The SPE has split the former "Management & Information" technical discipline into two new technical discplines:
- Data Science & Engineering Analytics
The SPE has split the former "Management & Information" technical discipline into two new technical discplines:
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Siyal, Amaar (Khalifa University of Science and Technology, Abu Dhabi) | Rahimov, Khurshed (Khalifa University of Science and Technology, Abu Dhabi) | AlAmeri, Waleed (Khalifa University of Science and Technology, Abu Dhabi) | Al-Shalabi, Emad W. (Khalifa University of Science and Technology, Abu Dhabi)
Abstract Different enhanced oil recovery (EOR) methods are usually applied to target remaining oil saturation in a reservoir after both conventional primary and secondary recovery stages. The remaining oil in the reservoir is classified into capillary trapped residual oil and unswept /bypassed oil. Mobilizing the residual oil in the reservoir is usually achieved through either decreasing the capillary forces and/or increasing the viscous or gravitational forces. The recovery of the microscopically trapped residual oil is mainly studied using capillary desaturation curve (CDC). Hence, a fundamental understanding of the CDC is needed for optimizing the design and application of different EOR methods in both sandstone and carbonate reservoirs. For sandstone reservoirs, especially water-water rocks, determining the residual oil saturation and generating CDC has been widely studied and documented in literature. On the other hand, very few studies have been conducted on carbonate rocks and less data is available. Therefore, this paper provides a comprehensive review of several important research studies published on CDC over the past few decades for both sandstone and carbonate reservoirs. We critically analyzed and discussed theses CDC studies based on capillary number, Bond number, and trapping number ranges. The effect of different factors on CDC were further investigated including interfacial tension, heterogeneity, permeability, and wettability. This comparative review shows that capillary desaturation curves in carbonates are shallower as opposed to these in sandstones. This is due to different factors such as the presence of high fracture density, presence of micropores, large pore size distribution, mixed-to-oil wetting nature, high permeability, and heterogeneity. In general, the critical capillary number reported in literature for sandstone rocks is in the range of 10 to 10. However, for carbonate rocks, that number ranges between 10 and 10. In addition, the wettability has been shown to have a major effect on the shape of CDC in both sandstone and carbonate rocks; different CDCs have been reported for water-wet, mixed-wet, and oil-wet rocks. The CDC shape is broader and the capillary number values are higher in oil-wet rocks compared to mixed-wet and water-wet rocks. This study provides a comprehensive and comparative analysis of CDC in both sandstone and carbonate rocks, which serves as a guide in understanding different CDCs and hence, better screening of different EOR methods for different types of reservoirs.
Siyal, Amaar (Khalifa University of Science and Technology, Abu Dhabi, UAE) | Ahmed, Shehzad (Khalifa University of Science and Technology, Abu Dhabi, UAE) | AlAmeri, Waleed (Khalifa University of Science and Technology, Abu Dhabi, UAE) | Al-Shalabi, Emad W. (Khalifa University of Science and Technology, Abu Dhabi, UAE)
Abstract It is widely recognized that the determination of true residual oil saturation to water (Sorw) is a critical factor in predicting waterflooding performance and implementing enhanced oil recovery (EOR) methods. A particular EOR method will provide a high prospective result if there is an overestimation of Sorw. The concept of capillary desaturation curve (CDC) is used to determine how much amount of oil can be recovered, when implementing a certain EOR technique. The objective of this study is to determine the true residual oil saturation to water (Sorw) for low permeability Indiana limestone outcrops using the centrifuge technique under reservoir conditions and further generate their CDCs. In this work, three carbonate Indiana limestone outcrops with low permeability range (4-8 mD) and representative fluid samples i.e., field oil, formation water, and seawater, were utilized. The CDC was then characterized for carbonate rocks by further reducing Sorw using surfactant flooding where three anionic surfactant formulations with different IFT values were selected. A systematic approach was followed starting with conventional core analysis followed by special core analysis. For the CDC generation via surfactant flooding, three surfactant formulations having different IFT values were selected through a preliminary screening. This study showed that there is no correlation between initial water saturation (Swi) and absolute permeability for the cores tested. In addition, variations in spontaneous oil recovery was noted among cores within the same range of rock permeability, which indirectly indicates the existence of heterogeneity within each rock. Furthermore, a true Sorw of 20-29% was achieved using the centrifuge method, which was confirmed during the surfactant flooding stage. Additionally, CDC studies indicated that a critical trapping number of 10 was achieved for the tested cores, which is higher than most of the reported values of 10 to 10 in the literature. Accordingly, the complete desaturation of mixed-to-oil wet carbonate rocks is quite challenging since it requires a further increase in trapping number, which could possibly be achieved using ultra-low IFT surfactants. This work presents a systematic and comprehensive approach for determining true Sorw and understanding microscopically trapped oil in carbonate rocks based on CDC. The produced results would be useful in EOR screening for future surfactant flooding pilots in carbonate rocks with low permeability.
