About the author: Xian, male, doctor, professor and doctoral supervisor. Graduate School of China Academy of Sciences, Yuquan Road 19, Beijing, 10049, 0/Kloc-0-88256466, 138 10002826, Juyw03 @163.
(School of Earth Sciences, Graduate School of Chinese Academy of Sciences, Beijing 100049)
Abstract: CBM and shale gas are important unconventional resources. At present, China's coalbed methane industry has achieved commercial production, but shale gas is still in the experimental stage. In some energy basins, coalbed methane and shale gas source rocks will coexist, and they may be adjacent or in near or far horizons. Although coalbed methane and shale gas are different in gas source and occurrence horizon, they have certain commonness in enrichment characteristics, migration process and development technology. The enrichment of coalbed methane mainly exists in coal seam in adsorption state, and the enrichment of shale gas exists in high carbon shale in adsorption or free state. Both coalbed methane and shale gas are stored in reservoirs with low porosity and permeability, and their exploitation technologies include evaluation technology, testing technology, drilling technology and reservoir reconstruction technology. If there are coalbed methane and shale gas in a basin at the same time, we can consider using the same well to exploit coalbed methane and shale gas at the same time, so as to improve its exploitation efficiency and promote the rapid development of unconventional natural gas industry.
Keywords: CBM and shale gas enrichment characteristics development technology, reservoir reconstruction
Enrichment characteristics of coalbed methane and shale gas and similarities and differences of mining technologies in China.
Ju Yan Chao Feng Fang Wen Jing
(School of Earth Sciences, Graduate School of Chinese Academy of Sciences, Beijing, 100049)
Abstract: CBM and shale gas are important unconventional resources. At present, the coalbed methane industry in China has achieved commercial production; However, shale gas production is still in the experimental stage. The source rocks of coalbed methane and shale gas will appear together in an energy basin, which may be adjacent to each other or in the near or far layer. Although coalbed methane and shale gas are different in source and occurrence horizon. There are some common situations, such as enrichment characteristics, migration procedures and development technologies. Coalbed methane is mainly enriched in coal seams in adsorbed state, while shale gas is enriched in high-carbon mudstone or shale in adsorbed or free state. Because both coalbed methane and shale gas are stored in reservoirs with low porosity and permeability, their mining technologies include evaluation, testing, drilling and reservoir stimulation. If coalbed methane and shale gas are in the same basin, they can be mined in the same well, thus improving their mining efficiency, and the unconventional natural gas industry will develop rapidly.
Keywords: coalbed methane; Shale gas; Enrichment characteristics; Develop technology; Reservoir stimulation
1 preface
With the sustained and rapid economic development in China, the demand for energy is increasing and the demand for natural gas is growing rapidly. It is predicted that the demand for 20156 billion m3 in 2005 will be156 billion m3, with a gap of about 56 billion m3, and the demand for 2020 will be 293 billion m3, with a gap of 100 billion m3 (Wang Yibing et al., 20 10). Unconventional natural gas such as coalbed methane and shale gas is an important target of oil and gas exploration in the world (Ross et al., 2008). It is very difficult to increase conventional oil and gas production in China, so developing unconventional resources such as coalbed methane and shale gas has become a realistic choice for sustainable energy development in China.
The exploration, development and utilization of coalbed methane and shale gas were first successful in the United States. Since 2006, the annual output of coalbed methane in the United States has been stable at more than 54 billion m3 (Li Wuzhong et al., 2008), and in 2009, the output of coalbed methane in the United States reached 54.2 billion m3. In 2009, there were nearly 98,590 shale gas production wells in the United States, and the annual output of shale gas was close to10 billion m3 (Cui Qing, 20 10). In 20 10, the proven shale gas reserves in the United States exceeded 60 trillion m3, and the output reached10 billion m3, accounting for 1/ of its total natural gas production. Coalbed methane and shale gas industries have become important energy industries in the United States. In addition to the United States, Canadian, Australian and China have also made breakthroughs in coalbed methane. By the end of 2009, China had built a CBM production capacity of 2.5 billion m3, and the annual surface CBM production exceeded 654.38+0 billion m3 (Xinhuanet, 2065.438+065.438+0). In 20 10 year, the surface coalbed methane drainage amount is1580 million m3. In addition to the United States, Canadian shale gas has also started large-scale production, and China, Australia and other countries have also started experimental research.
