J. Energy Resour. Technol. 1999;121(2):73-74. doi:10.1115/1.2795070.
Topics: Petroleum
Commentary by Dr. Valentin Fuster


J. Energy Resour. Technol. 1999;121(2):75-80. doi:10.1115/1.2795071.

Heat transfer can be of importance in the design of multiphase petroleum flowlines. However, heat transfer data for gas-liquid flows are available only for small-diameter pipes at low pressures. Moreover, existing prediction methods are largely not suited to petroleum pipeline conditions due to implicit use of simplistic two-phase flow models. In this work heat transfer estimation methods are derived for nonboiling gas-liquid flow in pipes of high Prandtl number liquids, such as crude oil. The methods are readily evaluated for engineering applications and are applicable to all flow regimes, except those with low liquid holdup. Comparison is made with literature data. Accuracies of ±33 percent are obtained in general. The methods explicitly couple with arbitrary prediction methods for two-phase flow pressure drop and liquid holdup. This explicit coupling makes plausible the hypothesis that predictions will be robust at conditions well beyond the range of the existing experimental data.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):81-85. doi:10.1115/1.2795072.

The environmental conditions typically encountered in deepwater petroleum operations lead to a number of production problems. The cold environment promotes particle formation and deposition of paraffinic compounds in a waxy crude system. These solids can build up inside flowlines and significantly decrease production rates. The wax deposition scale-up technique used for single phase has been applied to multiphase flow conditions. The resulting semi-empirical model is used to simulate multiphase transport of waxy crude oil through integration with a commercial steady-state pipeline simulation package. The wax model is a semi-empirical representation of the wax deposition phenomena. The model incorporates the effects of diffusion and shear through the wax deposition tendency. The key parameters are first measured in a laboratory flow loop.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):86-90. doi:10.1115/1.2795073.

The effect of drag-reducing agent (DRA) on multiphase flow in upward and downward inclined pipes has been studied. The effect of DRA on pressure drop and slug characteristics such as slug translational velocity, the height of the liquid film, slug frequency, and Froude number have been determined. Experiments were performed in 10-cm i.d., 18-m long plexiglass pipes at inclinations of 2 and 15 deg for 50 percent oil-50 percent water-gas. The DRA effect was examined for concentrations ranging from 0 to 50 ppm. Studies were done for superficial liquid velocities between 0.5 and 3 m/s and superficial gas velocities between 2 and 10 m/s. The results indicate that the DRA was effective in reducing the pressure drop for both upflow and downflow in inclined pipes. Pressure gradient reduction of up to 92 percent for stratified flow with a concentration of 50 ppm DRA was achieved in ±2 deg downward inclined flow. The effectiveness of DRA for slug flow was 67 percent at a superficial liquid velocity of 0.5 m/s and superficial gas velocity of 2 m/s in 15 deg upward inclined pipes. Slug translational velocity does not change with DRA concentrations. The slug frequency decreases from 68 to 54 slugs/min at superficial liquid velocity of 1 m/s and superficial gas velocity of 4 m/s in 15 deg upward inclined pipes as the concentration of 50 ppm was added. The height of the liquid film decreased with the addition of DRA, which leads to an increase in Froude number.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):91-95. doi:10.1115/1.2795074.

Multiphase oil/water/gas flow regime transition studies are carried out in a 10-cm i.d., 18-m long pipe at inclinations of ±2 deg at system pressures between 0 to 0.79 MPa. The results are compared to those of other researchers, and the effects of pressure, inclination, and liquid viscosity are shown. The water cut of the liquid has some effects on the transition from stratified to slug flow. Increasing the water cut results in the transition occurring at higher liquid velocity at the same gas velocity. Water cut has little effect on the slug/annular transition for low viscosity oil used. The system pressure has a moderate effect on the transition from stratified to slug and slug to annular. For the transition from stratified to slug, increasing the system pressure requires higher liquid velocity. The transition from slug to annular occurs at lower liquid velocity with increasing the system pressures. The inclination of the pipe has little effect on the transition from slug to annular flow. Increasing the inclination causes the transition to occur at approximately the same gas velocity at the same liquid velocity. The experimental results show a good agreement with Wilkens’ model.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):96-101. doi:10.1115/1.2795075.

Results from experiments conducted in downward liquid-gas flows in inclined, eccentric annular pipes, with water and air as the working fluids, are presented. The gas was injected in the middle of the test section length. The operating window, in terms of liquid and gas superficial velocities, within which countercurrent gas flow occurs at two low-dip angles, has been determined experimentally. The countercurrent flow observed was in the slug regime, while the co-current one was stratified. Countercurrent flow fraction and void fraction measurements were carried out at various liquid superficial velocities and gas injection rates and correlated to visual observations through a full-scale transparent test section. Our results indicate that countercurrent flow can be easily generated at small downward dip angles, within the practical range of liquid superficial velocity for drilling operations. Such flow is also favored by low gas injection rates.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):102-109. doi:10.1115/1.2795064.

