Oil Production Wells Research Articles

Effect of Jet Impingement Velocity and Angle on CO2 Erosion-Corrosion with and without Sand for API 5L-X65 Carbon Steel.

Most oil and gas production wells have plenty of corrosive species present along with solid particles. In such production environments, CO2 gas can dissolve in free phase water and form carbonic acid (H2CO3). This carbonic acid, along with fluid flow and with/without solid particles (sand or other entrained particles), can result in unpredictable severe localized CO2 corrosion and/or erosion–corrosion (EC). So, in this work, the CO2 EC performance of API 5L X-65 carbon steel, a commonly used material in many oil and gas piping infrastructure, was investigated. A recirculating flow loop was used to perform these studies at three different CO2 concentrations (pH values of 4.5, 5.0, and 5.5), two impingement velocities (8 and 16 m/s), three impingement angles (15°, 45°, and 90°), and with/without 2000 ppm sand particles for a duration of 3 h in 0.2 M NaCl solution at room temperature. Corrosion products were characterized using FE-SEM, EDS, and XRD. The CO2 EC rates were found to decrease with an increase in the pH value due to the increased availability of H+ ions. The highest CO2 erosion–corrosion rates were observed at a 45° impingement angle in the presence of solid particles under all conditions. It was also observed that a change in pH value influenced the morphology and corrosion resistance of the corrosion scales.

Improved capacitance model considering bottom-hole flowing pressure and interference between oil wells

With the continuous production of oil wells, the reservoir properties, such as permeability and porosity, are changing accordingly, and the reservoir heterogeneity is also enhanced. This development is vulnerable to the problem of the one-way advance of injected water and low efficiency of water flooding. The interwell connectivity between injection and production wells controls the flow capacity of the subsurface fluid. Therefore, the analysis of interwell connectivity helps to identify the flow direction of injected water, which is of great significance for guiding the profile control and water plugging in the later stage of the oilfield. In this study, based on the principle of mass conservation, a capacitance model considering the bottom-hole flowing pressure was established and solved by using the production dynamic data of injection–production wells. Then, the validity of the capacitance model was verified by numerical simulation, and the influences of well spacing, compression coefficient, frequent switching wells, injection speed, and bottom-hole flowing pressure on interwell connectivity were eliminated. Finally, a practical mine technique for inversion of connectivity between wells using dynamic data was developed. The advantage of this model is that the production dynamic data used in the modeling process are easy to obtain. It overcomes the shortcomings of previous models and has a wider range of applications. It can provide a theoretical basis for the formulation of profile control and water-plugging schemes in the high-water-cut period.

Optimization of MBR Field Integrated Production Model

MBR Field is an onshore field located at East Kalimantan. In this field, there are five clusters with 2 main stations and 1 supporting station. With the current pipe flow conditions, this field has several constraints which are ESP maximum motor loads, ESP maximum frequencies, and current maximum water injection plant capacity. First, modeling is done with deviations of less than 10 percent to reach matching conditions in several parameters such as upstream pressure, downstream pressure, liquid rate, water rate, oil rate, and gas rate. Afterward, the first optimization is done by increasing the ESP frequency, increasing choke bean size, and shutting-in relatively low oil production wells with high water cut. But due to the water production is almost exceeding the water injection plant capacity, then the second optimization is done. The second optimization is done by increasing choke bean size and shutting-in relatively low oil production wells with high water cut. Each optimization is then followed by an analysis of pressure and flowrates alterations and the existence of backpressure in unoptimized wells.

Compensator-vibration isolator

Nowadays electrical submersible pump (ESP) units remain the main type of formation fluid production equipment. There may be complicating factors during the operation of oil production wells equipped with such units. Complicating factors include the fall of the submersible unit downhole to the bottom of the production well. The damage from such falls is great. Despite the accumulated experience in this area, effective solutions need to be developed. In the work, the sources of vibration in the ESP are noted, and the places of breaks in the unit are indicated. The reasons for the occurrence of vibration processes in a submersible unit are summarized. The authors propose a design of a compensator intended for quenching vibration energy in a submersible unit. To evaluate the effectiveness of the compensator as part of an ESP unit, tests on industrial sites were conducted. At Open Joint-Stock Company Alnas, the test stand was a vertical well. Tests in Almetyevsk Central Base of Electric Submersible Production Service were carried out on a horizontal test stand section by section. Analysis of the results of the tests showed that the use of a compensator can reduce vibration levels.

