(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 221008; 2. Key Laboratory of Coalbed Methane Resources and Accumulation Process, Ministry of Education, Xuzhou, Jiangsu 22 1008)

Based on the study of geological background, coal reservoir characteristics, coalbed methane occurrence characteristics and gas-controlling geological factors in Hong En mining area, it is found that the coal structure, pore types and roof and floor lithology of coal seams with a depth of 500 ~ 1000 m in this area are all conducive to the adsorption and preservation of coalbed methane, but there are also unfavorable factors such as low permeability, low reservoir pressure and strong heterogeneity of coal seams. The average gas content of main coal seams is more than 8m3/t, and the gas content is most obviously controlled by buried depth and structure. Other conditions such as caprock, coal rank, coal thickness and hydrogeology are also beneficial to coalbed methane enrichment. Considering comprehensively, the development prospect of coalbed methane in this area is good, but poor permeability and complex structural conditions are the most unfavorable factors. We should explore the development mode of casing fracturing completion and ultra-short radius horizontal well, and cooperate with other stimulation measures such as high-energy gas fracturing and "virtual pay zone" to carry out coalbed methane development experiments.

Keywords: exploration and development of coalbed methane gas content in Hong En mining area

Project support: National "973" coalbed methane project (2009CB2 19605), major national science and technology projects (201ZX05034), key projects of National Natural Science Foundation (40730422) and Youth Science Fund project (40802034)

Author's brief introduction: Zhang Jinbo, born in 1987, male, from Nangong, Hebei Province, is a master's student in College of Resources and Earth Sciences, China University of Mining and Technology. His research interests are coalbed methane and gas geology. Tel: 187954262 12, email: xiaopo688@ 126. com

Coal reservoir and gas-bearing characteristics in Hong En mining area and suggestions for exploration and development

(Zhang Jinbo 1, 2 Wu Caifang 1, 2)

(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22 1008. China II. Key Laboratory of Coalbed Methane Resources and Accumulation, Xuzhou, Jiangsu 22 1008)

Abstract: Based on the study of geological background, coal reservoir characteristics, coalbed methane enrichment law and geological control factors in Hong En mining area, it is found that the coal structure, pore type, roof and floor lithology and other conditions are conducive to the adsorption and preservation of coalbed methane in the depth range of 500-1000 m. But there are also unfavorable factors such as low permeability, low reservoir pressure and strong heterogeneity of coal. The average gas content of main coal seams is more than 8 m3/ t, and the depth and structure are the most obvious controlling factors of coalbed methane. Other conditions, such as caprock, coal rank, coal seam thickness and hydrogeology, are also conducive to the enrichment of coalbed methane. Generally speaking, the development prospect of coalbed methane in this area is good. However, poor permeability and complex structural conditions may be the most important unfavorable factors. We should explore casing fracturing completion and short-radius horizontal well as other stimulation measures, such as high-energy gas fracturing and the concept of "virtual layer", to carry out coalbed methane development experiments.

Key words: Hong En mining area, coalbed methane, gas control factors, gas content, exploration and development.

Hong En mining area is located in Qujing city in the east of Yunnan province. The whole area is distributed in three directions: north, southeast and west, with a length of 53 kilometers and a width of 9-20 kilometers, covering an area of 620 square kilometers, including 485 square kilometers of coal-bearing area. The coal-bearing stratum is the Upper Permian Xuanwei Formation (P2x) in the Late Paleozoic, and the coalbed methane resources in the shallow part of 2000m are 6129 million m3, of which more than 82% are buried in the depth of 1.000m, which has a good prospect for coalbed methane development (Deng et al., 2004).

Predecessors have studied the basin structural characteristics, coalbed methane accumulation conditions and favorable block screening in this area (Deng et al., 2004; Wang Chaodong et al., 2004; Gui, 2004), thinks that Hong En mining area is one of the favorable blocks for coalbed methane exploration and development in eastern Yunnan and western Guizhou (Gui, 2004). On the basis of studying the occurrence characteristics of coalbed methane in Hong En mining area, this paper further discusses the exploration and development methods suitable for this area in order to provide ideas for coalbed methane development in this area.

