Fund Project: National 973 Project "Basic Research on Enrichment Mechanism of High Abundance Coal Seam and Improving Mining Efficiency" (2009CB2 19607).
About the author: Chen, male, from Taoyuan, Hunan, 1979, Ph.D., mainly engaged in oil and gas geology and coalbed methane geology research. Address: Langfang Branch of Wanzhuang Coalbed Methane Research Institute, Langfang, Hebei. Tel: 010-692135421379 30613041e-mail: cbmjimcoco @126.com.
(1. Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang 065007;
2. School of Resources and Environmental Engineering, Henan Polytechnic University, Jiaozuo 454000)
Abstract: The deformation of coal is closely related to the permeability of coalbed methane reservoir. It is of great guiding significance to find out the structure of coal-bearing rock mass and quantitatively evaluate the deformation of coal-bearing rock mass for predicting the permeability of coal reservoirs. Through a large number of field observations, combined with indoor scanning electron microscope, optical microscope and atomic force microscope, the deformation characteristics and spatial distribution of coalbed methane reservoirs in the south of Qinshui Basin are studied, and the internal relationship among structural characteristics, coal deformation degree and rock mass structure is discussed, and its causes are revealed. The results show that the macroscopic deformation of coal body in southern Qin is mainly brittle deformation, and most cleavage is filled with calcite, which makes little contribution to reservoir permeability. The deformation of coal and rock mass depends on the strength and structure of rock mass, especially the spatial distribution of soft coal development thickness and proportion is related to strength factor and fractal dimension. At the same time, it is found that the strike of coal-bearing joints and coal seam cracks is dominant, which is consistent with the direction of current principal stress field, and Sitou fault has limited influence on serious deformation of coal body. In the next step of coalbed methane development and construction, we should try to avoid laying wells in areas with strong soft coal development.
Keywords: soft coal brittle deformation intensity factor, coalbed methane rock mass structure
Deformation characteristics and formation mechanism of coal seam in southern Qinshui basin
Chen Zhenhong 1, Wang Yibing 1, Su Xianbo 2
(1. Langfang Branch of Petroleum Exploration and Development Research Institute of China Petroleum and Natural Gas Corporation, Langfang 065007; 2. Institute of Resources and Environmental Engineering; Henan Polytechnic University, Jiaozuo, China 454000)
Abstract: Coal deformation is the key control factor of coal reservoir permeability. Studying the structure of coal and quantitatively evaluating the deformation of coal is an important part of reservoir permeability prediction, which is of great significance. Through a lot of field survey, scanning electron microscope, optical microscope and atomic force microscope, the deformation characteristics, spatial distribution and evolution law of reservoir are studied. Formation mechanism of coalbed methane and its relationship with regional structure; Coal deformation and rock structure. That's all for discussion. The southern part of Qinshui basin is dominated by brittle deformation, with poor fracture permeability and calcium filling. Coal deformation depends on the strength and structure of rock mass. Especially soft coal thickness, soft coal rate and strength factor & fractal dimension. In addition, the strike of coal seam cracks is closely related to the occurrence of coal seam. The joints are mainly in NE-SW direction, which is the main stress field at present, and the debris of Sitou fault affects the deformation of coal. Therefore, in the future development of coalbed methane, we should try to avoid high-strength soft coal areas.
Keywords: coalbed methane; Rock mass construction; Brittle deformation; Power divisor; bituminous coal
introduce
As a special rock with low Young's modulus and high Poisson's ratio, the temperature and pressure required for its ductile deformation are far lower than those of inorganic rocks. It is precisely because of this special deformation behavior of coal that the deformation of coal is closely related to the permeability of coalbed methane reservoir and coal and gas outburst. The deformation of coal and rock mass during geological evolution is controlled by rock mass strength, tectonic stress field, temperature and boundary conditions [1~4]. In the same tectonic stress field, rocks with different lithologic strata or lithologic combinations show different mechanical properties and deformation characteristics of rock mass, that is, the dominant factor controlling the deformation of coal and rock mass in a local range is rock mass structure.
For anthracite reservoirs in southern Qin, cleavage is seriously closed or filled with minerals, and exogenous fractures are the channels for coalbed methane migration and production [5-8]. Exogenous fractures are the result of coal deformation, and fractured coal formed by medium coal deformation is the best permeable reservoir in this area. Therefore, according to the data of exploration and development stage of coalbed methane well, finding out the rock mass structure of coal-bearing rock series and quantitatively evaluating the deformation characteristics of coal rock mass can provide reference for permeability evaluation of coal reservoir, predict the permeability of undeveloped reservoir and provide basis for exploration and development deployment.
Figure 1 development characteristics of coal and rock cleavage in southern Qinshui basin
1 macroscopic deformation characteristics of coal and rock in southern Qinshui basin
There is usually a layer of soft coal with a thickness less than 1m in the lower part of No.3 coal seam of Shanxi Formation in southern Qin, which is mostly scaly mylonite with granular coal locally developed. In some areas, there are lenses of full-thickness mylonite, which are generally less than 20m×50m.
Through the observation of borehole coal core and underground coal wall, combined with logging response, it is found that the macroscopic deformation of coal body is mainly brittle deformation, and its main deformation signs are Griffith fracture in the early stage of cleavage formation (figure 1a) and calcite filling cleavage (figure 1b). The cause of cleavage is very complicated, which is generally considered to be the result of synsedimentary compaction, diagenesis, lateral paleotectonic stress, shrinkage and coalification [9~ 12].
