Research Papers

J. Energy Resour. Technol. 2016;139(3):031001-031001-7. doi:10.1115/1.4035020.

The increasing penetration of variable and unpredictable renewable energy sources into the liberalized electricity market brings about significant changes in the management strategies of fossil fuel power plants. These new operation modes cause remarkable effects on the lifetime of the plants. Consequently, the operators of the plants need to be assisted by effective procedures, which are able to define suitable production plans. In the present paper, the authors propose a dynamic model which can be used to estimate the effects of the variations of thermodynamic and mechanical parameters during transient operation, start-ups and shut-downs. To check the effectiveness of the model, a combined cycle plant with a three-pressure level heat recovery steam generator has been selected. The geometry of the components, the influence of the environmental conditions, and the control strategies are included in the model. In this way, the residual lifetime of the most critical components can be predicted.

Commentary by Dr. Valentin Fuster

Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2017;139(3):031201-031201-11. doi:10.1115/1.4035782.

Solar chimney or Trombe wall has been studied numerically and analytically. Analytical results available in the literature overestimate air flow rate by 46–97%. While insulated walls are used in the experiments, there might still be loss from the chimney walls, which is not usually considered in the available analytical models. It is found that the overestimation of air flow rate can be reduced to 3–14% by including heat losses from the glass and wall side of the chimney in the analytical model. The presently developed numerical model is validated against experimental data from literature. The conditions within which the analytical solution can give good approximate results regarding the air volume flow rate have been identified and discussed. We found that the analytical method simulates solar chimneys well for gap widths of up to 0.3 m and incident radiation above 500 W/m2. The present numerical results revealed that the optimum value of chimney gap width that maximizes the induced flow through the chimney is 0.3 m.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):031202-031202-13. doi:10.1115/1.4035423.

This paper offers tools and insights regarding wind farm layout to developers in determining the conditions under which it makes sense to invest resources into more accurately predicting of the cost-of-energy (COE), a metric to assess farm viability. Using wind farm layout uncertainty analysis research, we first test a farm design optimization model's sensitivity to surface roughness, economies-of-scale costing, and wind shear. Next, we offer a method for determining the role of land acquisition in predicting uncertainty. This parameter—the willingness of landowners to accept lease compensation offered to them by a developer—models a landowner's participation decision as a probabilistic interval utility function. The optimization-under-uncertainty formulation uses probability theory to model the uncertain parameters, Latin hypercube sampling to propagate the uncertainty throughout the system, and compromise programming to search for the nondominated solution that best satisfies the two objectives: minimize the mean value and standard deviation of COE. The results show that uncertain parameters of economies-of-scale cost-reduction and wind shear have large influence over results in the sensitivity analysis, while surface roughness does not. The results also demonstrate that modeling landowners' participation in the project as uncertain allows the optimization to identify land that may be risky or costly to secure, but worth the investment. In an uncertain environment, developers can predict the viability of the project with an estimated COE and give landowners an idea of where turbines are likely to be placed on their land.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):031203-031203-11. doi:10.1115/1.4035907.

In this paper, the aerodynamics of two vertical axis wind turbines (VAWTs) are discussed, on the basis of a wide set of experiments performed at Politecnico di Milano, Milan, Italy. A H-shaped and a Troposkien Darrieus turbine for microgeneration, featuring the same swept area and blade section, are tested at full-scale. Performance measurements show that the Troposkien rotor outperforms the H-shaped turbine, thanks to the larger midspan section of the Troposkien rotor and to the nonaerodynamic struts of the H-shaped rotor. These features are consistent with the character of the wakes shed by the turbines, measured by means of hot wire anemometry on several surfaces downstream of the models. The H-shape and Troposkien turbine wakes exhibit relevant differences in the three-dimensional morphology and unsteady evolution. In particular, large-scale vortices dominate the tip region of the wake shed by the H-shape turbine; these vortices pulsate significantly during the period, due to the periodic fluctuation of the blade aerodynamic loading. Conversely, the highly tapered shape of the Troposkien rotor not only prevents the onset of tip vortices, but also induces a dramatic spanwise reduction of tip speed ratio (TSR), promoting the onset of local dynamic stall marked by high periodic and turbulent unsteadiness in the tip region of the wake. The way in which these mechanisms affect the wake evolution and mixing process for the two classes of turbines is investigated for different tip speed ratios, highlighting some relevant implications in the framework of wind energy exploitation.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2016;139(3):032001-032001-8. doi:10.1115/1.4034514.