Siyal, Amaar (Khalifa University of Science and Technology, Abu Dhabi, UAE) | AlAmeri, Waleed (Khalifa University of Science and Technology, Abu Dhabi, UAE) | Al-Shalabi, Emad W. (Khalifa University of Science and Technology, Abu Dhabi, UAE) | Ahmed, Shehzad (Khalifa University of Science and Technology, Abu Dhabi, UAE) | Masalmeh, Shehadeh K (ADNOC Group) | Al-Sumaiti, Ali M (ADNOC Group)
Abstract Most of the oil remain trapped in the reservoir after both primary and secondary recovery stages. Enhanced oil recovery (EOR) techniques are usually implemented in the tertiary stage to recover the trapped oil. Accordingly, the inaccurate determination of residual oil saturation after waterflooding (Sorw) in the secondary stage affects the success and economics of the EOR processes in the tertiary stage. Thus, the capillary desaturation curve (CDC) is usually introduced as guidance to estimate the mobilized residual oil. The objectives of this study include determining the true Sorw for carbonate Indiana limestone outcrops under harsh conditions, then investigating the effect of trapping number, permeability, and initial oil saturation on Sorw, and finally characterizing the CDC for carbonate rocks by further reducing the Sorw using surfactant flooding. For this purpose, six carbonate Indiana limestone outcrop samples with different permeabilities (4-69 mD) and fluid samples i.e., field-representative oil, formation water, seawater, and surfactant solutions were utilized. The drainage process was performed systematically using a coreflooding system to establish initial water saturation by injecting heavy oil followed by crude oil and aging for two weeks. Afterward, all six cores were subjected to spontaneous imbibition using Amott cell. This was further followed by forced imbibition using both ultra-centrifuge and coreflooding systems for comparison purposes and achieving Sorw condition. Finally, forced imbibition was performed on all cores using coreflood to generate CDC using three different surfactants with varying IFT values. The results showed that all rock samples achieved initial water saturation (Swi) in the range of 18-32% with no correlation between Swi and rock permeability. In addition, spontaneous imbibition tests showed slight oil production which reflect the oil-wetness of these cores used. It was noted that this slight production varied among cores with the same rock permeability range, which indirectly indicating the existence of heterogeneity within each permeability range. Furthermore, Sorw of 20-30% was reached using ultra-centrifuge and coreflooding method, indicating no correlation of permeability with Sorw. Based on the CDC studies, the critical trapping number was in the range between 10 and 10 for the tested cores, which is higher than the reported values in literature (10 to 10). This work provides a new insight into the understanding of capillary trapping effect on residual oil using CDC in carbonates. The complications in carbonate rocks, including the complex nature of high heterogeneity, mixed-to-oil wettability, high temperature, and high salinity, render accurate determination of true Sorw is a challenge at lab-scale. Sorw determination and CDC characterization aid in EOR screening to find the effective and economically viable methods for production enhancement.
