Is rock core more valuable than gold? The "golden key" to the door of "finding oil and gas" is not called that for nothing!

The core is like a thick geological history textbook, recording the evolution of the strata for hundreds of millions of years and telling the story of layer by layer of sedimentation. It has become the golden key for petroleum geologists to explore the secrets of underground resources and open the door to finding oil and gas underground. Although new technologies and methods for detecting underground geological conditions continue to emerge, the role of cores is still irreplaceable by indirect detection methods. The small cores help us solve the mystery of the underground strata thousands of meters deep with their stories across time and space, and are the most valuable basic data for oilfield scientific research. Some experts say: "The value of cores is equal to or even exceeds the value of the same volume of gold."

Core acquisition

First, let us understand how the core is obtained from thousands of meters underground?

Drilling cores is like a doctor diagnosing a disease and taking pathological sections. Different positions are selected for slicing according to different diseases. Cores are cylindrical rock samples taken from the formation using annular core drill bits and other coring tools, or rock blocks taken from the well wall using a well wall coring device, based on the needs of geological exploration and oil field development research. Common coring techniques include conventional coring, closed coring, pressure-maintaining coring, horizontal closed coring, deep well dynamic coring technology, etc. The sizes of the cores obtained vary from 5 cm to 20 cm. These cores take you into a colorful microscopic world with the help of various experimental instruments.

The Secret of the Core

What mysterious information does this tiny rock core contain?

(1) Cores can tell us the age and paleoenvironment of strata. Just like identifying the age of a tree through tree rings, core identification can obtain iconic fossils of ostracods, ostracods, spores and pollen from billions of years ago, and recreate the entire process of biological-environment-climate evolution. So far, 1,263 species of paleontological fossils belonging to more than 20 categories, 445 genera and 1,263 species have been discovered, thus determining the geological age of the strata. In addition, the color and occurrence of cores can also tell us the paleoenvironment when the strata were formed. For example, red cores represent an oxidizing environment, and green represents a reducing environment. Horizontal bedding represents a weak hydrodynamic environment, and wavy bedding or cross-bedding represents a sedimentary environment with strong hydrodynamics. This is an important basis for oil and gas exploration and development.

(2) Cores can tell us how oil and gas are generated. Petroleum is mainly derived from a large amount of algae and other organic matter in ancient oceans and lakes, which evolved over millions, tens of millions, or even hundreds of millions of years under high temperature and high pressure. By analyzing the quantity, type, quality, and maturity of the kerogen in the cores, we can reveal the oil and gas generation potential of the underground rock formations. Using analytical methods such as chromatography, chromatography-mass spectrometry, infrared spectroscopy, and microscopic identification, we can clarify the generation characteristics of oil and gas, the stratum from which they came, and the dominant migration path, providing an important basis for the discovery of oil and gas reservoirs and the calculation of resource quantities.

(3) Cores can tell us where oil and gas are stored. Oil and gas are generally stored in rock pores that are tens to hundreds of the size of a human hair. By studying the pore structure between and within the skeleton particles in the core, we can know the size, shape, and connectivity characteristics of the oil and gas storage space and flow throats. Using optical, electronic, laser confocal microscopy observation and CT scanning, core analysis has achieved precise characterization from millimeter level to nanometer level and from two-dimensional to three-dimensional, which has important scientific guiding significance for oil field exploration and development.

(4) Cores can tell us how oil and gas flow underground. Oil and gas are stored in pore spaces ranging from a few nanometers to a few hundred microns inside the core. Under the action of pressure difference, they seep along the pores and throats. It is not as obvious as the flow of river water on the ground, and it is difficult to observe with the naked eye. Using core micro-displacement, oil-water relative permeability measurement, wettability evaluation, nuclear magnetic resonance scanning and other analytical techniques, we can accurately understand and evaluate the state of oil and gas occurrence and seepage characteristics, and provide reliable parameters for efficient oil and gas development.

The role of core

"A small space can reveal a big picture." Generations of oil workers have started their journey to conquer super basins by studying cores. Cores have witnessed the development of my country's oil industry, and cores have made outstanding contributions to the history of my country's oil industry. For example, the exploration and discovery of Daqing Oilfield and its efficient development are closely related to cores. From Songji 3 Well to Xushen 1 Well, and then to Guyeyouping 1 Well, cores have played an irreplaceable and important role in the discovery and development of oil and gas.

(1) Cores opened the door to my country's oil industry. In the early 20th century, foreign scholars generally believed that China was poor in oil. In 1941, Li Siguang, who had studied abroad, returned to his motherland after many hardships. He dared to question the "China is poor in oil" theory, studied hard, and measured the land of his motherland with his feet. He pioneered the idea of ​​terrestrial oil generation. In 1958, the Party Central Committee made a decision to shift the oil exploration strategy eastward and identified the Songliao Basin as the main battlefield for oil exploration. Oil exploration teams from all over the country gathered in the Songliao Basin and began to conduct a survey of the area. In April 1959, the first exploration well, Songji No. 3 Well, was drilled on the Gaotaizi uplift in Datong Town. Oil immersion and oil-bearing phenomena were seen in the cores taken from six consecutive sections at a depth of 1,050 meters. This important discovery based on cores confirmed that the Songliao Basin is rich in oil resources. On September 26, a black dragon of oil gushed out from the wellhead, shining with golden light in the sun, marking the discovery of the Daqing Oilfield and opening a new chapter in the history of China's petroleum industry. At the same time, it enriched and developed the theory of terrestrial oil generation and broke the conclusion of Western experts that China was oil-poor. The precious core of Songji Mitsui was also designated as a national first-class cultural relic and is currently preserved in the Daqing Oilfield Core Museum.