Effect of electromagnetic field on water-in-oil emulsion and calcium carbonate crystallization

The investigation was devoted to the effect of electromagnetic field on crystallization of calcium carbonate, structure of CaCO3 crystals generated from oversaturated aqueous solutions, and water-in-oil emulsion (crude oil) properties. XRD demonstrated that electromagnetic field caused a decrease in mean size of CaCO3 particles by 30 μm and prevalent aragonite formation. We detected that electromagnetic field application leads to decreased apparent viscosity of water-in-oil emulsion and inhibits influence of a non-ionogenic surfactant on stability and viscosity properties of crude oil. As a matter of practice, the method demonstrated its efficiency to prevent scale deposits in oil production wells.

Bypassing annular gas by means of jet device in oil wells with electric submersible pumping units

Nowadays the main equipment for oil production is the installation of electric centrifugal pumps. When operating oil wells with such installations, various complicating factors arise, in particular, the accumulation of free gas in the space between the outer surface of the tubing and the casing string. The experience of the ESP unit operation shows the negative impact of high gas pressure in the annular space on the efficiency of the unit. The technical solutions available today for the wells characterized by high GOR are the different types of valves, which have a number of operational shortcomings. To improve the efficiency of oil well operation, the authors suggest a jet device to bypass free gas above the dynamic level, thus reducing the gas pressure in annular space. The solution of the problem of reducing the negative effect of excess free gas in annular space is based on the study of experimental data from oilfields by using up-to-date information technology. The technology using a jet device in combination with a submersible electric centrifugal pump is proposed. The use of the jet apparatus enables reducing the gas pressure in the annular space of oil-producing wells even regardless of the discharge pressure and external ambient temperature, which increases the operating efficiency of the electric submersible pumping unit.

Tepidiphilus baoligensis sp. nov., a Novel Bacterium of the Family Hydrogenophilaceae Isolated from an Oil Reservoir.

A Gram-negative, aerobic, motile, non-spore-forming and rod-shaped bacterium, designated strain B18-69T, was isolated from oil-well production liquid in Baolige oilfield, China. The strain was able to grow at pH 6-9.5 (optimum at pH 7), in 0-4% (w/v) NaCl (optimum at 0.5-1%, w/v) and at 35-60°C (optimum at 55°C). Major cellular fatty acids were C16:0, C19:0 cyclo ω8c, C17:0 cyclo and C18:1 ω7c. The predominant respiratory quinone was ubiquinone 8. Major polar lipids were phosphatidylethanolamine (PE), phosphatidylglycerol (PG), diphosphatidylglycerol (DPG) and phosphatidylcholine (PC). Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain B18-69T was most closely related to Tepidiphilus margaritifer DSM 15129T (98.8% similarity). The draft genome of strain B18-69T was composed of 2,250,419bp, and the G+C content was 64.6mol%. Average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between strain B18-69T and T. margaritifer DSM 15129T were 90.9% and 68.9%, respectively. Genotypic and phenotypic features indicate that strain B18-69T represents a novel species of the genus Tepidiphilus, for which the name Tepidiphilus baoligensis sp. nov. is proposed. The type strain is B18-69T (= CGMCC 1.13573T = KCTC 62782T).

НОВЫЙ ПОДХОД К ОЦЕНКЕ РЕЗУЛЬТАТОВ ГИДРАВЛИЧЕСКОГО РАЗРЫВА ПЛАСТА (НА ПРИМЕРЕ БОБРИКОВСКОЙ ЗАЛЕЖИ ШЕРШНЕВСКОГО МЕСТОРОЖДЕНИЯ)