In this work, for the first time, an energy harvester based on the nonlinear dynamical response of a parametrically excited cantilever beam in contact with mechanical stoppers has been fabricated and tested; a 145% increase in the bandwidth at which energy can be effectively harvested has been observed. Experimental and theoretical investigations have been performed in order to assess the increased operating bandwidth of the energy harvester fabricated; for the experimental investigations, an electrodynamic shaker connected to a shaking table has been used to parametrically stimulate the energy harvesting device. Results showed that the parametric energy harvester without stoppers displayed a weak softening-type nonlinear response; however, with the addition of mechanical stoppers the energy harvester displayed a strong hardening-type nonlinear response which is ideal for capturing kinetic energy over larger bandwidths. The addition of mechanical stoppers on a parametrically excited cantilever beam has significant qualitative and quantitative effects on the nonlinear parametric energy harvesting; the energy harvesting bandwidth was increased in the range of 35–145% by adjusting the stoppers.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032002-032002-9. doi:10.1115/1.4035057.

A thermodynamic model and parametric analysis of a natural gas-fired power plant with carbon dioxide (CO2) capture using multistage chemical looping combustion (CLC) are presented. CLC is an innovative concept and an attractive option to capture CO2 with a significantly lower energy penalty than other carbon-capture technologies. The principal idea behind CLC is to split the combustion process into two separate steps (redox reactions) carried out in two separate reactors: an oxidation reaction and a reduction reaction, by introducing a suitable metal oxide which acts as an oxygen carrier (OC) that circulates between the two reactors. In this study, an Aspen Plus model was developed by employing the conservation of mass and energy for all components of the CLC system. In the analysis, equilibrium-based thermodynamic reactions with no OC deactivation were considered. The model was employed to investigate the effect of various key operating parameters such as air, fuel, and OC mass flow rates, operating pressure, and waste heat recovery on the performance of a natural gas-fired power plant with multistage CLC. The results of these parameters on the plant's thermal and exergetic efficiencies are presented. Based on the lower heating value, the analysis shows a thermal efficiency gain of more than 6 percentage points for CLC-integrated natural gas power plants compared to similar power plants with pre- or post-combustion CO2 capture technologies.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032003-032003-6. doi:10.1115/1.4035067.

Due to the increasing demand for clean and potable water stemming from population growth and exacerbated by the scarcity of fresh water resources, more attention has been drawn to innovative methods for water desalination. Capacitive deionization (CDI) is a low maintenance and energy efficient technique for desalinating brackish water, which employs an electrical field to adsorb ions into a high-porous media. After the saturation of the porous electrodes, their adsorption capacity can be restored through a regeneration process. Herein, based on a physical model previously developed, we conjecture that for a given amount of time and volume of water, multiple desalination cycles in a high flow rate regime will outperform desalinating in a single cycle at a low flow rate. Moreover, splitting a CDI unit into two subunits, with the same total length, will lead to higher desalination. Based on these premises, we introduce a new approach aimed at enhancing the overall performance of CDI. An array of CDI cells are sequentially connected to each other with intermediate solutions placed in between them. Desalination tests were conducted to compare the performance of the proposed system, consisting of two CDI units and one intermediate solution buffer, with a two-cascaded-CDI unit system with no intermediate solution. Experimental data demonstrated the improved performance of the buffered system over the nonbuffered system, in terms of desalination percentage and energy consumption. The new proposed method can lead to lower amount of energy consumed per unit volume of the desalinated water.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032004-032004-8. doi:10.1115/1.4035749.

