(Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)
About the author: Wang Hongyan, born in 197 1, male, from Xuzhou, Jiangsu, senior engineer, Ph.D., has been engaged in comprehensive geological research of new energy sources such as coalbed methane for a long time. Address: Box Petroleum Branch, No.44 Wanzhuang, Langfang City, Hebei Province, zip code: 065007.
Supported by National 973 Project (No.:2002CB2 1 1705).
There are great differences between high and low rank coalbed methane in reservoir physical properties, formation water salinity, coal adsorption and reservoir-forming process. Domestic scholars generally believe that the high rank coal seam underestimates the exploration prospect because of its high degree of evolution, undeveloped cleat and extremely low permeability, forming a "forbidden zone" for coalbed methane exploration. The geological conditions and tectonic activities of coal-bearing basins in China are much more complicated than those in the United States. The generation and enrichment of coalbed methane has its own characteristics. Most coal seams have undergone multi-stage and multi-directional stress field transformation after deposition, and the formation of most high-rank coals is related to magmatic thermal metamorphism events. Low-rank coal in northwest China is rich in coalbed methane resources, accounting for about 50% of the total resources in China. The natural gas genesis, physical characteristics, hydrogeological conditions, gas-bearing property and reservoir-forming process of high and low rank coals are obviously different from those of low rank coals and foreign high rank coals, and the reservoir-forming difference between high and low rank coals is very obvious. Under the matching conditions, they may form a high-yield and enriched area of coalbed methane and form a favorable area for coalbed methane exploration.
Coalbed methane, high and low coal rank
Comparison of CBM Reservoir-forming Performance of Different Coal Ranks in China
Wang Hongyan, Li Jingming, Li Jian, Zhao Qun
Liu Honglin, Li Guizhong, Wang Bo, Liu Fei
(Langfang Branch of PetroChina Exploration and Development Research Institute Langfang 065007)
Abstract: There are great differences in the characteristics of coalbed methane accumulation in different coal ranks in terms of reservoir physical properties, salinity of formation water, coal adsorption and coal accumulation history. It is generally believed that high-rank coal seams are called the forbidden zone for coalbed methane exploration because of their high metamorphic degree, undeveloped cleavage and low permeability. In fact, the exploration prospect of coalbed methane is underestimated. The characteristics of coalbed methane accumulation in China are much more complicated than those in the United States, mainly because most coal seams in China have experienced multi-stage and multi-directional stress transformation in history after deposition, and the formation of these coal seams is related to thermal events of magmatism. Low rank coal seams in northwest China are rich in coalbed methane resources, accounting for about 50% of the total coalbed methane resources in China. There are great differences between high rank coal and low rank coal in coalbed methane genesis, physical characteristics, hydrogeological conditions, gas content and reservoir-forming process, as well as at home and abroad. Under the matching geological conditions, both high rank coal and low rank coal may form favorable coalbed methane accumulation and exploration areas.
Keywords: coalbed methane; Advanced coal; Low rank coal
The coal resources of high rank coal in China are huge, and the coalbed methane resources account for 30% of the total coalbed methane resources in China [1]. Because the coal ranks in coal-bearing basins that have successfully explored coalbed methane in the United States are all middle and low ranks, domestic scholars generally believe that the exploration prospect is underestimated because of the high degree of coal seam evolution, undeveloped cleat and extremely low permeability. Therefore, it is of great scientific significance to study the reservoir-forming conditions of high-order coalbed methane and carry out comparative research on the reservoir-forming mechanism of high-order coalbed methane. In order to better study the characteristics of high coal rank reservoir formation, this paper focuses on the particularity of high coal rank reservoir formation by comparing high and low coal rank. In contrast, RO < 0.7% is defined as low rank coalbed methane reservoir, RO > 2% is defined as high rank coalbed methane reservoir, and RO > 0.7% ~ 2% is defined as middle rank coalbed methane reservoir.
1 The genesis of coalbed methane reservoirs in high and low rank is different. Primary and secondary thermal coalbed methane are dominant in high coal rank, and primary biogenic coalbed methane is dominant in low coal rank.
