Wang Hongyan, Li Jingming, Li Jian, Zhao Qun, Liu Honglin, Li Guizhong, Wang Bo, Liu Fei

(Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang, Hebei 065007)

About the authors: Wang Hongyan, born in 1971, male, native of Xuzhou, Jiangsu Province, senior engineer, Ph.D., has long been engaged in comprehensive geological research on new energy sources such as coalbed methane. Address: Petroleum Branch, Box 44, Wanzhuang, Langfang City, Hebei Province, Postal Code: 065007.

Supported by the National 973 Program (No. 2002CB211705).

Abstract Coalbed methane of high and low rank coals has great differences in reservoir physical properties, formation water salinity, coal adsorption and accumulation process. Domestic scholars generally believe that high-rank coal seams underestimate the exploration prospects due to their high degree of evolution, underdeveloped cleats, and extremely low permeability, thus forming a "forbidden zone" for coalbed methane exploration. The geological conditions and tectonic activities of coal-bearing basins in my country are much more complex than those in the United States. The generation and enrichment of coalbed methane have their own characteristics, and most coal seams have experienced multiple stages and multiple directions of stress field transformation after their deposition. , and the formation of most high-rank coals is related to magmatic thermal metamorphism events. Low-rank coal in northwest my country is rich in coalbed methane resources, accounting for 50% of the country's total resources. The gas origin, physical characteristics, hydrogeological conditions, gas content and accumulation process of high and low rank coals are obviously different from those of low rank coals and foreign high rank coals. The differences in the accumulation of high and low rank coals are very obvious. Under matching conditions, it is possible to form a high-yield enrichment area for coalbed methane, forming a favorable area for coalbed methane exploration.

Keywords Coal bed methane, high coal rank, low coal rank

ComParison on Accumulation Performance of CBM in Different Rank Coal Seams of China

Wang Hongyan, Li Jingming, Li Jian, Zhao Qun

Liu Honglin, Li Guizhong, Wang Bo, Liu Fei

（Langfang Branch of PetroChina Research Institute of Petroleum ExPloration & Development Langfang 065007）

Abstract: Accumulation performances of CBM are quite different in different rank coal seams such as reservoir physical features, salinity of formation water, absorption of coal and accumulation history of coal. It is generally understood that high rank coal seams are so called forbidden area for CBM exploration because of high metamorphic grade, undeveloped cleats and low permeability.In fact, the exploration prospects of CBM are underestimated.CBM accumulation performance of China has its own features which are much more complicated than that of the U.S.and the main reasons are that most of coal seams of China suffered from historical multiphase and multidirectional transformation of stress after sedimentation, moreover, formation of these coal seams were related to the thermal events of magmatism. There are rich CBM resources in low rank coal seams of northwest parts of China which accounts for 50 percent of total CBM resources of China.The cause of formation of CBM, physical features, hydrogeology conditions, gas contents and accumulation process are quite different between high rank and low rank coals as well as between domestic and overseas.Either high rank coal or low rank coal may form favorable CBM accumulation and prospection area under matching geological conditions.

Keywords: CBM; high rank coal; low rank coal

Coal resources of high rank coal in my country Huge, among which coalbed methane resources account for 30% of China’s total coalbed methane resources [1].

Since the coal ranks in coal-bearing basins with successful coalbed methane exploration in the United States are medium and low coal ranks, domestic scholars generally believe that high-rank coal seams have underestimated exploration due to their higher degree of evolution, underdeveloped cleats, and extremely low permeability of coal seams. Therefore, it is of great scientific significance to study the conditions for high-rank coalbed methane accumulation and to carry out comparative research on the accumulation mechanisms of high- and low-rank coalbed methane. In order to better study the characteristics of high coal rank reservoir formation, here we focus on exploring the particularity of high coal rank reservoir formation through the comparison of high and low coal ranks. For ease of comparison, Ro<0.7% is defined as a low-rank coalbed methane reservoir, Ro>2% is regarded as a high-rank coalbed methane reservoir, and Ro>0.7% to 2% is regarded as a medium-rank coalbed methane reservoir.