Актуальностьисследования обусловлена значительным вкладом объемов нефти, добытых в результате проведения на скважинах гидравлического разрыва пласта, в суммарную добычу. Правильная оценка результатов фактически проведенных мероприятий по гидравлическому разрыву позволит выработать четкие рекомендации по дальнейшему применению данного метода интенсификации добычи нефти для геолого-физических условий конкретных месторождений. Цель:оценить результаты проведения гидравлического разрыва пласта применительно к элементу системы разработки, в котором находится скважина – объект воздействия. Объекты: нефтедобывающие скважины – элемент системы разработки бобриковской залежи Шершневского нефтяного месторождения. Методы:геолого-промысловые исследования, корреляционный анализ, схематизация взаимодействия между скважинами. Результаты. Установлено, что проведение гидравлического разрыва пласта на скважине 221 Шершневского месторождения привело к изменению характера взаимодействия между скважинами в пределах всего элемента системы разработки, который стал работать как единая однонаправленно согласованная система. В результате гидравлического разрыва пласта произошло не просто перераспределение объемов дренирования, а возник синергетический эффект, когда проведение мероприятия в одной скважине привело к росту дебитов и согласованности работы всего элемента системы разработки. Вероятно, проведение гидравлического разрыва пласта в скважине 221 привело к существенному изменению фильтрационных параметров бобриковской залежи Шершневского месторождения в более значительных пределах, нежели объем зоны дренирования этой скважины, и на довольно большом участке залежи возникла целая система каналов с пониженными фильтрационными сопротивлениями, а не единичная трещина, как это принято в классическом представлении.

Pseudomonas Saliphila sp. nov., a Bacterium Isolated from Oil-Well Production Water in Qinghai Oilfield of China.

Strain 16W4-4-3T was isolated from the oil-well production water in Qinghai Oilfield, China. Cells were Gram-stain-negative, rod-shaped, catalase- and oxidase-positive, facultatively anaerobic and motile by single polar flagellum. The 16S rRNA gene sequences of strain 16W4-4-3T showed the highest similarities with Pseudomonas profundi M5T (98.8%), P. pelagia CL-AP6T (98.0%), P. salina XCD-X85T (97.7%), and P. sabulinigri J64T (97.5%). The phylogenetic trees based on multilocus sequence analyses with concatenating 16S rRNA, gyrB, rpoD and rpoB genes suggested that this strain should be affiliated to the genus Pseudomonas but remotely related from other species. In addition, whole genome analyses revealed that the digital DNA-DNA hybridization values and average nucleotide identities of strain 16W4-4-3T against its close relatives were all below 28.8% and 86.5%, respectively. Furthermore, the isolate had totally different whole cell protein profile as compared to those of other species. Major fatty acids were summed feature 8 (C18:1ω7c and/or C18:1ω6c), C16:0, summed feature 3 (C16:1ω7c and/or C16:1ω6c) and C17:0cyclo. Major isoprenoid quinone was ubiquinone (Q-9), and major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, and phosphatidylethanolamine. The DNA G + C content was 58.5mol%. Therefore, phenotypic, phylogenetic, genomic, chemotaxonomic, and proteomic traits showed that the isolate represented a novel species of the genus Pseudomonas, the name Pseudomonas saliphila sp. nov. is proposed. Type strain is 16W4-4-3T (= CGMCC 1.13350T = KCTC 72619T).

The study on multifunctional aminoalkylenephosphonate-based antiscalant for industrial water systems under harsh conditions

Mineral scale deposit is one of the major problems incessantly faced in the oil and gas producing wells and surface facilities, and if not well-handled can cause an unsuccessful petroleum production. To eradicate scale deposits, scale inhibitors (SIs) are commonly deployed into industrial water systems. Because of effectiveness and long squeeze lifetimes, organic phosphonate SIs are extensively used for preventing mineral scale formation in industrial water systems. However, they usually exhibit lower Ca2+ and Mg2+ tolerances that restrict their applications in harsh conditions where high inhibitory dosages are needed. In this paper, a multifunctional aminoalkylenephosphonate-based antiscalant having higher Ca2+ and Mg2+ tolerances is studied. The antiscalant was synthesized, characterized (FTIR and NMR spectroscopies), and evaluated in accordance with the NACE Standard Method (TM0374) for preventing CaSO4 and CaCO3 formation in the petroleum industry. The effects of inhibitory dosages, water hardness, temperature, incubation period, and pH were studied. We further tested the performance of the antiscalant in simulated field waters. Experimental results show that the multifunctional aminoalkylenephosphonate-based antiscalant has better inhibitory properties as compared to conventional aminoalkylenephosphonate-based SIs. The incorporation of the sulfonate group into the molecule significantly improved the solubility of the antiscalant and hence its tolerances to Ca2+ and Mg2+ ions. Also, it bolstered thermal stability of the inhibitor. The results from simulated field conditions are promising. The synthesized antiscalant performed superbly than the conventionally aminomethylenephosphonate antiscalants where Ca2+ and Mg2+ concentrations are challenging.

