Clausius–Duhem jump condition, the resulting motion as described by the solution of the Navier–Stokes equations—whether the interface is isentropic or nonisentropic—is thermodynamically admissible.

Commentary by Dr. Valentin Fuster

### Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2017;140(1):012201-012201-8. doi:10.1115/1.4037367.

Utilization of oxygenated fuels has proven to be able to significantly control diesel engine exhaust emissions. Presented in this paper is a new oxygenated fuel di-(2-methoxypropyl) carbonate (DMPC), which was produced through transesterification reaction using dimethyl carbonate (DMC) and propylene glycol monomethyl ether (PGMME) as reactants as well as potassium hydroxide (KOH) as catalyst. Its structure characterization was completed through analyses with Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), and GC-MS analytical techniques. Further study was made about the effect of the oxygenate addition to diesel fuel on chemicophysical properties, combustion performances, and exhaust emissions characteristics. Experimental results displayed that the oxygenated fuel is mutually soluble with diesel fuel in any proportion at ambient temperature around 25 °C. With DMPC introduced to diesel fuel, kinematic viscosity decreases linearly, smoke point increases linearly, and flash point declines remarkably even under low content 5 vol %. Results of combustion test carried out on a single cylinder, DI diesel engine running at 1600 rpm and 2000 rpm showed that CO can be reduced by up to 60.0%, smoke can be lessened by up to 90.2%, while NOx increases by 4.4–14.0% as 15 vol % and 25 vol % of the oxygenate was added to a diesel fuel. Engine in-cylinder peak pressure increases somewhat and ignition delay duration becomes a little shorter. Both engine in-cylinder pressure rising rate and heat release rate increase noticeably during the premixed combustion.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012202-012202-8. doi:10.1115/1.4037370.

The objective of this experimental work is to design an original microfluidic mixer for continuous emulsification of small fractions of water in a lipid phase. This system is aimed to be integrated on-line in the process so as to avoid the use of a surfactant. The currently targeted application is a better combustion of water-supplemented alternative biofuels in boilers, turbines, or internal combustion engines in general. Therefore, mean size of droplets of water in the emulsion should be 5–10 μm, and the water content should not exceed ∼20%. Microsystems developed in this work are designed so as to enhance different flow perturbations that are favorable for the emulsification process. The microchannels for the fluids admittance have different sections: 300 × 300 μm2 and 600 × 600 μm2. As a consequence, an impinging flow is developed at the crossing of the inlet microchannels of the two phases which has for effect a significant stretching of the fluids. Then, depending on the continuous phase, Rayleigh instabilities can be developed in the straight parts of the outlet channels (600 × 600 μm2) and/or the enhancement of fluid splitting is obtained; thanks to a singularity (bend) located in the same outlet channels. Two different continuous phases are tested (gasoil and sun flower oil) for which the flow rate is about (65–100 ml/min). The water fraction is varied in the range 7–24%. It is shown that the length of the outlet microchannels is a crucial parameter. Considering an oil phase with low viscosity, such as gasoil, a too long channel can promote coalescence. On the opposite, longer outlet channels are needed with more viscous fluids (like sunflower oil) in order to develop Rayleigh instabilities which is, in this case, the more efficient way to obtain emulsions in this kind of microsystem. On a general point of view, concerning the size of the water droplets, dispersion of water is much more efficient with this microsystem using gasoil rather than vegetable oil as the continuous phase. Considering the targeted application, emulsions with an average size of water droplets of about 10 μm were obtained with gasoil as the continuous phase.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012203-012203-9. doi:10.1115/1.4037483.

Sorption hysteresis characterization of CH4 and CO2 on lignite, bituminous coal, and anthracite were studied to improve the understanding of the interaction between gas molecules and different ranks of coal and further improve the precision of the adsorption methods in characterizing pore structure at low pressure. Pore structure of three ranks of coal was investigated with scanning electron microscopy (SEM) and nitrogen (N2) adsorption. Then, CH4 and CO2 sorption isotherms were measured using the gravimetric method under 288, 308, and 328 K. The N2 sorption isotherms show that a wide distribution of pore size existed in three coal samples, and with the process of coalification, the specific surface area (SSA) decreased and then increased, while the pore size of coal monotonically decreased. This is confirmed by SEM observation. The measured sorption isotherms were then decomposed into simultaneously running adsorption and absorption branches based on the assumption that the former is totally reversible and the latter completely irreversible. The reconstructed adsorption branches can be well described by both Langmuir model and Dubinin–Radushkevich (D–R) equation. The absorption, which represents the sorption hysteresis portion, increased with pressure, but decreased with temperature. The absorbed amount of gas increased with pressure, but the absorption of CO2 increased concavely with gas pressure while CH4 followed an upward exponential function. Also, the absorption varied with coal rank, following a U-shaped function. This study can provide new insights to CH4 and CO2 sorption hysteresis on coal and other organic geomaterials.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012204-012204-5. doi:10.1115/1.4037376.