In some energy basins, coalbed methane and shale gas source rocks will coexist, and they may be adjacent or in near or far horizons. In the geological process, the gases produced by biochemical or physicochemical actions will be stored in coalbed methane or shale gas reservoirs respectively. If different reservoirs are connected by faults or fractures, mixed reservoirs or very close reservoirs may be formed. Although coalbed methane and shale gas are different in gas source and occurrence horizon, they have certain commonness in enrichment characteristics, migration process and development technology. In the process of mining coalbed methane or shale gas, how to extract gas from two reservoirs which are close to each other? If the two reservoirs are far apart, can coalbed methane and shale gas be mined at the same time?
After years of exploration, experiments and research, the geological research of coalbed methane in China has made remarkable progress in the study of geological process and dynamic mechanism of coalbed methane occurrence, the study of coalbed methane storage system and accumulation and dispersion mechanism, and the study of field effect of economical and efficient development of coalbed methane reservoirs (Telly, 2003; Tang Dazhen et al., 2003); At the same time, important breakthroughs have been made in the selection of development technologies such as evaluation technology, drilling technology, fracturing technology and drainage technology (Li Jiachuan et al., 20 1 1). In recent years, some achievements have been made in shale gas exploration theory and technology (Cheng Keming et al., 2009; Nie Haikuan et al, 2010; Zhang Jinchuan et al., 2008).
The problems of coalbed methane in China are complicated geological conditions-low permeability, low pressure and low saturation, many problems in development theory and technology have not been solved, storage and transportation are difficult and utilization rate is low. Shale gas in China is still in the research stage and has not yet started trial production. There is little research on the percolation mechanism of shale gas (Andy Lau et al., 20 1 1). Therefore, we should strengthen the research on the basic theory of coalbed methane, further improve the understanding of coalbed methane, and improve the mining efficiency and resource utilization rate; It is necessary to strengthen the study of shale gas enrichment characteristics and seepage mechanism, form a systematic development technology system, and promote the development of shale gas industry.
On the basis of previous studies, this paper discusses the similarities and differences of enrichment characteristics and mining technologies of coalbed methane and shale gas. The purpose of this study is to explore the internal relationship, formation, evolution and enrichment mechanism of coalbed methane and shale gas, and the possibility of joint development of them. Therefore, the comparative study on the enrichment characteristics and mining technology of coalbed methane and shale gas is conducive to the development of unconventional natural gas geological theory suitable for China's geological conditions and the formation of unconventional natural gas industry in China as soon as possible.
2 concepts and evaluation methods of coalbed methane and shale gas
Coalbed methane, commonly known as gas, is a kind of gas resource coexisting with coal and coal. Its main component is methane with a content of 80%~99%, followed by a small amount of CO2, N2, H2, SO2 and C2H6. Coalbed methane mainly exists on the pore surface of coal seam or fills the gap of coal seam structure in adsorption state, and there is a small amount of free gas and dissolved gas in coal seam cracks and coal seam water. Coalbed methane in pores and fractures of coal seam and coal seam water form a special hydrodynamic system, and coalbed methane can be desorbed only when the reservoir pressure is lower than the desorption pressure.
Shale gas is the natural gas extracted from shale formation system rich in organic matter, located in dark shale or high carbon shale, and mainly exists in mudstone, shale and other formations with hydrocarbon generation ability in adsorption and free state. Although shale gas development has low productivity, it has the advantages of long mining life and long production cycle. Because of the wide distribution and large thickness of gas-bearing shale, shale gas resources are huge. Therefore, shale gas wells can produce gas stably for a long time, with a general production life of 30-50 years and even 80 years for the elderly (Xia et al., 2009; Li Shizhen et al, 20 10).
Coalbed methane and shale gas are both unconventional natural gas with the functions of self-generation, self-storage, adsorption and accumulation, and they are both related and different in conceptual characteristics. Table 1 gives a comparison of the conceptual characteristics of coalbed methane and shale gas.
