Author's brief introduction: Wang Bo, engineer of Langfang Branch of China Petroleum Exploration and Development Research Institute, mailing address: ,Email:wangbo69@petrochina.com.cn Coalbed Methane Institute of China Petroleum Langfang Branch, Box 44, Wanzhuang, Langfang City, Hebei Province Tel: 13784808 169.
(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22 10082. Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)
In order to promote the industrialization of low-rank coalbed methane, through the analysis of coal seam buried depth, gas content, single-layer thickness, total thickness and resource division, it is determined that the main controlling factors of low-rank coalbed methane enrichment area are coal-forming environment, gas source, structure, magmatic activity and hydrology. Combined with these main control factors, based on the analysis of a large number of experimental test data, the formation mechanism of coalbed methane enrichment area in Wang Ying-Liu Jia block of Fuxin basin is discussed by using analogy method and geostatistics method, and a fractured coalbed methane enrichment model with mixed causes of hydrodynamic and rock wall plugging is put forward. Looking for enrichment areas with similar enrichment patterns in the later stage of coalbed methane exploration and development may form high-yield low-rank coalbed methane.
Key words: hydrodynamic desorption enrichment model of gas source magma in high abundance and rich integrated coal environment
Reservoir-forming model of Wang Ying-Liu Jia coalbed methane enrichment area in Fuxin basin
Wang Bo 1, 2 Li Guizhong 2 Wang Yibing 2 Yang Jiaosheng 2 Chen Yanpeng 2 Deng Ze 3 Geng Meng 2
(1. School of Resources and Earth Sciences, China University of Mining and Technology; Technology, Xuzhou, Jiangsu 22 1008, China; 2. Langfang Branch of Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)
Abstract: In order to make a greater breakthrough in the industrialization of low-rank coalbed methane, based on the analysis of coal seam depth, gas content, coal seam thickness, total thickness and resource division, the main controlling factors of low-rank coalbed methane enrichment area were studied, including coal-forming environment, gas source, structure, magmatic activity and hydrology. In view of these main controlling factors, the formation mechanism of coalbed methane enrichment area in Wang Ying-Liu Jia block of Fuxin Basin was discussed by analogy. A hybrid hydrodynamic-dike plugging fractured coalbed methane enrichment model is proposed. If the enrichment areas with similar enrichment patterns are found in the future exploration and development of coalbed methane, high yield can be obtained.
Keywords: high abundance; Coalbed methane; Coal forming environment; Gas source; Magma; Fluid power; Desorption; Concentration model
1 Introduction
At present, Fenhe Basin is one of the most successful gas-bearing basins in the world to develop low-rank coalbed methane, with a coalbed methane resource of 3.34 trillion cubic meters. By the end of 2008, 20,000 wells had been drilled in this basin, and the coalbed methane production was nearly 654.38+0.6 billion m3, accounting for more than 20% of the coalbed methane production in the United States [654.38+0]. The division of high abundance areas is an important factor for the success of coalbed methane industrialization in Fenhe Basin, and Fuxin Basin is also the first low rank coal-bearing basin in China to make a breakthrough. At present, there are 52 wells in this basin, with an average gas production of 2,500m3/d and an annual commodity volume of nearly 20 million m3. The main exploration and development blocks are Wang Ying-Liu Jia block, Wulong block and Haizhou block. In this paper, the geological parameters and main controlling factors of coalbed methane enrichment area in Fenhe Basin are dissected, and the formation model of Wang Ying-Liu Jia coalbed methane enrichment area in Fuxin Basin is analyzed, which has guiding significance for the optimal development of other coalbed methane enrichment areas in Fuxin Basin.
Two main controlling factors of coalbed methane enrichment area in Fenhe basin, USA
2. 1 Basis for dividing enrichment areas
According to the buried depth, gas content, single thickness, total thickness and resource abundance of coal seam, combined with the present situation of coalbed methane exploration and development, and based on the geological conditions corresponding to different gas production, the following classification standards are formulated.