Geological background of 1

Structure and mechanical characteristics of 1. 1

Hong En mining area is located in the eastern margin of Kangdian ancient land of Yangtze plate, and its main body is a large-scale syncline structure with the axial NNE-nearly NNE direction, in which secondary and anticline fold structures are densely distributed, and from west to east, it is Hong En syncline, Niutoushan syncline and Guanping Daping syncline in turn. The axial direction is nearly north-south, all of which are tilted north and lifted south, extending 15 ~ 30km, with a distribution area of tens to hundreds of km2. The latest stratum exposed at the core of the syncline is the Middle Triassic Guanling Formation (T2g) or the Lower Triassic Yongningzhen Formation (T 1y), and the oldest stratum at the axis of the anticline is the Upper Permian Emeishan Basalt Formation (P2β) or the Lower Permian Maokou Formation (P 1m), and the dip angle of its wings is generally10. Compression-torsion, tension-torsion and strike faults are well developed. The main faults are Fuyuan-Mile fault, Guanping-Agang fault and Mile-Shizong fault. A series of minor faults or secondary faults with * * * yokes are derived from the main fault, and the stress field in the whole region is obviously tensile and torsional (Gui, 2004), as shown in figure 1.

1.2 sedimentary characteristics of coal seam

During the late Permian coal-forming period, the Paleo-Tethys oceanic crust continued to subduct eastward, and the Kangdian ancient land continued to rise into a denudation area. The eastern part of Yunnan was located in the western margin of the Sichuan-Guizhou-Yunnan fault depression sedimentary area in the plate. Taking the north-south ancient fault line of Ganluo Xiaojiang as the sedimentary boundary of the western margin, under the control of the seesaw mechanism of "rising west and falling east", the eastern wall continues to sink, forming a coal-bearing system dominated by river delta sedimentary system, with a total thickness of 205 ~ 335 m, an average thickness of 250m coal 18 ~ 73 layers and a total thickness of15.99 ~. General 1 1 ~ 13, recoverable thickness 10 ~ 3 1m, average 18m (Song Yang et al., 20 10).

Hydrogeological characteristics of 1.3

This area is located in the plateau mountainous area, mainly in the middle and low mountains, and is a typical karst landform area. Strong terrain cutting, developed valleys and good drainage conditions. The lower Triassic Kayitou Formation (T 1k), the lower Triassic Feixianguan Formation (T 1f) and the upper Permian Emeishan Basalt Formation (P2β) in coal measures and overlying strata are weak in water-abundance. The Lower Triassic Yongningzhen Formation (T 1y) and the Middle Triassic Guanling Formation (T2g) are mainly limestone, with developed lava fractures and strong water-rich properties, but the aquiclude or weak aquifer separated from the coal-bearing strata by hundreds of meters has little influence on the coal-bearing strata. Controlled by lithology, the fault zone is rich in water and weak in water conductivity. The unit water inflow (Q) in shallow weathering zone is 0.0104 ~ 0.0899L/s m, which is generally lower than 0.05 L/s·m, which is beneficial to the preservation of coalbed methane.

Generally speaking, the underground hydraulic connection of each hydrogeological unit is not strong, and the hydrogeological conditions are simple. Coal measures strata are mostly fractured and weak aquifers, and the recharge, diameter and discharge of groundwater are limited to shallow parts (vertical depth of 50m).

Figure 1 Structural Outline of Mining Area (Deng 2000)

1.4 coal and rock characteristics

The macroscopic coal and rock types in this area are mainly semi-bright and semi-dark briquettes, followed by dim briquettes. The maceral composition of coal and rock is between 75% and 89%. Among them, vitrinite is the main group, accounting for 58% ~ 82%, followed by inertinite group, accounting for 10% ~ 35%, hemivitrinite group is less, accounting for 4% ~1%,and chitin group content is very small. The coal body structure is mainly primary structure, which is generally uniform, quasi-uniform and banded, mainly medium and thin banded, followed by lineation.

