Review Article

J. Energy Resour. Technol. 2018;140(5):050801-050801-14. doi:10.1115/1.4038785.

The design and development of wind turbines is increasing throughout the world to offer electricity without paying much to the global warming. The Savonius wind turbine rotor, or simply the Savonius rotor, is a drag-based device that has a relatively low efficiency. A high negative torque produced by the returning blade is a major drawback of this rotor. Despite having a low efficiency, its design simplicity, low cost, easy installation, good starting ability, relatively low operating speed, and independency to wind direction are its main rewards. With the goal of improving its power coefficient (CP), a considerable amount of investigation has been reported in the past few decades, where various design modifications are made by altering the influencing parameters. Concurrently, various augmentation techniques have also been used to improve the rotor performance. Such augmenters reduce the negative torque and improve the self-starting capability while maintaining a high rotational speed of the rotor. The CP of the conventional Savonius rotors lie in the range of 0.12–0.18, however, with the use of augmenters, it can reach up to 0.52 with added design complexity. This paper attempts to give an overview of the various augmentation techniques used in Savonius rotor over the last four decades. Some of the key findings with the use of these techniques have been addressed and makes an attempt to highlight the future direction of research.

Commentary by Dr. Valentin Fuster

Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2017;140(5):051201-051201-17. doi:10.1115/1.4037813.

The splitter blades are very common to use for centrifugal compressor impellers to improve the compressor performance and manufacturing capability. In this study, a low-flow single-stage centrifugal compressor with a vaneless diffuser was used to investigate the location effects of the impeller splitter between two main blades. It is demonstrated that the splitter position provides an opportunity to improve the compressor performance and reduce the operational cost. The splitter location optimizations were performed numerically, and the optimal splitter location was identified. A prototype was built for the impeller with optimal splitter position. The performance tests were performed, and test results are compared with numerical analyses. The studies indicated that splitter positions have impacts on the compressor stage performances. The studies showed that the traditional splitter located in the middle of the two main blades is not the optimal location for aerodynamic performance. The splitter location optimization provided the opportunity to improve the centrifugal compressor performance further.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):051202-051202-13. doi:10.1115/1.4038119.

A fast-growing worldwide interest is directed toward green energies. Due to the huge costs of wind farms establishment, the location for wind farms should be carefully determined to achieve the optimum return of investment. Consequently, researches have been conducted to investigate land suitability prior to wind plants development. The generated data from the sensors detecting a potential land can be very huge, fast in generation, heterogeneous, and incomplete, which become seriously difficult to process using traditional approaches. In this paper, we propose Trio-V Wind Analyzer (WA) that handles data volume, variety, and veracity to identify the most suitable location for wind energy development in any study area using a modified version of multicriteria evaluation (MCE). It utilizes principal component analysis (PCA) and our proposed Double-Reduction Optimum Apriori (DROA) to analyze most of the environmental, physical, and economical criteria. In addition, Trio-V WA recommends the suitable turbines and proposes the adequate turbines’ layout distribution, predicting the expected power generated based on the recommended turbine’s specifications using a regression technique. Thus, Trio-V WA provides an integral system of land evaluation for potential investment in wind farms. Experiments indicate 80% and 95% average accuracy for land suitability degree and power prediction, respectively, with efficient performance.

Topics: Turbines , Wind , Wind farms , Design
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):051203-051203-6. doi:10.1115/1.4038485.

This paper aims to increase the efficiency of the maximum power point tracking (MPPT) methods by using an integral sliding mode voltage regulator (ISMVR). The ISMVR is applied to one of the frequently used MPPT methods called as tip speed ratio (TSR). The proposed method presents a fast and robust tracking capability. Also, there is no need to know about the parameters of the generator in order to generate the control signal. The ISMVR presents considerably simple control structure due to the fact that the authors used only the boost converter (BC) and controller parameters for the control signal. Additionally, the performances of the proposed improved TSR-MPPT method based on ISMVR are compared to the TSR-MPPT method based on conventional sliding mode voltage regulator (CSMVR) under the same conditions. The dynamic performance, robustness, and fast approximation of the offered method are proved with the simulations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):051204-051204-12. doi:10.1115/1.4038918.

