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### Research Papers

J. Energy Resour. Technol. 2018;141(4):041001-041001-11. doi:10.1115/1.4041724.

Diffusion coefficient of carbon dioxide (CO2), a significant parameter describing the mass transfer process, exerts a profound influence on the safety of CO2 storage in depleted reservoirs, saline aquifers, and marine ecosystems. However, experimental determination of diffusion coefficient in CO2-brine system is time-consuming and complex because the procedure requires sophisticated laboratory equipment and reasonable interpretation methods. To facilitate the acquisition of more accurate values, an intelligent model, termed MKSVM-GA, is developed using a hybrid technique of support vector machine (SVM), mixed kernels (MK), and genetic algorithm (GA). Confirmed by the statistical evaluation indicators, our proposed model exhibits excellent performance with high accuracy and strong robustness in a wide range of temperatures (273–473.15 K), pressures (0.1–49.3 MPa), and viscosities (0.139–1.950 mPa·s). Our results show that the proposed model is more applicable than the artificial neural network (ANN) model at this sample size, which is superior to four commonly used traditional empirical correlations. The technique presented in this study can provide a fast and precise prediction of CO2 diffusivity in brine at reservoir conditions for the engineering design and the technical risk assessment during the process of CO2 injection.

Commentary by Dr. Valentin Fuster

### Research Papers: Energy Storage/Systems

J. Energy Resour. Technol. 2019;141(4):041901-041901-10. doi:10.1115/1.4042234.

Lithium-ion battery (LIB) utilization as energy storage device in electric and hybrid-electric vehicles, wind turbine systems, a number of portable electrical devices, and in many other application fields is encouraged due to LIB small size alongside high energy density. Monitoring of LIB health state parameters, calculation of additional LIB operating parameters, and the fulfillment of safety requirements are provided through battery management systems. Prediction of remaining useful lifetime (RUL) of LIB and state-of-health (SoH) estimation are identified as still challenging and not completely solved tasks. In this contribution, previous works on RUL/SoH estimation, mainly relied on modeling of underlying electrochemical processes inside LIB, are compared with newly developed approach. The proposed approach utilizes acoustic emission measurements for LIB aging indicators estimation. Developed model for RUL estimation is closely related to frequency spectrum analysis of captured acoustic emission (AE) signal. Features selected from AE measurements are considered as model inputs. The novelty of this approach is the opportunity to estimate RUL/SoH of LIB without necessity to capture some intermediate variables, only indirectly related to RUL/SoH (charging/discharging currents, temperature, and similar). The proposed approach provides the possibility to obtain reliable information about current RUL/SoH without the knowledge about underlying physical processes occurred in LIB. Experimental data sets gathered from LIB aging tests are used for model establishment, training, and validation. The experimental results demonstrate the applicability of the novel approach.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(4):041902-041902-11. doi:10.1115/1.4042280.

Thermoelectric generators (TEGs) are used to produce electricity utilizing two energy reservoirs. Despite the extensive research conducted on thermoelectric (TE) modules, their efficiencies are still low; therefore, any contribution to increase the efficiency of TE modules is valuable. It is known that the efficiency of individual TE modules depends on the temperature difference between their hot and cold faces. In practical applications employing an array of TE modules, the temperature distribution along the flow direction varies, which adversely affects system's efficiency. In this study, it is aimed to attain a homogeneous temperature distribution along a number of TE pieces by focusing on the structure of TEG heat exchanger. The proposed design includes an intermediate layer of liquid that plays a key role in keeping the temperature distribution homogeneous and at the desired temperature difference level. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed for analyzing the circulation of liquid layer and the thermal behavior in the system. Results show decrease in temperature deviation both on cold and hot sides of TE modules, while the decrease is more on the latter. With more homogeneous temperature distribution along the TE surfaces, it is possible to tune the system to operate TE modules in their optimum temperature differences. It is illustrated that the heat transfer rate is increased by 11.71% and the electric power generation is enhanced by 19.95% with the proposed heat exchanger design. The consumption of pumping power has taken into account in the efficiency calculations.

Commentary by Dr. Valentin Fuster

### Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2018;141(4):042001-042001-9. doi:10.1115/1.4041842.

