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### Research Papers: Petroleum Wells-Drilling/Production/Construction

J. Energy Resour. Technol. 2008;130(3):033101-033101-7. doi:10.1115/1.2955558.

This paper presents an innovative undersea blowout preventer (BOP) using shape-memory alloy (SMA). The new device using SMA actuators could easily be implemented into existing conventional subsea control system so that they can work solely or as a backup of other methods. Most important, the innovative all-electric BOP will provide much faster response than its hydraulic counterpart and will improve safety for subsea drilling. To demonstrate the feasibility of such a device, a proof-of-concept prototype of a pipe RAM type BOP with SMA actuation has been designed, fabricated, and tested at the University of Houston. The BOP actuator uses strands of SMA wires to achieve large force and large displacement in a remarkably small space. Experimental results demonstrate that the BOP can be activated and fully closed in less than $10s$. The concept of this innovative device is illustrated, and detailed comparisons of the response time for hydraulic and nitinol SMA actuation mechanisms are included. This preliminary research reveals the potential of smart material technology in subsea drilling systems.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2008;130(3):033102-033102-11. doi:10.1115/1.2955560.

The coning problems for vertical wells and the ridging problems for horizontal wells are very difficult to solve by conventional methods during oil production from reservoirs with bottom water drives. If oil in a reservoir is too heavy to follow Darcy’s law, the problems may become more complicated for the non-Newtonian properties of heavy oil and its rheology. To solve these problems, an innovative completion design with downhole water sink was presented by dual-completion in oil and water columns with a packer separating the two completions for vertical wells or dual-horizontal wells. The design made it feasible that oil is produced from the formation above the oil water contact (OWC) and water is produced from the formation below the OWC, respectively. To predict quantitatively the production performances of production well using the completion design, a new improved mathematical model considering non-Newtonian properties of oil was presented and a numerical simulator was developed. A series of runs of an oil well was employed to find out the best perforation segment and the fittest production rates from the formations above and below OWC. The study shows that the design is effective for heavy oil reservoir with bottom water though it cannot completely eliminate the water cone formed before using the design. It is a discovery that the design is more favorable for new wells and the best perforation site for water sink (Sink 2) is located at the upper 1/3 of the formation below OWC.

Topics: Water , Reservoirs , Wells
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2008;130(3):033103-033103-6. doi:10.1115/1.2955562.

A new-type energy-recovering hydraulic workover rig is researched. The basic structure and working theory of this rig are introduced. This rig decreases its equipped power remarkably compared with the conventional rig, and can recover the potential energy released by the tubing string when lowered. With infinite force-grade number, this rig can obtain the maximum lifting efficiency and energy-recovering rate. The dynamics model of lifting the tubing string is deduced and a simulation analysis is conducted. Through simulation some conclusions are obtained: (1) the greater the throttle valve path area the higher the tubing string lifting velocity, (2) the lighter the tubing string the higher the tubing string lifting velocity, and (3) the greater the displacement of the variable pump motor the higher the tubing string lifting velocity. The actual measurement results prove that the dynamics model and simulation results are basically right.

Commentary by Dr. Valentin Fuster

### Research Papers: Deep-Water Petroleum

J. Energy Resour. Technol. 2008;130(3):031501-031501-7. doi:10.1115/1.2955484.

Current tank tests have been performed on two equal-diameter, flexible cylinders in tandem with various end spacings or separation distances. While the cylinders experienced transverse vortex-induced vibration in only the first mode, the test Reynolds numbers were well into the transition range, and the drag crisis was clearly observed on the upstream cylinder. Both acceleration and drag were measured to provide some interesting correlations between vibration, drag, and end spacing for each of the two cylinders.