A Case Study on Budgetary Control Application in the Ministry of Interior, UAE

Oil prices have been on a roller-coaster ride over the last few years. This thesis examines the evaluation of budgetary control due to fluctuation of petroleum price. During the past 5 years, production of crude petroleum had been increasing steadily in the UAE. In 2014, ADNOC produced crude petroleum from onshore fields, including the Murban field, and from offshore fields, including the Lower Zakum, the Umm Shaif, and the Upper Zakum fields. The country used enhanced oil recovery (EOR) technologies to extract crude petroleum and natural gas from many of its oil-producing wells. The specific research objectives conducted in this study are to investigate effects on budget during petroleum price fluctuation and other objectives are, to examine impact of fiscal policy of UAE on the budget of UAE, to investigate effects of petroleum production on the budget of UAE and to investigate effects of OPEC regulation on petroleum production. The study was conducted in the ministry of interior, United Arab Emirates to undermine the evaluation of the budgetary control due to the fluctuation of petroleum price. Since the main source of revenue for the Government is petroleum so, the public sector budgets heavily depends on the global petroleum price and the fluctuation of it has a direct impact on the public sector operation. Ministry of interior being the key public sector is also within the strategic concern of the government revenue and its implication on its budgetary affairs. Based on the findings of the study the main recommendations made for the study are firstly, it is very common phenomenon for the oil price to fluctuate, hence alternative source of income to be generated so that dependency only from oil price for budget for the ministry of interior can be minimized. Secondly, to introduce new elements in the fiscal policy of the country as alternate source of government revenue likes looking into public revenue and expenses. Concentration on invisible exports to be considered. Like tourism, education, health care etc. Thirdly, reform of OPEC policy’s on petroleum production per day quota need to be relooked due to persistent fall of the oil price as to adjust the supply and demand. The contribution from the researcher there is not cost elimination but there is an in built process now for cost minimization. Secondly, impact of the fiscal policy has a direct impact on the budget. As fiscal policy consist of taxation and government expenditure, so the government now should look for alternate indirect tax to increase the inflow for the government as a measure to reduce the negative burden on petroleum price fluctuations.

Development of Mathematical Model and Technological Process Flow Diagram of Delivery of Reagent in Initial Treatment of Oil Production of Wells in Separator Tanks

Development of Mathematical Model and Technological Process Flow Diagram of Delivery of Reagent in Initial Treatment of Oil Production of Wells in Separator Tanks

Identification of potential CO2 leakage pathways and mechanisms in oil reservoirs using fault tree analysis

AbstractGeological storage of CO2 technologies has become an important and effective way to reduce the greenhouse gas emissions, especially when it is combined with CO2 enhanced oil recovery (EOR), which can not only trap CO2 but also enhance oil recovery. However, the risk of CO2 leakage has always been a prominent issue. In this paper, the mechanisms and pathways of CO2 leakage during geological storage in oil reservoirs were analyzed using fault tree analysis (FTR). Besides, monitoring technologies were discussed and deployed in a CO2 EOR demonstration project. The analysis results showed that the sealing failures of oil producer and CO2 injector wells, like well cement failure and casing failure, are the main reasons for the CO2 leakage, which has been observed in the oil field monitoring project. The monitoring results indicated that there is no large‐scale CO2 leakage, while relatively high and abnormal CO2 concentration in soil gas near some wellbores are observed, which indicates there is some leakage of CO2 through incomplete cement ring and well casing string. FTR results provide guidelines for monitoring and preventing of CO2 leakage during geological storage in oil reservoirs. The near‐surface monitoring methods, especially the soil gas monitoring technologies, can effectively detect the leakage of CO2, and are a proper method for CO2 leakage monitoring. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