The average ash content of raw coal in each coal seam is 16% ~ 29%, and the ash content in the lower part (below the coal of En 2 1) and the upper part (above the coal of En 7 1) is higher in the longitudinal direction, while the ash content in the middle coal seam is lower. On the plane, the ash content increases from southeast to northwest. The total sulfur content of raw coal in each seam is generally 0.50% ~ 6.80%, with an average of 0. 16% ~ 5.30%, which belongs to ultra-low sulfur and high sulfur coal. In the vertical direction, the middle part of coal measures is lower, and the upper part, especially the lower part, is higher. Generally, it decreases from the southeast (sea) to the northwest (land) on the plane, which is positively or negatively correlated with haze, as shown in Figure 2.

Fig. 2 Variation curve of ash and total sulfur content of coal in Hong En mining area.

The average reflectance of vitrinite ranges from 65438 0.278% to 65438 0.699%, and the main component is coking coal-lean coal. Controlled by plutonic metamorphism and magmatic thermal metamorphism, the old coal rank rises vertically with the change of coal seam level. On the plane, the metamorphic degree of each coal seam increases from the northwest to the southeast of the mining area, but in the east of the mining area near the main fault, the variation range tends to increase.

2 coal reservoir characteristics

2. Fracture characteristics of1hole

The development of pores and cracks in coal seam directly affects the permeability of coal seam and is one of the main factors that determine the migration and output of coalbed methane. Coal seam cracks can be divided into three types: micro-cracks, endogenous cracks (cleavage) and exogenous cracks. Among them, cleavage has the greatest contribution to coal seam permeability, and it can be divided into two types: face cleavage and end cleavage. The former extends farther and has stronger continuity than the latter. According to the observation of coal seam in the mining area, it is found that the width of surface cleat is 0. 1 ~ 1.0 mm, the length is 20 ~ 85 mm, the width of end cleat is 0.05~0.45mm, and the length is 1.5 ~ 5.4 mm, which indicates that there are endogenous cracks in the coal seam in this area and the cleat is more open.

Coal seam porosity is the storage place of adsorbed gas, which is not only the key factor to determine the gas-bearing property of coal seam, but also the channel of coalbed methane seepage. According to the measured data of some coal mines in this area, the porosity in this area is between 0.7% and 4.4%, with an average of 2.8%, and the porosity is low. Micropores and transition pores are dominant, accounting for 63.8% of the total pore volume and more than 98% of the total specific surface area. It shows that coal has a strong adsorption capacity, which is beneficial to the storage of coalbed methane, but not conducive to the diffusion and seepage of coalbed methane (Song Yang et al., 20 10).

2.2 Reservoir pressure

At present, there is no well test pressure data of coalbed methane parameter wells in this mining area. According to the conversion of water head height data, the coal reservoir pressure in the mining area is between 0.255 MPa and 2.002 MPa, and the pressure gradient is between 4.412 and 8.920 kpa/m, which belongs to low pressure and ultra-low pressure reservoir state. However, from the measured data, the actual gas content in the shallow part of this area is often greater than the theoretical gas content, and even there are overpressure reservoirs in some gas-rich areas, which shows that the reservoir heterogeneity in this area is strong and the distribution and control factors of reservoir pressure need to be further studied.

