Qinshui basin

Qinshui basin is the basin with the highest degree of coalbed methane exploration and development in China, and the coal-bearing strata are mainly Taiyuan Formation of Upper Carboniferous and Shanxi Formation of Lower Permian. Thick coal seam, stable distribution, high degree of thermal evolution and large gas production; The cleavage of coal reservoir is developed, the cracks at the intersection of structural lines are developed, and the coalbed methane production conditions are good; The effective thickness of overlying coal seam is large, the hydrodynamic conditions are good, and the preservation conditions of coalbed methane are favorable; The geological resources of coalbed methane in the basin are 39 500.42× 108m3, the recoverable resources are11216.22×108m3, and the first and second types of resources account for nearly 50% of the geological resources in the basin respectively.

(1) Overview

Qinshui Basin is located in the southeast of Shanxi Province, with a latitude of 35 ~ 38 N and an longitude of112 00' ~1/3 50'. It is oval in shape, with its long axis extending in the northeast direction, with a width of about 120km from east to west and a length of about 330km from north to south. The basin is surrounded by Taihang Mountain, Wuwangshan Mountain, Zhongtiao Mountain, Taiyue Mountain and other mountains, with an altitude of more than 700 m, and the terrain is undulating, mostly with obvious loess landforms.

Qinshui Basin is the basin with the highest degree of coalbed methane exploration and development in China. Since 1990s, China Coal Geology Bureau, Sino-US joint venture Jindan Energy Research and Development Company, China United Company, China Petroleum Company, Asia-America Continental Coal Company and many other units have successively conducted coalbed methane exploration experiments here. So far, more than 350 coalbed methane wells have been completed and remarkable exploration results have been achieved. Throughout the exploration history of coalbed methane in Qinshui basin, it can be divided into two stages, namely, the pilot test stage of exploration and evaluation and the test stage of development and utilization.

(2) Characteristics of coal seam, coal rock and coal quality

1. Coal seam characteristics

Qinshui basin was controlled by the subsidence of coal-accumulating depression basin in North China platform in late Paleozoic. In the process of sedimentary evolution, coal accumulation occurred many times in Taiyuan period and Shanxi period, which laid a solid material foundation for the generation and storage of coalbed methane. The coal-bearing strata in Qinshui Basin are mainly Taiyuan Formation of Upper Carboniferous and Shanxi Formation of Lower Permian. There are only thin coal seams or coal lines in benxi formation and Xiashihezi Formation, so it is meaningless to evaluate coalbed methane.

Taiyuan Formation is characterized by K 1 sandstone as the bottom and K7 sandstone as the upper boundary, which is thick in the north and thin in the south. Coal-bearing layer 4 ~ 14 is 16, 15, 13, 12,1,10, from bottom to top. The lower 15 coal seam is thick and laterally stable, which is one of the most important coal seams in this area. The coal seam thickness of the whole group is 0.4 ~ 19.4m, with an average of 6.36m

Shanxi Formation is bounded by K7 limestone and Taiyuan Formation, and the upper boundary is the bottom of K8 sandstone. The change trend of thickness is thick in the north and thin in the south. There are 2 ~ 7 layers of coal-bearing strata, which are No.5, No.4, No.3, No.2 and 1 from bottom to top, with No.3 coal seam as the main one. The total thickness of this group of coal seams is 0.25 ~11.51m, with an average of 4.94 meters.

2. Quality characteristics of coal and rock

The macroscopic coal and rock types in Qinshui Basin can be divided into four types: bright, semi-bright, semi-dark and dim, in which the coal and rock types of Shanxi Formation are mainly semi-bright and semi-dark coal, and the coal seams of Taiyuan Formation are mainly semi-bright and semi-bright. In the horizontal direction, the content of bright briquette and semi-bright briquette in the main coal seams of Shanxi Formation and Taiyuan Formation gradually increased from north to south, while the content of semi-dull briquette gradually decreased.

