Supported by major national science and technology projects (201kloc-0/zx05034), national 973 coalbed methane project (2009CB2 19605), key national natural science foundation project (40730422) and youth science fund project (40802032).
About the author: Zeng Jiayao (1987-), female, from Dafang County, Guizhou Province, studied at the School of Resources and Earth Sciences of China University of Mining and Technology (Xuzhou) with a master's degree, and her research direction is coalbed methane exploration and development. Mailing address: Room 302, Unit 5, 1st floor, Research Institute, Nanhu Campus, China University of Mining and Technology, Xuzhou, Jiangsu. Tel: 1895 2246792, email: jiaoyanghaha @126.com.
(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22 10082. Key Laboratory of Coalbed Methane Resources and Accumulation Process, Ministry of Education, Xuzhou, Jiangsu 22 1008)
Abstract: The permeability of coal reservoir is one of the important factors restricting the development of coalbed methane. On the basis of in-depth study on the permeability characteristics of coal reservoirs in western Guizhou-eastern Yunnan, combined with a large number of coalfield geological exploration data, the main geological factors controlling permeability in the study area are expounded. The research shows that the permeability of the whole study area gradually decreases from east to west, and the permeability of Jina coalfield in western Guizhou is much higher than that in other areas. Among many factors affecting permeability, regional tectonic stress, development of coal seam cracks, buried depth of coal seam and thickness of coal seam play an important role in controlling coal seam permeability.
Keywords: coal seam permeability, tectonic stress, coal seam depth and coal seam thickness.
Study on permeability characteristics and geological control factors of coal reservoirs in Qianxi-East Yunnan area
Zengjiayao 1, 2 Wu Caifang 1, 2
(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22 1008. Key Laboratory of Coalbed Methane Resources and Accumulation Process in Xuzhou, Jiangsu Province 22 1008)
Abstract: Coal seam permeability is one of the key factors restricting the development of coalbed methane. Based on the analysis of coal seam permeability characteristics and referring to coalfield geological exploration data, the main geological factors affecting coal seam permeability in western Guizhou-eastern Yunnan are expounded. The results show that the permeability of the whole region shows a downward trend from east to west, and the permeability of Zhina coal mine in western Guizhou is obviously higher than that in other regions. Among all the factors affecting permeability, regional tectonic stress, coal seam cracks, coal seam buried depth and coal seam thickness have significant control effects.
Keywords: coal seam; Permeability; Tectonic stress; Buried depth of coal seam; coal seam thickness
introduce
There are abundant coalbed methane resources in western Guizhou, which mainly occur in the syncline structure of Liupanshui coalfield and Zhina coalfield. The resources in methane-rich areas with methane content exceeding 8m3/t account for more than 90% of the total resources in Guizhou Province. The coalbed methane resources in eastern Yunnan are 450 billion m3, accounting for 90% of the total coalbed methane resources in Yunnan Province.
The permeability of coal reservoir is one of the most important indexes to measure the exploitability of coalbed methane (Telly et al., 2000). On the premise that the gas source of coalbed methane has been determined, the permeability of coal reservoir is one of the key factors restricting the success or failure of coalbed methane resource development. In the process of dehydration and depressurization, with the desorption, diffusion and emission of coalbed methane, the effective stress effect, coal matrix shrinkage effect and gas slippage effect make the permeability of coal reservoir change dynamically. In-depth analysis of permeability distribution characteristics and its geological control factors is of great theoretical and practical significance for optimizing favorable zones and development measures of coalbed methane.
1 coal seam permeability characteristics
1. 1 well test permeability of coal seam
According to statistics, there are well test data of 9 coalbed methane wells at 19 level in Guizhou province (table 1). Two parameter wells of coalbed methane in Zhina coalfield are located in Hua Le exploration area of Bide syncline. The depth of the tested coal seam is less than 600m, and the well test permeability is relatively high, ranging from 0.1074 to 0.5002 MD, with an average of 0.2797mD, which belongs to the medium permeability coal seam and has favorable conditions for commercial development. There are 7 CBM exploration wells in Liupanshui coalfield, all of which are distributed in Panguan syncline and Qingshan syncline in the southeast. The well test permeability of coal seam is 0.0004~0.4800mD, most of which is lower than 0.02mD, with an average of 0.074 1mD, which is much lower than that of Zhina coalfield and belongs to ultra-low permeability coal seam.
Table 1 Well Test Results of CBM Wells in Qianxi Area
sequential
1.2 Distribution characteristics of coal seam permeability
According to the statistical results in Table 1, based on the experimental coal seam with a buried depth of less than 650m, the regional distribution law of upper Permian coal seam permeability in western Guizhou (even in eastern Yunnan) is very obvious, showing a decreasing trend from east to west as a whole. For example, the average well test permeability of Bede syncline coal seam in Zhinan coalfield is 0.2797mD, that of Jinzhuping exploration area in Panguan syncline in Liupanshui coalfield and Mayidong exploration area in Qingshan syncline is about 0. 15mD, and that of Hong En, Laochang, Xuanwei syncline or coalfield in eastern Yunnan is only 0.0904 MD On the one hand, this regional distribution law is that the coal seams are different due to structural changes after the coal accumulation period.