Fluid flow inside heterogeneous structure of dual porosity reservoirs is presented by two coupled partial differential equations (PDE). Finding an analytical solution for the diffusivity equations is tedious or even impossible in some circumstances due to the heterogeneity of dual porosity reservoirs. Therefore, in this study, orthogonal collocation method (OCM) is proposed for solving the governing equations in dual porosity reservoirs with constant pressure outer boundary. Since no analytical solution has been proposed for this system, validation is carried out by comparing the OCM-obtained results for “dual porosity reservoirs with circular no-flow outer boundary” with both exact analytical solution and real field data. Sensitivity analyses reveal that the OCM with 13 collocation points is a good candidate for prediction of pressure transient response (PTR) in dual porosity reservoirs. OCM predicts the PTR of a real field draw-down test with an absolute average relative deviation (AARD) of 0.9%. Moreover, OCM shows a good agreement with the analytical solution obtained by Laplace transform (AARD = 0.16%). It is worth noting that OCM requires a smaller computational effort. Thereafter, PTR of dual porosity reservoirs with a constant production rate in the wellbore and constant pressure outer boundary is simulated by OCM for wide ranges of operating conditions. Accuracy of OCM and its low required computational time justifies that this approximate method can be considered as a practical candidate for pressure transient analysis in dual porosity reservoirs.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042002-042002-12. doi:10.1115/1.4042003.

This work presents a theoretical thermodynamic study of a compression–absorption cascade refrigeration system using R134a and a lithium bromide–water solution as working fluids. First and second law of thermodynamics analyses were carried out in order to develop an advanced exergetic analysis, by splitting the total irreversibility and that of every component. The potential for improvement of the system is quantified, in the illustrated base-case 55.4% of the irreversibility is of avoidable nature and it could be reduced. The evaporator is the component that shows a significant potential for improvement, followed by the cascade heat exchanger, the compressor, and finally, the generator. The results of the advanced exergetic analysis can be very useful for future design and experimentation of these kinds of systems.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042003-042003-8. doi:10.1115/1.4042004.

Climatic change illustrates the need to new policy of load management. In this research, a special design of thermal energy storage (TES) system, with an appropriate storage medium that is suitable for residential and commercial buildings has been constructed and commissioned. Direct contact heat transfer is a significant factor to enhance the performance of TES. Numerous experimental runs were conducted to investigate the clathrate formation and the characteristics of the proposed TES cooling system; in addition, the effect of using nanofluid particles Al2O3 on the formation of clathrate under different operating parameters was evaluated. The experiments were conducted with a fixed amount of water 15 kg, mass of refrigerant to form clathrate of 6.5 kg, nanofluid particles concentration ranged from 0.5% to 2% and the mass flux of refrigerant varied from 150 to 300 kg/m2 s. The results indicate that there is a significant effect of using nanoparticles concentration on the charging time of the clathrate formation. The percentage of reduction in charging time of about 22% was achieved for high nanoparticles concentration. In addition, an enhancement in charging time by increasing the refrigerant flow rate reaches 38% when the mass flux varied from 200 to 400 kg/m2 s. New correlation describing the behavior of the temperatures with the charging time at different nanoparticles concentrations is presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042004-042004-13. doi:10.1115/1.4041898.

This paper reports the comprehensive thermodynamic modeling of a modified combustion gas turbine plant where Brayton refrigeration cycle was employed for inlet air cooling along with evaporative after cooling. Exergetic evaluation was combined with the emission computation to ascertain the effects of operating variables like extraction pressure ratio, extracted mass rate, turbine inlet temperature (TIT), ambient relative humidity, and mass of injected water on the thermo-environmental performance of the gas turbine cycle. Investigation of the proposed gas turbine cycle revealed an exergetic output of 33%, compared to 29% for base case. Proposed modification in basic gas turbine shows a drastic reduction in cycle's exergy loss from 24% to 3% with a considerable decrease in the percentage of local irreversibility of the compressor from 5% to 3% along with a rise in combustion irreversibility from 19% to 21%. The environmental advantage of adding evaporative after cooling to gas turbine cycle along with inlet air cooling can be seen from the significant reduction of NOx from 40 g/kg of fuel to 1 × 10−9 g/kg of fuel with the moderate increase of CO concentration from 36 g/kg of fuel to 99 g/kg of fuel when the fuel–air equivalence ratio reduces from 1.0 to 0.3. Emission assessment further reveals that the increase in ambient relative humidity from 20% to 80% causes a considerable reduction in NOx concentration from 9.5 to 5.8 g/kg of fuel while showing a negligible raise in CO concentration from 4.4 to 5.0 g/kg of fuel.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042005-042005-6. doi:10.1115/1.4042085.

