(China Petrochemical Zhongyuan Oilfield Branch Archives Management Office)
Core is the most important basic geological data for oil and gas exploration and development in oilfield. Due to the change of natural conditions and the influence of human factors, the core will gradually lose its original true appearance after being brought to the surface. Digitization of cores can permanently and completely preserve the original image information of cores, and the construction of digital core library is an indispensable and beneficial supplement to physical core library. The construction of digital core library starts from core digitalization, and aims at resource sharing and improving the efficiency of oil and gas exploration and development, so as to realize systematic, scientific and digital core data management. The construction of digital core database mainly includes the observation of digital cores, the retrieval of core related databases, and the establishment of core description and interpretation databases. Strengthening the construction of digital core bank is of great significance for consolidating the foundation of scientific research and production and improving the level of oil and gas exploration and development.
Keywords: digital core library, oil and gas exploration and development archives, physical archives, digital archives
Core is the most intuitive, authentic and reliable original data obtained by drilling and coring, which reflects the geological characteristics of the stratum and is a valuable asset of the oilfield. The core itself contains rich geological information closely related to oil and gas exploration and development, and the observation and analysis of the core is the basis of various studies on oil and gas exploration and development in oil fields. Through the observation and analysis of the core, we can understand the characteristics of paleontology, determine the stratigraphic age and make stratigraphic correlation. We can know the lithology, color and structure of the formation, thus inferring its formation environment and predicting favorable reservoir development areas. Grasping the oil-bearing and petrophysical properties of the formation can study the relationship between reservoir lithology, physical properties, electrical properties and oil-bearing properties. Especially at present, with the deepening of oil and gas exploration, it is more and more difficult to explore, and there are fewer and fewer large structural reservoirs that are easy to find. Subtle reservoirs dominated by strata and lithology have become the main direction of oil and gas exploration. Exploration of subtle oil and gas reservoirs requires a deeper understanding of lithology and strata, and observation and analysis of cores are particularly important.
1 the necessity of core digitization
After the core is brought to the surface, due to the change of natural conditions and the influence of human factors, with the passage of time, the core will inevitably gradually lose its original true appearance.
On the one hand, the core is in a stable state in the environment of high underground temperature and high pressure, but after being brought to the surface, it is constantly weathered and eroded due to the change of natural conditions, which gradually changes the color, physical and chemical characteristics and oil content of the core. In particular, some cores with special lithology are more vulnerable to damage, such as rocks with weak cementation are more likely to be broken and weathered, and salt gypsum rocks are more likely to deliquesce and dissolve. In addition, in the cold areas of northwest and northeast China, frost heaving weathering caused by water freezing in the core is the main reason for accelerated weathering of the core, which is called "the first killer of the core" [1].
On the other hand, in the process of core observation, the core is easy to be damaged and dislocated due to repeated movement, especially the core in important intervals becomes incomplete due to repeated sampling. The uniqueness of core data makes it impossible to copy and regenerate, and once it is destroyed, it will cease to exist, causing irreparable losses.
In addition, although the traditional extensive core management model can basically meet the needs of scientific research and production, its management model has fallen behind the times, and there are some drawbacks in the use, protection and enjoyment of achievements, which is not conducive to the in-depth development of research work. Especially in order to see a well or a short section of cores, researchers need to rummage through a large number of cores in the core library, which is labor intensive and inefficient, and the cold weather in winter is not suitable for core observation, which also brings great inconvenience to scientific research and production.
With the continuous development and progress of computer network technology and the gradual popularization and application of information technology, it is possible to transmit large-capacity core comprehensive data information, which correspondingly creates good conditions for the management and preservation of core information. In order to effectively rescue the original core data, protect the integrity and authenticity of its original image information, and give full play to the role of cores as precious geological data in oilfield production, scientific research and management, it is imperative to digitize cores. After the core is digitized, it can not only completely and timely preserve the original image information of the core, but also make up for the loss defects caused by weathering, crushing and sampling of the core, so as to achieve the purpose of permanently preserving the core image. Moreover, through the construction of digital core library and network informatization, the remote and networked application of core graphic information materials can be realized, and the sharing of core resources can be realized. The vast number of scientific researchers will no longer be restricted by weather, time and place, and can observe and study cores conveniently and quickly, which not only improves the work efficiency and the utilization rate of core data, but also excavates the scientific value of core resources to a greater extent and promotes the development of oilfield exploration and development.