1 The formation causes of high and low coal rank coalbed methane reservoirs are different. High coal rank is dominated by primary and secondary thermogenic coalbed methane, while low coal rank coal is dominated by primary biogenic coalbed methane

< p>Coalbed methane has two types: biological origin and thermal origin. Primary biogenic gas refers to the coalbed methane formed by the degradation of organic matter under the action of microorganisms in the early stage of coalification (diagenesis stage); secondary biogenic gas refers to medium-low rank coal that has undergone metamorphism (Ro<1.5+ %) coalbed methane formed under the action of microorganisms after uplift; primary thermogenic gas refers to coalbed methane formed during the metamorphism of organic matter; if primary thermogenic gas undergoes desorption-diffusion-migration-reaggregation, it is secondary Thermogenic coal bed methane.

High coal rank coalbed methane reservoirs are mainly primary and secondary thermogenic coalbed methane. The coalbed methane reservoir in the southern Qinshui Basin is represented. The coal seams in the Qinnan area are mainly high-rank anthracite, with Ro=2.2% to 4.0%. The coalbed methane is mainly of thermal origin. The δ13C of coalbed methane methane is generally small, ranging from -26.6‰ to -36.7‰, and becomes larger as the burial depth increases. This is due to the fractionation of isotopes caused by the desorption, diffusion, and migration of coalbed methane. This kind of secondary thermogenic coalbed methane is very common at home and abroad. The stagnation zone is less affected by desorption-diffusion-migration fractionation and basically maintains its original state. It can be seen that the origin of coalbed methane in the Qinnan coalbed methane reservoir has a spatial zoning phenomenon: secondary thermogenic coalbed methane exists in the shallow runoff zone, and primary thermogenic gas exists in the deep stagnation zone.

The immature low-coal-rank coalbed methane reservoirs are mainly protist biogenic coalbed methane, and the representative coalbed methane reservoirs are located in the Powder River Basin in the United States. The coal of the Tertiary Fort Union Formation in the Powder River Basin is lignite (Ro = 0.3% to 0.4%) in most areas. High-volatile bituminous coal exists in deep parts and has not reached the maturity to produce large amounts of thermogenic methane. Its methane δ13C value is -60.0‰～-56.7‰, and its δD value is -307‰～-315‰. It shows that the gas is mainly biogenic and is mainly formed through the microbial fermentation metabolic pathway [2].

The origin of coalbed methane in mature coalbed methane reservoirs with low coal rank is very complex, including secondary biological origin, primary and secondary thermal origin. Coalbed methane of these three origins exists in the San Juan and Uinta basins in the United States. The Ro of the Cretaceous Fuxin Formation coal in the Fuxin Basin, my country, is between 0.6% and 0.72%. According to the analysis of isotopes and coalbed methane components, the coalbed methane in this area is mainly of secondary thermal origin, followed by secondary biological origin. 2 The adsorption capacity of coal thus plays a decisive influence on the gas content of coal bed methane. The higher the coal rank, the greater the amount of coalbed methane generated. As the coal rank increases, the adsorption capacity has experienced three stages: low-high-low, reaching a maximum value when Ro=3.5% [3].

High-rank coalbed methane reservoirs have the highest gas content. The gas content of Qinnan coalbed methane reservoirs is generally 10 to 20m3/t, and can reach up to 37m3/t. In addition to the influence of coal rank, preservation conditions also play a role.

The gas content of low-coal-rank immature coalbed methane reservoirs is generally low. For example, the coalbed methane content in the Powder River Basin is generally 0.78 to 1.6m3/t, with a maximum of no more than 4m3/t. The gas content of mature coalbed methane reservoirs with low coal rank is relatively high. The gas content of the Ferron coalbed methane reservoir in the Ferron sandstone section of the 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 to 10m3/t. The sealing capacity of the roof and floor of the coal seams in low-rank coalbed methane reservoirs is lower than that of high-rank coalbed methane reservoirs due to weak diagenesis. Therefore, groundwater dynamic sealing is particularly important for low coal rank coalbed methane reservoirs. Low-rank coalbed methane reservoirs have very low gas content, so they must develop extremely thick coal seams to increase the abundance of coalbed methane resources. High permeability results in a large drainage radius for single wells, so that they can have commercial development value.