This comprehensive investigation has been executed to compare the thermodynamic performance and optimization of LiCl–H2O and LiBr–H2O type absorption system integrated with flat-plate collectors (FPC), parabolic-trough collectors (PTC), evacuated-tube collectors (ETC), and compound parabolic collectors (CPC). A model of 10 kW is analyzed in engineering equation solver (ees) from thermodynamic perspectives. Solar collectors are integrated with a storage tank which fueled the LiCl–H2O and LiBr–H2O vapor absorption system to produce refrigeration at 7 °C in evaporator for Gujarat Region of India. The main objective includes the evaluation and optimization of critical performance and design parameters to exhibit the best working fluid pair and collector type. Optimum heat source temperature corresponding to energetic and exergetic aspects for LiCl–H2O pair is lower than that of LiBr–H2O pair for all collectors. Simulation shows that FPC has lowest capital cost, exergetic performance wise PTC is optimum, and ETC requires lowest collector area. On the basis of overall evaluation, solar absorption cooling systems are better to be powered by ETC with LiCl–H2O working fluid pair.

Commentary by Dr. Valentin Fuster

Research Papers: Energy From Biomass

J. Energy Resour. Technol. 2017;140(5):051801-051801-9. doi:10.1115/1.4038313.

Chlorine plays an important role in the slagging and corrosion of boilers that burn high-chlorine content biomass. This research investigated the emissions of hydrogen chloride (HCl) gas from combustion of biomass in a fixed bed, as functions of the mass air flow rate through the bed and of the moisture content of the fuel. The biomass burned was corn straw, either raw or torrefied. Results showed that increasing the air flow rate through the bed increased the release of HCl gas, as a result of enhanced combustion intensity and associated enhanced heat release rates. When the airflow through the bed was increased by a factor of six, the amount of fuel-bound chlorine converted to HCl nearly tripled. Upon completion of combustion, most of the chlorine remained in the biomass ashes, with the exception of the highest air flow case where the fraction of chlorine released in HCl equaled that captured in the ashes. HCl emissions from torrefied biomass were found to be lower than those from raw biomass. Finally, drying the biomass proved to be beneficial in drastically curtailing the generation of HCl gas.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2017;140(5):052001-052001-15. doi:10.1115/1.4038236.

Boiler's efficiency is one of the important performance indicators of boiler. To keep track of operation cost, efficiency needs to be calculated with adequate accuracy by employing effective mathematical tools. In this work, a new modification in conventional mathematical formulation of efficiency is presented based on time-varying efficiency using time-varying operational variables of boiler. This modification was accomplished using indirect method of efficiency by applying experimental data of variables for certain time span. Moreover a second-order dynamic model of flue gas temperature (FGT) has been derived to construct the mathematical formulation of efficiency only in terms of available inputs. The resulting input–output-based model proved to be in quite agreement with efficiency calculated from experimental data. After modeling, influence of variations in air to fuel ratio (AFR) and fuel flow rate (FFR) upon efficiency has been discussed and it has been shown that time-varying efficiency covers deeper aspect of dynamic relation between efficiency and other input of boiler especially AFR and FFR. Moreover, it has been established that efficiency interacts with the dynamics of boiler, and in this respect, a dynamic relation between combustion process and boiler dynamics has been constructed via efficiency.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052002-052002-13. doi:10.1115/1.4037369.

This paper presents the approach of thermoeconomic analysis of centralized cold generation in trigeneration system integrated with steam-powered absorption chillers (ACs). The analysis was conducted for real back-pressure combined heat and power (CHP) unit BC-50 and single-effect absorption refrigerators using water and lithium bromide as the working fluids. It has been assumed that the heating medium supplied to the chiller generator is technological steam from the existing steam bleeding. The calculations take into account changes of energy demand for heating and cooling for each month of the year. Mathematical simulation models of cogeneration and trigeneration systems have been developed with the commercial program for power plant simulation EBSILON Professional. System effects of heat and electricity cogeneration and cogeneration with additional cold production have been calculated compared to separate production of heat, electricity, and cold (replaced heating plant and power unit). The effect of trigeneration has been assessed quantitatively by the coefficient of the increasing cogeneration effects, which has been calculated as a ratio of chemical energy savings of fuels to the demand for heat by the consumers in the cases of trigeneration and cogeneration. This paper includes also analysis of economic effectiveness of a trigeneration system with ACs for cold agent production. The results of economic calculations show that an acceptable payback period of approximately 13 yr for a CHP and absorption system may be achieved. Discounted payback (DPB) is equal to the half of assumed operating time of the system. Sensitivity analysis shows that the most important impact on profitability is the selling price of cold and the purchase of fuel—hard coal.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052003-052003-16. doi:10.1115/1.4038387.