This study aimed to investigate the effectiveness of the full-scale internal circulation (IC) reactor in biodegrading of municipal solid waste (MSW) fresh leachate under mesophilic conditions, where the anaerobic process stability, biogas yield, and sludge granulation were intensively investigated. The effects of operational parameters on the influent organic loading rate (OLR), chemical oxygen demand (COD) removal efficiency, alkalinity (ALK), pH, volatile fatty acids (VFAs) accumulation, and effluent recirculation were also studied. The results showed that the reactor operated stably and effectively. The COD removal efficiency and biogas yield could be maintained at (92.8 ± 2.0)% and (0.47 ± 0.05) m3/kg CODremoval, respectively, with the influent OLR (24.5 ± 0.9) kg COD/(m3 d) and hydraulic retention time (HRT) 2.7d during the stable operation phase. Meanwhile, this study demonstrated that 1.5–3.0 m/h would be the optimal Vup for the reactor, corresponding to the effluent recirculation of 32.5–78.0 m3/h. Moreover, it was found that the content of extracellular polymeric substances (EPS) in the anaerobic sludge increased from 50.3 to 140.7 mg/g volatile suspended solids (VSS), and the sludge had good granular performance during the reactor operation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042006-042006-9. doi:10.1115/1.4042231.

This paper presents an analysis of the transient operation of a micro combined heat and power (CHP) system, equipped with both thermal and electric storage units and connected with both electric and district heating grids. Analysis is carried out by means of a simulation model developed by the authors for reproducing the transient behavior of micro-CHP systems operating within a microgrid. The prime mover considered in this paper is an internal combustion reciprocating engine. A residential user, characterized by electric and thermal energy demand during one representative summer day, is analyzed by using literature data. The transient response of each component is evaluated separately to quantify the relative deviation (RD) between the user-demand and micro-CHP system transient response. Therefore, this paper provides a measure of the RD over 1 day in terms of the energy required by the user versus the energy provided to the user itself.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042007-042007-7. doi:10.1115/1.4042025.

In this work, the methanol synthesis on a commercial industrial catalyst in a novel cylindrical radial flow packed-bed reactor is investigated. The adiabatic and nonadiabatic cylindrical radial flow reactors were proposed and modeled in this research. The proposed configuration has been compared with conventional reactor for methanol production. It leads to higher methanol production and lower pressure drop, with the same catalyst consumption. Furthermore, the results show that the nonadiabatic radial flow packed-bed reactor has a higher methanol content compared with the adiabatic one. The improvement in methanol production was studied by optimizing the essential parameters such as inlet temperatures of the feed and cooling water as well as the number of cooling tubes. The nonlinearity and complexity of the reactor models make the traditional optimization methods ineffective and improbable. Therefore, the process was optimized by genetic algorithm (GA) method, which is one of the most powerful methods. The optimum values for the number of cooling tubes, feed and cooling water temperatures were 308, 507.6 K, and 522.43 K, respectively. The optimization results showed that a new reactor design could be proposed to reduce the cost of methanol synthesis.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042008-042008-13. doi:10.1115/1.4042087.

In this study, the thermal and operational characteristics of a 400 m3/day mechanical vapor compression desalination (MVCD) system that uses a water-injected twin-screw compressor have been studied and presented. A mathematical model of the MVCD system has been developed including mass and energy conservation equations, heat transfer equations, as well as thermophysical correlations. The effects of the MVCD system design and operation parameters on the system performance are analyzed and discussed. The effect of different boiling-point elevation correlations on the specific area is investigated. The brine and distillate preheaters' areas are studied as a function of inlet seawater temperature. The effect of the injection pressure on system performance is studied. Results show that the optimal injection point is close to the beginning of the compression process. Using this optimum injection pressure, the reduction in power consumption was found to be about 7.3% for high compression ratios. The effects of the brine and feed salinity on system performance are also analyzed. It is found that the specific heat transfer area strongly depends on the brine salinity, especially at temperature differences less than 6 °C. It increases by 44% and 32% at a temperature difference of 4 and 6 °C, respectively. The compressor inlet volume flowrate increases by 9% when the brine salinity increases from 50,000 to 150,000 ppm at all brine boiling temperatures considered. The feed-to-distillate ratio increases rapidly with rising feed salinity, while it decreases with rising brine salinity.

Commentary by Dr. Valentin Fuster

### Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2018;141(4):042201-042201-8. doi:10.1115/1.4041841.