The design, construction, and experimental evaluation of a cascade thermoacoustic engine are presented in this paper. The system was designed and built under the constraint of an inexpensive device to meet the energy needs of the people based in remote and rural areas. From the cost and straightforward system point of view, the air at atmospheric pressure was applied as a working fluid, and the main resonator tubes were then constructed of conventional polyvinyl chloride (PVC) pipes. Such device consists of one standing-wave unit and one traveling-wave unit connected in series. This topology is preferred because the traveling-wave unit provides an efficient energy conversion, and a straight-line series configuration is easy to build and allows no Gedeon streaming. The system was designed to operate at a low frequency of about 57 Hz. The measured results were in a reasonably good agreement with the predicted results. So far, this system can deliver up to 61 W of acoustic power, which was about 17% of the Carnot efficiency. In the further step, the proposed device will be applied as the prime mover for driving the thermoacoustic refrigerator.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032005-032005-10. doi:10.1115/1.4035750.

Intermittency of sustainable energy or waste heat availability calls for energy storage systems such as thermal batteries. Thermochemical batteries based on a reversible solid–gas (MgCl2–NH3) reactions and NH3 liquid–gas phase change are of specific interest since the kinetics of absorption are fast and the heat transfer rates for liquid–vapor phase change are high. Thus, a thermochemical battery based on reversible reaction between magnesium chloride and ammonia was studied. Two-dimensional experimental studies were conducted on a reactor in which temperature profiles within the solid matrix and pressure and flow rates of gas were obtained during discharging processes. A numerical model based on heat and mass transfer within the salt and salt–gas reactions was developed to simulate the NH3 absorption processes within the solid matrix, and the results were compared with experimental data to determine dominant heat and mass transfer processes within the salt. It is shown that for high permeability salt beds, the reactor uniformly adsorbs gaseous ammonia until the bed reaches the equilibrium temperature, then adsorbs gas near the cooled boundaries as the reaction front moves inward. In that mode, the heat transfer is the dominant factor in determining reaction rates.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2016;139(3):032201-032201-8. doi:10.1115/1.4034858.

The influence of nanoparticles' dispersion on the physical properties of aviation fuel and its spray performance has been investigated in this work. To this end, the conventional Jet A-1 aviation fuel and its mixtures with alumina nanoparticles (nanofuel) at different weight concentrations are investigated. The key fuel physical properties such as density, viscosity, and surface tension that are of importance to the fuel atomization process are measured for the base fuel and nanofuels. The macroscopic spray features like spray cone angle and sheet breakup length are determined using the shadowgraph technique. The microscopic spray characteristics such as droplet diameter, droplet velocity, and their distributions are also measured by employing phase Doppler anemometry (PDA) technique. The spray performance is measured at two nozzle injection pressures of 0.3 and 0.9 MPa. The results show that with the increase in nanoparticle concentrations in the base fuel, the fuel viscosity and density increase, whereas the surface tension decreases. On the spray performance, the liquid sheet breakup length decreases with increasing nanoparticle concentrations. Furthermore, the mean droplet diameters of nanofuel are found to be lower than those of the base fuel.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032202-032202-5. doi:10.1115/1.4035023.

The present work shows that how the angle of an air swirler vane affects the combustion characteristics of liquid fuels such as flame temperature, radiation heat flux, combustion efficiency, and pollutants' emission. It finds out an optimum angle of vane based on flame characteristics. Three vanes with angles of 0 deg, 40 deg, and 80 deg which induced low and high-swirl intensities in air stream were investigated, and the combustion characteristics of flame were quantified. The flame temperature was measured by an S-type thermocouple, and a Testo 350 XL gas analyzer was used to determine the CO and NO pollutant concentrations. Also, gravity method was used to gauge the soot concentration along the furnace, and a SBG01 water cooled heat flux sensor determined the flame radiation. The results indicate that the angle of the swirler vane has significant effects on temperature, combustion efficiency, and NO and CO pollutants' emission. Most importantly, there is an optimum angle for the swirler vane. At the optimum angle, the optimum combination of the contact area and time maximizes the mixing rate of the inlet air and the fuel jet. Consequently, at the optimum angle, the mean temperature, radiation heat flux, and combustion efficiency are higher than at small and large swirl angles and soot, CO and NOx emissions are at their minimum states.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032203-032203-10. doi:10.1115/1.4034979.