2.3 Permeability

In 2004, Yunnan Coalfield Geology Bureau cooperated with Zhonglian CBM Company to drill two CBM parameter wells in the south of Hong En mining area. According to the obtained well test data, the permeability of EH-0 1 well 9# and 16# coal seams are 0.0 16mD and 0.0045mD respectively, and the permeability of EH02 well 9#, 16# and 2 1# coal seams are respectively The permeability of coal seam is poor, and the permeability difference between two wells in 16# coal seam is one order of magnitude, showing strong heterogeneity. With the increase of buried depth, the permeability increases. According to the analysis of coring data, this may be due to the change of coal structure. 9# and 16# coal seams are seriously damaged by structure, and there are many mylonite coal structures; The coal body structure of the lower coal seam is more complete than that of the upper coal seam, mainly composed of primary structure and fracture structure, with good permeability.

3 Gas-containing and gas-controlling factors

3. 1 gas bearing property

The content of coalbed methane in this area is high, and it increases with the increase of buried depth. The methane content of 9# coal seam is 3.72 ~ 14.54 m3/t (dry base is ash-free and can be burned; The same below); The average is 8.68m3/t, the methane content in 16# coal seam is 3.92 ~ 2 1.98m3/t, and the average methane content in 1 0.20m3/t.21# coal seam is 4.50 ~/kloc-0.

Table 1 Statistical Table of Gas Content and Gas Composition of Some Coal Seam in Hong En Mining Area

From the plane, the distribution of coalbed methane in this area is obviously controlled by syncline and anticline fold structure. Generally, with the increase of buried depth, the methane content in the axis of the two wings of syncline increases, and the relationship between them is logarithmic. The contour distribution of gas content is basically consistent with the contour trend of coal seam floor (Figure 3).

Figure 3 Isogram of Gas Content in No.9 Coal Seam in Mining Area (Deng, 2000)

Seen from the vertical direction, according to the different components, it can be divided into three zones from top to bottom (Nie et al., 2007):

① Nitrogen-bearing layer: N2≥70%, CO2≤20%, CH4≤ 10%, and the buried depth of coal seam is generally around 0 ~ 100m.

② Nitrogen and methane zone: N2

③ Methane layer: CH4≥70%, generally buried depth >; 140m.

3.2 Gas control factors

3.2. 1 buried depth

Buried depth can affect coalbed methane enrichment from two aspects: first, with the increase of buried depth of coal seam, reservoir pressure increases and the adsorption capacity of coal to methane increases, but the relationship between them is not simple linear. In shallow coal seam, the gradient of methane content changes with the buried depth, and the deeper the buried depth, the less the influence on methane content in coal seam. Second, with the increase of buried depth, the preservation conditions of coalbed methane gradually become better. Shallow coal seams often suffer from weathering and erosion, which makes the coalbed methane generally shallow at 140m, and the methane content decreases with the depth becoming shallow. The depth of gas weathering zone is different under different geological background. For example, in Laochang mining area adjacent to Hong En mining area, the maximum depth of weathering zone can reach more than 600m, while in Qingshuigou mine field of Hong En mining area, the depth of weathering zone is only 50 ~ 80 m due to the fault shielding.

Considering only the buried depth, the development depth of coalbed methane in Hong En mining area is generally 250 ~ 1000m. When it is less than 250 m, it is difficult to completely avoid the influence of wind oxidation zone. Because the permeability of deep coal seams below1000 m is extremely low, it is very difficult to mine coalbed methane.

3.2.2 Tectonic geological conditions

The influence of tectonic geological conditions on gas-bearing property is a very complicated problem. After Himalayan movement, the coal-bearing basins formed in this area were destroyed, forming some discontinuous folds and some fault blocks, mainly syncline or syncline structure, with less anticline, which is beneficial to the preservation of coalbed methane.

The data study shows that the coalbed methane content in this area increases with the increase of buried depth, and the gas-rich structures are generally located at the high points of secondary folds and fault zones, that is, the coalbed methane content tends to increase from the syncline axis to the two wings; The enrichment zone appears at the intersection of closed faults and the core of anticline, or on the fault blocks of horst, such as several high-value areas where the gas content in No.9 coal seam in the east of Laoshitou mine field and Qingshuigou mine field is >:11m3/t. In addition, in some small anticline and tensile fracture zones, especially in areas where coal seam is squeezed, structural cracks develop and coal seam suddenly thickens, the gas emission is doubled.