The centrifugal compressors are widely used in industrial applications. The design, manufacturing, and installation are all critical for the compressor performance. Many studies have been carried out in the past to optimize the compressor performance during compressor design. The manufacturing tolerances and installation errors can cause the performance drop. There are many compressor performance distortions that are not fully understood due to manufacturing and facilities. In this paper, an asymmetrical radial clearance of the impeller due to manufacturing and installation is studied in detail for the performance impacts. The numerical studies and experiments indicated that the asymmetric radial clearance impacts the compressor flow field structure and performance. Experimental results suggested that the manufacturing and installation cause asymmetric radial clearance which decreased the compressor performance in whole operating range. The numerical analysis demonstrated that the impeller asymmetric clearance impacts performance near the design pressure ratio more than other pressure ratios. The numerical studies showed that the maximum clearance location of asymmetric clearance might impact the compressor performance. The proper asymmetricity of diffuser verse the volute may benefit the compressor performance. The excellent compressor performances for centrifugal compressors especially for small centrifugal compressors not only need to have a good aerodynamic design but also need to control manufacturing and installation carefully.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):052004-052004-8. doi:10.1115/1.4038963.

A generalized methodology for pinch point design and optimization of subcritical and transcritical organic Rankine cycles (ORCs) using both wet and dry fluids is adopted in this study. The presented algorithm can predict the pinch point location in evaporator and condenser simultaneously and optimize the evaporator pressure for best performance with various heat source and sink conditions. Effects of pinch point temperature difference (PPTD), isentropic efficiency, subcooling, superheating and regenerator on the energy and economic performances are discussed for selected working fluids. System yields similar optimum design for both maximum power generation and minimum capital cost per unit power. At optimum condition, ammonia is best in terms of higher thermal efficiency and lower component size, R152a is best in terms of higher net power output and heat recovery efficiency (11.1%), and toluene is best in terms of lower capital cost and cost per unit power output (7060 $/kW). Effect of heat source and sink parameters on both energy and economic performances is significant. Contour plots are presented to select the best ORC design parameters for available heat source condition. PPTD and expander isentropic efficiency have significant effect on performances. However, the effect of subcooling, superheating and regenerator depends on working fluid.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):052005-052005-17. doi:10.1115/1.4038847.

This paper presents experimental investigations conducted to understand the influence of water-soluble drag-reducing polymers (DRPs) in single- and two-phase (stratified wavy) flow on flow-field characteristics. These experiments have been presented for water and air–water flowing in a horizontal polyvinyl chloride 22.5-mm ID, 8.33-m long pipe. The effects of liquid flow rates and DRP concentrations on streamlines and the instantaneous velocity were investigated by using particle image velocimetry (PIV) technique. A comparison of the PIV results was performed by comparing them with the computational results obtained by fluent software. One of the comparisons has been done between the PIV results, where a turbulent flow with DRP was examined, and the laminar–computational fluid dynamic (CFD) prediction. An agreement was found in the region near the pipe wall in some cases. The results showed the powerfulness of using the PIV techniques in understanding the mechanism of DRP in single- and two-phase flow especially at the regions near the pipe wall and near the phases interface. The results of this study indicate that an increase in DRP concentrations results in an increase in drag reduction up to 45% in single-phase water flow and up to 42% in air–water stratified flow.

Commentary by Dr. Valentin Fuster

Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2017;140(5):052201-052201-9. doi:10.1115/1.4038376.