In this paper, catalyzed pyrolysis of scrap tires was studied in order to identify the influence of catalysts on gas composition during the main thermal range of the decomposition process. The aim is related to gas fraction optimization in terms of yield, composition, and distribution during the pyrolysis process. This is an original work using for the first time powder catalysts (MgO, Al2O3, CaCO3, and zeolite ZSM-5) uniformly distributed on a single layer of oyster shells (OSs) particles. The catalyst/tires mass ratio was kept for all the tests at 1/30. Depending on used catalyst, pyrolysis products yields ranged from 39 to 42 wt.% for char, from 26 to 38 wt.% for oils, and from 16 to 30 wt.% for gas. Compared to the thermal pyrolysis, it was found that the liquid yield increases in the presence of MgO/OS, while the use of Al2O3/OS decreases it significantly. The gas yield grows in the presence of Al2O3/OS ranging from 24.6 wt.% (thermal pyrolysis) to 30.6 wt.%. On the other hand, ZSM-5/OS and CaCO3/OS did not bring significant changes in products yield, but there are considerable influences on the evolution of gas composition during the tires decomposition. Also, two important advantages of using these new catalytic systems are identified. These relate to the formation of gaseous species throughout the waste decomposition, thus harmonizing the calorific value for the entire thermal range, and the disappearance of heavy molecules in liquid fractions, simplifying or canceling further upgrading processes.

Topics: Catalysts , Pyrolysis , Tires
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032204-032204-6. doi:10.1115/1.4035828.

Dual-fuel reactivity-controlled compression ignition (RCCI) combustion can yield high thermal efficiency and simultaneously low NOx and soot emissions. Although soot emissions from RCCI are very low, hydrocarbon (HC) emissions are high, potentially resulting in higher than desired total particulate matter (PM) mass and number caused by semivolatile species converting the particle phase upon primary dilution in the exhaust plume. Such high organic fraction PM is known to be highly sensitive to dilution conditions used when collecting samples on a filter or when measuring particle number using particle sizing instruments. In this study, PM emissions from a modified single-cylinder diesel engine operating in RCCI and conventional diesel combustion (CDC) modes were investigated under controlled dilution conditions. To investigate the effect of the fumigated fuel on the PM emissions, 150 proof hydrous ethanol and gasoline were used as low reactivity fuels. The data reveal that PM from RCCI combustion is more sensitive to the variation of dilution conditions than PM from single fuel conventional diesel combustion. RCCI PM primarily consisted of semivolatile organic compounds and a smaller amount of solid carbonaceous particles. The fumigated fuel had a significant effect on PM emissions' characteristics for RCCI combustion. Hydrous ethanol fueled RCCI PM contained a larger fraction of volatile materials and was more sensitive to the variation of dilution conditions compared to the gasoline fueled RCCI mode.

Commentary by Dr. Valentin Fuster

Research Papers: Oil/Gas Reservoirs

J. Energy Resour. Technol. 2016;139(3):032801-032801-10. doi:10.1115/1.4034857.

In this paper, a reservoir simulation study was conducted for the characterization and prediction of gas breakthrough during the development of cyclic steam and gas stimulation (CSGS) for a horizontal well. A new concept named the gas breakthrough coefficient (GBC) was proposed to characterize the gas breakthrough degree quantitatively, and a regression model and two calibration curves were established to predict the gas breakthrough degree. The method of foam plugging to inhibit gas breakthrough was also discussed. It was found that the gas breakthrough degree could be well characterized by the GBC and distinguished as four types: weak, moderate, strong, and severe. The regression model and calibration curves could also be used to predict the gas breakthrough degree under different reservoir and development conditions. Foam plugging was found to be effective to inhibit gas breakthrough when the gas breakthrough degree was moderate or strong.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2016;139(3):032901-032901-8. doi:10.1115/1.4034810.