Classification criteria of enrichment areas: buried depth of coal seam 159~657m, single thickness of coal seam greater than 12m, total thickness greater than 60m, gas content greater than 2.34m3/t and resource abundance greater than 200m3/km2.
Classification standard of relative enrichment area: the buried depth of coal seam is mainly 9 1.2~ 159m and 657~9 12m, the single thickness of coal seam is greater than 10m, the total thickness is greater than 30.4m, and the gas content is greater than1.88m3/t.
2.2 The main controlling factors for the formation of enrichment areas
Based on the analysis of geological characteristics, reservoir physical properties and structural evolution of coalbed methane in Fenhe Basin, it is considered that there are three main controlling factors for coalbed methane enrichment in Fenhe Basin: favorable sedimentary environment, short geological and historical evolution time after coal seam formation and favorable hydrogeological environment for biogas generation.
2.2. 1 favorable sedimentary environment
Since Paleogene, a large number of sediments in Fenhe Basin have flowed into floodplains, estuaries and swamps in the newly formed Fenhe Basin. Paleogene rocks began from the "lowest stable lignite bed" directly produced on the latest dinosaur fossils. The Paleogene rocks covering most areas of Fenhe Basin are the Youningbao Formation in Paleocene and the Wosaqi Formation in Eocene. The period of Youningbao and Wasacz was characterized by periodic deposition of coastal environment, which periodically suffered from rising and sinking. In the stable period, a large area of coal-forming swamp developed, and coal seams with wide distribution and large thickness were deposited.
2.2.2 After coal formation, the geological evolution time is short, the compaction is weak and the physical properties are good.
The sedimentary rocks in Fenhe Basin include a series of thick Paleozoic and Mesozoic rocks dominated by marine facies, and a series of thin late Cretaceous and Cenozoic continental rocks.
The initial continental deposits in the late Cretaceous were called Lance Formation in Wyoming and Hulkrick Formation in southeastern Montana. These two strata are alternately composed of thick layered sandstone, dark clay and shale. This group thickens from152 ~ 204m in Bighorn County, Montana to 760m in Convers County, Wyoming. Although there is evidence of Lalai movement in rocks of this era in other areas, there is no evidence of such orogeny in Fenhe Basin during Lance period.
Therefore, after the formation of thick coal seams widely developed in Paleogene and Neogene in Fenhe Basin, no major tectonic movement occurred, the overall compaction was weak, and the physical properties of coal reservoirs were good. The matrix pores of coal reservoirs in Fenhe Basin are well developed, and the porosity is 1.5%~ 10%. The coal reservoir in Fenhe Basin has good physical properties and high permeability. The fracture permeability of coal seam is 32~550mD, and the matrix permeability is 0.01~ 20 MD. The coal seams in most areas of Fenhe Basin also belong to negative pressure reservoirs. The pressure gradient of coal reservoir is 0.6~0.7MPa/ 100m, and the reservoir gas is mostly saturated, the saturation is 90%~ 100%, and there is a proper amount of free gas in coal matrix and cleavage (fracture), which indicates that there is external gas supply in Fenhe basin.
2.2.3 Hydrogeological environment conducive to biogas generation.
The coal of Paleogene Fort Union Formation in Fenhe Basin is mostly lignite (Ro=0.3%~0.4%), with high volatile bituminous coal in the deep part, which is not mature enough to produce a large amount of thermogenic methane. Its methane δ 13C value is -60.0 ‰ ~-56.7 ‰, and δ D value is -307‰~-3 15‰. It shows that biogas is dominant [2].
Comparative analysis of deuterium (δ2H) and oxygen (δ 18O) isotopic composition between groundwater in Fenhe basin and global precipitation line, combined with the results of tritium isotope analysis, shows that the genesis of coal seam water in Lianbao is atmospheric action, and its age is earlier than 1952 (Figure 1)[3]. It shows that the atmospheric precipitation entered the stratum during the geological history. Through qualitative observation, especially the formation of uncertain complex mixture and the analysis of saturated hydrocarbon by full ion chromatography, it shows that the Lianbao coal seam in Fenhe Basin has undergone certain biodegradation. These chromatograms are controlled by bimodal distribution, and UCM (unresolved complex mixture) increases, which qualitatively shows biodegradation. Meanwhile, the unimodal distribution of coal shows an increase in thermal maturity (Figure 2).