3 The essence of the difference in physical properties between high and low coal ranks is the dualism of physical property changes. The degree of metamorphism is high, the matrix is ??dense, and the physical permeability of the coal seam is low

Qinnan coalbed methane with high coal ranks The reservoir permeability is (0.1~5.7)×10-3μm2, generally not exceeding 2×10-3μm2. Coal seam pores are mainly micropores and transition pores, with mesopores and macropores being rare. The porosity ranges from 1.15% to 7.69%, generally <5%, and has little contribution to permeability [4]. Cleats are severely closed or filled and contribute little to permeability. Structural fractures are major contributors to permeability.

This pore and fissure development characteristic determines that it is difficult for coalbed methane to desorb from the matrix pores and diffuse into the fissures. The adsorption time is long, the time to reach the peak production is short, and the time for stable low production is long [5].

The matrix porosity of low-coal-rank immature coalbed methane reservoirs is high, and the proportion of macropores is high, which contributes to the reservoir permeability. The low cleat density controls the reservoir. The main factor of permeability is structural fractures; the main contributors to the permeability of low-coal-rank mature coal-bed methane reservoirs are cleats and structural fractures; high-coal-rank coal-bed methane reservoirs have serious cleats due to low matrix porosity and mostly micropores. Closed or filled with minerals, the major contributors to permeability are therefore structural fractures. The permeability of low-coal-rank coal-bed methane reservoirs is generally greater than that of high-coal-rank coal-bed methane reservoirs.

For the convenience of comparison, lignite from the Tuha Basin and anthracite from the Qinshui Basin are used for simulation work. Due to its low degree of evolution and undeveloped cracks, lignite mainly exhibits pore type. As the coal rank increases, coal seam fissures develop and the matrix becomes dense, mainly showing fissure type [6].

Figure 1 Relationship between high and low coal rank migration and accumulation pressure difference and system pressure

A pressure difference of 0.14MPa can break through under high pressure conditions of anthracite coal; under low pressure conditions, a pressure difference of 0.50MPa can achieve breakthrough Breakthrough; as the pressure decreases, the pressure difference between transport and accumulation increases. It shows that the expansion physical properties of anthracite depressurized matrix decrease, and the shrinkage physical properties of pressurized matrix increase.

For Tuha Basin lignite, the simulation results are opposite. Under high pressure, a pressure difference of 0.08MPa can break through, and under low pressure, a pressure difference of 0.03MPa can break through. The depressurization matrix expansion physical properties of lignite increase. The shrinkage properties of the pressurized matrix are reduced. The dualism of reservoir physical property changes reflects the essence of changes in coal reservoir characteristics as coal bed methane continues to be mined and formation pressure continues to decrease (Figure 1).

4 Tectonic thermal events and tectonic stress fields play a decisive role in the physical properties of coal seams

Reservoir structure and structural changes caused by magma intrusion play a role in increasing coal bed methane storage space, which is called The storage function of magma intrusion. The thermal baking of magma volatilizes the organic matter in the coal, leaving many dense groups of round or tubular pores, which increases the porosity of the reservoir; the coal matrix shrinks, producing shrinkage cracks; the dynamic extrusion of magma intrusion produces The superposition of exogenous fissures and endogenous fissures (cleats) changes the nature and scale of coal seam fissures, increases the degree of fissures, and enhances permeability.

The wall distance of natural fractures in coal reservoirs plays a key role in controlling the original permeability. The natural fracture wall distance is a function of the magnitude and direction of in-situ stress. There are two completely opposite effects on the influence of the principal stress difference of the tectonic stress field on the rock fracture wall distance and permeability. When the maximum principal stress direction of the tectonic stress field is consistent with the development direction of the dominant fracture group in the rock formation, the fracture surface is essentially subject to relative tension. The greater the principal stress difference, the stronger the relative tension effect, which is more conducive to the increase in fracture wall distance and Increase in penetration rate. When the maximum principal stress direction is perpendicular to the development direction of the dominant fracture group in the rock formation, the fracture surface is squeezed. The greater the principal stress difference, the stronger the squeeze effect, the fracture wall distance will be reduced or even closed, and the permeability will be reduced. In other words, tectonic stress essentially affects the original permeability of the reservoir by controlling the opening and closing of natural fractures.