A relatively simple, general, and very flexible method to design complex, three-dimensional hole trajectories can be obtained by using a 3D extension for Bézier curves. This approach offers superior results in terms of coding, use, and flexibility compared to other methods using double-arc, cubic functions, spline-in-tension functions, or constant curvature. The mathematics is surprisingly simple, and the method can be used to obtain trajectories for any of the four typical end conditions in terms of inclination and azimuth, namely: free-end, set-end, set-inclination/free azimuth, and free-inclination/set-azimuth. The resulting trajectories are smooth, with continuous and smooth change of curvature and toolface, better exploiting the expected delivery of modern rotary steerable deviation tools, particularly the point-the-bit and the push-the-bit systems. With the relevant parameters at any point of the trajectory (curvature and toolface angle) an automated system can steer the hole toward the defined targets in a smooth fashion. The beauty of the method is that the description of the trajectory is obtained with one single expression that handles the three space coordinates, instead of working with three separate coordinate functions. It uses a generalization of the well-known 2D Bézier curve. The concept is easy to understand, and implementation even using spreadsheets is straightforward. Besides, the conditions at both ends (coordinates and inclination/azimuth for set ends) the trajectory curve has up to two independent parameters. By playing suitably with these parameters, one can obtain a curve that favors the reduction of drag and torque during drilling, tripping, and casing running. In addition to the formulation for trajectory calculation, the paper presents the expressions to calculate the inclination, azimuth, curvature, and toolface at any point along the trajectory. Proper numerical examples illustrate the various end-conditions. The method can be used during the hole planning cycle as well as during the hole drilling for automatic and manual steerage.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032902-032902-10. doi:10.1115/1.4034808.

This paper presents an efficient production optimization scheme for an oil reservoir undergoing water injection by optimizing the production rate for each well. In this approach, an adaptive version of simulated annealing (ASA) is used in two steps. The optimization variables updating in the first stage is associated with a coarse grid model. In the second step, the fine grid model is used to provide more details in final solution search. The proposed method is formulated as a constrained optimization problem defining a desired objective function and a set of existing field/facility constraints. The use of polytope in the ASA ensures the best solution in each iteration. The objective function is based on net present value (NPV). The initial oil production rates for each well come from capacity and property of each well. The coarse grid block model is generated based on average horizon permeability. The proposed optimization workflow was implemented for a field sector model. The results showed that the improved rates optimize the total oil production. The optimization of oil production rates and total water injection rate leads to increase in the total oil production from 315.616 MSm3 (our initial guess) to 440.184 MSm3, and the recovery factor is increased to 26.37%; however, the initial rates are much higher than the optimized rates. Beside this, the recovery factor of optimized production schedule with optimized total injection rate is 3.26% larger than the initial production schedule with optimized total water injection rate.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032903-032903-8. doi:10.1115/1.4035021.

The application of equilibrium thermodynamics in the study of thermal plant performance under real operating conditions is a constant challenge. In this paper, an analysis of a reservoir pressure piston working between two linear flow resistances is performed by considering the friction of the piston cylinder system on the walls. The proposed model is developed to obtain the optimum power output and speed of the piston in terms of first law efficiency. If the friction on the piston–cylinder assembly is neglected, the expressions obtained are consistent with those presented in the literature under laminar regime. It was also demonstrated that for both laminar and turbulent regimes with overall size constraints, the power delivered can be maximized by balancing the upstream and downstream flow resistances of the piston. This paper also evaluated the influence of the overall size constraints and flow regime on the performance of the piston cylinder. This analysis is equivalent to evaluate the irreversibilities in an endo-irreversible Carnot heat engine with heat loss resistance between the engine and its heat reservoirs. The proposed model introduced some modifications to the results obtained from the recent literature and led to important conclusions. Finally, the proposed model was applied to calculate the lost available work in a turbine operating at steady flow conditions with an ideal gas as working fluid.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;139(3):032904-032904-7. doi:10.1115/1.4035022.