The rate-controlled constrained-equilibrium (RCCE) model reduction scheme for chemical kinetics provides acceptable accuracies in predicting hydrocarbon ignition delays by solving a smaller number of differential equations than the number of species in the underlying detailed kinetic model (DKM). To yield good approximations, the method requires accurate identification of the rate controlling constraints. Until recently, a drawback of the RCCE scheme has been the absence of a systematic procedure capable of identifying optimal constraints for a given range of thermodynamic conditions and a required level of approximation. A recent methodology has proposed for such identification an algorithm based on a simple algebraic analysis of the results of a preliminary simulation of the underlying DKM, focused on the degrees of disequilibrium (DoD) of the individual chemical reactions. It is based on computing an approximate singular value decomposition of the actual degrees of disequilibrium (ASVDADD) obtained from the DKM simulation. The effectiveness and robustness of the method have been demonstrated for methane/oxygen ignition by considering a C1/H/O (29 species/133 reactions) submechanism of the GRI-Mech 3.0 scheme and comparing the results of a DKM simulation with those of RCCE simulations based on increasing numbers of ASVDADD constraints. Here, we demonstrate the new method for shock-tube ignition of a natural gas/air mixture, with higher hydrocarbons approximately represented by propane according to the full (53 species/325 reactions) GRI-Mech 3.0 scheme including NOx formation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052202-052202-12. doi:10.1115/1.4038625.

New drilling techniques have increased availability and decreased costs of oil and gas. The decreased costs have caused an increase in drilling activity. The well sites have a large power demand that is typically met by diesel engines for the drilling derrick, fracking pumps, and electrical power. Dual fuel retrofit kits are being increasingly used at well sites to reduce operating costs and the amount of fuel trucked in to the site. Natural gas (NG) is cheaper compared to diesel and can be delivered to the site by the pipeline limiting the disturbance to surrounding communities due to diesel truck loads. The purpose of this work is to examine the performance of a typical dual fuel retrofit kit commissioned for field operation on a 6.8 L tier II diesel engine. After the baseline commissioning, the mechanisms limiting further substitution were clearly identified as engine knock similar to end gas auto-ignition in spark-ignited engines and governor instability. Two methods are examined for their ability to increase substitution limits by adjusting the start of injection timing (SOI) and the intake air manifold temperature. Retarding the SOI is able to delay the onset of knock at high loads and therefore increase the substitution level by around 4% at full load. At high loads, lowering the air manifold temperature is able to increase the substitution levels by around 10%. Preheating the intake air was able to increase low load substitution levels by 10% as well.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):052203-052203-10. doi:10.1115/1.4039549.

Cycle-to-cycle variability (CCV) is detrimental to IC engine operation and can lead to partial burn, misfire, and knock. Predicting CCV numerically is extremely challenging due to two key reasons. First, high-fidelity methods such as large eddy simulation (LES) are required to accurately resolve the in-cylinder turbulent flow field both spatially and temporally. Second, CCV is experienced over long timescales and hence the simulations need to be performed for hundreds of consecutive cycles. Ameen et al. (2017, “Parallel Methodology to Capture Cyclic Variability in Motored Engines,” Int. J. Engine Res., 18(4), pp. 366–377.) developed a parallel perturbation model (PPM) approach to dissociate this long time-scale problem into several shorter time-scale problems. This strategy was demonstrated for motored engine and it was shown that the mean and variance of the in-cylinder flow field was captured reasonably well by this approach. In the present study, this PPM approach is extended to simulate the CCV in a fired port-fuel injected (PFI) spark ignition (SI) engine. The predictions from this approach are also shown to be similar to the consecutive LES cycles. It is shown that the parallel approach is able to predict the coefficient of variation (COV) of the in-cylinder pressure and burn rate-related parameters with sufficient accuracy, and is also able to predict the qualitative trends in CCV with changing operating conditions. It is shown that this new approach is able to give accurate predictions of the CCV in fired engines in less than one-tenth of the time required for the conventional approach of simulating consecutive engine cycles.

Commentary by Dr. Valentin Fuster

Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2017;140(5):052901-052901-7. doi:10.1115/1.4038405.