A numerical study has been carried out to investigate the impact of adding syngas into JP-8 fuel. A new chemical mechanism has been assembled from existing mechanism of JP-8 and syngas and has been examined by comparing with the experimental data from literatures. The mechanism was then applied to Cantera zero-dimension constant internal energy and constant volume model and one-dimensional (1D) freely propagating flame model to calculate the ignition delay time and laminar burning speed, respectively. The simulations were carried out over a large range of temperature (700–1000 K), blending ratio (0–20% syngas), and H2/CO ratio (10/90 to 50/50). Simulation results showed that the blending syngas with JP-8 will slightly increase the ignition delay time and laminar burning speed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012205-012205-10. doi:10.1115/1.4037372.

Research focused in the present paper to evaluate the combustion, performance, and emission characteristics of refined biodiesel (refined biofuel) such as sunflower oil methyl ester (SOME) with the partial addition of n-butanol (butanol) in it. Various characteristics of butanol–SOME blends with varying volume percentage of butanol such as 5, 10, 15, and 20 in butanol–SOME blends were compared with the characteristics of neat SOME (100%) and neat diesel (100%). It is investigated that with an increase in butanol content from 5% to 20% in butanol–SOME blends at full load condition, brake-specific fuel consumption, and NOx emissions were increased by 11% and 43%, respectively, while brake thermal efficiency (BTE) was decreased by 8%. At full load condition, for all the selected fuels hydrocarbon (HC) emissions were found to be negligible, i.e., less than 0.12 g/kWh. Carbon monoxide (CO) emissions at full load condition for the four butanol–SOME blends were observed to be four to six times more than observed CO emissions in case of neat SOME and neat diesel. Various characteristics of all the selected fuels were compared in order to finalize the promising alternate sustainable renewable fuel. Thus, study reports the solution for increase in demand and price of shortly diminishing conventional diesel fuel which is widely used for power generation and also to reduce the serious issues concerned with environmental pollution due to usage of neat diesel.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012206-012206-9. doi:10.1115/1.4037685.

A coupled computational fluid dynamics (CFD)/discrete element method (DEM) is used to simulate the gas–solid two-phase flow in a laboratory-scale spouted fluidized bed. Transient experimental results in the spouted fluidized bed are obtained in a special test rig using the high-speed imaging technique. The computational domain of the quasi-three-dimensional (3D) spouted fluidized bed is simulated using the commercial CFD flow solver ANSYS-fluent. Hydrodynamic flow field is computed by solving the incompressible continuity and Navier–Stokes equations, while the motion of the solid particles is modeled by the Newtonian equations of motion. Thus, an Eulerian–Lagrangian approach is used to couple the hydrodynamics with the particle dynamics. The bed height, bubble shape, and static pressure are compared between the simulation and the experiment. At the initial stage of fluidization, the simulation results are in a very good agreement with the experimental results; the bed height and the bubble shape are almost identical. However, the bubble diameter and the height of the bed are slightly smaller than in the experimental measurements near the stage of bubble breakup. The simulation results with their experimental validation demonstrate that the CFD/DEM coupled method can be successfully used to simulate the transient gas–solid flow behavior in a fluidized bed which is not possible to simulate accurately using the granular approach of purely Euler simulation. This work should help in gaining deeper insight into the spouted fluidized bed behavior to determine best practices for further modeling and design of the industrial scale fluidized beds.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012207-012207-10. doi:10.1115/1.4037686.