There are two kinds of coalbed methane: biological and thermal. Primary biogas refers to coalbed methane formed by the degradation of organic matter under the action of microorganisms in the early stage of coalification (diagenetic stage); Secondary biogas refers to the coalbed methane formed by microorganisms after the metamorphism of middle and low rank coal (RO < 1.5+%) rises. Primary thermogenic gas refers to coalbed methane formed by organic matter during metamorphism; If the primary thermogenic gas undergoes desorption-diffusion-migration-reaggregation, it is secondary thermogenic coalbed methane.
High rank coalbed methane reservoirs are mainly primary and secondary thermogenic coalbed methane. Take the coalbed methane reservoir in the south of Qinshui Basin as the representative. High rank anthracite is the main coal seam in Qinnan area, with RO = 2.2% ~ 4.0%, and coalbed methane is mainly thermal. The methane δ 13C of coalbed methane is generally small, ranging from-26.6 ‰ to-36.7 ‰, which increases with the increase of buried depth. This is due to isotope fractionation caused by desorption-diffusion-migration of coalbed methane. This kind of coalbed methane caused by secondary heat is very common at home and abroad. The stagnant area is less affected by desorption-diffusion-migration fractionation and basically remains the same. It can be seen that the genesis of coalbed methane in coalbed methane reservoirs in southern Qin exists zoning phenomenon in space: there are secondary thermogenic coalbed methane in shallow runoff zone and primary thermogenic gas in deep stagnation zone.
Immature low rank coalbed methane reservoirs are mainly biogenic coalbed methane, and the representative coalbed methane reservoirs are located in Fenhe Basin, USA. Most of the tertiary Lianbao Formation coal in Fenhe Basin is 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 δD value is -307 ‰ ~-3 15 ‰. It shows that biogas is the main gas, which is mainly formed by microbial fermentation and metabolism [2].
The genesis of coalbed methane in low rank mature coalbed methane reservoir is very complicated, including secondary biological genesis, primary and secondary thermal genesis. These three types of coalbed methane exist in the San Juan and Yinta basins in the United States. Ro = 0.6% ~ 0.72% of Cretaceous Fuxin Formation coal in Fuxin Basin, China. According to isotope and coalbed methane composition analysis, coalbed methane in this area is mainly caused by secondary heat, followed by secondary biogenesis.
There is a great difference in coal adsorption capacity between high and low coal rank, and the coal adsorption capacity and gas content in high coal rank area are large.
The degree of coal metamorphism determines the production of coalbed methane and the adsorption capacity of coal, which plays a decisive role in the gas content of coalbed methane. The higher the coal rank, the greater the coalbed methane production. With the increase of coal rank, the adsorption capacity experienced three stages: low-high-low, and reached the maximum when Ro=3.5% [3].
High rank coalbed methane reservoir has the highest gas content. The gas content of coalbed methane reservoirs in southern Qin is generally 10 ~ 20m3/t, and the highest gas content can reach 37m3/t ... Besides the influence of coal rank, preservation conditions also play a certain role.
The gas content of immature coalbed methane reservoirs with low rank is generally low. For example, the coalbed methane content in Fenhe Basin is generally 0.78 ~ 1.6m3/t, and the maximum is not more than 4m3/t ... The mature coalbed methane reservoir with low rank has high gas content. The gas content of Feilong coalbed methane reservoir in ferron sandstone section of Upper Cretaceous in central Utah is 0.37 ~14.3m3/t, generally 5 ~10m3/t. The coalbed methane content in Fuxin Basin is generally 8 ~ 65438+100m3/t. Due to weak diagenesis, the top of low-rank coalbed methane reservoir, Therefore, dynamic plugging of groundwater is particularly important for low-rank coalbed methane reservoirs. Due to the low gas content of low rank coalbed methane reservoir, it is necessary to develop extremely thick coal seams to make coalbed methane resources rich, and high permeability makes the oil drainage radius of single well large, which is of commercial development value.
The essence of the difference of physical properties between high and low coal ranks is the duality of physical properties, high metamorphic degree, dense matrix and low permeability of coal seam.
