Fund Project: National Science and Technology Major Special Demonstration Project 62(20092×05062)

About the author: Lin Liang, born in 1983, male, engineer, master, graduated from China University of Mining and Technology (Beijing) in 2009, and now works in the international cooperative exploration department of China United Coalbed Methane Co., Ltd., engaged in oil-gas basin analysis and research on coalbed methane exploration and utilization. 0 10-6429888 1,atlan-tics@foxmail.com

(China United Coalbed Methane Co., Ltd. Beijing 1000 1 1)

Through the implementation of the national major scientific and technological special project "Development of large oil and gas fields and coalbed methane" and "Demonstration Project of Carboniferous and Permian coalbed methane exploration and development in Ordos Basin", a large number of coalfield data were collected, pilot production wells of coalbed methane were built, and the porosity and permeability development characteristics of coalbed methane reservoirs in Liulin area were studied. The results show that the porosity of coal and rock in this area is mainly influenced by coalification degree, maceral, mineral content and coal structure. The permeability of coal seam changes greatly, and the permeability is relatively low, which has strong heterogeneity; Generally speaking, the permeability tends to decrease from northeast to southwest, and the permeability of coal seam in Taiyuan Formation is lower than that in Shanxi Formation.

Keywords: coalbed methane in Liulin block with variable porosity and permeability

Porosity and permeability characteristics of coalbed methane block in Liulin, Shanxi Province

Lin Liang Yao Yong Huang Xiaoming

(China United Coalbed Methane Co., Ltd., Beijing 1000 1 1, China)

Liulin Demonstration Project, a demonstration project for exploration and development of Carboniferous and Permian coalbed methane in Ordos Basin, is one of the major national science and technology projects of "Development Planning of Large Oil and Gas Fields and Coalbed Methane". In order to study the porosity and permeability characteristics of coal seam reservoirs in this area, we collected a lot of coalfield data and many parameters, and implemented production wells. The results show that the porosity of coal is mainly influenced by coalification degree, maceral, mineral content and coal shape. The permeability of coal is relatively low, and it changes greatly, showing a decreasing trend from northeast to southwest. The coal permeability of Taiyuan Formation is lower than that of Shanxi Formation.

Keywords: Liulin block; Coalbed methane; Porosity; pervade

Liulin is located in the west of Shanxi Province, in the middle of Hedong coalfield, adjacent to Shilou North Block in the south and Yangjiaping Block in the east. Administrative divisions belong to Mucun Town, Xuecun Town, Zhuangshang Town, Gao Jiagou Township and Jia Jiayuan Township in Liulin County, Luliang City. Geographical coordinates: east longitude110 44' 00 "~/kloc-0 53' 00", north latitude 37 2 1' 00 "~ 37 3 1' 00 "

1 regional geological background

Hedong coalfield is mainly located in the west slope of Lvliang, a north-south structural belt on the east bank of the Yellow River, and belongs to the east of the "north-south structural belt on both sides of the Yellow River" pointed out by Li Siguang. The coalfield is generally a monoclinic structure inclined to the west, belonging to the west wing of anticlinorium, Lvliang. On the monocline, there are secondary folds and meridional or Xinhua Xia fault structures [1].

Liulin area is located in the west of Liu Li mining area in the middle of Hedong coalfield, adjacent to Shilou north block in the south and Sanjiao block in the north, and structurally located in the south wing of Shibi structure in the eastern margin of Ordos Basin. The strata in the northern part of the study area are inclined to the west, and gradually turn to the southwest from the south, which is generally a monoclinal structure inclined to the west or southwest. The occurrence of stratum is gentle, and the dip angle is about 3 ~ 8. On the background of nose structure, there are secondary small folds with weak fluctuation, and the fluctuation height is generally less than 50 m. Faults in the area are not developed, and graben composed of north-south normal fault of Jucaita and its derived small faults is developed only in the northern part of the block. There is no collapse column or magmatic activity on the surface [2].