This work highlights the ability of the computational singular perturbation (CSP) method to calculate the significant indices of the modes on evolution of species and the degree of participation of reactions. The exploitation of these indices allows us to deduce the reduced models of detailed mechanisms having the same physicochemical properties. The mechanism used is 16 species and 41 reversible reactions. A reduction of these 41 reactions to 22 reactions is made. A constant pressure application of the detailed and reduced mechanism is made in OpenFOAM free and open source code. Following the Reynolds-averaged Navier–Stokes simulation scheme, standard k–ε and partial stirred reactor are used as turbulence and combustion models, respectively. To validate the reduced mechanism, comparison of numerical results (temperature and mass fractions of the species) was done between the detailed mechanism and the simplified model. This was done using the DVODE integrator in perfectly stirred reactor. After simulation in the computational fluid code dynamic (CFD) OpenFOAM, other comparisons were made. These comparisons were between the experimental data of a turbulent nonpremixed diffusion flame of type “DLR-A flame,” the reduced mechanism, and the detailed mechanism. The calculation time using the simplified model is considerably reduced compared to that using the detailed mechanism. An excellent agreement has been observed between these two mechanisms, indicating that the reduced mechanism can reproduce very well the same result as the detailed mechanism. The accordance with experimental results is also good.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042202-042202-7. doi:10.1115/1.4041725.

Nitromethane is extensively used in drag races and in glow plug unmanned aerial vehicle (UAV) engines. However, it has not been analyzed in the combustion literature enough. Nitromethane has a low stoichiometric air–fuel ratio; it can be blended with gasoline and used in larger quantities to enhance the power output of the internal combustion (IC) engines. This could find potential use in burgeoning UAV industry. The present investigation aims at experimentally determining the laminar burning speeds of nitromethane—gasoline blends at different equivalence ratios. Tests were conducted at both ambient conditions and at elevated temperatures and pressures. A constant volume combustion chamber (CVCC) was constructed and instrumented to carry out the investigation. The pressure rise in the chamber due to combustion was acquired and analyzed to determine the laminar burning speeds. The results showed that with an increase in the nitromethane concentration in gasoline, the laminar burning speeds for all the initial conditions also increased. With the rise in initial temperatures, the laminar burning speeds were observed to increase. However, a drop was observed with a rise in the initial pressures for all the blends. The obtained results for pure gasoline were compared with existing literature. A good match was observed. The investigation also aims at providing vital experimental data, which can be used for computational fluid dynamics validation studies later.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042203-042203-7. doi:10.1115/1.4042086.

This study attempts to identify the optimum dosing level of aqueous aluminum oxide nanofluid in diesel to improve combustion and engine performance and also to overcome the engine emission issues especially, the oxide of nitrogen, smoke, and the particulate matter. The aqueous aluminum oxide (aluminum oxide nanoparticle aqueous 5 wt % suspension) is used as a nanofluid. The dosing level of nanofluid is varied from 30 cc to 60 cc in steps of 10 cc for the performance study. Fuel blend properties such as calorific value, density, kinematic viscosity, and flash point are determined using ASTM standard test methods. Among all blends, the D+50AN showed a maximum improvement of about 5.9% in brake thermal efficiency (BTE) and remarkable reduction in NOx, smoke, HC, and CO as 15.6%, 22.34%, 31.82%, and 13.79%, respectively, at maximum rated power output.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042204-042204-14. doi:10.1115/1.4042068.

Intake generated flows are known to have a fundamental influence on the combustion both in spark ignition (SI) and compression ignition engines. This study experimentally investigated the tumble flow structures inside a cylinder of gasoline direct injection (GDI) engine utilizing a stereoscopic time-resolved particle image velocimetry (PIV). The experiments were conducted in a GDI engine head for a number of fixed valve lifts and 150 mmH2O pressure difference across the intake valves. A tumble flow analysis was carried out considering different vertical tumble planes. In addition, the proper orthogonal decomposition (POD) identification technique was applied on the PIV data in order to spatially analyze the structures embedded in the instantaneous velocity data sets. The results showed that the flow was dominated by a strong tumble motion in the middle of cylinder at high valve lifts (8–10 mm). Moreover, it is worth pointing out that, because of the complexity of the flow at the high valve lifts, the flow energy was distributed over a higher number of POD modes. This was confirmed by the need of a higher number of POD modes needed to reconstruct the original velocity field to the same level of fidelity.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042205-042205-9. doi:10.1115/1.4042005.