In the coalbed methane reservoir with high coal rank in southern Qin, the reservoir permeability is (0. 1 ~ 5.7) × 10-3 μ m2, generally not exceeding 2× 10-3μm2. Pores in coal seam are mainly micropores and transition holes, with few mesopores and macropores. Porosity is between 1. 15%-7.69%, generally less than 5%, which hardly contributes to permeability [4]. Cleavage is closed or filled seriously, and its contribution to permeability is weak. Structural fractures are the main contributors to permeability. The development characteristics of pores and fractures determine that coalbed methane is difficult to diffuse from matrix pores to fractures, and the adsorption time is long, the time to reach the peak output is short, and the time to stabilize low production is long [5].
Low rank immature coalbed methane reservoir has high matrix porosity and high proportion of macropores, which contributes to reservoir permeability. Because of the low cleat density, the main factor controlling reservoir permeability is structural fracture. Cleavage and structural fractures are the main contributing factors to the permeability of mature coalbed methane reservoirs with low coal rank; Because the matrix porosity of high rank coalbed methane reservoir is low, and most of them are micropores, the fractures are seriously closed or filled with minerals, so the main contributor to permeability is structural fractures. The permeability of low rank coalbed methane reservoir is generally greater than that of high rank coalbed methane reservoir.
In order to facilitate comparison, lignite from Turpan-Hami basin and anthracite from Qinshui basin are used for simulation. Because of the low evolution degree of lignite and undeveloped cracks, lignite is mainly porous. With the increase of coal rank, cracks develop in coal seam, and the matrix becomes dense, mainly in the form of cracks [6].
Figure 1 Relationship between migration and accumulation pressure difference of high and low coal steps and system pressure
Under the high pressure of anthracite, the pressure difference of 0. 14MPa can be broken; The pressure difference of 0.50MPa can be broken under low pressure; With the decrease of pressure, the pressure difference between migration and accumulation increases. The results show that the expansion performance of anthracite matrix under reduced pressure decreases, while the shrinkage performance of anthracite matrix under pressure increases.
For lignite in Turpan-Hami basin, the simulation results are contrary, the pressure difference of 0.08MPa can be broken under high pressure and 0.03MPa can be broken under low pressure, which increases the expansibility of lignite depressurization matrix and decreases the shrinkage of pressurized matrix. The duality of the change of reservoir physical properties reflects the nature of the change of coal reservoir characteristics with the continuous exploitation of coalbed methane and the continuous decline of formation pressure (Figure 1).
4. Tectonic thermal events and tectonic stress fields play a decisive role in coal seam physical properties.
The change of reservoir structure and structure caused by magmatic intrusion increases the storage space of coalbed methane, which is called the storage function of magmatic intrusion. The thermal roasting of magma volatilizes the organic matter in coal, leaving many dense circular or tubular pore groups, which improves the porosity of the reservoir. Coal matrix shrinks, resulting in shrinkage cracks; The dynamic compression of magma intrusion leads to the superposition of exogenous cracks and endogenous cracks (cleavage), which changes the nature and scale of coal seam cracks, increases the degree of cracks and enhances permeability.
The wall spacing of natural fractures in coal reservoirs plays a key role in controlling the original permeability. The wall spacing of natural fractures is a function of the magnitude and direction of geostress, and there are two situations in which the principal stress difference of tectonic stress field has an opposite effect on the wall spacing and permeability of rock fractures. When the direction of the maximum principal stress in the tectonic stress field is consistent with the development direction of the dominant fracture groups in the rock stratum, the fracture surface is essentially subjected to relative tension. The greater the principal stress difference, the stronger the relative tension effect, which is more conducive to the increase of fracture wall spacing and permeability. However, when the direction of maximum principal stress is perpendicular to the development direction of dominant fracture groups in rock strata, the fracture surface is squeezed. The greater the principal stress difference, the stronger the extrusion effect, the smaller the distance between the walls of cracks or even the closed ones, and the lower the permeability. That is to say, tectonic stress essentially affects the original permeability of reservoir by controlling the opening and closing degree of natural fractures.
5 the difference of hydrogeological conditions on the control of coalbed methane accumulation in high and low coal rank, the stagnant water area of high coal rank is gas-rich area
The formation of high total salinity area in strata is reflected in closed sedimentary environment, semi-arid paleoclimate, poor water leakage condition, good sealing condition and continuous concentration of formation water. At the same time, due to the fault activity, the formation water with high salinity migrates upward through the fault, resulting in the vertical distribution of salinity and the emergence of high-value areas. Therefore, the salinity of formation water is an important index to reflect the migration, accumulation, preservation and enrichment of coalbed methane.