5 Differences in the control of hydrogeological conditions on high and low coal rank coalbed methane accumulation. Areas with high coal rank stagnant water are gas-rich areas

In areas with high total salinity of formations The formation is reflected as a closed sedimentary environment, the paleoclimate is semi-arid, the water leakage conditions are poor, the sealing conditions are excellent, and the formation water is continuously concentrated. At the same time, due to fault activity, formation water with high salinity migrates upward through faults, resulting in vertical distribution of salinity and the emergence of high-value areas. Therefore, the salinity of formation water is an important indicator reflecting the migration, accumulation, preservation and enrichment of coalbed methane.

The northern section of the Jinhui fault zone on the eastern boundary of the Qinshui Basin plays an obvious lateral water blocking role on the Middle Ordovician aquifer group. The middle section has strong hydraulic conductivity and hydrodynamic conditions, and the southern section has underground water flow. The conditions are extremely poor and it does not conduct water. The southern boundary consists of the eastern water-conducting section, the middle water-resisting section, and the western water-conducting section. Especially the water-resisting nature of the middle section plays an important role in the preservation and enrichment of coalbed methane in Jincheng. The western boundary is bounded by Anze, the northern section is a water-blocking boundary, and the southern section is composed of conductive faults. There are four important hydrogeological boundaries inside. Among them, the Sitou fault is a closed fault with extremely poor water and gas conductivity; in the Daning-Panzhuang-Fanzhuang area between the Sitou fault and the southern section of the Jinhuo fault in the middle and southern Qinshui Basin, Shanxi The equipotential surface of the aquifers of the Taiyuan Formation and the Taiyuan Formation is obviously higher than that of the areas on the east and west sides of the fault. The groundwater obviously seals the coalbed methane in the coal seam in the form of hydrostatic pressure. In Zhengzhuang and its surrounding areas on the west side of the Sitou fault, the intensity of groundwater flow may be weak, which is more conducive to the preservation of coalbed methane [7].

The high coal rank groundwater stagnation area is the best place for coal bed methane accumulation, but recent exploration and research have shown that there are exceptions for low coal rank coal bed methane reservoirs, especially immature low coal rank coal bed methane reservoirs. .

The total salinity of the Paleozoic formation water in the coalbed methane reservoir in the Tuha Basin is 20,000 to 160,000 mg/L, and the average salinity is 109,300 mg/L, which is higher than that of seawater (35,000 mg/L ) is more than 3 times concentrated and has the characteristics of high mineralization.

The gas content of low-rank lignite in the Tuha Basin has been tested to be less than 2m3/t. At a depth of >300m, the coal seam thickness is greater than 50m. The water salinity is so high and the gas content is so low, which is much lower than people's imagination. Previous exploration work has proven that high coal rank exploration indicates that high salinity corresponds to good preservation conditions.

The experiment uses water-type saturated brine and distilled water with different salinities for simulation to study the adsorption capacity of lignite to coalbed methane under the conditions of water with different salinities. The saturated brine simulation shows that the gas content reaches 2m3/t when the formation pressure reaches 1.7MPa, and the distilled water simulation shows that the gas content reaches 2m3/t when the formation pressure reaches 2.5MPa. The higher the salinity, the smaller the pressure decrease, the faster the formation pressure gradient decreases, and the lower the reservoir pressure, resulting in a decrease in adsorption capacity, an increase in gas saturation, and a large amount of gas desorption and loss.