This research investigated the effects of the specific primary (under-fire) air flowrate ($m˙$air) on the combustion behavior of a 50–50 wt % blend of raw corn straw (CS) and raw pinewood wastes in a fixed-bed reactor. This parameter was varied in the range of 0.079–0.226 kg m−2 s−1, which changed the overall combustion stoichiometry from air-lean (excess air coefficient λ = 0.73) to air-rich (excess air coefficient λ = 1.25) and affected the combustion efficiency and stability as well as the emissions of hazardous pollutants. It was observed that by increasing $m˙$air, the ignition delay time first increased and then decreased, the average bed temperatures increased, both the average flame propagation rates and the fuel burning rates increased, and the combustion efficiencies also increased. The emissions of CO as well as those of cumulative gas phase nitrogen compounds increased, the latter mostly because of increasing HCN, while those of NO were rather constant. The emissions of HCl decreased but those of other chlorine-containing species increased. The effect of $m˙$air on the conversion of sulfur to SO2 was minor. By considering all of the aforesaid factors, a mildly overall air-rich (fuel-lean) (λ = 1.04) operating condition can be suggested for corn-straw/pinewood burning fixed-bed grate-fired reactors.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(4):042206-042206-12. doi:10.1115/1.4042824.

This work investigates the performance of film-cooling on trailing edge of gas turbine blades using unsteady three-dimensional numerical model adopting large eddy simulation (LES) turbulence scheme in a low Mach number flow regime. This study is concerned with the scaling parameters affecting effectiveness and heat transfer performance on the trailing edge, as a critical design parameter, of gas turbine blades. Simulations were performed using ANSYS-fluentworkbench 17.2. High quality mesh was adapted, whereas the size of cells adjacent to the wall was optimized carefully to sufficiently resolve the boundary layer to obtain insight predictions of the film-cooling effectiveness on a flat plate downstream the slot opening. Blowing ratio, density ratio, Reynolds number, and the turbulence intensity of the mainstream and coolant flow are optimally examined against the film-cooling effectiveness. The predicted results showed a great agreement when compared with the experiments. The results show a distinctive behavior of the cooling effectiveness with blowing ratio variation as it has a dip in vicinity of unity which is explained by the behavior of the vortex entrainment and momentum of coolant flow. The negative effect of the turbulence intensity on the cooling effectiveness is demonstrated as well.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2019;141(4):042207-042207-6. doi:10.1115/1.4042826.

The paper presents physicochemical properties of pyrolysis oil (PO) blends obtained from pyrolysis of rubber and spent tires mixed with selected heavy fuel oil (HFO) and the effect of PO properties on physicochemical properties of the final heavy heating oil. On the basis of physicochemical properties determinations, one sample of PO was selected, which was characterized by the best properties from the point of view of technological application. In the next step, physicochemical properties for the selected sample of heavy heating fuel oil consisting of 25% PO and 75% HFO were determined. It was found that the most important property of tire-derived PO is the content of gasoline, i.e., light hydrocarbons with a boiling point below 180 °C, which determine the ignition temperature of the obtained fuel blends. This property determines also the amount of PO that can be added to HFO, on the order of 30 wt % and more. The lower content of light hydrocarbons, the greater the amount of PO can be used to compose HFO. A positive aspect of the use of tire derive PO for the composing of heavy heating fuel is about a threefold decrease in kinematic viscosity, lowering the flow temperature and a significant reduction in ash content. Other properties of the modified HFO remained virtually unchanged and the fuel obtained as a result of blending meets the requirements of the relevant standard.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2018;141(4):042901-042901-16. doi:10.1115/1.4041839.

In order to lessen the computational time in fractured oil reservoir simulations, all fractures are usually assumed to be as one equivalent fracture at the center or around the model. This, specially, has applications in industrial engineering software, where this assumption applies. In this study, using two general contradictory examples, it is shown that ignoring a fracture network and assuming an equivalent single-fracture has no logical justification and results in a considerable error. The effect of fracture aperture on composition distribution of a binary and a ternary mixture was also investigated. These mixtures were C1 (methane)/n-C4 (normal-butane) and C1 (methane)/C2 (ethane)/n-C4 (normal-butane), which were under diffusion and natural convection. Governing equations were numerically solved using matlab. One of the main relevant applications of this study is where permeability and temperature gradient are the key difference between reservoirs. Compositional distribution from this study could be used to estimate initial oil in place. Using this study, one can find the optimum permeability, namely the permeability at which the maximum species separation happens, and the threshold permeability (or fracture aperture), after which the convection imposes its effect on composition distribution. It is found that the threshold permeability is not constant from reservoir to reservoir. Also, one can find that full mixing happens in the model, namely heavy and light densities of top and bottom mix up together in the model. Furthermore, after maximum separation point, convection causes unification of components.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042902-042902-9. doi:10.1115/1.4041843.