Commentary by Dr. Valentin Fuster

### Research Papers: Hydrates/Coal Bed Methane/Heavy Oil/Oil Sands/Tight Gas

J. Energy Resour. Technol. 2008;130(3):032501-032501-14. doi:10.1115/1.2956978.

Numerical modeling of gas hydrates can provide an integrated understanding of the various process mechanisms controlling methane $(CH4)$ production from hydrates and carbon dioxide $(CO2)$ sequestration as a gas hydrate in geologic reservoirs. This work describes a new unified kinetic model which, when coupled with a compositional thermal reservoir simulator, can simulate the dynamics of $CH4$ and $CO2$ hydrate formation and decomposition in a geological formation. The kinetic model contains two mass transfer equations: one equation converts gas and water into hydrate and the other equation decomposes hydrate into gas and water. The model structure and parameters were investigated in comparison with a previously published model. The proposed kinetic model was evaluated in two case studies. Case 1 considers a single well within a natural hydrate reservoir for studying the kinetics of $CH4$ and $CO2$ hydrate decomposition and formation. A close agreement was achieved between the present numerical simulations and results reported by Hong and Pooladi-Darvish (2003, “A Numerical Study on Gas Production From Formations Containing Gas Hydrates  ,” Petroleum Society’s Canadian International Petroleum Conference, Calgary, AB, Jun. 10–12, Paper No. 2003-060). Case 2 considers multiple wells within a natural hydrate reservoir for studying the unified kinetic model to demonstrate the feasibility of $CO2$ sequestration in a natural hydrate reservoir with potential enhancement of $CH4$ recovery. The model will be applied in future field-scale simulations to predict the dynamics of gas hydrate formation and decomposition processes in actual geological reservoirs.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2008;130(3):032502-032502-11. doi:10.1115/1.2956979.

Continuing concern about the impacts of atmospheric carbon dioxide $(CO2)$ on the global climate system provides an impetus for the development of methods for long-term disposal of $CO2$ produced by industrial and other activities. Investigations of the $CO2$-hydrate properties indicate the feasibility of geologic sequestration $CO2$ as gas hydrate and the possibility of coincident $CO2$ sequestration/$CH4$ production from natural gas hydrate reservoirs. Numerical studies can provide an integrated understanding of the process mechanisms in predicting the potential and economic viability of $CO2$ gas sequestration, especially when utilizing realistic geological reservoir characteristics in the models. This study numerically investigates possible sequestration of $CO2$ as a stable gas hydrate in various reservoir geological formations. As such, this paper extends the applicability of a previously developed model to more realistic and relevant reservoir scenarios. A unified gas hydrate model coupled with a thermal reservoir simulator (CMG STARS) was applied to simulate $CO2$-hydrate formation in four reservoir geological formations. These reservoirs can be described as follows. The first reservoir (Reservoir I) is similar to tight gas reservoir with mean porosity 0.25 and mean absolute permeability $10mD$. The second reservoir (Reservoir II) is similar to a conventional sandstone reservoir with mean porosity 0.25 and mean permeability $20mD$. The third reservoir (Reservoir III) is similar to hydrate-free Mallik silt with mean porosity 0.30 and mean permeability $100mD$. The fourth reservoir (Reservoir IV) is similar to hydrate-free Mallik sand with mean porosity 0.35 and mean permeability $1000mD$. The Mallik gas hydrate bearing formation itself can be described as several layers of variable thickness with permeability variations from $1mDto1000mD$, and is addressed as a separate part of this study. This paper describes numerical methodology, model input data selection, and reservoir simulation results, including an enhancement to model the effects of ice formation and decay. The numerical investigation shows that the gas hydrate model effectively captures the spatial and temporal dynamics of $CO2$-hydrate formation in geological reservoirs by injection of $CO2$ gas. Practical limitations to $CO2$-hydrate formation by gas injection are identified and potential improvements to the process are suggested.

Commentary by Dr. Valentin Fuster

### Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2008;130(3):032201-032201-8. doi:10.1115/1.2955563.