A leak-off test (LOT) is a verification method to estimate fracture pressure of exposed formations. After cementing each casing string, LOT is run to verify that the casing, cement and formation below the casing seat can withstand the wellbore pressure required to drill for the next casing string safely. Estimated fracture pressure from the test is used as the maximum pressure that may be imposed on that formation. Critical drilling decisions for mud weights, casing setting depths, and well control techniques are based upon the result of a LOT. Although LOT is a simple and inexpensive test, its interpretation is not always easy, particularly in formations that give nonlinear relationships between pumped volume and injection pressure. The observed shape of the LOT is primarily controlled by the local stresses. However, there are other factors that can affect and distort LOT results. Physically the LOT, indeed, reflects the total system compressibility, i.e., the compressibility of the drilling fluid, wellbore expansion, or so-called borehole ballooning, and leak (filtration) of drilling fluids into the formation. There is, however, no mathematical model explaining the nonlinear behavior. Disagreement on determining or interpreting actual leak-off pressure from the test data among the operators is common. In this paper, a mathematical model using a well-known compressibility equation is derived for total system compressibility to fully analyze nonlinear LOT behavior. This model accurately predicts the observed nonlinear behavior in a field example. The model also predicts the fracture pressure of the formation without running a test until formation fracture.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):110-116. doi:10.1115/1.2795065.

The power section of a positive displacement drill motor (PDM) consists of a steel rotor and a tube with a molded elastomeric lining (stator). Power section failures are typically due to the failure of the stator elastomer. Stator life depends on many factors such as design, materials of construction, and downhole operating conditions. This paper focuses on the stator failure mechanisms and factors affecting stator life. An analytical method for predicting the effect of various design and operating parameters on the strain state and heat build-up within elastomers is discussed. The effect of parameters such as rotor/stator design, downhole temperature, drilling fluid, stator elastomer properties, motor speed, and motor differential pressure on the stator life is discussed. Nonlinear finite element analysis is used to perform thermal and structural analysis on the stator elastomer. Data from laboratory accelerated life tests on power section stators is presented to demonstrate the effect of operating conditions on stator life.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):117-121. doi:10.1115/1.2795066.

During 1996 and 1997, a steam and water separator used in the nuclear power industry was tested with hydrocarbon fluids to evaluate its potential for use in the hydrocarbon production industry. Prior to testing with hydrocarbon fluids, a nondimensional parameter was developed from a simple model of the second-stage centrifugal separator to correlate existing liquid separation efficiency data for this separator using steam and water. This paper outlines the development of the nondimensional correlating parameter and presents comparisons of liquid separation efficiency with steam and water and hydrocarbon fluids using this parameter.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):122-130. doi:10.1115/1.2795067.

A higher-order numerical procedure is applied to simulate typical transient phenomena in natural gas transportation. Reliable modeling and prediction of transients features in transmission pipelines are desirable for optimal control of gas deliverability, design and implementation of active controls, and modeling of operational behavior of network peripheral equipment (e.g., chokes, valves, compressors, etc.). As an alternative to the method of characteristics (MOC) that had been widely used for modeling these systems, higher-order total variation diminishing (TVD) methods are used to model some transient problems. This class of methods has the capability of capturing fine-scale phenomena, and they do provide a better resolution of frontal discontinuities. In this study, the TVD schemes are utilized in conjunction with upwind methods. Also, in order to ensure a stable time-stepping scheme over a wide range of Courant-Friedrich-Lewy (CFL) number, a special Runge-Kutta method is employed as the base solution algorithm to integrate the highly nonlinear, hyperbolic equations which govern the transportation of natural gas in pipelines. The overall procedure is stable, robust, and accurate when applied to solve practical problems with dynamic pressure waves.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):131-136. doi:10.1115/1.2795068.

In a laboratory-scale circulating fluidized bed combustor (CFBC), which mainly consists of quartz-glass, the relative importance of the radicals, generated by the combustion process, on the N2 O and NO formation and destruction paths are studied. The CFBC unit is electrically heated and operating conditions can be nearly independently changed over a wide range; e.g., the bed temperature was varied between 700 and 900°C. The radicals’ importance on the destruction reactions of N2 O has been investigated under CFBC conditions by a recently developed iodine-addition technique to suppress the radical concentrations. Additionally, CO, CH4 and H2 O have been added to study their influence and to change the pool of radicals. Time-resolved concentration changes at the top of the riser have been measured by using a high-performance FT-IR spectrometer in combination with a low-volume, long-path gas cell. The FT-IR analysis is focused on the carbon-containing species, viz., CO2 CO, CH4 NO2 and other hydrocarbons, as well as on the nitrogen-containing species, viz., NO, NO2 , N2 O, and HCN. In the continuous combustion tests, petroleum coke has been burned in the CFBC. Concentration profiles and concentration changes at the top of the riser have been measured. Iodine has been added and the bed temperature and the initial fuel particle size are varied. With the knowledge of the N2 O destruction reactions, the relative importance of the radicals on N2 O and NO formation reactions has been identified and is discussed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1999;121(2):137-141. doi:10.1115/1.2795069.

Absorption heat pump technology may be improved by new cycle configurations by new working fluids. In this study, the effect of hypothetical working fluids on performance improvement is explored. The performance of two cycles is studied using three fluid property sources for ammonia/water, i.e., curve-fit experimental data, an ideal solution model, and the Peng-Robinson equation of state model. The models require only minimal fundamental thermodynamic property data for the two pure components. This allows investigation into the influence of each fundamental property on cycle performance, providing insight into desirable properties for new absorption fluid pairs. Variations of fundamental fluid properties are used as input to the models, showing that the volatilities of the refrigerant and absorbent have the greatest effect on cycle performance.

Commentary by Dr. Valentin Fuster

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