Drilling mud should be properly designed to build an effective filter cake on the formation face during the drilling process. This filter cake should be removable to allow the oil and gas production. The need for removal increases when the liftoff pressure is high or when the formation drawdown is extremely low. An effective filter cake removal design includes the knowledge of the filter cake composition along the horizontal section. This paper, for the first time, introduces material balance model to predict the composition of the filter cake along the length of the lateral of an actual horizontal well drilled in a sandstone formation. The model is based on the material balance of two sources of solids: the first one is the drilling fluid solids and the second one is the drilled-formation solids. The mud used to drill the rock was contaminated by the drilled-formation solids. The parameters used to construct the model were composition of the mud and formation, efficiency of each separation stage, rate of penetration (ROP), and mud circulation rate. The model was validated with actual mud samples collected from different locations along the horizontal section of a sandstone formation. The model showed that the sand content in the filter cake is affected by ROP, rock composition, mud composition and volume, and efficiency of sand separation equipment. We came up with several correlations that can be used to design the drilling fluid operations in horizontal well to avoid the formation of irremovable filter cake.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032905-032905-9. doi:10.1115/1.4035746.

This paper reports the findings of an investigation into the molecular structures and properties of three asphaltene samples, namely, an asphaltene sample extracted from Buton Oil Sand (Indonesia), and two asphaltene samples extracted from vacuum residues from Liaohe Refinery (China) and Vene Refinery (Venezuela), respectively. The average molecular structural parameters, including the average polycyclic aromatic hydrocarbon (PAH) size, average side chain length, and average molecular weight (AMW), of the three asphaltene samples were estimated using data from nuclear magnetic resonance (NMR) in combination with distortionless enhancement by polarization transfer (DEPT), and then compared against each other. The molecular weight distributions (MWDs) of the three asphaltene samples were measured using a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The results indicated that the island molecular architecture predominated in all three asphaltenes and the average polycyclic aromatic hydrocarbon size was found to be six rings. The average molecular weight of the Buton asphaltene sample was found to be ca. 800 Da while those of the two petroleum asphaltene samples were approximately 600 Da. In comparison, the Buton asphaltene sample contained a much higher level of oxygen and sulfur, but a lower aromaticity than those of the two petroleum asphaltene samples. The use of liquid NMR in combination with DEPT was shown to provide an effective method for characterization and estimation of the molecular structures of asphaltenes, supported by MALDI-TOF mass spectra.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032906-032906-10. doi:10.1115/1.4035552.

Radial jet drilling (RJD) is an efficient approach for improving the productivity of wells in low permeability, marginal and coal-bed methane (CBM) reservoirs at a very low cost. It uses high-pressure water jet to drill lateral holes from a vertical wellbore. The length of the lateral holes is greatly influenced by the frictional resistance in the hole deflector. However, the hole deflector frictional resistance and structure design have not been well studied. This work fills that gap. Frictional resistances were measured in a full-scale experiment and calculated by numerical simulation. The structure of the hole deflector was parameterized and a geometric model was developed to design the hole deflector track. An empirical model was then established to predict the frictional resistance as a function of the hole deflector structure parameters and an optimization method for designing the hole deflector was proposed. Finally, four types of hole deflectors were optimized using this method. The results show good agreement between the numerical simulation and the experimental data. The model error is within 11.6%. The bend radius R and exit angle β are the key factors affecting the performance of the hole deflector. The validation test was conducted for a case hole deflector (5½ in. casing). The measured frictional resistance was decreased from 31.44 N to 23.16 N by 26.34%. The results from this research could serve as a reference for the design of hole deflectors for radial jet drilling.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032907-032907-12. doi:10.1115/1.4035747.