Table 1 conceptual comparison between coalbed methane and shale gas
The enrichment of coalbed methane and shale gas has many characteristics such as gas source and storage medium. Evaluation of these characteristics requires many methods (Feng et al., 20 10), some methods are only applicable to coalbed methane reservoirs, some methods are only applicable to shale gas reservoirs, and some methods are both applicable. Table 2 lists some important evaluation methods.
Table 2 Important methods for evaluating coalbed methane and shale gas reservoirs
(revised by Feng et al. on 20 10)
3 Enrichment characteristics of coalbed methane and shale gas
Both coalbed methane and shale gas are self-generated and self-stored, and adsorbed into unconventional natural gas reservoirs. Shale thickness in shale gas-rich areas is often large, fractures are developed, and thermal evolution degree is appropriate, such as Barnett shale in the United States (Bowker, 2007; Zhao et al., 2007; Polastro, 2007). There are many similarities and obvious differences in their enrichment characteristics. The enrichment characteristics of coalbed methane reservoir and shale gas reservoir are compared from the aspects of source rocks, generation and evolution characteristics, reservoir and distribution characteristics, seepage and migration characteristics. Table 3 lists some similarities and differences in the enrichment characteristics of the two.
Table 3 Similarities and differences of enrichment characteristics between coalbed methane and shale gas
4 Enrichment mechanism of coalbed methane and shale gas
Coalbed methane (CBM) is the residual part of natural gas in source rocks formed during coal formation, and the coal seam is both a gas source rock and a gas storage interval. The natural gas formed in the process of coalification is concentrated in situ or migrated in short distance, mainly through the adsorption of coal seam (Scholl,1980; Tadashi et al., 1995), which is a typical adsorption and enrichment mechanism. The gas storage capacity of coal is related to its lithofacies composition, metamorphic degree, temperature and pressure. Therefore, coalbed methane is very different from conventional natural gas storage in terms of reservoir-forming mode, dynamic type and reservoir-forming characteristics (Zhang Jinchuan et al., 2008). Because coalbed methane is mainly adsorbed, the content of adsorbed gas is usually above 80%, and the ratio of free gas to dissolved gas is very small, so there is no need for the usual trap. As long as there are good caprock conditions, considerable formation pressure can be maintained, and gas reservoirs can be formed in both high and low parts of reservoir structure (Chu Huili et al., 20 10).
Shale gas enrichment mechanism has typical "mixing" characteristics. According to different enrichment conditions, shale gas enrichment can be divided into typical adsorption enrichment mechanism, piston enrichment mechanism or displacement enrichment mechanism. The first stage is the generation and adsorption of natural gas, and its reservoir-forming mechanism is the same as that of coalbed methane (Zhang Jinchuan et al., 2003); The second stage occurs at the peak of anger; With the continuation of shale gas generation process, when the maximum adsorbed gas provided by shale organic particles is not enough to meet the demand of generating natural gas accumulation, free natural gas begins to appear. With the continuation of gas generation process, natural gas gradually forms high pressure in the formation, leading to the formation of small-scale fractures along the weak surface of shale, and natural gas begins to migrate and accumulate in the fractures in a free state. Because shale pores and micro-fractures have the characteristics of fine pore throat, the displacement of formation water by free natural gas is a piston-type integral displacement enrichment mechanism. If the natural gas production continues to increase, the natural gas will be displaced and migrated through the large-hole channel, and the natural gas will move up and down in the water, which indicates the displacement and enrichment mechanism in the fracture system (Xu Bo, 2009).
Both coalbed methane and shale gas are produced in energy basins. Coal and rock of coalbed methane source rocks are formed in a swamp environment suitable for plant growth, and shale gas source rocks are formed in deep lake facies or lake central facies (Law, 2002). After sedimentation and burial diagenesis, faults and fractures are produced in rocks affected by structural changes, which leads to the connection of pores and fractures between different horizons. After organic matter is buried and deteriorated, organic carbon begins to produce gas. With the development of metamorphism, the maturity of oil and gas is getting higher and higher, and the gas production is also increasing. Most of the generated gas is adsorbed in different reservoirs such as coal seam and shale, and some of it will migrate along faults and fractures. If the coalbed methane reservoir and shale gas reservoir are adjacent or very close, they may form two adjacent or similar gas reservoirs. When the two gas reservoirs are adjacent or similar due to gas migration, there may even be a mixed gas reservoir of coalbed methane and shale gas.