In-cylinder flows in internal combustion (IC) engines have always been a focus of study in order to gain better understanding of fuel–air mixing process and combustion optimization. Different conventional experimental techniques such as hot wire anemometry (HWA), laser Doppler anemometry (LDA), and numerical simulations have been grossly inadequate for complete understanding of the complex 3D flows inside the engine cylinder. In this experimental study, tomographic particle imaging velocimetry (PIV) was applied in a four-valve, single-cylinder optical research engine, with an objective of investigating the in-cylinder flow evolution during intake and compression strokes in an engine cycle. In-cylinder flow seeded with ultra-fine graphite particles was illuminated by a high energy, high frequency Nd:YLF laser. The motion of these tracer particles was captured using two cameras from different viewing angles. These two-directional projections of flowfield were used to reconstruct the 3D flowfield of the measurement volume (36 × 25 × 8 mm3), using multiplicative algebraic reconstruction technique (MART) algorithm. Captured images of 50 consecutive engine cycles were ensemble averaged to analyze the in-cylinder flow evolution. Results indicated that the in-cylinder flows are dependent on the piston position and spatial location inside the engine cylinder. The randomness of air-flow fields during the intake stroke was very high, which became more homogeneous during the compression stroke. The flows were found to be highly dependent on Z plane location inside the engine. During the intake stroke, flows were highly turbulent throughout the engine cylinder, and velocities vectors were observed in all directions. However, during the compression stroke, flow velocities were higher near the injector, and they reduced closer to the valves. Absolute velocity during compression stroke was mainly contributed by the out of plane velocity (Vz) component.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2017;140(1):012901-012901-12. doi:10.1115/1.4037366.

In recent years, colloidal gas aphron (CGA) fluids have been much attended by researchers for their possible application in infill drilling, due to their pore blockage ability. In this study, the possible synergistic effect of silica nanoparticle hydrophobicity in the presence of sodium dodecyl sulfate (SDS), as a surface active agent, on enhancement of properties of CGA fluids was experimentally investigated. Results revealed that the hydrophobicity of nanoparticles, adsorbed at the bubble interface, plays an important role in improving stability and blockage ability at low as well as high pressure/temperature conditions, low shear rate viscosity (LSRV), and return permeability ability of CGA dispersion measured in a special radial sand pack apparatus at different levels of surfactant concentration. It was observed that partially hydrophobic SiO2 nanoparticles (nanosilica coated with KH550-Silane) yield a better performance than both strongly hydrophilic and hydrophobic nanoparticles (silicon dioxide nanopowder coated with 2 wt. % Silane) which confirms what is expected from the particle detachment theory. Optimal SDS concentrations equal to 0.25 wt. % for strongly hydrophilic, and 0.33 wt. % for both strongly hydrophobic and partially hydrophobic SiO2 nanoparticles were also found, which maximize the improving effect of CGA fluids. The superiority of the aphronized fluid improved by partially hydrophobic nanoparticles of SiO2 to CGA fluid stabilized only by surfactant makes the CGA fluids attractive for some industrial and drilling applications.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012902-012902-9. doi:10.1115/1.4037480.

Routine measurement of hydraulic diffusivity of ultralow permeability rocks, such as shale, is a prolonged process. This study explores the effects of a sorptive characteristic of the porous medium on hydraulic diffusivities of shale rocks. The examined rock types include Mancos Shale, Catoosa Shale, Eagle Ford Shale, and core samples from the Gulf of Mexico. First, the adsorption isotherms of the selected shale rocks were obtained. Then, the hydraulic properties of the selected shale rocks were determined using Shale/Fluid Interaction Testing Cell, which employs pore pressure transmission technique. The experimental results show that the moisture content of shale is correlated with water activity using a multilayer adsorption theory. It is found that the adsorption isotherms of various shale formations can be scaled using their respective cation exchange capacity (CEC) into a single adsorption curve. Analyzing the transient pore pressure response in the downstream side of shale sample allows calculating the transport coefficients of shale samples. Hydraulic properties of shales are obtained by matching the pore pressure history with one-dimensional coupled fluid flow model. The experimental results indicate that sorptive properties can be inversely related to the hydraulic diffusivity of shale rocks. It is found that with an increase in the magnitude of sorption potential of shale, the hydraulic diffusivity decreases. This study is useful for shale characterization and provides a correlation, which can have various applications including, but not limited to, wellbore stability prediction during well planning.

Topics: Pressure , Sorption , Rocks , Shales , Fluids
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012903-012903-12. doi:10.1115/1.4037374.