The vitrinite content of Shanxi Formation in Qinshui Basin is between 45% and 70%, and the inertinite content is between 20% and 36%. The vitrinite content of Taiyuan Formation is between 65% and 80%, and the inertinite content is 16% ~ 30%. The volatile matter in the coal seam of Shanxi Formation is between 7.03% and 38.92% (higher in some areas), with an average value of 17.23%. The volatile matter in the main coal seams of Taiyuan Formation is generally between 8.98% and 265438 0.39%, with an average value of 65438 04.36%. The ash content of Taiyuan coal group is 4.8 ~ 25.49%, with an average of 13.26%. Huoxian and Qinyuan have the highest ash content, and Xishan coalfield has the lowest ash content of 15 JHJ coal. The ash content of Jinmei Coal Group is generally 2.6% ~ 24. 15%, with an average of1.11%,which is slightly lower than that of Taiyuan Coal Group. The general trend of ash content is high in the west and low in the east.

Qinshui Basin has relatively complete coal types, ranging from gas coal to anthracite, but mainly metamorphic bituminous coal and anthracite. It is an important area of Carboniferous-Permian high metamorphic coal in North China, with the largest anthracite reserves and the largest distribution area. From the plane distribution of coal types in the whole basin, coking coal and gas coal are dominant in the west, lean coal and lean coal are dominant in the east, lean coal, lean coal and anthracite are dominant in the north, and anthracite is dominant in the south.

(3) Gas-bearing characteristics

The overall variation characteristics of coal seam gas content in Shanxi Formation and Taiyuan Formation in Qinshui Basin are that the gas content gradually increases from the periphery of the basin to the interior of the basin, from about 6m3/t at the edge of the basin to 26 ~ 30 m3/t at the axial part of the basin, which reflects that the gas content increases with the increase of the overlying effective stratum thickness under the background of high coal rank (Figure 6- 10, Figure 6- 165438+) The first two areas are lean coal (Rom axl.73%) and lean coal (Romax 1.8% ~ 2.4%), and Yangcheng mining area is anthracite (Romax4. 1%).

Fig. 6- 10 the relationship between effective thickness and gas content in Han Zhuang mine field of Shouyang mining area

Figure 6- 1 1 Relationship between effective thickness and gas content of No.3 coal seam of Shanxi Formation in Tunliu minefield of Lu 'an mining area

Fig. 6- 12 relationship between effective thickness of overlying coal seam and gas content in Jincheng panzhuang mine field

(4) CBM reservoir-forming conditions

1. Thick coal seam, stable distribution, high degree of thermal evolution, large gas production and high gas content.

The total thickness of coal seams is mostly more than 5m, and the thickness of No.3 coal in Shanxi Formation and No.9 coal in Taiyuan Formation of Carboniferous-Permian is stable and widely distributed in the basin. The metamorphic degree of coal in Qinshui basin is generally high, and the Ro value is generally between1.5% and 4.5%. Coal rank is mainly anthracite Ⅲ, lean coal and lean coal. According to the results of thermal simulation experiment, when the coal evolves from lignite to lean coal, the gas production has reached 1.4m 3/t, and the anthracite ⅲ has reached 280m3/t, which has far exceeded the adsorption capacity of the coal seam itself.

2. The cleavage of coal reservoir is developed, the cracks at the intersection of structural lines are developed, and the production conditions of coalbed methane are favorable.

Generally, two groups of cleats are developed in coal seams in the basin. The cleavage strike of No.3 working face is between N15 and 66 E, and the end cleavage strike is between N5 and 84 W; The cleavage strike of 15 working face is between N20 and 30 E, and the end cleavage strike is between N3b and 88BW. Cleavage density: No.3 coal is between 173 ~ 604 strips /m, and the regional distribution law is cleavage development from north to south; 15 coal is between 530 and 580 strips/m, with dense cleat and even distribution in the basin.