Thermal–chemical flooding (TCF) is an effective alternative to enhance heavy oil recovery after steam injection. In this paper, single and parallel sand-pack flooding experiments were carried out to investigate the oil displacement ability of thermal–chemical composed of steam, nitrogen (N2), and viscosity breaker (VB), considering multiple factors such as residual oil saturation (Sorw) postwater flood, scheme switch time, and permeability contrast. The results of single sand-pack experiments indicated that compared with steam flooding (SF), steam-nitrogen flooding, and steam-VB flooding, TCF had the best displacement efficiency, which was 11.7% higher than that of pure SF. The more serious of water-flooded degree, the poorer of TCF effect. The improvement effect of TCF almost lost as water saturation reached 80%. Moreover, the earlier TCF was transferred from steam injection, the higher oil recovery was obtained. The parallel sand-pack experiments suggested that TCF had good adaptability to reservoir heterogeneity. Emulsions generated after thermal–chemical injection diverted the following compound fluid turning to the low-permeable tube (LPT) due to its capturing and blocking ability. The expansion of N2 and the disturbance of VB promoted oil recovery in both tubes. As reservoir heterogeneity became more serious, namely, permeability contrast was more than 6 in this study, the improvement effect became weaker due to earlier steam channeling in the high-permeable tube (HPT).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052902-052902-8. doi:10.1115/1.4038381.

Lost circulation is a serious problem which always exists in the petroleum industry. Wellbore strengthening by lost circulation materials (LCMs) is a commonly applied method for mitigating lost circulation. This paper presents a hydraulic fracturing apparatus to investigate the effect of material type, concentration, and particle size distribution (PSD) of LCMs on wellbore strengthening behavior. In addition, the characteristics of pressure curves in the fracturing process are analyzed in detail. The results showed that the fracture pressure of the artificial core can be increased by LCMs, and there exists an optimum concentration of LCMs for the maximum wellbore strengthening effect. The LCMs with wide PSD can significantly increase the fracture pressure. However, some LCMs cannot increase or even decrease the fracture pressure; this is resulting from the LCMs with relatively single PSD that makes the quality of mud cake worse. The representative pressure curve in the fracturing process by drilling fluids with LCMs was divided into five parts: the initial cake formation stage, elastic plastic deformation stage, crack stability development stage, crack instability development stage, and unstable plugging stage. The actual fracturing curves were divided into four typical types due to missing of some stages compared with the representative pressure curve. In order to strengthen the wellbore in effective, good LCMs should be chosen to improve the maximum pressure in the elastic plastic deformation stage, extend the stable time of pressure bearing in the crack stability development stage, and control the crack instability development stage.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052903-052903-9. doi:10.1115/1.4038382.

This study investigates the effect of using date seed-based additive on the performance of water-based drilling fluids (WBDFs). Specifically, the effects of date pit (DP) fat content, particle size, and DP loading on the drilling fluids density, rheological properties, filtration properties, and thermal stability were investigated. The results showed that dispersion of particles less than 75 μm DP into the WBDFs enhanced the rheological as well as fluid loss control properties. Optimum fluid loss and filter cake thickness can be achieved by addition of 15–20 wt % DP loading to drilling fluid formulation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052904-052904-15. doi:10.1115/1.4038386.

A power-law mixing rule has been developed to determine apparent diffusion coefficient of a binary gas mixture on the basis of molecular diffusion coefficients for pure gases in heavy oil. Diffusion coefficient of a pure gas under different pressures and different temperatures is predicted on the basis of the Hayduk and Cheng's equation incorporating the principle of corresponding states for one-dimensional gas diffusion in heavy oil such as the diffusion in a pressure–volume–temperature (PVT) cell. Meanwhile, a specific surface area term is added to the generated equation for three-dimensional gas diffusion in heavy oil such as the diffusion in a pendant drop. In this study, the newly developed correlations are used to reproduce the measured diffusion coefficients for pure gases diffusing in three different heavy oils, i.e., two Lloydminster heavy oils and a Cactus Lake heavy oil. Then, such predicted pure gas diffusion coefficients are adjusted based on reduced pressure, reduced temperature, and equilibrium ratio to determine apparent diffusion coefficient for a gas mixture in heavy oil, where the equilibrium ratios for hydrocarbon gases and CO2 are determined by using the equilibrium ratio charts and Standing's equations, respectively. It has been found for various gas mixtures in two different Lloydminster heavy oils that the newly developed empirical mixing rule is able to reproduce the apparent diffusion coefficient for binary gas mixtures in heavy oil with a good accuracy. For the pure gas diffusion in heavy oil, the absolute average relative deviations (AARDs) for diffusion systems with two different Lloydminster heavy oils and a Cactus Lake heavy oil are calculated to be 2.54%, 14.79%, and 6.36%, respectively. Meanwhile, for the binary gas mixture diffusion in heavy oil, the AARDs for diffusion systems with two different Lloydminster heavy oils are found to be 3.56% and 6.86%, respectively.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052905-052905-10. doi:10.1115/1.4038622.

