Carlos F. Torres

Global Optimization of Gas Allocation to a Group of Wells in Artificial Lift Using Nonlinear Constrained Programming

Continuous flow gas lift is one of the most common artificial lift methods widely used in the oil industry. A continuous volume of high-pressure gas is injected as deep as possible into the tubing, to gasify the oil column, and thus facilitate the production. If there is no restriction in the amount of injection gas available, sufficient gas can be injected into each oil well to reach maximum production. However, the injection gas available is generally insufficient. An inefficient gas allocation in a field with limited gas supply reduces the revenues, since excessive gas injection is expensive due to the high gas prices and compressing costs. Therefore, it is necessary to assign the injection gas into each well in optimal form to obtain the field maximum oil production rate. The gas allocation optimization can be considered as a maximization of a nonlinear function, which models the total oil production rate for a group of wells. The variables or unknowns for this function are the gas injection rates for each well, which are subject to physical restrictions. In this work a nonlinear optimization technique, based on an objective function with constraints, was implemented to find the optimal gas injection rates. A new mathematical fit to the gas-lift performance curve (GLPC) is presented and the numeric results of the optimization are given and compared with those of other methods published in the specialized literature. The GLPC can be either measured in the field, or alternatively generated by computer simulations, by mean of nodal analysis. The optimization technique proved fast convergence and broad application.

New comb-like poly(n-alkyl itaconate)s with crystalizable side chains

A series of poly(mono n-alkyl itaconate)s, poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s with n ¼ 12; 14, 16, 18 and 22 have been prepared by radical polymerization. NMR studies point out that poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s are mainly syndiotactic polymers whereas poly(mono n-alkyl itaconate)s are obtained as almost atactic polymers. The characterization performed by DSC, solid state 13 C CP/MAS NMR and X-ray diffraction, indicates that the side chains of poly(mono n-alkyl itaconate)s and poly(methyl n-alkyl itaconate)s derivatives with more than 12 carbon atoms are able to crystallize in hexagonal lattices. In the case of poly(di n-alkyl itaconate)s, when the side chains contain 12 or more carbon atoms, they are able to crystallize also in hexagonal lattices. q 2003 Elsevier Science Ltd. All rights reserved.

Experimental Investigation of Horizontal Gas–Liquid Stratified and Annular Flow Using Wire-Mesh Sensor

Stratified and annular gas–liquid flow patterns are commonly encountered in many industrial applications, such as oil and gas transportation pipelines, heat exchangers, and process equipment. The measurement and visualization of two-phase flow characteristics are of great importance as two-phase flows persist in many fluids engineering applications. A wire-mesh sensor (WMS) technique based on conductance measurements has been applied to investigate two-phase horizontal pipe flow. The horizontal flow test section consisting of a 76.2mm ID pipe, 18m long was employed to generate stratified and annular flow conditions. Two 16 16 wire configuration sensors, installed 17 m from the inlet of the test section, are used to determine the void fraction within the cross section of the pipe and determine interface velocities between the gas and liquid. These physical flow parameters were extracted using signal processing and cross-correlation techniques. In this work, the principle of WMS and the methodology of flow parameter extraction are described. From the obtained raw data time series of void fraction, cross-sectional mean void fraction, time averaged void fraction profiles, interfacial structures, and velocities of the periodic structures are determined for different liquid and gas superficial velocities that ranged from 0.03m/s to 0.2m/s and from 9m/s to 34m/s, respectively. The effects of liquid viscosity on the measured parameters have also been investigated using three different viscosities. [DOI: 10.1115/1.4027799]

Hairy-rod random copoly(β,l-aspartate)s containing alkyl and benzyl side groups

Hairy random copoly(β,l-aspartate)s were prepared by anionic ring opening polymerization of mixtures of (S)-4-octadecoxycarbonyl and (S)-4-benzyloxycarbonyl 2-azetidinones at 3:1, 1:1 and 1:3 molar ratios. The three resulting copolymers had molar ratios of octadecyl to benzyl units of 68:32, 44:56 and 12:88, respectively. They all were found to adopt the rigid α-helix-like conformation characteristic of poly(β,l-aspartate)s, but only the first one displayed crystallization of the alkyl side chains. This copolymer showed thermochromic behavior with color changes taking place when heated above the side chain melting temperature. These results evidenced the ability of the benzyl group to be accommodated within the layered structures of comb-like poly(β,l-aspartate)s and to modify the temperature range in which chromatic changes are observed in these systems.