Figure 1 Isotopic Composition of Deuterium (δ2H) and Oxygen (δ 18O) in Groundwater of Fenhe Basin
3 Formation model of Wang Ying-Liu Jia coalbed methane enrichment area in Fuxin basin
3. 1 sedimentary environment controls extremely thick coal seam.
Peat swamp facies in fan delta accumulated stably and continuously in the axis of synsedimentary anticline for a long time, and formed a thick coal seam in Fuxin basin, and the thickness of coal seam in the axis of synsedimentary anticline was relatively large [4]. Taking Wang Ying-Liu Jiafu coal belt as an example, the total thickness of Taiping Formation coal seam in this belt is more than 45m after merging with the axis of the same sedimentary anticline. Among them, the total thickness of Taishang coal seam is more than 20m, the total thickness of Taishang coal seam is more than 20m and the maximum thickness of Sunjiawan coal seam can reach more than 25m; The maximum thickness of medium-thick coal seam can reach more than 15m. These thick coal seams provide good reservoir conditions for the generation and enrichment of coalbed methane, and are the material basis of coalbed methane enrichment zone, which makes coalbed methane resources show the characteristics of "small but fat".
3.2 Magmatic activity has improved reservoir physical properties.
3.2. 1 Transformation of coal reservoir physical properties by magmatic activity
When magma invades the rock wall, the dynamic destruction and baking of coal seam are uneven. With the change of the distance from the rock wall, the coal seam structure changes in a strip shape, and the coal seam permeability changes, thus controlling the storage and migration of coalbed methane in [5- 10] section. In a block, the zoning phenomenon of natural coke-high metamorphic broken coal-structural coal-normal coal is formed on both sides of the rock wall in turn. The closer to the rock wall, the higher the metamorphic degree of coal, the more developed the joints, the higher the porosity and the better the permeability of coal. In particular, the columnar natural coke, which is close to the rock wall, has columnar joints, porosity hundreds of times higher than that of ordinary coal, good permeability and large pores, and is not only a good storage space for coalbed methane, but also a good migration channel. However, in the "structural coal" belt formed outside the high metamorphic broken coal, the primary structure of coal is completely destroyed and the permeability is extremely low. The thickness of this zone is generally more than 2m, which has a good lateral sealing effect on coalbed methane. The vitrinite reflectance and volatile organic matter of coal in the rock wall affected areas of BL8-2, BL8-5 and BL 14 were tested and studied, and it was determined that the rock wall affected width was about 5~ 12 times of the rock wall thickness. Between the rock wall and the "tectonic coal" zone, the main storage mode of coalbed methane in high metamorphic fractured coal and natural coke is free gas. Adsorption gas and free gas coexist in high metamorphic coal, but the content of free gas is high.
The structural cracks caused by magmatic activity and the existence of rock walls and bedrock at present have improved the permeability of coal reservoirs. According to the test data of coal samples in Wang Ying coal field of Fuxin basin, the average permeability of coal and rock perpendicular to the direction of coal bedding in this area is142.3x10-3 μ m2, and the permeability of parallel bedding is 214.0x10-3 μ m2. The average permeability perpendicular to the cleavage direction of coal seam is 75.3× 10-3μm2, and the average permeability parallel to the cleavage direction is 356.9× 10-3μm2 (table 1).
Fig. 2 Total ion flow chromatogram of hydrocarbons in Fort Union coal seam of Fenhe Basin.
