J. Energy Resour. Technol. 2001;123(3):181-186. doi:10.1115/1.1383974.

Precession, which is the rolling motion of the drillstring on the walls of the borehole, is investigated by considering the various possible motions of a rotating disk in a circular hole. Due to the simplicity of the model, closed-form results are derived on the stability of precession and on the evolution of impacting motions towards precession. It is found that increasing coefficients of friction and contact damping have a favoring effect on precession, while the stiffness and damping of the drillstring bending mode have an opposing effect.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):187-193. doi:10.1115/1.1386390.

We present a procedure that uses nonlinear optimization theory to plan complex, three-dimensional well paths and path corrections while drilling. The problem of hitting a 3-D target is posed as seeking a profile that optimizes some well-defined objective function (the optimality criterion) subject to equality and inequality constraints. The well path is idealized to contain a finite combination of turn and straight sections. Operational restrictions translate into inequality constraints, and target restrictions translate into equality constraints. Several optimality criteria may be chosen, and appropriate choices are discussed. In this work, we choose optimization with respect to user preferred parameters as the criterion. The resulting nonlinear optimization problem is solved using a sequential gradient-restoration algorithm (SGRA), with scaling and optimal step-size selection. The optimization problem formulation and the solution procedure are described. The procedure is robust, efficient, and clearly superior to trial-and-error heuristic techniques that are commonly used to plan well paths today. A computer program based on this technique has been developed and successfully used. Two examples are included to illustrate the procedure. It is concluded that nonlinear optimization is a powerful and versatile mathematical tool that can be used for planning better, optimal well paths, and can be extended to several other drilling and production problems.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):194-199. doi:10.1115/1.1377894.

The use of core-annular flow pattern may be attractive as an artificial lift method in heavy oil wells. This flow pattern can be induced by the lateral injection of relatively small quantities of water, in order to get a lubricated oil core along the pipe. Frictional pressure drop measurements for upward vertical core flow in a 1-in. pipe, using a 17.6-Pa.s, 963-kg/m3 oil and water at room temperature reported a decrease by over 1000-fold with respect to single-phase oil flow, being comparable to the flow of water alone in the pipe at mixture flow rate. The total pressure drop was reduced by over 45-fold. The frictional pressure drop model proposed includes both irreversible and buoyancy terms. The model was adjusted to fit our data and shows excellent agreement with laboratory data available.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):200-204. doi:10.1115/1.1385382.

Solids transport in multiphase systems falls under the umbrella of “flow assurance.” Unlike issues such as waxes and hydrates, solids transport has received relatively little attention to date. This is especially true for solids transport in high-viscosity fluids such as Venezuelan crude, where viscosities around the 300–400-cP mark are commonly encountered. This paper describes some experiments performed on the BP Amoco 6-in. multiphase flow test facility located at Sunbury. These looked at the transport of field representative sand through a pipeline dip. Several fluids were selected for these experiments to examine the influence of liquid viscosity on the results. These were water, oil, and two different carboxymethylcellulose solutions (150 and 300 cP). These experiments showed that, in slug flow, water and low-viscosity oil were able to transport the sand uphill, whereas neither high-viscosity solution was able to transport the solids. This feature was examined in comparison to the model for solids transport in near-horizontal pipes discussed in this paper. Three-phase flow experiments (water-oil-air) were also performed to investigate the effect of oil or water prewetting of the solids on solids transport. If prewetted by water, the sand could not be moved by oil slugs. Once water was added to the system, the sand became increasingly mobile.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):205-213. doi:10.1115/1.1386389.

We have investigated porosity and permeability damage around perforations using a combination of transient analysis and X-ray CT. The method applied allowed us to perform the entire experiments on samples under simulated in-situ stress conditions and to map variations in permeability along the length of the core as well as with radial distance from the perforation. Berea (10.2-cm (4-in.) dia) cores saturated with low-viscosity silicone oil were perforated using conventional-shaped charges (6-g HMX) and API RP43 procedures by using 6.88-MPa (1000-psi) effective stress and 5.16-MPa (750-psi) and 2.61-MPa (350-psi) underbalance. Low-permeability Torrey Buff Sandstone was also perforated using 5.16-MPa (750-psi) underbalance. After sufficiently flowing the perforations, higher-viscosity silicone oil was injected. The movement of fluids was tracked using X-ray CT to measure the local velocity of the viscous fluid front at different locations along the perforation. Results of these tests were compared in terms of permeability and porosity damage. Quantitative analysis on Berea cores show, for the specific charge and test conditions used, that damage extends approximately 2 cm (0.78 in.) from the center of the perforation. Comparison of tests performed with 2.41-MPa (350-psi) and 5.16-MPa (750-psi) underbalance show a clear increase in permeability near the tunnel wall at the higher underbalance. A zone of somewhat-reduced permeability exists at approximately 1.7 cm from the perforation center in the latter case. Porosity profiles calculated show that porosity is almost uniform out from the tunnel and there is no compacted zone near the tunnel wall in liquid-saturated cores. However, there is a high-porosity zone from the tunnel wall out about 2 mm. This may be due to a region of circumferential partings and small cracks that lead to high porosity or due to the possible artifacts discussed in the paper. Qualitative results have also been obtained for a tight sandstone for which underbalance was insufficient to remove debris from the perforation tunnel. CT images reveal that the plugged tunnel acts as a conduit for fluid flow, showing that the plugging material has significantly higher permeability than the surrounding rock.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):214-220. doi:10.1115/1.1385517.