The strata in and around this block are the Middle Ordovician Fengfeng Formation (O2f); Middle Carboniferous benxi formation (C2b) and Upper Carboniferous Taiyuan Formation (C3T); Shanxi Formation of Lower Permian (P 1s) and Xiashihezi Formation (P1x); Upper Permian Upper Shihezi Formation (P2s) and Shiqianfeng Formation (P2SH); Lower Triassic liujiagou formation (T 1l) and heshanggou formation (t1h); Cenozoic Neogene Pliocene (N2); Quaternary Middle Pleistocene (Q2), Upper Pleistocene (Q3) and Holocene (Q4). There are coal seams 14 in this area, including 5 layers in Shanxi formation, numbered from top to bottom as 1, 2, 3, 4(3+4) and 5 # coal seam; Taiyuan Formation consists of 9 layers, numbered from top to bottom as No.6, No.6, No.7, No.7, No.8+9, No.9, 10, 10 and10 [2]. Among them, No.2, No.3, No.4 (3+4) and No.5 coal seams of Shanxi Formation and No.8+9 and 10 coal seams of Taiyuan Formation are the main minable coal seams [3].

2 coal reservoir pore characteristics

Coal porosity refers to the space that is not filled with solid matter, which is an important part of coal structure and closely related to the storage performance and permeability of coal reservoirs. Generally speaking, with the increase of coal rank, the total pore volume in coal decreases exponentially, and the general rule is that micropores and pores increase, while macropores and micropores decrease [4].

There are many schemes to divide pores, and B.B. Khodot's scheme is generally adopted, that is, macropores are larger than 1000nm, mesopores 1000~ 100nm, micropores 10nm and micropores are smaller than10 nm.

According to the pore volume percentage of coal reservoirs in the eastern margin of Ordos Basin, the pore volume percentage is between 26.06% and 66.78%, with an average of 48.75%, and the micropores are between14.89% and 39.39%, with an average of 27.47%. Macropores are the second, ranging from 5.56% to 44.24%, with an average of16.43%; The mesopores are the weakest, ranging from 2.35% to 32.98%, with an average of 7.33%. The pore distribution of coal reservoirs in different regions and horizons is very different [5].

According to the data of Yangjiaping Well Group (Table 1), the coal seam pores in Liulin area are mainly small pores, which generally account for 40%~55% of coal seam pores. In addition, there are many micro-pores and macro-pores, and the mesopores are the least developed. The average total pore content is between 0.0258 and 0.041.3cm3/g, and the pore development is general. Under the condition of 4MPa effective overburden pressure, the average total porosity of No.8 coal seam in Liulin area is 7. 18%, that of No.5 coal seam is 3.45%, and that of No.4 coal seam is 3.90%, among which No.8 coal seam is the best.

Table 1 Pore Development of Different Coal Seam in Liulin Area (Note: Yangjiaping Well Group Data)

Generally speaking, the total pore volume in Liulin area is generally between (1 48 ~ 547 )×10-4cm3/g, with an average value of about 323 × 10-4cm3/g ... As shown in figure1,the pore volume distribution is mainly small pores and micropores.

Figure 1 Comparison of Pore Volume Ratio of Various Pores in Liulin Area

The specific surface area of total holes for coal seam mercury injection in Liulin area is between 0.103cm3/g and 0.413cm3/g. Small holes and micropores account for the absolute advantage of the specific surface area of total holes, while the ratio of large holes to middle holes is very small, and the ratio of transition holes is slightly higher than that of micropores.

3 coal reservoir permeability characteristics

The permeability of No.3+No.4 coal seam in the study area is 0.0 1~2.8mD, with an average of 0.6mDFL-EP 1 well, which is relatively high. The permeability of No.5 coal seam ranges from 0.06- 1.59 MD, with an average value of 0.7mD8+9 coal seam ranges from 0.005-24.8 MD, with an average value of 4.8md. On the whole, the permeability of No.8+9 coal seam is obviously higher than that of No.3+4 and No.5 coal seams, and the permeability of each horizon is characterized by high in the north and low in the south [6] (Figure 2).

The permeability of coal and rock varies greatly in the plane, and the permeability is relatively low in the west because of the deep burial of coal seam, which shows that coal seam has strong heterogeneity. Generally speaking, the permeability tends to decrease from northeast to southwest, and the permeability of Taiyuan Formation is higher than that of Shanxi Formation.