The permeability of coal seam is also related to the development degree of structural fractures. There are three groups of tectonic lines in the basin, namely, NNE, NNE and NEE, which represent the tectonic movements in different periods. Fracture development zones are formed at the intersection of structural lines in different periods, which greatly improves the permeability of coal reservoirs.

In the area where cleavage and structural cracks are developed, the permeability of coal seam is greatly improved, forming a high permeability area, which is beneficial to the production of coalbed methane. The well test permeability of coal seam in panzhuang mine field in the south of the basin is 65438±0.53m·D, and it is as high as 3 ~ 5m·D in some areas.

3. The effective thickness of the overlying coal seam is large, the hydrodynamic conditions are good, and the coalbed methane preservation conditions are favorable.

Because the Yanshan and Himalayan tectonic movements did not cause strong structural deformation in this area, the weathering and denudation were not strong, and the effective thickness of overlying coal seam remained in the periphery and center of the basin, especially in Fan Zhuang block. The thickness of the direct mudstone caprock of No.3 coal reached more than 50 meters, so the gas content is still high.

According to the data of borehole pumping test, the hydrogeological conditions of different horizons in Qinshui coalfield are very different. From the calculated hydrostatic pressure gradient of each main aquifer, it can be seen that the hydrostatic pressure gradient of coal-bearing aquifer is 0.2 1 ~ 0.62 MPa/hm, with an average value of 0.35 ~ 0.48 MPa/hm；; The average hydrostatic pressure gradient of Shangshihe Formation aquifer is 0.62MPa/hm, and that of Quaternary unconsolidated aquifer is 0.82MPa/hm. These different data show that there is no obvious hydraulic connection between aquifers in the basin, and the coal measure is an almost independent and closed hydrological system, which is beneficial to the preservation of coalbed methane (Table 6- 12).

Table 6- 12 Statistical Table of Aquifer Pressure in Qinshui Basin (Zhang, 2002)

(5) CBM resources

The geological resources of coalbed methane from the weathered zone to the shallow part with a buried depth of 2 000m in Qinshui Basin are 39 500.42× 108m3, the resource abundance is 1.46× 108m3/km2, and the recoverable resources are112. The geological resources of coalbed methane in Taiyuan Formation of Upper Carboniferous and Shanxi Formation of Lower Permian are 23 397.99× 108m3,16102.43×108m3 respectively, accounting for 59.23% and 40.77% of the total geological resources.

According to regional statistics, the geological resources of coalbed methane in Qinshui, Huoxi and Xishan gas-bearing areas are 361765 438+0.39×108m3, 2,535.30×108m3 and 793.73× 108m3, respectively, accounting for the total. The recoverable resources of coalbed methane in Qinshui, Huoxi and Xishan gas-bearing zones are 9 677.54× 108m3, 1 042.59× 108m3 and 496. 10× 108m3, respectively, accounting for 86.5% of the total recoverable resources.

According to the depth statistics, the buried depth of coal seam is 1 000m, which is relatively shallow, namely 1 000~ 1 500m+0 500 m and 1 500 ~ 2 000 m respectively, and the geological resources of coalbed methane are 20 808.87×/kloc respectively. The recoverable resources of coalbed methane with shallow burial 1 000m and1000 ~1500m are 6219.14×108m3 and 4 997.08×/kloc-0, respectively.

The primary resources of this gas-bearing basin group are 18 467.38× 108m3, accounting for 46.75% of the total geological resources. Class II resources are 20 503.22× 108m3, accounting for 565 438+0.965 438+0% of the total geological resources; Class III resources are 529.8 1× 108m3, accounting for 1.34% of the total geological resources (tables 6- 13 and 6- 14).

Table 6- 13 Summary of CBM Resource Calculation in Qinshui Basin

Table 6- 14 Table of CBM Resources in Gas-bearing Zones in Qinshui Basin