Table 1 Absolute Permeability Test Data Sheet of Coal and Rock Methane Gas in Wang Ying Minefield, Fuxin Basin
3.2.2 Sealing function of rock wall
The magma in the deep underground invades and pierces the reservoir, which constitutes a barrier to prevent the coalbed methane from continuing to migrate, and can also form a barrier with the overlying strata. Its plugging mechanism is equivalent to conventional oil and gas piercing trap and compound trap. For example, after the coal formation in Wang Ying mine field, the Paleogene magma invaded strongly, and more than 30 rock walls spread all over the region, penetrating through coal seams and overlying sediments. The bedrock laterally comes from the rock wall, and the bedrock meets or penetrates the adjacent rock wall, which divides the mine field into several secondary gas storage units [1].
3.2.3 Production characteristics of coalbed methane
Magmatic activity makes exogenous fractures develop and become a channel for coalbed methane desorption, which accelerates the desorption rate of coalbed methane in Fuxin basin. At the same time, because the pore structure of low rank coal reservoir itself is mainly macropores, the desorption characteristics of coalbed methane in Fuxin basin have the characteristics of medium and high rank coal and low rank coal. Taking the gas production curve of Well LJ-6 in Liu Jia Block of Fuxin Basin as an example, it shows that Well LJ-6 in Liu Jia Block of Fuxin was put into production in 2003 (Figure 3), with an initial daily gas production of 4500m3, and it began to decline after four years of stable production. At present, the daily gas production is 2780m3, the cumulative output is 662× 104m3, the recovery ratio is 26.6%, and the expected recovery ratio is 50%.
Fig. 3 Drainage and Production Curve of Well LJ-6 in Liu Jia, Fuxin.
3.3 Genetic types and hydrodynamic plugging of coalbed methane
3.3. 1 Genetic types of coalbed methane
According to the composition of coalbed methane and the methane isotope value of -50.42‰~-44.75‰ (Figure 4), it shows that the genesis of coalbed methane in Wang Ying-Liu Jia is complex, including secondary biological genesis and thermal genesis, and it is secondary thermal genesis [13- 19].
The coalbed methane is dominated by hydrocarbons, in which the volume fraction of methane is relatively high, ranging from 87.58% to 98.03%, with an average of 93.54%. The volume fraction of heavy hydrocarbons is low, ranging from 0 to 2.22%, with an average of 0.63%, which is a typical dry gas. The δ13c11kloc-0/of biogas is -58.00‰~-44.70‰, indicating the existence of biogas. Ro is 0.3%~ 1.5%, which is suitable for the generation of secondary biogas. The Ro of Fuxin Formation in the shallow part of the basin is 0.42%~0.62%, and that of Shahai Formation in the deep part is 0.70%~ 1.67%, which is beneficial to the generation of biogas. Geothermal data in this area show that the highest geothermal temperature in Fuxin basin is 70℃ in the range of 1800m, and the geothermal temperature in the range of 1500 m is generally 30~60℃, which is just at the temperature of 0~75℃ where methanogens live. The comparison of hydrogen isotopes between coal seam water and coalbed methane (Figure 5) shows that biogas tends to be reduced by CO2, during which the ancient sedimentary water medium of coal seam is continuously transformed by atmospheric precipitation, which also confirms the contribution of hydrodynamic force to the initial biogas generation.
Fig. 4 δ 13CCH and CCH/(CC2H6+CC3H8) diagrams of coalbed methane.
Fig. 5 deuterium isotope comparison diagram of coal bed water and coalbed methane
Fig. 6 hydrogen and oxygen isotopic composition of drainage water in Fuxin basin
Hydraulic sealing action
When the local groundwater goes from shallow to deep along the coal seam, the upward diffusion of gas in the coal seam will be blocked, leading to the accumulation of coalbed methane. The characteristics of hydraulic plugging and gas control are common in asymmetric syncline or monocline [20,265,438+0]. Under certain pressure difference, coalbed methane seeps from high pressure area to low pressure area, or from deep to shallow. The pressure drop desorbs coalbed methane, so it is the escape zone of coalbed methane in the outcrop and shallow part of coal seam. If the aquifer or coal seam receives outcrop recharge, the groundwater will go from shallow to deep along the layer, and the upward diffusion gas in the coal seam will be blocked, leading to the accumulation of coalbed methane.