Two constrained equilibrium codes, GNASA and GSTANJAN, have been developed which determine the composition of constrained equilibrium gas mixture. These codes use NASA and STANJAN equilibrium programs as the basis for generalized equilibrium routines. Gas mixture composition is determined by minimizing Gibbs free energy of the mixture subject to any specified constraints in addition to elemental constraints. Performances of these two codes have been compared to each other, and it has been found that GSTANJAN converges over a wider range of constraints, while the convergence of GNASA is limited. These codes have been applied in nonequilibrium evolution process of hydrogen-oxygen mixture. The nonequilibrium process has been modeled by using only two constraints in addition to elemental constraints. The results are in good agreement with detailed kinetic solution.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):221-227. doi:10.1115/1.1385518.

Results are presented on the formation of a diffusion flame in a methane nonpremixed jet following the propagation of partially premixed combustion. An initially nonreacting turbulent methane jet (Re=2700) in quiescent air is ignited at a downstream location (x/d=70). High-speed video images (125 and 250 Hz) were obtained that chart the evolution of the combustion process. Partially premixed flame propagation is witnessed as the combustion front moves upstream (downward) toward the nozzle exit. As the front propagates, the blue (premixed) character of the flame is diminished, the combustion region narrows, and the transition to diffusion-limited combustion along the stratified methane/air layer takes place. Before reaching the nozzle exit, axial wisps of blue flame emission are witnessed along the jet-edge near the fuel/air interface (i.e., at larger radii than the eventual diffusion flame boundary). Luminosity from soot is first apparent just upstream of an axisymmetric flame bulge as the diffusion flame forms, and within 100 ms, soot attains levels present in the steady-state turbulent diffusion flame. Images are presented portraying the phenomenon, and three regimes are proposed to characterize the propagation of combustion.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):228-235. doi:10.1115/1.1383973.

NOx (i.e., NO and NO2) and N2O are known as harmful pollutants. In fluidized bed combustion these are formed from the nitrogen in the fuel. To develop effective primary measures reducing the emissions, more knowledge on the mechanism of formation and destruction ongoing in fluidized beds has to be obtained. In this work, a detailed chemistry model is combined with a two-phase model for a stationary fluidized bed to calculate the emissions of a single fuel particle in a laboratory-scale stationary fluidized bed. The single particle model consists of a simple model for the H2O release during drying, a model for the volatiles composition, and a model for the nitrogen chemistry during char combustion. The detailed reaction mechanism consists of a homogeneous part, heterogeneously catalyzed reactions on the bed material, and radical recombination reactions on the solids’ surface. The results confirm that devolatilization and char combustion are of nearly equal importance for NO and N2O formation. During devolatilization, NO is formed from HCN and NH3, while N2O is formed almost exclusively from HCN. During char combustion, NO is mostly formed by heterogeneous oxidation of char nitrogen, while N2O is formed from homogeneous oxidation of HCN. On the other hand, there is also a back coupling of NO on the homogeneous burnout of the carbon containing species, by sensitizing the oxidation of CH4.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):236-241. doi:10.1115/1.1385519.

This research effort involved experimentally testing an advanced-cycle, ammonia-water absorption chiller with a cooling capacity of 17.6 kW (5 refrigeration tons (RT)). The system was a generator-absorber heat exchange (GAX) cycle and was sized for residential and light commercial use, where very little absorption equipment is currently used. The components of the cycle were assembled with instrumentation, including flow meters, pressure transducers, and thermocouples. The findings of the research were cycle cooling load and coefficient of performance (COP), as well as many component heat duties and working fluid state points throughout the cycle. The COP of the chiller at essentially full load was measured at 0.68. A simulation of the GAX cycle was performed with a computer program that predicted the heat duties of each component and the cooling load of the cycle. The simulation of the GAX cycle and experimental testing compared closely. Existing market research shows that significant business opportunities exist for a GAX heat pump or chiller with a cooling COP of 0.70 or greater. The work performed in this study consisted of testing a GAX cycle with a COP that approached the target value of 0.70 and identified improvements that must be made to reach the target COP value.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2001;123(3):242-249. doi:10.1115/1.1377895.

Experiments were conducted for ammonia-water falling film absorption in a plate heat exchanger with offset strip fins. The objectives of this paper were to analyze combined heat and mass transfer during the ammonia-water absorption process under different inlet subcooling modes, and to obtain heat transfer coefficients (Nusselt number). This paper examined the effects of the inlet subcooling modes, the inlet concentration difference, liquid Reynolds number, and vapor Reynolds number on the heat transfer performance. Inlet liquid concentrations were set at 0, 5, 10, and 15 percent in mass of ammonia, while inlet vapor concentration ranged from 64.7 to 83.6 percent. Experiments were conducted in three ways according to the inlet subcooling conditions, i.e., Case A (Tv>Tl), Case B (Tv∼Tl), and Case C (Tv<Tl). In Case A, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption was confirmed in the experiments, which resulted in a lower absorption performance. The heat transfer coefficient increased as the inlet subcooling increased in all cases. The effect of inlet subcooling on heat transfer performance was more significant in Case A than in Cases B and C. The inlet subcooling had more significant effect on the heat transfer performance than the inlet concentration difference. Nusselt number increased as liquid and vapor Reynolds numbers increased. The vapor velocity should be maximized to increase absorption performance in cocurrent ammonia-water absorption process. The parametric analysis provides fundamental understandings of the ammonia-water absorption process, and thus gives a guideline for heat exchanger compactness in ammonia-water absorption systems.

Commentary by Dr. Valentin Fuster


J. Energy Resour. Technol. 2001;123(3):250. doi:10.1115/1.1406948.
Topics: Patents , Turbines
Commentary by Dr. Valentin Fuster

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