2 Digital Core Library Construction
The construction of digital core library starts from core digitalization, and aims at resource sharing and improving the efficiency of oil and gas exploration and development, so as to realize systematic, scientific and digital core data management. There have been many attempts to design and develop digital core library software based on network computer technology [2 ~ 6], and in the current international continental drilling projects, the scanned core images have been recycled through the network, greatly improving the work efficiency of scientists at home and abroad [7], and the construction of digital core library has already met the basic conditions. The construction of digital core library should mainly include the observation of digital cores, the retrieval of core related databases and the establishment of core description and interpretation databases.
2. 1 observation of digital core
The core itself is a visual entity. In the early days of oil and gas exploration in China, people tried to save core images in the form of photos or videos for scientific researchers to use, which played a certain role in the exploration and development of oil fields, but its disadvantages were long imaging period, short storage time and extremely inconvenient transmission, publication and printing. Color core image scanning analysis technology [8], which appeared after 1990s, uses color core scanner to collect, transmit and store core surface image information, which not only expands the preservation means of core files, but also provides high-quality core image data, which greatly supports the observation and description of cores and the research of oil and gas exploration and development.
Scanning the core image after the core is taken out of the barrel can preserve the integrity of the original image information of the core to the greatest extent, and make up for the damage to the core caused by later sampling and analysis to some extent. According to the natural sequence of cores from barrels, single core images are spliced from top to bottom to form longitudinal columnar core images of each barrel of cores, which are divided according to depth marks, and detailed descriptions of cores are pasted to corresponding positions respectively to make graphic core maps. In order to further observe the cores, all cores can be vertically sliced and polished, and then the image information of the sliced surface can be carefully collected, so that the researchers can see the core images more clearly.
Through the development and application of color core image scanning analysis technology and network technology, the digitalization and network management of core image data can be realized. High-resolution core images basically meet the requirements of conventional core observation and research, and realize on-line core observation and research, which is beneficial to the further exchange and utilization of core data and greatly improves the utilization rate of core data.
2.2 core related database query
The core scanning image gives an intuitive image, and researchers can observe the characteristics of rock color, lithology, sedimentary structure and oil content, but its physical properties, exact mineral composition and accurate rock naming can only be determined by further testing and analysis. Therefore, it is necessary to connect the oil field exploration and development database with the digital core database, and combine coring data with graphic data such as analysis and testing, logging, oil testing and drilling into a whole information system, so as to combine macro-core observation with micro-test analysis and related petroleum geological data to truly, intuitively and comprehensively reflect the geological characteristics of cores and provide support for oil field exploration, development, production and research.
2.3 Establishment of core description and interpretation database
The core description and interpretation database should be constructed as a dynamic database. Because everyone's major, research direction, experience and level are different, the amount of information extracted from the core is different, and there are also differences in the description and understanding of the same core, especially in further judging the formation environment of the core and inferring its geological process, which is normal in scientific research. Therefore, the digital core library system should provide a platform for everyone to learn and communicate with each other, and core observers can express their different views and opinions through this platform. Because it takes some time to publish the research results of core observers, the file managers of digital core libraries should keep in touch with core observers and supplement their research results to the database in time. On the one hand, it reflects the important role of core observation and analysis in oilfield exploration and development, respects the labor achievements of core observers, and on the other hand, it provides reference for later core observers, so as to achieve the purpose of learning from each other and improving together.
3 Conclusion
The construction of digital core library is an important part of the construction of "digital oilfield" in oil fields, an inseparable part of the construction of national digital archives and an indispensable and beneficial supplement to the physical core library. In recent years, major oilfields in China [1, 8 ~ 14] have generally begun to try to digitize cores, among which Changqing Oilfield built the largest digital core library in Asia on 201[9]. However, because the construction of digital core banks requires a lot of money and multidisciplinary technology, it is difficult. At present, the construction of digital core banks in China is still in the primary stage. However, in order to further explore the value of core as a valuable first-hand information in exploration and development, it is necessary to further strengthen the construction of digital core bank, which is of great significance to consolidate the foundation of scientific research and production and improve the level of oil and gas exploration and development.
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