(2) Cores have played an indispensable role in understanding and developing oil fields. In the early stages of the development of the Daqing Oilfield, due to a lack of data and experience, it was believed that the underground reservoir was a stable homogeneous lake-phase deposit. For this reason, large-cut-distance row water injection was adopted for development. After water injection, water seepage soon occurred. In order to find the root cause of the contradiction, researchers analyzed and compared tens of thousands of meters of cores and finally saw the light, unveiling the mystery of the underground large-scale continental river-delta reservoir. They also created a development model of "layered mining, classified adjustment, and succession of stable production", which provided technical support for the oilfield to maintain an annual output of 50 million tons for 27 consecutive years, creating a world miracle.

After water injection development, nearly 60% of the reserves are still trapped underground. In order to further improve the recovery rate, after thousands of core displacement experiments and theoretical calculations, we pioneered the polymer viscoelastic oil recovery theory and the theory of matching relationship between low acid value crude oil and surfactants, and pioneered the chemical drive technology. From indoor research to pilot tests, expanded tests, and then to industrial promotion, the recovery rate has been continuously surpassed from 40% for water drive to 50% for polymer drive and then to 60% for composite drive, making it possible to significantly increase the recovery rate from impossible to possible.

(3) Cores have played an important role in the breakthrough of shale oil exploration and development. Many people are familiar with shale oil. The United States has achieved energy independence by relying on the revolution of shale oil and gas. Unlike the marine shale oil that has been successfully developed abroad, my country's shale oil is mainly continental shale oil. Its distribution is unstable, its maturity is low, its permeability is poor, and its mining cost is high. Shell, BP and other international companies have conducted preliminary evaluation research on Gulong shale oil. Their answer is: no similar shale oil with commercial development value has been found globally, and the risk of exploration and development is very high. As for the exploration of shale oil, Daqing oil workers have never given up the dream of finding oil in shale. Based on the basic exploration theory, they boldly explored and deployed the Guye 1 well in the center of the lake basin where shale is widely distributed. They continuously cored 449 meters. The research shows that shale has good storage capacity and the pores are continuously rich in oil. In order to verify the oil production capacity of shale, the Ancient Oil Ping-1 Well was deployed in the oil-rich section, with a daily oil and gas equivalent of 40.9 tons, achieving a revolutionary breakthrough from the "production" of oil in continental shale, which is comparable to the "Songji Sanjing". Through the experimental analysis and research of 50,000 cores, the traditional understanding of poor reservoir properties, poor oil content, poor fluidity and poor compressibility was broken through, and the "original source and reservoir in situ accumulation" theory was innovatively proposed. The estimated shale oil resources in the northern Songliao Basin are more than 10 billion tons, showing a large scene of vertical multi-layer and full-basin oil content, becoming an important strategic continuation of the construction of a century-old oil field, and realizing the dream of "finding Daqing under Daqing".

Cores are once again showing their charm in the development and utilization of new energy. Comprehensive studies of a large number of cores have shown that in addition to rich oil and gas resources, the Songliao Basin also contains huge green new energy such as geothermal energy deep underground. Through the core research of Songke 1 and Songke 2, the first scientific drilling wells in the Cretaceous continent in my country, huge dry hot rock formations with a depth of 4,000 to 5,000 meters and dating back 170 million years were discovered. There are high temperatures above 200°C here, which is called the "underground boiler". It is the largest hot basin on land in my country, with total resources equivalent to 1.1 trillion tons of standard coal, which is equivalent to my country's standard coal consumption in the past 30 years. Huge thermal energy is a precious green and clean energy, and it is the main battlefield for the future coordinated low-carbon development of multiple resources in oil fields.

Conclusion

Time passes slowly, and the stone speaks slowly. As the "golden key" of scientific exploration, the core has witnessed the evolution of the earth and recorded every breath of the earth's history; as the "telescope" for exploring the secrets deep in the strata, the core provides a scientific "ruler" for oil workers to explore and develop resources with its unique value and importance; as the "treasure map" for finding oil and gas, the core opens the door to the treasure of underground oil and gas resources, and the core also witnesses the development and growth of my country's oil industry. Let us cherish these ancient "stones" and continue to explore the mysteries of the deep earth. The core will surely lead us to move towards more difficult, broader and deeper fields, and create new glory in continued struggle.

Author: Wen Jing, Shi Lei, Zhang Hong, Wu Jiayi (Exploration and Development Research Institute of Daqing Oilfield Co., Ltd.)