Ensemble Kalman filter (EnKF) is one of the widely used optimization methods in petroleum engineering. It uses multiple reservoir models, known as ensemble, for quantifying uncertainty ranges, and model parameters are updated using observation data repetitively. However, it requires a large number of ensemble members to get stable results, causing huge simulation time. In this study, we propose a sampling method using principal component analysis (PCA) and K-means clustering. It excludes poor ensemble with different geological trends to the reference so we can improve both speed and reliability of future predictions. A representative model, which is selected from candidate models of each cluster, has a role to choose proper ensemble for EnKF. For applying EnKF to channelized reservoirs, we compare cases with using 400, randomly picked 100, sampled 100 using Hausdorff distance, and sampled 100 by the proposed method. The proposed method shows improvements over the other cases compared. It gives stable uncertainty ranges and well-updated reservoir parameters after the assimilations. Randomly selected 100 ensemble members predict wrong reservoir performances, and 400 ensemble members exhibit too large uncertainty ranges with long simulation times. Even though more ensemble members are utilized, they provide worse results due to disturbance by improperly designed models. We confirm our sampling strategy in a real field case, PUNQ-S3, and it reduces simulation time as well as improves the future predictions for efficient and reliable history matching.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032908-032908-7. doi:10.1115/1.4035748.

Oil shale in-situ retorting is a reasonable development technology. However, the ground water may flow into fractures in oil shale layer that impact the process of oil shale in-situ retorting. This paper introduced oil shale in-situ fracturing-nitrogen injection exploitation and a method of dynamic pressure balance between the ground water and high pressure nitrogen to keep the oil shale layer without ground water in the process of oil shale in-situ fracturing-nitrogen injection exploitation. Theoretical basis of dynamic pressure balance between ground water and nitrogen was established through analyzing pressure relationship between ground water and nitrogen in the fractures and field experiment was conducted according to the method. The field experiment results showed that nitrogen pressure maintained high level in the fractures during the stage of building pressure balance of nitrogen and ground water and pushed ground water out of the oil shale layer. Then, nitrogen pressure in the fractures reduced and maintained stable because part of nitrogen in the fractures flowed out from the production well and flow conductivity of fractures enhanced. After the balance between the ground water and high pressure nitrogen was established, water yield of production well reduced more than 85%. It explained that the balance has function of sealing up.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032909-032909-10. doi:10.1115/1.4035751.

Production forecast of steam-assisted gravity drainage (SAGD) in heterogeneous reservoir is important for reservoir management and optimization of development strategies for oil sand operations. In this work, artificial intelligence (AI) approaches are employed as a complementary tool for production forecast and pattern recognition of highly nonlinear relationships between system variables. Field data from more than 2000 wells are extracted from various publicly available sources. It consists of petrophysical log measurements, production and injection profiles. Analysis of a raw dataset of this magnitude for SAGD reservoirs has not been published in the literature, although a previous study presented a much smaller dataset. This paper attempts to discuss and address a number of the challenges encountered. After a detailed exploratory data analysis, a refined dataset encompassing ten different SAGD operating fields with 153 complete well pairs is assembled for prediction model construction. Artificial neural network (ANN) is employed to facilitate the production performance analysis by calibrating the reservoir heterogeneities and operating constraints with production performance. The impact of extrapolation of the petrophysical parameters from the nearby vertical well is assessed. As a result, an additional input attribute is introduced to capture the uncertainty in extrapolation, while a new output attribute is incorporated as a quantitative measure of the process efficiency. Data-mining algorithms including principal components analysis (PCA) and cluster analysis are applied to improve prediction quality and model robustness by removing data correlation and by identifying internal structures among the dataset, which are novel extensions to the previous SAGD analysis study. Finally, statistical analysis is conducted to study the uncertainties in the final ANN predictions. The modeling results are demonstrated to be both reliable and acceptable. This paper demonstrates the combination of AI-based approaches and data-mining analysis can facilitate practical field data analysis, which is often prone to uncertainties, errors, biases, and noises, with high reliability and feasibility. Considering that many important system variables are typically unavailable in the public domain and, hence, are missing in the dataset, this work illustrates how practical AI approaches can be tailored to construct models capable of predicting SAGD recovery performance from only log-derived and operational variables. It also demonstrates the potential of AI models in assisting conventional SAGD analysis.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):032910-032910-10. doi:10.1115/1.4035905.