The gas–liquid cylindrical cyclone (GLCC) is a simple, compact, and low-cost separator, which provides an economically attractive alternative to conventional gravity-based separators over a wide range of applications. Over the past 22 years, more than 6500 GLCCs have been installed around the world by the petroleum and related industries. However, to date no systematic study has been carried out on its structural integrity. The GLCC inlet section design is a key parameter, which is crucial for its performance and proper operation. This paper presents finite element analysis simulation results aimed at investigating the effect of various parameters on the inlet section structural integrity. Finally, recommendations on design modifications are presented, directed at strengthening the inlet section.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(5):052906-052906-11. doi:10.1115/1.4038623.

For liquid-loading gas wells, effective deliquification operation is needed but current liquid-lifting technology is not able to meet the requirements of high efficiency as well as low cost especially in large-deviated wells. This paper proposes a hybrid deliquification technology combining plunger lift, chemical foamer injection, and down-hole monitoring to unload liquid in deviated gas wells. The system comprised multipart plunger body, deployment-retrieving integrating assembly (DRIA) and operation canisters. By means of flexible plunger body, the system performs deliquification normally in deviated wellbore. The operation canisters are carried with plunger body through tubing onto the bottom of deviated section to operate in terms of four modes: long-term down-hole monitoring, foamer injection, mobile data acquisition, and wireless data exchange with the wellhead. The key components of DRIA and injection valve are made of improved disintegrating alloy with the rating temperature of 100 °C, compressive strength of 370 MPa, and disintegrating rate of 170.9 mg/(cm2 h) characterized by lab test. Field trials were successfully performed in two liquid-loading tight gas wells, and the maximal deviated angle of the wells was 68 deg. It indicates that the new technology is a cost-effective way contributing to automatic production and management of mature gas wells in the remote area instead of traditional rigid plunger and wire-line logging.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):052907-052907-15. doi:10.1115/1.4038845.

Modeling fractured reservoirs, especially those with complex, nonorthogonal fracture network, can prove to be a challenging task. This work proposes a general integral solution applicable to two-dimensional (2D) fluid flow analysis in fractured reservoirs that reduces the original 2D problem to equivalent integral equation problem written along boundary and fracture domains. The integral formulation is analytically derived from the governing partial differential equations written for the fluid flow problem in reservoirs with complex fracture geometries, and the solution is obtained via solving system of equations that combines contributions from both boundary and fracture domains. Compared to more generally used numerical simulation methods for discrete fracture modeling such as finite volume and finite element methods, this work only requires discretization along the boundary and fractures, resulting in much fewer discretized elements. The validity of proposed solution is verified using several case studies through comparison with available analytical solutions (for simplified, single-fracture cases) and finite difference/finite volume finely gridded numerical simulators (for multiple, complex, and nonorthogonal fracture network cases).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):052908-052908-7. doi:10.1115/1.4038846.

Nanotechnology has had revolutionary effects in various fields of industry such as electronics, pharmaceuticals, and biomaterials. However, upstream oil industry has been noticeably slow in adopting the emerging technologies. This is mainly due to the exceptionally large investments needed to implement novel technologies in this industry. However, the projections for the increasing global energy demand require that oil and gas industry inevitably move toward adopting the emerging technologies. The high risk associated with enormous investments required for this aim necessitates measured and well-researched energy policies, with regard to the implementation of nanotechnology in the oil and gas industry. This paper presents a concise summary of the research reported in the literature on the potential benefits of nanotechnology in upstream oil industry. These applications were categorized into ten groups, and presented to a pool of experts, who judged on their relative importance with respect to various decision-making criteria. All this information was then compiled into a single matrix, which indicates the priority of each investment alternative with respect to every criterion in the form of a raw number. Finally, using a decision-making software package, a dynamic analytic hierarchical process (AHP) analysis was performed, providing a route to customized investment policies.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):052909-052909-10. doi:10.1115/1.4038783.