Permeability of coal reservoir is the dominant factor to control methane productivity in coal seam. Permeability generally refers to well test permeability, which is obtained from well test data. Due to the limited exploration wells in the study area, the permeability data of coalbed methane well test is very limited. According to the available data, the permeability in Liulin area is between 0.0 1 ~ 100 MD, and the permeability in the south is better than that in the north. The permeability of coalbed methane reservoir is affected by the structure of coal, the development degree of fracture system and in-situ stress. In addition, the change of external conditions, especially the effective stress effect and matrix shrinkage effect caused by the change of reservoir pressure, also has a great influence on the permeability of coal and rock:

1. The Liulin demonstration area and its surrounding areas are dominated by middle coal, and the very developed cracks are the main controlling factors of permeability. The cracks are distributed in the east-west direction, and the cracks at the end intersect with them obliquely. On the plane, the two groups of cracks are mainly regular rhombic grids, followed by irregular grids, and isolated ones are rare.

The development of macropores, especially fractures, determines the permeability of reservoirs under the original formation conditions. The development degree of cracks mainly refers to the density (or spacing), length, width and width of cracks. The greater their values, the better the permeability of coal seam. The development degree of fault system is closely related to the composition of coal, the metamorphic degree of coal and tectonic stress. Bright briquette, medium metamorphic bituminous coal (such as fat coal, coking coal and lean coal) and low ash coal are the most favorable conditions for forming a large number of cracks. The coal in Liulin area is mainly semi-bright coal, and the coal rank is mainly coking coal, which is beneficial to the formation of cracks. Statistical surface crack density shows that the crack density is relatively high, and most cracks are not filled, which greatly expands the permeability of coal [6].

2. Coal seam is a kind of natural gas reservoir which is very sensitive to geostress. Usually, the geostress field is decomposed into vertical stress and horizontal stress. The vertical stress is caused by the weight of overlying strata. The permeability of coal seam fracture system is a function of effective stress, which is a function of the difference between vertical force and formation pressure. Both vertical stress and formation pressure increase linearly with the increase of buried depth. Because the density of rock stratum is much greater than that of fluid in pores, it can be seen that the effective stress increases with the increase of depth, and the permeability of fracture system decreases with the increase of depth. The buried depth of coal seam in Liulin area gradually increases from east to west to south, for example, the buried depth of No.4 coal seam increases from 200m in the east to 1250m in the southwest, and the permeability tends to decrease under the action of geostress.

3. The tectonic stress field and its associated joint development characteristics in the demonstration area are one of the main factors controlling the permeability of coal reservoirs. The change of joints in the south is small, but it is large in the middle, which indicates that the joints in different directions in the middle area may be distributed in a network, forming a high permeability stratum distribution area. At the same time, statistical data show that the permeability in the middle of the demonstration area is better than that in the east and west sides. Affected by Yanshan movement, the stratigraphic faults in Liulin area spread to the northeast; The fracturing results of No.3+4 coal seam in FL-EP 1 Jingshanxi Formation show that the fracture direction is still NNW, which is consistent with the main fracture direction of coal seam.

Fig. 2 Permeability of 4, 5 and 8+9 coal seams in Liulin area

4 conclusion

The coal measures strata in Liulin mining area are Upper Carboniferous benxi formation (C2b), Upper Carboniferous Taiyuan Formation (C3t) and Lower Permian Shanxi Formation (P 1s). Among them, the coal seams with coalbed methane exploration potential in the mining area are 8+9+ 10 coal in the bottom of Taiyuan Formation of Upper Carboniferous and 3+4+5 coal in Shanxi Formation of Lower Permian.

The macroscopic coal and rock types of the two groups of coal seams are mainly semi-bright coal and semi-dark coal, supplemented by bright coal and dark coal, with high vitrinite content, mainly coking coal. Small pores are the main part of coal seam pores, generally accounting for 40%~55% of coal seam pores. In addition, micropores and macropores are well developed, while mesopores are least developed. The total pore volume is generally between (148 ~ 547 )×10-4cm3/g, with an average value of about 323x10-4cm3/g. The total pore specific surface area of mercury is between 0.103 ~ 0.4/kloc.

The permeability of coal and rock varies greatly in the plane, and the permeability is relatively low in the west because of the deep burial of coal seam, which shows that coal seam has strong heterogeneity. Generally speaking, the permeability tends to decrease from northeast to southwest, and the permeability of Taiyuan Formation is higher than that of Shanxi Formation.

Considering the conditions of coal seam thickness, coal quality, porosity and permeability, Liulin area has the conditions for coalbed methane accumulation and has the potential for large-scale development.

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