In a word, the permeability and sealing of the structure determine whether coalbed methane is enriched or lost. Whether it is syncline, anticline, monocline or fault with poor permeability, the degree of coalbed methane escape or migration is poor, and the gas-rich parts are obviously controlled by buried depth, and the gas-rich areas are easily concentrated in the axis of syncline and the downward bending part of monocline; When the permeability is good, it is beneficial to the migration of coalbed methane. Local enrichment areas are formed at structural high points with good sealing, such as the axis of secondary anticline with no open fault at the top and the height of closed fault blocks.

3.2.3 metamorphic degree of coal

With the increase of coal rank, the gas content in coal seam presents the stage evolution characteristics of sharp increase → slow increase → sharp increase → sharp decrease (Fu et al., 2007). The coal rank in this area is mainly middle rank coking coal and lean coal, which is in the second jump stage of coalification, and the gas content increases slowly with the coal rank. The porosity and pore specific surface area of coal body are further increased, and the gas production and adsorption capacity are strong, which is beneficial to the enrichment of coalbed methane. The distribution of coal rank has obvious regularity. The coal rank increases from northwest to southeast, and the older the vertical coal rank, the higher the metamorphic degree (Yi, 2007).

3.2.4 Coal seam thickness

Hong En mining area has typical multi-seam characteristics, mainly thin coal seam-medium thick coal seam, and the thickness of single layer is generally less than 5m. The lithology of interlayer between coal seams is mainly mudstone and sandy mudstone. When mining, the coal seams with close distance can be regarded as a coal seam group (Yi, 2007), which has two advantages: First, the cumulative thickness becomes larger, which increases the recoverable amount of coalbed methane. Second, the interlayer between coal seams, mainly composed of sand and mudstone, can play a good supporting role for each mining layer, and is also convenient for maintaining coal seam pressure and increasing sealing performance. When reinforcing the reservoir, one coal seam group can be treated uniformly, or the interlayer in the middle of the coal seam group can be directly fractured according to the concept of "virtual production layer", which may get better results.

3.2.5 Coal seam roof and floor and overburden

The lithology and fracture development of coal seam roof and floor have great influence on coal seam gas content. Compared with sandstone and limestone, mudstone and sandy mudstone have strong sealing effect on coal seam, which is beneficial to the preservation of coalbed methane. The gas content of coal seam with faults or cracks on the roof and floor is much lower than that of coal seam with faults or cracks on the roof and floor. According to the statistics of this area, the same coal seam and the roof and floor with similar lithology have two situations: tensile fault cutting and no fault cutting, and the gas content of the latter coal seam is 3 ~ 12 times that of the former.

The upper Permian coal-bearing strata in this area are mainly argillaceous rocks and argillaceous siltstones, and the underlying strata are Emeishan basalt, which is nearly several hundred meters thick and the coalbed methane is well preserved.

hydrogeological condition

According to lithology, karst fracture development and water abundance, the mining area can be divided into six sets of aquifers from bottom to top, but there is little hydraulic connection between layers. The shallow outcrop area is fissure diving, and the groundwater alternates strongly. However, due to the influence of formation lithology, the vertical depth is generally less than 50m, and it gradually turns into weak fracture confined water during the deep runoff of syncline.

Hydrogeological data show that the groundwater conditions in Laotai mine field, Qingshuigou mine field in the south of the middle section and exploration area in the south of the middle section are good, and the groundwater migrates from the two wings to the oblique core, which forms a hydraulic plugging effect on coalbed methane and has high gas content. In addition, the confined water areas of No.7 and No.9 mine fields are also beneficial to the preservation of coalbed methane.