Gas-condensate productivity is highly dependent on the thermodynamic behavior of the fluids-in-place. The condensation attendant with the depletion of gas-condensate reservoirs leads to a deficiency in the flow of fluids moving toward the production channels. The impairment is a result of condensate accumulation near the production channels in an immobility state until reaching a critical saturation point. Considering the flow phenomenon of gas-condensate reservoirs, tight formations can be inevitably complex hosting environments in which to achieve economical production. This work is aimed to assess the productivity gas-condensate reservoirs in a naturally fractured setting against the effect of capillary pressure and relative permeability constraints. The severity of condensate coating and magnitude of impairment was evaluated in a system with a permeability of 0.001 mD using an in-house compositional simulator. Several composition combinations were considered to portray mixtures ascending in complexity from light to heavy. The examination showed that thicker walls of condensate and greater impairment are attained with mixture containing higher nonvolatile concentrations. In addition, the influence of different capillary curves was insignificant to the overall behavior of fluids-in-place and movement within the pores medium. A greater impact on the transport of fluids was owed to relative permeability curves, which showed dependency on the extent of condensate content. Activating diffusion was found to diminish flow constraints due to the capturing of additional extractions that were not accounted for under Darcy's law alone.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Energy Resour. Technol. 2016;139(3):034501-034501-8. doi:10.1115/1.4034759.

The main purpose of this paper is to analyze and compare the influence of nozzle size, uneven gravel packing, packer leakage, and dynamic production process on the inflow control effect. First, a new mathematical model of Inflow control devices (ICDs) completed horizontal well is proposed which has two new features. One feature is that the annulus between the sand control screen and the formation is considered. Therefore, the influence of uneven gravel packing can be simulated by adjusting the permeability distribution along the annulus. The other feature is that it accounts for packer leakage by introducing a new parameter named “leakage factor” into the model. Then, the inflow control efficiency is defined and used to quantitatively characterize the inflow control effect, and the influences of nozzle size, uneven gravel packing, packer leakage, and dynamic production process on inflow control efficiency are analyzed. The results show that the nozzle size and packer leakage have the biggest influence on the inflow control efficiency, and the influence of gravel packing is negligible unless the permeability of the packed gravel along the wellbore is extremely heterogeneous.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(3):034502-034502-6. doi:10.1115/1.4035951.

A very important aspect in highly inclined wellbores is the mechanical friction. For extended reach drilling (ERD) and through tubing extended reach drilling (TTERD) this can be a limiting factor. Friction caused by the contact between the drill string and the well casing or borehole is dependent on the drilling weight and fluid properties. Drilling fluids play an important role in determining mechanical friction. The use of oil-based drilling fluids with higher lubricity can reduce torque and drag behavior and minimize stick and slip. Reducing mechanical friction will improve drilling efficiency in general, and will in particular enable longer reach for ERD wells. This paper presents results from experimental laboratory tests where the mechanical friction has been investigated. Friction behavior was investigated for different drilling fluids; water-based and oil-based drilling fluids both with and without solid particles. A pin on disk setup was used for these experiments where a spherical ended steel pin was slid against a rotating disk made of granite. The test results show that the mechanical friction in general is smaller with oil-based than water-based drilling fluids in the presence of solid particles.

Commentary by Dr. Valentin Fuster

Expert View

J. Energy Resour. Technol. 2016;139(3):034701-034701-3. doi:10.1115/1.4034859.

Anthropogenic heat generation in the world has been shown to be non-negligible, as it was a previous misconception. The scientific contribution of the current work is to urge scientists and engineers to develop technologies to reject heat from engineering systems to outer space. Outer space acts as a definitive heat sink since a statistical average temperature may be assigned to it as 3 K. This temperature is a lot lower than the average temperature anywhere on Earth, at any time of the year. Until recently, the concept was well known but not systematically developed nor advanced using modern engineering knowledge. Looking at recent figures of heat generated associated with power plants worldwide, a theoretical potential exists to reduce the amount of anthropogenic heat rejected in the world's environment by very significant amounts. Outer space is the ultimate sink for man's heat from engineered systems, and the upper limit is comfortably very large to not be of any concern at the present time.

Commentary by Dr. Valentin Fuster

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