Characterizing Slug/Churn Flow Using Wire Mesh Sensor

A wire mesh sensor (WMS) is a device used to investigate multi-phase flows. The WMS measures the instantaneous local electrical conductivity of multiphase flows at different measuring points. There is a significant difference in the electrical conductivity of the employed fluids (in this work air and water, conductivity of water is much higher than that of air). Using the difference in the electrical conductivity, the WMS provides the local void fraction. The WMS utilized in this work includes two identical planes of parallel 16×16 grid of wires. The separation distance between these two planes is 32 mm. The WMS was installed in a 76.2 mm (3-inch) diameter vertical pipe to extract information such as void fraction distribution, structure velocity, and slug/churn flow structure. The superficial gas (air) velocity (VSG) ranged from 10 to 38.4 m/s. Liquid (water) superficial velocities (VSL) of 0.30, 0.46, 0.61 and 0.76 m/s were employed. To study the effects of viscosity on the slug/churn flow structure, Carboxyl Methyl Cellulose (CMC) was added to water to increase the liquid viscosity without altering its density. Each experiment was performed for 60 seconds. An operation frequency for the WMS of 10 kHz (totaling 600,000 frames of void fraction measurement per experiment) was used for all experiments.Copyright

Experimental Study of Vertical Gas-Liquid Pipe Flow for Annular and Liquid Loading Conditions Using Dual Wire-Mesh Sensors

In gas well production, liquid is produced in two forms, droplets entrained in the gas core and liquid film flowing on the tubing wall. For most of the gas well life cycle, the predominant flow pattern is annular flow. As gas wells mature, the produced gas flow rate reduces decreasing the liquid carrying capability initiating the condition where the liquid film is unstable and flow pattern changes from fully co-current annular flow to partially co-current annular flow. The measurement and visualization of annular flow and liquid loading characteristics is of great importance from a technical point of view for process control or from a theoretical point of view for the improvement and validation of current modeling approaches. In this experimental investigation, a Wire-Mesh technique based on conductance measurements was applied to enhance the understanding of the air-water flow in vertical pipes. The flow test section consisting of a 76 mm ID pipe, 18 m long, was employed to generate annular flow and liquid loading at low pressure conditions. A 16×16 wire configuration sensor is used to determine the void fraction within the cross-section of the pipe. Data sets were collected with a sampling frequency of 10,000 Hz. Physical flow parameters were extracted based on processed raw measured data obtained by the sensors using signal processing. In this work, the principle of Wire-Mesh Sensors and the methodology of flow parameter extraction are described. From the obtained raw data, time series of void fraction, mean local void fraction distribution, characteristic frequencies and structure velocities are determined for different liquid and gas superficial velocities that ranged from 0.005 to 0.1 m/s and from 10 to 40 m/s, respectively. In order to investigate dependence of liquid loading phenomenon on viscosity, three different liquid viscosities were used. Results from the Wire-Mesh Sensors are compared with results obtained from previous experimental work using Quick Closing Valves and existing modeling approaches available in the literature.Copyright