Lignite with low coal rank has low adsorption capacity, and the pressure change is not obvious. The higher the salinity, the lower the adsorption capacity and the smaller the gas content. During the geological history, the salinity continues to increase. High salinity results in reduced adsorption capacity, resulting in reduced formation pressure gradient, low reservoir pressure, increased gas saturation, and large amounts of gas desorption and loss. High metamorphism tends to high salinity, indicating good preservation conditions, which means weak hydraulic alternation and good coalbed methane preservation conditions.

6 The difference between high and low coal rank coalbed methane reservoirs is mainly reflected in the difference in the accumulation process. The accumulation process of high coal rank coalbed methane is complicated

Immature low coal rank coalbed methane reservoirs The history of Tibet is simple [8]. Coal seams generally undergo only one uplift after formation. However, the supply, migration, discharge and stagnation of groundwater now play a decisive role in the adjustment and transformation of coalbed methane reservoirs. Gas has been generated from the formation of coal seams to today, which has an impact on the composition and isotope characteristics of coal seam methane. However, the current structural pattern and groundwater occurrence status are the key to affecting the generation of coalbed methane, and are also the key to controlling the accumulation of coalbed methane. It can be seen that the generation of coalbed methane is continuous.

The accumulation process of mature low-coal rank coalbed methane reservoirs is relatively simple and is dominated by plutonic metamorphism. Even if there is the influence of magmatic activity, it is only contact metamorphism and the scope of influence is limited. The current structural pattern and groundwater occurrence status are the controlling factors for the adjustment and transformation of coalbed methane reservoirs. The generation of coalbed methane coexists in stages and continuously. The period of maximum burial depth and degree of thermal evolution determines the characteristics of thermogenic coalbed methane. Therefore, the formation of thermogenic coalbed methane has stages [9]. As the coal seam rises to a 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 continuous. The current existence state of groundwater not only affects the generation of secondary biogas but also affects the migration of thermogenic gas.

The formation process of high coal rank coalbed methane reservoirs is complex. Regardless of the existence of secondary hydrocarbon generation, regional magma 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 evolution level, coalbed methane will no longer be generated and the coalbed methane reservoir will enter the adjustment and transformation stage.

7 Conclusion

The accumulation characteristics of high coal rank coalbed methane reservoirs in China are mainly concentrated in eight aspects: ①The origin of coalbed methane is mainly primary and secondary thermogenic coalbed methane; ② High-rank coal seams have large adsorption capacity and high gas content; ③ The stagnant water area is a gas-rich area; ④ The coal seam matrix is ??dense, the permeability is low, and the cleat crack stress is sensitive; ⑤ Tectonic thermal events have a greater impact on the physical properties of the coal seam; ⑥ Requires continuous Drainage and depressurization mining, large-scale fracturing; ⑦ branch well technology, greatly increasing single well production; ⑧ complex reservoir formation process.

The characteristics of low coal rank coalbed methane reservoirs in China are mainly concentrated in six aspects: ① The formation of coalbed methane is mainly biodegradable gas (primary and secondary); ② The degree of coal evolution is low and the gas content is small. , with high gas saturation; ③ Low coal rank basin edge slow flow late biogas accumulation; ④ Coal seam cleat fissures are not developed, the matrix is ??loose, permeability is high, and stress is insensitive; ⑤ Mainly plutonic thermal metamorphism, tectonic thermal The impact of the incident is small; ⑥ Low coal rank self-relief mining mechanism; ⑦ Shaft mining technology, small-scale fracturing; ⑧ The accumulation process is simple, with one more settlement and one adjustment.

It can be seen that high-rank coalbed methane reservoirs have three significant advantages:

(1) High degree of coal metamorphism, large amount of gas, strong coal adsorption capacity, and large gas content;

(2) Tectonic thermal events and tectonic stress fields have a greater impact on the physical properties of coal seams. Tectonic thermal events promote the large-scale generation of coalbed methane and improve the physical properties of the reservoir. Tectonic stress affects the opening and closing of natural fissures. Control and affect the original permeability of the reservoir;

(3) Coalbed methane preservation conditions are good in areas with stagnant water and high salinity, and coalbed methane preservation and drainage are used to reduce pressure and mine.

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