Guo, Hu (China Univerisity of Petroleum, Beijing) | Dou, Ma (Yanchang Petroleum Xunyi Project Department) | Hanqing, Wang (Xi'an Petroleum University) | Wang, Fuyong (China University of Petroleum, Beijing) | Yuanyuan, Gu (China University of Petroleum, Beijing) | Yu, Zhaoyan (China University of Petroleum, Beijing) | Yansheng, Wang (China University of Petroleum, Beijing) | Li, Yiqiang (China University of Petroleum, Beijing)
Abstract After decades of development, great progress has been made in capillary number theory and it has important but often incorrect application in EOR. Investigation into progress on capillary number theory and some misuse of capillary number theory helps to make better use of it. Latest progress concerning with capillary number theory and its application in chemical EOR is reviewed by studying the experiments data and checking its model hypothesis. Classic Capillary Desaturation Curves (CDC) are summarized and new CDC is introduced. Typical classical CDC showed larger capillary number lead to lower residual oil saturation and when capillary number increased to a certain critical value(first critical value), the residual oil saturation could drop to a minimum value even zero. CDC shapes were different in water-wet and oil-wet media. Recovery can be improved by increasing flooding rate, though invalid and impractical, displacement phase viscosity or/and reducing oil/water interfacial tension, which are actually adopted by chemical flooding. Guided by this theory and also first critical capillary number value requirement, it lead to the pursuit of low interfacial tension to largest extent and the requirement of ultra-low interfacial tension (10mN/m) in surfactant screening. However, experiments data showed that residual oil saturation was not always decreasing as capillary number increased. After capillary number increased to a certain value (second critical value), the residual oil saturation may increase or decrease as capillary number increase. What was more, the final residual oil saturation was quite more than zero and this CDC was regarded as the new CDC. Experiments in heavy oil laboratory tests showed that smaller injection rate lead to higher recovery which seemed contrary to capillary theory.
Abstract Capillary number (Ca), defined as dimensionless ratio of viscous force to capillary force, is one of the most important parameters in enhanced oil recovery (EOR). The ratio of viscous and capillary force is scale-dependent. At least 33 different Cas have been proposed, indicating inconsistencies between various applications and publications. The most concise definition containing velocity, interfacial tension and viscosity is most widely used in EOR. Many chemical EOR applications are thus based on the correlation between residual oil saturation (ROS) and Ca, which is also known as capillary desaturation curve (CDC). Various CDCs lead to a basic conclusion of using surfactant to reduce interfacial to ultra-low to get a minimum ROS and maximum displacement efficiency. However, after a deep analysis of Ca and recent new experimental observations, the traditional definition of Ca was found to have many limitations and based on misunderstandings. First, the basic object in EOR is a capillary-trapped oil ganglia thus Darcy's law is only valid under certain conditions. Further, many recent tests reported results contradicting previous ones. It seems most Cas cannot account for mixed-wet CDC. The influence of wettability on two-phase flow is important but not reflected in the definition of the Ca. Then, it is certainly very peculiar that, when the viscous and capillary forces acting on a blob are equal, the current most widely used classic Ca is equal to 2.2* 10. Ideally, the condition Ca ∼ 1 marks the transition from capillary dominated to viscous-dominated flow, but most Cas cannot fulfill this expectation. These problems are caused by scale dependent flow characterization. It has been proved that the traditional Ca is of microscopic nature. Based on the dynamic characterization of the change of capillary force and viscous force in macroscopic scale, a macroscopic Ca can well explain these complex results. The requirement of ultra-low IFT from microscopic Ca for surfactant flood is not supported by macroscopic Ca. The effect of increasing water viscosity to EOR is much higher than reducing IFT. Realizing the microscopic nature of the traditional Ca and using CDCs based on the more reasonable macroscopic Ca helps to update screening criteria for chemical flooding.