Experimental Investigation of Three-Phase Low-Liquid-Loading Flow

Although many different studies have been conducted on gas/ liquid multiphase flow, only a very small number of three-phase flow studies, especially for low-liquid-loading flows, can be found. These studies are mainly experimental, and focused on two-phase flow in small-diameter pipelines. The coexistence of thin films of water along with oil in production systems is very commonly observed in wet-gas pipelines. The existence of the second liquid phase influences all of the flow characteristics. The three-phaseflow behavior can be considered as a combination of gas/liquid and oil/aqueous phase interactions. Meng et al. (2001) conducted two-phase-flow experiments for oil/air flow in a 2-in.-ID pipe. They observed a surprising decrease in liquid holdup and pressure gradient when the vSL was increased. They attributed this decrease to the increase in droplet entrainment. They also developed a correlation for interfacial friction factor. Fan (2005) used two experimental facilities with IDs of 2 and 6 in., respectively, to conduct two-phase water/air low-liquid-loading experiments. Fan observed stratified smooth and stratified wavy flow patterns in his experiments with the 6-in.-ID facility. With the 2-in.-ID facility, in addition to stratified flow patterns, an annular flow pattern was observed. Fan used the acquired experimental data to develop new closure relationships for mechanistic modeling. These closure relationships included wetted-wall fraction, liquid-wall friction factor, and interfacial friction factor. Later, Dong (2007) modified the 6-in.-ID facility of Fan (2005) to conduct low-liquid-loading three-phase-flow experiments. Water, air, and oil with a viscosity of 13 cp were the flowing fluids. This is a relatively high oil viscosity compared with the commonly observed values in wet-gas pipelines, and the results may not be representative for wet-gas pipeline systems. The distribution of oil and water in liquid phase for different flowing conditions was observed and categorized. In addition, a model comparison was provided for flow characteristics. Recently, Gawas (2013) used the same 6-in.-ID facility of Dong (2007) to investigate the characteristics of three-phase low-liquidloading flow. Gawas conducted his experiments by use of an oil with a viscosity of 1.3 cp for different values of water cut, and developed correlations for entrainment of liquid droplets in gas phase for twoand three-phase flows. He also analyzed the droplet-size distribution and developed a correlation for interfacial wave celerity. In addition, several studies have been conducted in other research centers to analyze low-liquid-loading flow. A summary of these studies is presented in Gawas (2013). In the current study, the facility of Gawas (2013) is used. The main objective of this research is to study low-liquid-loading threephase flow, and the targeted flow parameters are liquid holdup, water holdup, wave pattern, and pressure gradient. The experimental results for different flow characteristics are analyzed and evaluated to improve understanding of the flow phenomena. In addition, the commonly used models are evaluated by use of the acquired experimental data.

Synthesis of ε-caprolactone-b-l-lactide block copolymers by mean sequential polymerization, using diphenylzinc as initiator

Block copolymers of ε-caprolactone (CL) and l-lactide (l-LA) were synthesized by sequential polymerization using diphenylzinc as initiator. The composition of the copolymers was adjusted changing the comonomers in ratio. Copolymers were characterized by 1H-NMR, 13C-NMR, DSC, and GPC. Results indicate that poly(ε-caprolactone)-b-poly(l-lactide) (PCL-b-PLA) block copolymers had a narrow molecular weight distribution and well-controlled sequences without random placement.

CFD Simulations of Low Liquid Loading Multiphase Flow in Horizontal Pipelines

Low Liquid Loading is a very common occurrence in wet gas pipelines where very small amounts of liquid flow along with the gas, mainly due to condensation of hydrocarbon gases and water vapor. The effects of low liquid loading on different flow characteristics, and flow assurance issues such as pipe corrosion prove the necessity of analyzing the flow behavior in more depth. In this study, CFD simulations are conducted for a horizontal pipe where liquid and gas are supplied at separate constant rates at the inlet. The liquid is introduced at the bottom to help shorten the developing section. The simulations are conducted with Ansys Fluent v14.5 using Volume Of Fluid (VOF) as the multiphase model. The analysis targets, mainly, the shape of the interface, velocity fields in both liquid and gas phases, liquid holdup, and shear stress profile. On the other hand, experiments are conducted in a 6-inch ID low liquid loading facility with similar testing condition. Experiments are conducted with water or oil as the liquid phase for a liquid volume fraction range of 0.0005–0.0020 of the inlet stream. For all cases, several flow parameters are measured including liquid holdup and interface wave characteristics. A comparison is conducted between CFD simulation results, model predictions, and experimental results, and a discussion of the sources of discrepancy is presented. Overall, the results help understand the low liquid loading flow phenomenon.Copyright

Modeling and simulation of the production process of electrical energy in a geothermal power plant

Abstract In this work, the study, modeling and simulation of custom designed geothermal power plant production processes are presented. A model based on the thermodynamical steam power cycles theory is developed using directed graphs, in order to consider different types of geothermal power plant structural models to be studied. The model includes steady state forms of the thermodynamical mass and energy conservation laws for each one of the considered equipments, in order to study the mass and energy interactions among them. A computational implementation of the IAPWS-IF97 formulation for the calculation, also by steady state forms, of the physical properties of the water is also presented, thus providing a reliable and accurate method to calculate them in execution time. The analysis of both topological and thermodynamical feasibility of the proposed input models is also presented, as well as the analysis of the efficiency in terms of both generated and invested heat and power. The results are compared against reference results in the literature; these are presented in relation to the proposed models, by reporting the variations of the mentioned water properties and by stating the results of the production processes in terms of both mechanical power generation, and efficiency of the proposed structural models.