The northern section of Jinchao fault zone in the eastern boundary of Qinshui Basin has obvious lateral water blocking effect on the Middle Ordovician aquifer formation, the middle section has strong hydraulic conductivity and hydrodynamic conditions, and the southern section has extremely poor groundwater runoff conditions and does not conduct water. The southern boundary consists of the eastern water-conducting section, the middle water-blocking section and the western water-conducting section, especially the water-blocking property of the middle section, which plays an important role in the preservation and enrichment of coalbed methane in Jincheng area. The western boundary is bounded by Anze, the northern section is a water-blocking boundary, and the southern section is composed of water-conducting faults. There are four important hydrogeological boundaries. Sitou fault is a closed fault with poor water and gas conductivity. In Daning-panzhuang-Fan Zhuang area between Sitou fault and southern section of Hawking fault in central and southern Qinshui basin, the aquifer equipotential surfaces of Shanxi Formation and Taiyuan Formation are obviously higher than those on the east and west sides of the fault, and groundwater obviously blocks coalbed methane in coal seam in the form of hydrostatic pressure. In Zheng Zhuang and its vicinity on the west side of Sitou fault, the intensity of groundwater runoff may be weak, which is more conducive to the preservation of coalbed methane [7].
High rank groundwater stagnant area is the best place for coalbed methane accumulation, but recent exploration and research show that low rank coalbed methane reservoirs, especially immature low rank coalbed methane reservoirs, are also exceptions.
The total salinity of Paleozoic formation water of coalbed methane reservoir in Shaerhu area of Turpan-Hami basin is 20000 ~ 160000 mg/L, and the average salinity is 109300mg/L, which is more than three times that of seawater (35000mg/L). The gas content of low rank lignite in Turpan-Hami basin is less than 2m3/t, the depth is more than 300m, the thickness of coal seam is more than 50m, the water salinity is so high and the gas content is so low, which is far below people's imagination. Previous exploration work has proved that high coal rank exploration shows that high salinity corresponds to good preservation conditions.
Saturated brine and distilled water with different salinity were used to simulate the experiment, and the adsorption capacity of lignite to coalbed methane under different salinity water conditions was studied. The simulation of saturated brine shows that the gas content reaches 2m3/t when the formation pressure reaches 1.7MPa, and the simulation of distilled water shows that the gas content reaches 2m3/t when the formation pressure reaches 2.5MPa. The higher the salinity, the smaller the pressure drop, the faster the formation pressure gradient decreases, and the lower the reservoir pressure, which leads to the decrease of adsorption capacity, the increase of gas saturation and the desorption loss of a large number of gases.
The adsorption capacity of low rank lignite is low, and the pressure change is not obvious. The higher the salinity, the lower the adsorption capacity and the smaller the gas content. Salinity has been increasing in geological history. High salinity will reduce adsorption capacity, formation pressure gradient, reservoir pressure, gas saturation and gas desorption. High metamorphism tends to high salinity, indicating that the preservation conditions are good, which means that hydraulic erosion is weak and the coalbed methane preservation conditions are good.
The difference between high-rank and low-rank coalbed methane reservoirs is mainly reflected in the difference of reservoir-forming process, and the reservoir-forming process of high-rank coalbed methane is complicated.
The history of immature low-rank coalbed methane reservoirs is very simple [8]. Generally speaking, the coal seam only experienced one uplift after its formation. However, the recharge, migration, discharge and stagnation of groundwater play a decisive role in the adjustment and transformation of coalbed methane reservoirs. Gas has been generated since the formation of coal seam, which has an influence on the composition and isotopic characteristics of coalbed methane. However, the current structural pattern and groundwater occurrence state are the key factors that affect the formation of coalbed methane and control reservoir formation. It can be seen that the generation of coalbed methane is sustainable.
The reservoir-forming process of mature low-rank coalbed methane reservoirs is relatively simple, dominated by plutonic metamorphism, and even if there is magmatic activity, it is only contact metamorphism with limited influence. The current tectonic framework and groundwater occurrence state are the controlling factors for the adjustment and reconstruction of coalbed methane reservoirs. The generation period and persistence of coalbed methane coexist. The maximum burial period and thermal evolution degree determine the characteristics of thermogenic coalbed methane. Therefore, the formation of thermogenic coalbed methane has stages [9]. When the coal seam rises to the depth where microorganisms can move, secondary biogas begins to be generated and continues to this day. It can be seen that the generation of secondary biogas is sustainable. The existence of groundwater not only affects the generation of secondary biogas, but also affects the migration of thermogenic gas.