4 Suggestions on exploration and development

4. 1 exploration direction

As an important part of CBM enrichment area in eastern Yunnan and western Guizhou, predecessors have done a lot of work on CBM resource evaluation and exploration direction in Hong En mining area. Gui et al. conducted in-depth research on coal-bearing basins in eastern Yunnan and western Guizhou from the perspective of coalbed methane system, and considered that the mining area was the key experimental area for coalbed methane exploration and development in eastern Yunnan.

Considering the geological conditions, resource occurrence, reservoir physical properties, traffic and market conditions and many other factors in this area, it is considered that Laotai minefield, middle south survey area, No.7 minefield, Qingshuigou minefield (east), No.9 minefield, 10 minefield and Daping survey area are the best blocks for coalbed methane exploration and development in Hong En mining area (Nie et al., 2007).

4.2 Development suggestions

At present, the area is still in the exploration and test stage, with only a few parameter wells and production test wells, and large-scale commercial development has not yet been realized. According to the mining information and the successful experience of other areas in China, the following suggestions are put forward for the future development of coalbed methane in this area:

(1) The coal seams in this area are rich in gas content and resources, but there are many coal seams and the thickness of single layer is not large. Therefore, it is necessary to consider the drilling and completion methods suitable for multi-seam mining, with appropriate reservoir strengthening measures and stimulation measures. According to the production experience at home and abroad, casing fracturing completion method and ultra-short radius horizontal well are suitable for mining coalbed methane in multi-seam environment and should be the main development methods of coalbed methane in this area. Open hole completion development mode can also be explored in areas with small local geostress, high coal strength and good permeability, but special attention should be paid to risks. Multi-branch horizontal well technology is suitable for single thick coal seam, and it is difficult to drill, so it is not recommended to be widely used in coalbed methane development in this area.

(2) The geological and structural conditions in this area are complex, and small tensile and torsional faults are extremely developed. The shallow coal seam is strongly reformed by tectonic movement, and the tectonic coal is relatively developed, with poor permeability and low gas content, which is not suitable for mining coalbed methane. Deep coal seam (500 ~ 1000 m) is mainly primary or fractured, with relatively good permeability, high gas content and small in-situ stress, which is suitable for coalbed methane mining.

(3) Compared with Jincheng, the coal rank in this area is low, the coal strength is low, the water absorption is strong, and it is easy to soften after water absorption. Therefore, the applicability of hydraulic sand fracturing completion technology widely used in other fields needs further study. According to the previous gas well fracturing results of Zhonglian CBM Company (Wang Jianzhong, 20 10), the gas production effect is not good, so other stimulation measures such as high-energy gas fracturing and "virtual reservoir" should be considered to improve the success rate of CBM development.

5 conclusion

(1) On the whole, the coal seam roof and floor in Hong En mining area have good sealing property, strong coal seam pore adsorption capacity and simple hydrogeological conditions. Groundwater migrates along the syncline core in two wings, which has a hydraulic sealing effect on coal seam methane. Although limited by complex structural conditions, the coal seam has strong heterogeneity and poor permeability, but the gas content, permeability and coal structure are positively related to the buried depth, so it is suitable for mining coalbed methane in 500 ~ 1000 m coal seam.

(2) Buried depth and tectonic conditions are two main factors controlling gas content in this area. Under the background that the general trend is to increase with the increase of buried depth, the content of coalbed methane is obviously controlled by structural conditions, which is enriched in areas with good sealing conditions and lost in areas with poor sealing conditions. Hydrogeological conditions, roof and floor lithology and coal adsorption capacity are also important factors affecting the gas content in this area.

(3) The coal reservoir in this area has strong heterogeneity, poor permeability, low coal strength, small coal seam thickness and many layers. Considering the development risk and investment cost, it is not suitable to use open hole completion and multi-branch horizontal wells to mine coalbed methane. Casing fracturing completion and ultra-short radius horizontal wells have strong adaptability to coal reservoirs and are suitable as the main development methods of coalbed methane development in this area.

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