Because of the correlation between the buried depth of coal reservoir and the effective stress of corresponding strata, the deeper the buried depth, the greater the effective stress and the lower the permeability (Fu et al., 2003; Zhou Weiyuan, 1990), there seems to be no obvious distribution trend of coal seam permeability at the horizon (table 1). Such as Hua Le exploration area 1602 well, Liangshan exploration area QH 1 well, Jinzhuping exploration area Gm2 well and Ant Cave exploration area MY0 1 well, the permeability tends to decrease with the increase of coal seam buried depth. However, in Well MY03 in Mayidong exploration area, Well QH3 in Liangshan exploration area and Well 3603 in Hua Le exploration area, the coal seam level decreases and the well test permeability tends to increase.
2 geological factors affecting coal seam permeability
There are many factors affecting coal seam permeability, such as tectonic stress field, coal seam buried depth, coal reservoir thickness, coal reservoir pressure, coal body structure, coal quality characteristics, coal rank, natural fractures, etc., all of which have different degrees of influence on coal seam permeability, which can be the result of the comprehensive action of many factors or the main role of one factor.
2. 1 Influence of tectonic stress field on coal seam permeability
Cross-basement faults in Qianxi-East Yunnan control fold fault zones in different directions, which are combined into various structural styles such as arc, diamond and triangle, forming a unified regional structural framework (Figure 1). Among them, Zhina coalfield is located in Baixing triangle structure, and Liupanshui coalfield is mainly composed of Faer rhombic structure and Panxian triangle structure, and the tectonic stress field is extremely complicated (Figure 1). For triangular structures, the stress difference is the largest at the three vertex angles, followed by the edge, and decreases towards the inside of the triangle. The structural deformation is stronger at the top and edge of the corner and weaker in the middle, which accords with the regional distribution law of coal structure in Jina coalfield. Therefore, it is speculated that there may be two central belts with relatively complete coal structure in the south-central part of Liupanshui coalfield, namely the rhombic structural belt in the middle of Faer and the triangular structural belt in Panxian county in the south. Among them, the structural uplift in Faer rhombic structural area is strong, and the preservation conditions of coal-bearing strata are poor, only scattered. Therefore, the zones with good coal seam permeability in western Guizhou may be located in two zones: one is in the middle of zhina coalfield, such as Shuigonghe syncline, Zhuzang syncline and Niuchang syncline; Second, the central belt of Panguan syncline in the south of Liupanshui coalfield is located in the north of Panxian county.
The physical properties of coal seams in Qianxi-East Yunnan are closely related to geostress, especially the coal structure, coal seam permeability and coal reservoir pressure. The geostress field is controlled by the regional tectonic background. This control effect is reflected in the level of geostress gradient, which is an important geological reason for the regional distribution difference of coal seam permeability.
Figure 1 Schematic Diagram of Structural Framework in Western Guizhou
Enever et al. (1997) found that the change of coal seam permeability is exponentially related to the change of in-situ stress by studying the correlation between coal seam permeability and effective stress in Australia (Zhou Weiyuan, 1990):
K/K0=e3C△δ
Where: K/K0 is the ratio of permeability to initial permeability under specific stress conditions; C is the coefficient of pore compressibility of coal; △△ is the effective stress from the initial to a certain stress state.
According to the 36-level well test data of Qianxi-East Yunnan CBM well 18 well, the minimum principal stress (closing pressure) gradient in the geostress field decreases and the coal seam permeability increases correspondingly, which shows a negative power exponential relationship. In addition, the permeability decreases with the increase of in-situ stress and the damage degree of primary structure of coal seam. The minimum principal stress gradient in the area gradually increases from east to west, reaching Bide syncline 17~2 1kPa/m in Na coalfield, Qingshan syncline 12~27kPa/m in Liupanshui coalfield, Panguan syncline 2 1~33kPa/m in Liupanshui coalfield and Laochang mining area in eastern Yunnan. The closer to the direction of Kangdian ancient land, the higher the minimum principal stress.
2.2 Influence of Coal Seam Buried Depth on Permeability
The density of rock strata is much higher than that of fluid in pores, which leads to a great increase in vertical stress. Fu et al. (200 1) think that the permeability of coal reservoir decreases exponentially with the increase of buried depth. On the other hand, it also reflects the influence of geostress on the permeability of coal reservoir, that is, with the increase of buried depth, the gravity of overlying strata will enhance the compression of cracks and increase the effective stress, but it is not conducive to the development of cracks in coal reservoir, thus reducing the permeability.