5. Development technology of coalbed methane and shale gas
The key technology of coalbed methane and shale gas development is firstly evaluation technology. The performance, gas content, distribution range and abundance of source rocks (reservoirs) are evaluated by geological and logging methods, and the favorable areas for reservoir performance and development are determined. Testing technology, testing reservoir parameters, such as gas content, adsorption performance, microfractures, permeability, etc. Reservoir reconstruction technologies such as fracturing technology and horizontal drilling technology refer to drilling multiple horizontal intervals from horizontal wellbore, which is very beneficial to the technical transformation of low permeability reservoirs.
CBM development technologies include: (1) drilling technology, including drilling and completion technology. Such as horizontal well drilling technology, air underbalanced drilling technology and reservoir protection drilling technology, are the keys to form coalbed methane holes economically, efficiently and quickly; (2) Reservoir reconstruction technology: CBM reservoir belongs to low porosity and low permeability reservoir, and it needs to be reconstructed before commercial production. Reservoir reconstruction measures are important measures to improve coalbed methane production, and fracturing technology is an important technology for reservoir reconstruction, such as clean fracturing hydraulic fracturing technology, hydraulic sand fracturing technology, nitrogen foam fracturing technology and other stimulation reconstruction technologies, and underground microseismic fracturing fracture monitoring tests; (3) Drainage technology, the technology of pumping coalbed methane from underground to the ground; (4) Low-pressure gathering and transportation technology of coalbed methane field, including centralized compressor station and decentralized skid-mounted liquefaction device.
The development of shale gas is inseparable from reservoir reconstruction technology. Barnett shale in the United States began to produce gas only after hydraulic fracturing (Zhao et al., 2007). Technical progress promotes the development of shale gas horizontal wells. 90% of new wells in Barnett shale gas reservoir are horizontal wells (Feng et al., 20 10). Reservoir fracturing and repeated fracturing technologies (Zou Neng et al., 20 1 1) have greatly increased shale gas production and played a decisive role in the commercial exploitation of shale gas.
Both coalbed methane and shale gas are unconventional natural gas, and their development technologies have many similarities. If coalbed methane and shale gas exist in a basin at the same time, and if the same well can exploit coalbed methane and shale gas at the same time, the gas production and exploitation life of a single well will be improved compared with single gas production. Therefore, the economic benefits of natural gas production enterprises can be improved.
6 conclusion and understanding
Both coalbed methane and shale gas are unconventional natural gas, and they have many similarities in reservoir characteristics, enrichment mechanism, mining technology and so on, but there are also obvious differences between them.
(1) Both coalbed methane and shale gas are unconventional natural gas, which have the functions of self-generation, self-storage, adsorption and accumulation. Through gas source, gas composition, gas genesis, occurrence state and occurrence mode, the relationship and difference between them in conceptual characteristics are compared. There are different methods to evaluate the characteristics of coalbed methane and shale gas reservoirs, some methods are only suitable for coalbed methane reservoirs, some methods are only suitable for shale gas reservoirs, and some methods are both suitable.
(2) There are some similarities between coalbed methane and shale gas in enrichment characteristics, migration process and development technology, but they are different in gas source, occurrence horizon and preservation conditions. The enrichment of coalbed methane mainly exists in coal seam in adsorption state, and the enrichment of shale gas exists in high carbon shale in adsorption or free state. The enrichment of coalbed methane requires proper caprock conditions and hydrogeological conditions, while the enrichment of shale gas does not require additional caprock conditions and hydrogeological conditions.
(3) Coalbed methane enrichment mainly concentrates natural gas by adsorption, which is a typical adsorption and enrichment mechanism; Shale gas enrichment mechanism has typical "mixing" characteristics. According to different enrichment conditions, shale gas enrichment can be divided into typical adsorption enrichment mechanism, piston enrichment mechanism or displacement enrichment mechanism.
(4) Both coalbed methane reservoir and shale gas reservoir are low porosity and low permeability reservoirs, and reservoir reconstruction and permeability enhancement technologies, such as horizontal well technology and reservoir fracturing technology, need to be adopted during exploitation. If there are coalbed methane and shale gas in a basin at the same time, it can be considered to exploit coalbed methane and shale gas simultaneously with the same well to improve its exploitation efficiency.
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