Groundwater blockage in Fuxin Basin mainly occurs in Wang Ying-Liu Jia area. The surface runoff in this area is Wangying River and Xiwa River, tributaries of Xihe River. Groundwater receives obvious surface precipitation, and surface water is poured into cracks with different widths and changes underground, forming a water fence. Coalbed methane in deep coal seam moves upward and is preserved by underground water that moves downward. Judging from the hydrogen and oxygen isotope values of formation water, atmospheric precipitation is easy to enter the deep gas reservoir along the rock wall fracture zone or water-conducting fault, forming hydrostatic pressure plugging, which enriches and integrates coalbed methane (Figure 6).
3.4 Formation mode of coalbed methane enrichment area
Based on the above theory, the reservoir-forming model of Wang Ying-Liu Jia coalbed methane enrichment area can be summarized as a fractured coalbed methane enrichment model with mixed causes of hydrodynamic and rock wall plugging (Figure 7).
Fig. 7 CBM enrichment model diagram of hydrodynamic-rock wall plugging mixed genetic fracture
4 conclusion
(1) The main parameters for dividing coalbed methane enrichment areas in Fenhe Basin of the United States are buried depth, gas content, single thickness of coal seam, total thickness and resource abundance, and the geological factors controlling these parameters are sedimentary environment, tectonic evolution and hydrogeological conditions.
(2) Peat swamp facies in fan delta of Fuxin Basin accumulated stably and continuously in the axis of synsedimentary anticline for a long time, forming thick coal seam, and the thickness of coal seam in the axis of synsedimentary anticline was the largest, which provided a material basis for coalbed methane enrichment.
(3) Magmatic activities provide the gas generation and gas storage capacity of coal reservoirs, and at the same time, the existence of structural cracks and existing rock walls and bedrock improves the permeability of coal reservoirs, which is a favorable condition for high coalbed methane production in Wang Ying-Liu Jia block.
(4) Hydrodynamic biodegradation of methanogenic bacteria and thermogenic gas generated by magmatic activity make the genesis of coalbed methane in Wang Ying-Liu Jia block diversified, and hydrodynamic and rock wall can block coalbed methane reservoir.
(5) The formation model of coalbed methane enrichment area in Wang Ying-Liu Jia block is a fractured coalbed methane enrichment model caused by the mixing of hydrodynamic and rock wall plugging. In the process of exploration and development in the future, we can look for a CBM enrichment model similar to that in Wang Ying-Liu Jia block by analogy with geological conditions and main controlling factors, so as to promote a greater breakthrough in the industrialization of low-rank CBM.
refer to
Scott Montgomery. 1999. bode river basin in Wyoming: an expanding prospect of coalbed methane (CBM) [J].AAPG announcement, 83 (8): 1207~ 1222
[2] Rice, Flores, Strick and Ellis. 2008. Chemical and stable isotopic evidence of water/rock interaction and coalbed methane biogenesis, United Fort Formation in Fenhe Basin, Wyoming and Montana [J]. International Journal of Coal Geology, 76: 76~85.
[3] Romeo Flores, Cynthia Rice, Gary Strick, Augusta Walden and Margaret Ellis. 2008. Methane production pathway of coalbed methane in Fenhe Basin, USA: geological factors [J]. International Journal of Coal Geology, 76: 52~75
[4] Michael Formolo, Anna Martini, Steven Page. 2008. Biodegradation of sedimentary organic matter related to coalbed methane in Fenhe and San Juan basins [J]. Journal of China Geo University. International Journal of Coal Geology, 76: 86~97.
Tian Shuhua. 1990. Synsedimentary structure of coal-accumulating basin in eastern Fuxin and the formation relationship between alluvial fan at the foot of mountain and thick coal seam [J]. Journal of 562 Comprehensive Brigade of Chinese Academy of Geological Sciences, 65 ~ 76
Zhu Zhimin, Yan Jianfei, Shen Bing, Zhou Jiayun. 2007. Analysis of multi-energy mineral accumulation in Fuxin Basin from "tectonic thermal events" [J]. Progress in Earth Science, 2(5):468~479.