The rheological properties of bentonite suspensions depend on the chemical composition and the contained dominant element, such as calcium (Ca), potassium (K), and sodium (Na). Na-bentonite type is the one used in drilling fluids, because it has good dispersion stability, high swelling capacity, and outstanding rheological properties. Ca-bentonite generally has bad rheological performance; however, it can be activated by sodium to be used in drilling fluids, since there are huge unutilized Ca-bentonite resources. Many previous attempts of activation of Ca-bentonite were not feasible, upgrading required addition of many extra additives or sometimes mixed with commercial Na-bentonite to improve its properties. In this paper, a process of integrated beneficiation method is designed to efficiently remove the nonclay impurities and produce pure Ca-bentonite. An upgraded Ca-bentonite was produced using a combined thermochemical treatment in a wet process by adding 4 wt % of soda ash (Na2CO3) while heating and stirring. The new thermal treatment optimized and described in this study greatly improved the sodium activation and ions exchange process and improved bentonite properties. The thermochemically upgraded Ca-bentonite outperformed the rheological properties of the commercial bentonite. And when tested in a typical drilling fluid formulation at high temperature, the investigations showed an identical behavior of the commercial drilling grade bentonite. Moreover, the results obtained showed that the thermochemically upgraded Ca-bentonite has higher yield point/plastic viscosity (YP/PV) ratio than commercial Na-bentonite when mixed with the drilling fluid additives. Higher YP/PV ratio is expected to enhance the hole cleaning and prevent most of the drilling problems.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;141(4):042903-042903-9. doi:10.1115/1.4041840.

During the drilling operations, optimizing the rate of penetration (ROP) is very crucial, because it can significantly reduce the overall cost of the drilling process. ROP is defined as the speed at which the drill bit breaks the rock to deepen the hole, and it is measured in units of feet per hour or meters per hour. ROP prediction is very challenging before drilling, because it depends on many parameters that should be optimized. Several models have been developed in the literature to predict ROP. Most of the developed models used drilling parameters such as weight on bit (WOB), pumping rate (Q), and string revolutions per minute (RPM). Few researchers considered the effect of mud properties on ROP by including a small number of actual field measurements. This paper introduces a new robust model to predict the ROP using both drilling parameters (WOB, Q, ROP, torque (T), standpipe pressure (SPP), uniaxial compressive strength (UCS), and mud properties (density and viscosity) using 7000 real-time data measurements. In addition, the relative importance of drilling fluid properties, rock strength, and drilling parameters to ROP is determined. The obtained results showed that the ROP is highly affected by WOB, RPM, T, and horsepower (HP), where the coefficient of determination (T2) was 0.71, 0.87, 0.70, and 0.92 for WOB, RPM, T, and HP, respectively. ROP also showed a strong function of mud fluid properties, where R2 was 0.70 and 0.70 for plastic viscosity (PV) and mud density, respectively. No clear relationship was observed between ROP and yield point (YP) for more than 500 field data points. The new model predicts the ROP with average absolute percentage error (AAPE) of 5% and correlation coefficient (R) of 0.93. In addition, the new model outperformed three existing ROP models. The novelty in this paper is the application of the clustering technique in which the formations are clustered based on their compressive strength range to predict the ROP. Clustering yielded accurate ROP prediction compared to the field ROP.

Commentary by Dr. Valentin Fuster

### Discussion

J. Energy Resour. Technol. 2018;141(4):045501-045501-4. doi:10.1115/1.4041844.

Mathematical methods such as empirical correlations, analytical models, numerical simulations, and data-intensive computing (data-driven models) are the key to the modeling of energy science and engineering. Accrediting of different models and deciding on the best method, however, is a serious challenge even for experts, as the application of models is not limited only to estimations, but to predictions and derivative properties. In this note, by combining meaningful metrics of accuracy and precision, a new metric for determining the best-in-class method was defined.

Commentary by Dr. Valentin Fuster