A severe lost circulation event is usually associated with emanation and propagation of pre-existing or drilling induced fractures from the wellbore. To combat lost circulation and prevent further fracture propagation, a thorough understanding of the stress state in the near-wellbore region with fractures is imperative. However, it is not yet fully understood how temperature variation during the invasion of mud affects pre-existing or newly initiated fractures. A three-dimensional (3D) finite element (FE) analysis was conducted in this study to simulate the transport processes and state of stresses in the near-wellbore region during invasion of mud into fractures. To account for thermal effects, a thermo-poroelasticity model was coupled with flow and heat transfer models in the fractures. This study included a series of sensitivity analyses based on different formation properties and mud loss conditions to delineate the relative importance of different parameters on induced thermal stresses. It also evaluated potential risks of reinitiating fractures under various downhole conditions. The results demonstrate how the stresses redistribute as nonisothermal invasion of mud takes place in fractures. It shows that a temperature difference between the formation rock and the circulating muds can facilitate fracture propagation during invasion of mud. These results due to temperature change can also diminish the enhanced hoop stresses provided by wellbore strengthening (WBS) and other lost circulation prevention methods. Such information is vital to successful lost circulation management. The conclusions of this study are particularly relevant when a substantial temperature difference exists between circulating fluids and surrounding rock, as commonly seen in high-pressure, high-temperature, and deepwater wells.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Energy Resour. Technol. 2017;140(5):054501-054501-6. doi:10.1115/1.4038465.

Blooey line is a discharge pipe, used to conduct gas to keep drilling rock dust and cuttings away from the drilling rig, reducing the fire hazard and transporting the cuttings to a suitable distance from the well. In this paper, the blooey line's flow capacity and erosion mechanism have been investigated by numerical and experimental method. The model of blooey line, which is commonly used in Sichuan district, China, is established by using a computational fluid dynamics (CFD) method. And, the distribution of pressure field and velocity field in the blooey line are investigated by the CFD model. And, the effect of gas flow rate on impact force and erosion is also discussed. Compared with the simulation results, an experimental apparatus of the blooey line has been conducted under the mechanical similarity principle. The impact force and pressure on the elbows are measured under different gas flow rates. The numerical simulation and experimental method proposed in this paper can provide a reference for layout optimization and flow capacity calculation of blooey line in gas drilling.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(5):054502-054502-10. doi:10.1115/1.4039022.

In order to study the effects of particle size on the determination of pore structure in shale, the outcrop of Ordovician Wufeng (WF) and Silurian Longmaxi shale (LMX) samples from Sichuan basin were chosen and crushed into various particle sizes. Then, pore structure was analyzed by using low-pressure gas adsorption (LPGA) tests. The results show that the pore of shales is mainly composed of slit-type pores and open pores. The specific surface areas of shale are mainly contributed by micropores, while the largest proportion of the total pore volume in shale is contributed by mesopores. With the decreasing of particle size, the specific surface area of both samples is decreased, while average pore diameter and the total pore volume are increased gradually. The influences of particle size on the pore structure parameters are more significant for micropore and macropore, as the particle sizes decrease from 2.36 mm to 0.075 mm, the volume of micropores in Longmaxi shale increases from 0.283 cm3/100 g to 0.501 cm3/100 g with an increment almost 40%, while the volume of macropores decreases from 0.732 cm3/100 g to 0.260 cm3/100 g with a decrement about 50%. This study identified the fractal dimensions at relative pressures of 0–0.50 and 0.50–0.995 as D1 and D2, respectively. D1 increases with the decrease of particle size of shale, while D2 shows an opposite tendency in both shale samples.

Commentary by Dr. Valentin Fuster

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