Pulverizers play a pivotal role in coal-based thermal power generation. Improper coal fineness or drying reflects a qualitywise deterioration. This results in flame instability, unburnt combustible loss, and a propensity to slagging or clinker formation. Simultaneously, an improper air-coal ratio may result in either coal pipe choking or flame impingement, an unbalanced heat release, an excessive furnace exit gas temperature, overheating of the tube metal, etc., resulting in reduced output and excessive mill rejects. In general, the base capacity of a pulverizer is a function of coal and air quality, conditions of grinding elements, classifier, and other internals. Capacity mapping is a process of comparison of standard inputs with actual fired inputs to assess the available standard output capacity of a pulverizer. In fact, this will provide a standard guideline over the operational adjustment and maintenance requirement of the pulverizer. The base capacity is a function of grindability; fineness requirement may vary depending on the volatile matter (VM) content of the coal and the input coal size. The quantity and the inlet temperature of primary air (PA) limit the drying capacity. The base airflow requirement will change depending on the quality of raw coal and output requirement. It should be sufficient to dry pulverized coal (PC). Drying capacity is also limited by utmost PA fan power to supply air. The PA temperature is limited by air preheater (APH) inlet flue gas temperature; an increase in this will result in efficiency loss of the boiler. Besides, the higher PA inlet temperature can be attained through the economizer gas bypass, the steam coiled APH, and the partial flue gas recirculation. The PA/coal ratio, a variable quantity within the mill operating range, increases with a decrease in grindability or pulverizer output and decreases with a decrease in VM. Again, the flammability of mixture has to be monitored on explosion limit. Through calibration, the PA flow and efficiency of conveyance can be verified. The velocities of coal/air mixture to prevent fallout or to avoid erosion in the coal carrier pipe are dependent on the PC particle size distribution. Metal loss of grinding elements inversely depends on the YGP index of coal. Besides that, variations of dynamic loading and wearing of grinding elements affect the available milling capacity and percentage rejects. Therefore, capacity mapping is necessary to ensure the available pulverizer capacity to avoid overcapacity or undercapacity running of the pulverizing system, optimizing auxiliary power consumption. This will provide a guideline on the distribution of raw coal feeding in different pulverizers of a boiler to maximize system efficiency and control, resulting in a more cost effective heat rate.

Topics: Coal , Flow (Dynamics)
Commentary by Dr. Valentin Fuster

### Research Papers: Heat Energy Generation/Storage/Transfer

J. Energy Resour. Technol. 2008;130(3):032401-032401-9. doi:10.1115/1.2955479.

Global climate change mitigation requires the fossil fuel consumption substantially reduced. Space heating is an energy-consuming sector. Despite the fact that the thermal efficiency of current space heating systems has achieved a value higher than 85%, corresponding to lower than $40kg$ c.e./GJ, there is still a big potential for energy conservation. In order to realize the full potential, investigations of heating systems should appeal to reversibility/exergy analysis made on total energy concept basis. This paper starts with an introduction of the concept “reversible mode of heating,” leading the readers think of space heating in terms of reversibility. Right after, a systematic reversibility analysis on a “mine to home” basis is conducted to reveal the impact of any irreversibility of all subsystems or devices involved in the total energy system of heating on the fuel/monetary specific consumption of unit end-use heat. The paper points out that although combined heat and power (CHP) and electrically driven heat pump are both of “reversible mode,” the former is far more favorable in terms of energy conservation. The recently ascent decentralized energy system provides the best circumstances for CHP implementation. The demand-side improvement is a topic of most importance but frequently neglected. This study reveals that, if properly engineered, this improvement together with adopting a direct type of heat grid might lower the fuel specific consumption of end-use heat of CHP to a level as low as $13–9kg$ c.e./GJ.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Transport/Pipelines/Multiphase Flow

J. Energy Resour. Technol. 2008;130(3):033001-033001-13. doi:10.1115/1.2955483.

Two-phase slug flow in horizontal and near horizontal pipes is a common occurrence in many engineering applications and industrial operations. The objective of this study is to experimentally investigate the effects of separator pressure fluctuations on terrain slugging and slug flow characteristics along and downstream of a hilly terrain pipeline. A further objective is to numerically simulate the flow behavior using a transient multiphase flow simulator to match the simulation predictions with the experimental data. Experimental results revealed that during the separator pressure decline, slug initiation is promoted due to the increase in slip velocity, which enhances the slug initiation mechanisms at the lower elbow. On the other hand, during the separator pressure increase, the analyses show slug suppression. In terms of slug flow characteristics, the mean slug velocity, mean slug length, and maximum slug length increased during the separator pressure decline condition and decreased during the separator pressure increase condition. Furthermore, separator pressure has a significant decreasing effect on slug frequency, maximum slug length, and slug length variance downstream of the hilly terrain section. The statistical analysis shows mixed results of decreasing and increasing trends on mean slug lengths under the fluctuated separator pressure when compared with constant separator pressure conditions. The numerical simulation results showed a close match of liquid holdup downstream of the lower elbow and a fair match at the lower elbow. Furthermore, the model was successful in matching the pressure fluctuation at the lower elbow of the experimental data.

Commentary by Dr. Valentin Fuster