Although the relationship between coal seam permeability and buried depth is discrete, the trend of negative power exponent is very obvious. At the same time, the permeability tends to decrease from east to west under the condition that the buried depth of tested coal seams is similar (500 ~ 700 meters) (Figure 2). The turning depth of the negative exponential relationship between permeability and coal seam buried depth is about 600m, and the corresponding permeability is about 0.05mD. Once the coal seam permeability is lower than 0.05 MD, there is no definite relationship between permeability and buried depth, which shows that the extremely low permeability is not only related to coal seam buried depth, but also related to other factors, and other factors have great influence on coal seam permeability. It is difficult to develop surface coalbed methane, such as Panguan syncline and Hong En syncline in eastern Yunnan. Qingshan syncline shows the opposite trend, but with the increase of buried depth, the permeability of coal seam shows an increasing trend, and the methane content of coal seam in mining area has a certain distribution law on the plane, showing the general trend of "high in the north and low in the south, high in the east and low in the west, deep and shallow" (Peng Lun et al., 20 10). This is because the hydraulic connection between Qingshan syncline area and the outside world is weak, and hydraulic plugging leads to high gas content in coal seam, complete coal structure and good permeability, which has good development potential of coalbed methane.
Fig. 2 Relationship between coal seam permeability and buried depth in Qianxi-East Yunnan region
2.3 the relationship between coal seam permeability and reservoir pressure
When the buried depth of coal seam increases, the vertical geostress leads to the increase of reservoir pressure and the decrease of effective stress, and the elastic expansion of coal body leads to the decrease of fracture width and permeability. There is a negative logarithmic relationship between coal reservoir pressure and coal seam permeability in the study area, which is inevitably related to the control of reservoir pressure by coal seam buried depth. For example, when the reservoir pressure is between 5 and 5-7 MPa, the distribution of coal seam permeability is relatively discrete, and there is no specific trend (Figure 3).
Fig. 3 Relationship between coal seam permeability and coal reservoir pressure in Qianxi-East Yunnan.
Fig. 4 Relationship between coal seam permeability and coal seam thickness in Qianxi area
2.4 the influence of coal seam thickness on permeability
Telly et al. (2000) found that the Carboniferous-Permian coal seam in North China is bounded by the permeability of 0.5mD, and the distribution law of coal seam thickness and permeability presents two opposite trends. When the permeability is less than 0.5mD, the thickness of coal seam increases and the overall permeability increases. When the permeability is greater than 0.5mD, the permeability decreases with the increase of coal thickness.
As far as the coal seam with permeability greater than 0.03mD in Qianxi area is concerned, with the thickening of coal seam, the permeability tends to decrease (Figure 4), which is negatively related to the coal thickness and fracture development density, and the comprehensive effect of various geological factors during peat accumulation period plays an important control role. However, when the permeability is less than 0.03mD, there is a positive correlation between the thickness of coal seam and the permeability, which obviously cannot be explained by the above principle, indicating that other factors play a more important control role, such as coal structure, fracture opening and closing, fracture development density and so on under the control of coal rank and coal rock composition.
2.5 The influence of other factors on permeability
When the permeability is relatively small, the relationship between the buried depth of coal seam, the pressure of coal reservoir and the thickness of coal seam and the permeability is not a simple linear relationship, which shows that the permeability of coal reservoir is also controlled by other factors such as the porosity, fracture structure and coal body structure of coal seam.
From northeast to southwest, the porosity of the study area shows a double-peak feature of first increasing, then decreasing and then increasing. The porosity of coal reservoir is low, and the permeability increases with the increase of porosity. Porosity is significantly affected by regional metamorphism. With the increase of maximum vitrinite reflectance, porosity first increases and then slowly decreases. The development of pores in Panguan syncline coal reservoir is beneficial to the storage and seepage of coalbed methane. Secondly, some reservoirs in Zhina coalfield are well developed, and most coal reservoirs are extremely developed, which is very beneficial to the storage of coalbed methane, but poor pore connectivity is not conducive to the seepage and migration of coalbed methane. Gemu syncline and eastern Yunnan coal reservoir have similar pore development, many pore types, great differences, strong heterogeneity and relatively good reservoir performance, but it is not conducive to coalbed methane seepage migration on the whole.
The coal structure of different coalfields in Guizhou Province is quite different. Generally speaking, the coal structure of Liupanshui coalfield is broken, such as the structural coal of Panguan syncline; The coal body structure in zhina coalfield is relatively complete, such as the primary structure of most coal seams in Shuigonghe syncline. The difference of overall structure is an important reason why the permeability of coal seam in Zhina coalfield is much higher than that in Liupanshui coalfield.
3 Conclusion
To sum up, the coal seam permeability in Qianxi-East Yunnan is affected by many factors, such as tectonic stress, coal seam buried depth, coal reservoir pressure, coal seam thickness and so on, among which tectonic stress is the most important factor affecting coal seam permeability.
The permeability of (1) coal seam increases with the decrease of the minimum principal stress gradient in the local stress field.
(2) The permeability of coal seam in Qianxi-East Yunnan area decreases exponentially with the increase of coal seam buried depth. Affected by this, the relationship between coal reservoir pressure and coal seam permeability is negative logarithmic.
(3) Under the overall control of structural stress on the permeability of coal reservoir, there are many factors such as cracks, reservoir pressure, coal seam thickness and hydrogeological conditions, and other factors play a more important role under similar structural stress conditions.
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