РАЗРАБОТКА И ПРИМЕНЕНИЕ СТАЦИОНАРНОГО СКВАЖИННОГО ПРИБОРА В СИСТЕМЕ УПРАВЛЕНИЯ ПРОЦЕССОМ НЕФТЕДОБЫЧИ

Актуальность исследования заключается в получении достоверных измеренных технологических параметров (как минимум давления и плотности добываемой продукции) в скважине, полученных в режиме реального времени, для качественного управления нефтедобывающей скважиной и повышения эффективности ее эксплуатации. Цель: разработка стационарного скважинного прибора для измерения давления на приеме и выкиде насоса, внутри насосно-компрессорных труб и межтрубного пространства нефтедобывающей скважины; составление математической модели получения необходимых технологических параметров для решения основных задач при добыче нефти; применение стационарного скважинного прибора в предлагаемой системе управления процессом нефтедобычи. Объект – нефтедобывающая скважина. Методы: моделирование и оценка работы предлагаемой системы управления процессом нефтедобычи с применением разработанного стационарного скважинного прибора в программе Matlab/Simulink. Результаты. Разработан стационарный скважинный прибор для измерения давления на приеме и выкиде насоса, внутри НКТ и межтрубного пространства скважины и показана его конструкция; представлена математическая модель получения необходимых технологических параметров для решения основных задач при добыче нефти; предложена система управления с применением разработанного прибора; поставлена и решена задача создания эффективной системы управления процессом нефтедобычи. Выводы. Получен положительный результат при моделировании системы управления процессом нефтедобычи с применением разработанного скважинного прибора для измерения давления в нескольких контрольных точках ствола нефтедобывающей скважины в программе Matlab/Simulink. Реализация разработанного стационарного скважинного прибора в предлагаемой системе управления процессом нефтедобычи обеспечивает точность и быстродействие вывода скважины на стационарный режим работы, его стабилизацию и увеличение отбора добываемой продукции.

In depth metagenomic analysis in contrasting oil wells reveals syntrophic bacterial and archaeal associations for oil biodegradation in petroleum reservoirs

Microbial biodegradation of hydrocarbons in petroleum reservoirs has major consequences in the petroleum value and quality. The identification of microorganisms capable of in-situ degradation of hydrocarbons under the reservoir conditions is crucial to understand microbial roles in hydrocarbon transformation and the impact of oil exploration and production on energy resources. The aim of this study was to profile the metagenome of microbial communities in crude oils and associated formation water from two high temperature and relatively saline oil-production wells, where one has been subjected to water flooding (BA-2) and the other one is considered pristine (BA-1). The microbiome was studied in the fluids using shotgun metagenome sequencing. Distinct microbial compositions were revealed when comparing pristine and water flooded oil wells in contrast to the similar community structures observed between the aqueous and oil fluids from the same well (BA-2). The equal proportion of archaea and bacteria together with the greater anaerobic hydrocarbon degradation potential in the BA-1 pristine but degraded reservoir contrasted with the predominance of bacteria over archaea, aerobic pathways and lower frequency of anaerobic degradation genes in the BA-2 water flooded undegraded well. Our results suggest that Syntrophus, Syntrophomonas, candidatus Atribacteria and Synergistia, in association with mainly acetoclastic methanogenic archaea of the genus Methanothrix, were collectively responsible for the oil biodegradation observed in the pristine petroleum well BA-1. Conversely, the microbial composition of the water flooded oil well BA-2 was mainly dominated by the fast-growing and putatively aerobic opportunists Marinobacter and Marinobacterium. This presumable allochthonous community introduced a greater metabolic versatility, although oil biodegradation has not been detected hitherto perhaps due to in-reservoir unfavorable physicochemical conditions. These findings provide a better understanding of the petroleum reservoir microbiomes and their potential roles in biogeochemical processes occurring in environments with different geological and oil recovery histories.

Cluster Discriminant Prediction of Oil Well Production Based on Mahalanobis Distance

The cost of oil field drilling engineering is very high. Effective well selection and formation selection will avoid waste of energy and resources. Aiming at the problem of well selection and reservoir selection in oilfield, this paper presents a clustering discriminant prediction method for oil well production based on Mahalanobis distance. Based on the field data of Xinjiang oilfield, the main control factors with greater influence are selected after data pretreatment in this paper. Then we randomly divide the data into training data set and prediction data set, and establish a discriminant model of oil well classification based on Mahalanobis distance. Finally, the model is used to predict the production of oil wells in the forecasting data set, and the prediction accuracy reaches 87.5%.