The accumulation process of high rank coalbed methane reservoir is complicated. Whether there is secondary hydrocarbon generation or not, regional magmatic thermal metamorphism is a necessary condition for the formation of high-rank coalbed methane reservoirs. The formation of coalbed methane has obvious stages. After reaching the highest degree of evolution, coalbed methane is no longer generated and enters the stage of adjustment and transformation of coalbed methane reservoir.
7 conclusion
The reservoir-forming characteristics of high-rank coalbed methane reservoirs in China are mainly concentrated in eight aspects: ① The genesis of coalbed methane is mainly primary and secondary thermal coalbed methane; (2) High-rank coal seams have large adsorption capacity and high gas content; ③ Stagnant water area is gas-rich area; ④ The coal seam has dense matrix, low permeability and sensitive cleavage fracture stress; ⑤ Tectonic thermal events have great influence on coal seam physical properties; ⑥ Continuous drainage and depressurization mining and large-scale fracturing are needed; ⑦ Multi-branch well technology greatly improves the single well production; ⑧ The accumulation process is complicated.
The reservoir-forming characteristics of low-rank coalbed methane reservoirs in China are mainly concentrated in six aspects: ① the genesis of coalbed methane is mainly biodegradable gas (primary and secondary); ② Low coal evolution degree, low gas content and high gas saturation; (3) Biogas accumulation in the late stage of slow flow at the edge of low rank basin; ④ Cleavage cracks in coal seam are undeveloped, the matrix is loose, the permeability is high, and the stress is insensitive; ⑤ It is dominated by plutonic thermal metamorphism and less affected by tectonic thermal events; ⑥ Self-rescue mining mechanism of low coal rank; ⑦ Vertical shaft mining technology, small fracturing; ⑧ The accumulation process is simple, with one more settlement and one more adjustment.
It can be seen that the high rank coalbed methane reservoir has three obvious advantages:
(1) Coal has high metamorphic degree, large gas production, strong coal adsorption capacity and large gas content;
(2) Tectonic thermal events and tectonic stress fields have great influence on the physical properties of coal seams. Tectonic thermal events promoted the formation of a large number of coalbed methane and improved the physical properties of the reservoir. Tectonic stress affects the original permeability of reservoir by controlling the opening and closing degree of natural fractures.
(3) The coalbed methane in stagnant water and high salinity areas has good preservation conditions, which can be preserved and discharged under reduced pressure.
refer to
[1] Zhao Qingbo et al. 200 1. Research and progress of coalbed methane exploration in China, Xuzhou: China University of Mining and Technology.
[2] Scott R. 1993. Composition and source of coalbed methane in selected basins in the United States. 1993 international conference on coalbed methane, 209~222
Sang Shuxun, Fan Bingheng, Telly, et al. 1999. Storage and enrichment conditions of coalbed methane. Petroleum and natural gas geology, 20 (2): 104 ~ 107.
Fu, Terry, jiang bo, et al. 2001a. Coal cleat compression experiment and numerical simulation of permeability. Journal of Coal, 26 volumes, 6 issues: 573 ~ 577 pages.
Liu Honglin, Wang Hongyan and Zhang Jianbo. 2000. Calculation of coalbed methane adsorption time and analysis of its influencing factors. Petroleum Experimental Geology, 22(4)
[6] Wang Hongyan, Liu Honglin, Zhao Qingbo etc. 2005. Enrichment and integration law of coalbed methane. Beijing: Petroleum Industry Press.
Wang Hongyan et al. 200 1. Hydrogeological characteristics of coalbed methane reservoirs in southern Qinshui basin. Coalfield geology and exploration.
Su Xianbo, Jiangfeng Chen, Sun Junmin. Geology, exploration and development of coalbed methane. Beijing: Science Press.
[9]Scott A.R.2002. Hydrogeological factors affecting gas content distribution in coal seam. International Journal of Coal Geology, 50:363~387