Mathematical formulations have been proposed and verified to determine dynamic dispersion coefficients for solutes flowing in a circular tube with fully developed laminar flow under different source conditions. Both the moment analysis method and the Green's function are used to derive mathematical formulations, while the three-dimensional (3D) random walk particle tracking (RWPT) algorithm in a Cartesian coordinate system has been modified to describe solute flow behavior. The newly proposed formulations have been verified to determine dynamic dispersion coefficients of solutes by achieving excellent agreements with both the RWPT results and analytical solutions. The differences among transverse average concentration using the Taylor model with and without dynamic dispersion coefficient and center-of-mass velocity are significant at early times but indistinguishable when dimensionless time ($tD$) approaches 0.5. Furthermore, compared to solutes flowing in a 3D circular tube, dispersion coefficients of solutes flowing in a two-dimensional (2D) parallel-plate fracture are always larger for a uniform planar source; however, this is not always true for a point source. Solute dispersion in porous media represented by the tube-bundle model is greatly affected by pore-size distribution and increases as standard deviation of pore-size distribution ($σ$) increases across the full-time scale.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012904-012904-14. doi:10.1115/1.4037368.

To have an acceptable accuracy for water flooding projects, proper history matching is an important tool. Capacitance resistance model (CRM) simulates water flooding performance based on two tuning parameters of time constant and connectivity. Main advantages of CRM are its simplicity and fastness; furthermore, it needs only some field-available inputs like injection and production flow rates. CRM is reliable if producers receive the injection rate signal; in other words, duration of history matching must be enough so that the rate signal of injection is sensed in producers. It is a shortcoming of CRM that the results might not be accurate as a result of short history. In the common CRM, time constant is considered to be a static parameter (constant number) during the history of simulation. However, time constant is a time-dependent function that depends on the reservoir nature. In this paper, a new model has been developed as it decreases model dependency on the history matching length by shifting time axis. This new definition adds a rate shift constant to the model mathematics. Moreover, a new model is considering dynamic time constants. This new model is called dynamic capacitance resistance model (DCRM). Two reservoir models have been simulated to analyze the performance of DCRM, and, as a result, it is found that the static time constant is an erroneous assumption. Finally, the accuracy of the results has been improved since the degree-of-freedom of the CRM increased in the new version.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012905-012905-12. doi:10.1115/1.4037682.

Excessive drill stem (DS) vibration while rotary drilling of oil and gas wells causes damages to drill bits and bottom hole assemblies (BHAs). In an attempt to mitigate DS vibrations, theoretical modeling of DS dynamics is used to predict severe vibration conditions. To construct the model, decisions have to be made on which beam theory to be used, how to implement forces acting on the DS, and the geometry of the DS. The objective of this paper is to emphasize the effect of these assumptions on DS vibration behavior under different, yet realistic, drilling conditions. The nonlinear equations of motion were obtained using Hamilton's principle and discretized using the finite element method. The finite element formulations were verified with uncoupled analytical models. A parametric study showed that increasing the weight on bit (WOB) and the drill pipe (DP) length clearly decreases the DS frequencies. However, extending the drill collar length does not reveal a clear trend in the resulting lateral vibration frequency behavior. At normal operating conditions with a low operating rotational speed, less than 80 RPM, the nonlinear Euler–Bernoulli and Timoshenko models give comparable results. At higher rotational speeds, the models deliver different outcomes. Considering only the BHA overestimates the DS critical operating speed; thus, the entire DS has to be considered to determine the critical RPM values to be avoided.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012906-012906-9. doi:10.1115/1.4037712.