The Humbly Grove, Herriard and Hester's Copse fields, UK Onshore

Abstract The Humbly Grove Field has, for the UK, a unique development history. It was discovered as an oilfield in May 1980 and produced as an oilfield until 2000 along with small satellite fields Herriard (developed) and Hester's Copse (not developed). Peak production of 2219 bopd was achieved during July 1986 but, by October 1988, the rate had fallen to around 1000 bopd, a rate that was more or less maintained until October 1995 after which the production fell rapidly. At this point the decision was taken to reconfigure the field as a gas storage facility. Significant renewed pressure depletion occurred between 2000 and 2005, following which first cushion and then storage gas was injected into two reservoirs: the Middle Jurassic, Great Oolite Group and the uppermost Triassic, Rhaetian Westbury Formation. Gas storage operations commenced in 2005 and the reservoirs have undergone cyclical gas injection and gas withdrawal since that date. The cyclical injection of gas and re-pressuring of the Great Oolite reservoir causes mobile oil to be swept towards dedicated oil production wells. This operates effectively as an enhanced oil recovery scheme. The co-produced liquid hydrocarbons provide a valuable secondary income stream for the field.

Эффективность бурения и заканчивания наклонно-направленных нефтедобывающих скважин в Восточной Сибири через эволюцию горизонтального участка – от одиночных стволов к конструкции “березовый лист” в связи с детализацией геологического строения залежей УВ

Эффективность бурения и заканчивания наклонно-направленных нефтедобывающих скважин в Восточной Сибири через эволюцию горизонтального участка – от одиночных стволов к конструкции “березовый лист” в связи с детализацией геологического строения залежей УВ

Incorporation of Frailties Into a Non-Proportional Hazard Regression Model and Its Diagnostics for Reliability Modeling of Downhole Safety Valves

In this paper, our proposal consists of incorporating frailty into a statistical methodology for modeling time-to-event data, based on non-proportional hazards regression model. Specifically, we use the generalized time-dependent logistic (GTDL) model with a frailty term introduced in the hazard function to control for unobservable heterogeneity among the sampling units. We also add a regression in the parameter that measures the effect of time, since it can directly reflect the influence of covariates on the effect of time-to-failure. The practical relevance of the proposed model is illustrated in a real problem based on a data set for downhole safety valves (DHSVs) used in offshore oil and gas production wells. The reliability estimation of DHSVs can be used, among others, to predict the blowout occurrence, assess the workover demand and aid decision-making actions.

Slip behaviours and evaluation of interpretative model of oil/water two-phase flow in a large-diameter pipe

Mixtures of oil and water are common in oil-producing wells. When the oilfield has a mid-late exploitation status, the production of oil and water gradually decreases and the comprehensive water content sharply increases with time. An accurate evaluation of the volumetric flow rate of each phase in the pipe is of importance for diagnosing production problems, developing an engineering design, and guiding reservoir management activities. In this study, the slip behaviours we investigated based on the high-quality experimental data obtained from a multiphase flow loop with a large-diameter (inner diameter: 5.5 inch) vertical pipe with low flow rates (<30 m3/d). Furthermore, the interpretation accuracies of five interpretative models of vertical oil/water flow ware comprehensively evaluated. The results show that the slip trends to increase with decreasing flow rates. However, it was found that, when the flow rate is less than 7 m3/d, the flow rate minimally affects the slip. Additionally, the variation tendency of the slip velocity as a function of the water holdup is not stable, hence, a reasonable interpretative model should be selected according to the water holdup. The slip velocity gradually increased, and was found to be nearly linear, and the slip model proposed by Hasan and Kabir (1999) demonstrated the best adaptability under the conditions of the tested data and a water holdup of less than 0.9. The slip velocity exponentially increased with increasing water holdup and the modified drift-flux model proposed by Wu et al. (1992) was found to be more appropriate under the condition that the water holdup exceeds 0.9. Overall (i.e. for all of the test data), the Hasan model (1999) yielded better processing results for the vertical oil/water flow system under the condition of low flow rates. The analysis in this study provides better insight into the slip between the oil and water phases in a two-phase flow system with a low flow rate. In addition, the interpretative model with the best predictive capability is highlighted in this paper.