Zhang Junbao. 2003. Analysis of geological influencing factors of coalbed methane in Liujia District of Fuxin coalfield [J]. Journal of Liaoning University of Engineering Science, 22(6):764~766
Meng Zhaoping, Peng Suping and He Rixing. 1997. Influence of geological structure of mine field in Liujia District of Fuxin City on coal reservoir and prediction of favorable blocks [J]. China Coalfield Geology, 9(3):36~39.
Jiang Fuxing, Jiang Jinhua, etc. 2002. Shallow gas accumulation and magmatic activity in Wang Ying mine field [J]. Journal of Liaoning University of Engineering Science: Natural Science Edition 2 1(4):50 1~503.
[10] Zhang Junbao, He Yumei. 2003. Activity law of diabase in Liujia district of Fuxin coalfield and its influence on coalbed methane occurrence [J]. Coal mining, 8(3):2 1~22
[1 1] Wang Yulin, Guo Qiang, Zhao Zhongying, Wei Hengfei, Wang Wei. 2009. Enrichment of coalbed methane by magmatic activity in rift basin-taking Liaohe Basin and Fuxin Basin as examples [J]. Natural gas industry, 29 (7): 1 19.
[12], Zhao, Gao Zhanwu, Zhou Rui. 1998. research on intrusive rocks in Wang Ying mine field of Fuxin basin [J]. journal of Liaoning university of engineering technology (natural science edition), 17(5):466~47 1
Zhao Qingbo, Chen Gang, Li Guizhong. 2009. Enrichment and high yield law, mining characteristics and applicable technologies for exploration and development of coalbed methane in China [J]. Natural gas industry, 29(9): 13~ 19.
[14] Li Wuzhong, Yong Hong, Li Guizhong.2010. Carbon isotope characteristics of methane in coalbed methane and its fractionation effect [J]. Natural gas industry, 30 (11):14 ~16.
Liu, Xu Yongchang. 1999. Two-stage fractionation model and mechanism of carbon isotope evolution of coal-type gas [J]. Geochemistry, 28(4):359~366.
Don Yixiu's TV program. 2000. Discussion on stable carbon isotope distribution and genesis of coalbed methane in China [J]. Journal of China University of Mining and Technology, 29 (2):113 ~119.
[17] Gao Bo, Zhang Jianbo et al., 2002. Distribution characteristics and controlling factors of methane carbon isotope in coalbed methane [J]. Coalfield Geology and Exploration, 30(3): 14~ 17.
[18] Duan Lijiang, Tang Shuheng, Liu Honglin, etc, 2008. Influence of coal reservoir properties on carbon isotope fractionation of methane [J]. Journal of Geology, 82 (10):1330 ~1334.
[19] Hu, Li Jin, Ma Chenghua et al., 2007. Characteristics and significance of carbon isotope fractionation of pyrolysis gas of high-rank coal in Qinshui coalbed methane field [J]. Frontier of Earth Science, 14(6):267~272.
[20]Whiticar M . j . 1999。 Hydrocarbon isotope systematics of bacterial formation and methane oxidation [J]. Chemical Geology,161:291~ 314.
Zhu Zhimin, Shen Bing, Zhu Feng, Lu Aiping. 2006. Mechanism of groundwater on coalbed methane system [J]. The 6th International Symposium on Coalbed Methane,114 ~119.
Liu Junjie [22]. Relationship between groundwater and coalbed methane occurrence and migration in Wang Ying mine field [J]. Journal of Coal Science, 23 (3): 225 ~ 230.
[23] Wang Yi, Jiang Yi, Liu L, et al. 2009. Physical simulation of hydrodynamic conditions of high rank coalbed methane accumulation. Mining Science and Technology (China), 19 (4):435~440
[24] Tang Shaohui, Wang Yongbo, Zhang Dasheng .2007. Comprehensive evaluation of coalbed methane reservoir characteristics in Turpan-Hami basin. Journal of China University of Mining and Technology, 17 (4): 52 1~525.