Drilling fluid with proper rheology, strong shale, and hydrate inhibition performance is essential for drilling ultralow temperature (as low as −5 °C) wells in deepwater and permafrost. In this study, the performance of drilling fluids together with additives for ultralow temperature wells has been evaluated by conducting the hydrate inhibition tests, shale inhibition tests, ultralow temperature rheology, and filtration tests. Thereafter, the formulation for a highly inhibitive water-based drilling fluid has been developed. The results show that 20 wt % NaCl can give at least a 16-h safe period for drilling operations at −5 °C and 15 MPa. Polyalcohol can effectively retard pore pressure transmission and filtrate invasion by sealing the wellbore above the cloud point, while polyetheramine can strongly inhibit shale hydration. Therefore, a combination of polyalcohol and polyetheramine can be used as an excellent shale stabilizer. The drilling fluid can prevent hydrate formation under both stirring and static conditions. Further, it can inhibit the swelling, dispersion, and collapse of shale samples, thereby enhancing wellbore stability. It has better rheological properties than the typical water-based drilling fluids used in onshore and offshore drilling at −5 °C to 75 °C. In addition, it can maintain stable rheology after being contaminated by 10 wt % NaCl, 1 wt % CaCl2, and 5 wt % shale cuttings. The drilling fluid developed in this study is therefore expected to perform well in drilling ultralow temperature wells.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012907-012907-9. doi:10.1115/1.4037687.

The effects of CO2 injection pressure (P$CO2$) on CO2 dispersion and the mechanism of CO2–CH4 displacement in a shale sampled from Changning of China were studied. Results indicated that Coats–Smith dispersion–capacitance model gave a reasonable simulated result to the breakthrough curves of CO2 under different injection pressures. The shapes of CO2 breakthrough curves became more asymmetrical with the increase of CO2 injection pressure. A higher CO2 injection pressure caused early CO2 breakthrough and reduced the recovery of CH4 at CO2 breakthrough (Rpipeline-$CH4$), but improved the ultimate displaced CH4 amount (Rultimate-$CH4$). With the increase of CO2 injection pressure, dispersion coefficient (Kd) increased nearly exponentially. A larger Kd led to a lower Rpipeline-$CH4$ and a longer transition zone. With the increase of CO2 injection pressure, the flowing fraction (F) in pore space decreased nearly linearly and more CO2 diffused into stagnant region to replace adsorbed CH4 in a shale, which resulted in a larger Rultimate-$CH4$. The mass transfer coefficient (Km) between the flowing and stagnant regions increased with the increase of CO2 injection pressure, which led to a smaller F and larger Rultimate-$CH4$. CO2 diffusion provided major contribution to CO2 dispersion at lower injection pressure, and mechanical mixing of CO2–CH4 offered predominant contribution to CO2 dispersion at higher injection pressure. Larger mechanical mixing accelerated the mixing of CO2–CH4, which was unfavorable for Rpipeline-$CH4$. Lower CO2 injection pressure was conductive to gain higher Rpipeline-$CH4$.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(1):012908-012908-12. doi:10.1115/1.4037713.

The study here presents laboratory testing results of Class F fly ash geopolymer for oil well cementing applications. The challenge reported in literature for the short thickening time of geopolymer ash has been overcome in this study, where more than 5 h of the thickening time is achievable. API Class H Portland cement used a controller on all the tests conducted in this work. Tests conducted in this research include unconfined compressive strength (UCS), shear bond strength, thickening time, shrinkage, free water, and cyclic and durability tests. Results indicate temperature as a crucial factor affecting the thickening time of geopolymer mix slurry. UCS testing indicates considerably higher compressive strength after one and fourteen days of curing for geopolymer mixtures. This indicates gaining strength with time for geopolymer mixture, where time retrogression effects are observed for Portland cements. Results also indicate higher shear bond strength for geopolymer mix that can better tolerate debonding issues. Additionally, more ductile material behavior and higher fracture toughness were observed for optimum geopolymer mixes. Tests also show applicability of these materials for deviated wells as a zero free water test was observed.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Energy Resour. Technol. 2017;140(1):014501-014501-7. doi:10.1115/1.4037536.

Bundled wind–thermal generation system (BWTGS) is an effective way to utilize remote large–scale wind power. The optimal generation maintenance schedule (GMS) for BWTGS is not only helpful to improve the system reliability level but also useful to enhance the system economic efficiency and extend the lifetime of components. This paper presents a model to optimize the GMS for BWTGS. The probabilistic production simulation technique is employed to calculate the system costs, and a sequential probabilistic method is utilized to capture the sequential and stochastic nature of wind power. A hybrid optimization algorithm (HOA) based on the simulated annealing (SA) and multipopulation parallel genetic algorithm (GA) is developed to solve the proposed model. Case studies demonstrate the effectiveness of this proposed model. Effects of the reliability deterioration of thermal generating units (TGUs) and the pattern of BWTGS transmission power are also investigated.

Commentary by Dr. Valentin Fuster