Another sign of brittle deformation in coal is exogenous fracture. When the external cracks are not developed, the coal body maintains its original structure; When exogenous cracks develop, coal is destroyed as broken coal. The core of this kind of coal is often fragments, but the fragments have strength.
2 Micro-deformation characteristics of coal and rock in Qinnan area
With the help of scanning electron microscope, the microscopic pore structure of coal is systematically observed, and it is found that the cleavage of coal and rock is filled with calcite (Figure 2a), or the cleavage is closed (Figure 2b), and the matrix pores (pores) are developed (Figure 2c).
Fig. 2 Microscopic characteristics of coal and rock in Qinnan area (scanning electron microscope)
Under the optical microscope, the signs of brittle deformation of coal are mainly some exogenous cracks (Figure 3).
Fig. 3 Exogenous cracks and reflections of coal and rock in southern Qin under optical microscope, × 15.
Under the scanning electron microscope, the signs of ductile deformation in coal are mainly folds, residual spots and SC structure (Figure 4).
It is worth noting that the identification of brittle-ductile deformation of coal and rock is related to the observation scale, and brittle deformation can still be found in the microscopic view of observed ductile deformation (Figure 5). However, it is difficult to observe ductile deformation under ultra-microscopic conditions.
3 spatial distribution of coal and rock deformation in Qinnan area
The observation of coal core and logging response show that the soft coal in Fan Zhuang block in southern Qin area is generally developed in the lower part of coal seam, separated from the upper hard coal by a gangue layer, with a thickness of 0~ 1. 15m, with an average of 0.7m, accounting for 0~0. 177 and an average of 0.1/kloc-.
The thickness and ratio of soft coal in Guxian area in the north are the highest, especially from G 12-9 to G7- 12. The thickness of soft coal exceeds 1m, and the ratio exceeds 0. 15, and the development degree of soft coal in the east gradually decreases. The main controlling factor of its relative development is the influence of folds, and the development of soft coal basically develops along the axis of anticline. Sitou fault has not seriously affected the coal structure. The thickness and proportion of soft coal in wells G4-7, G2-7 and G2-6 near Sitou fault are not as high as the fold axis, and the NW fold is most closely related to the development of soft coal. Guxian area is the most developed area of soft coal in Fanzhuang block, and the coalbed methane wells distributed in this area are closest to Sitou fault, which shows that Sitou fault has more or less influence on coal deformation.
The development of soft coal in Fanzhuang area is controlled by NW folds, and the thickness is generally less than 1m, and the ratio is mostly below 0. 15. The most developed area is located in the fold axis (F 14- 13, F 13- 14), and the lowest is in the wing (F65438).
Fig. 4 Microscopic signs of coal ductile deformation (SEM)
Fig. 5 ductile-brittle deformation mark (SEM) of coal.
Fig. 6 contour map of soft coal thickness in Fanzhuang block in southern Qin area
Soft coal is the least developed area in Puchi Yuxi, and its thickness is generally less than 1m, mostly below 0.5m, and its ratio is mostly below 0. 1. Soft coal also develops along the northwest fold axis, but the distribution of soft coal is complicated due to the superposition of nearly east-west folds.
Generally speaking, the bituminous coal in Guxian is the most developed in the whole Fanzhuang block, followed by Fanzhuang and Puchi Yuxi. The development degree of soft coal is most closely related to NW-trending folds, mostly located in the axis of folds. Sitou fault has certain influence on coal structure, but it is not serious.
4 Cause analysis
4. 1 Control of rock mass structure on coal and rock deformation
In addition to the boundary fault-Sitou fault, there are three groups of small faults in the study area, which are nearly north-south, nearly east-west and northeast. Folds are very developed and can be roughly divided into two types: northwest and near east-west The formation of these folds is closely related to the rock mass structure. The formation of fold is strictly controlled by the strength and structure of rock mass, and the low strength factor and fractal rock mass which are prone to strong deformation are located in the axis of fold. In Yuxi area of Puchi, the intensity factor is the highest, which absorbs stress through faults and forms dense folds. The folds in Guxian area with the lowest strength factor are not as developed as those in Yuxi area in Puchi, which is the result of soft coal formed in this area due to the shear absorption stress along the coal seam.
Coal seam has lower Young's modulus and higher Poisson's ratio. Compared with other strata, it can reach a deeper deformation degree under the action of relatively low temperature and weak tectonic stress in the statistical interval of coal-bearing rock series. Therefore, the evolution information of tectonic stress field recorded in coal seam is more detailed and comprehensive than its surrounding rock. Comparing the spatial distribution of soft coal thickness and proportion with strength factor, fractal dimension and fold, it is found that:
Ⅰ: Low strength factor and low fractal dimension area with thick formation. The coal-bearing rock series is mainly ductile deformation, which is located in the multi-fold axis, and the deformation degree of coal body is deep. Shear along the coal seam causes ductile deformation of coal body and forms "soft coal" to reduce structural stress.
Ⅱ: The high fractal dimension area with high strength factor and low formation thickness is generally located in the fold axis. The coal-bearing rock series is mainly brittle deformation, and soft coal is not developed.
ⅲ: regionally, Guxian area has the lowest strength factor and fractal dimension, but folds and faults are not developed, and the only way to absorb structural stress is to form soft coal along the shear deformation of coal seam. Therefore, Guxian area is the most developed area of bituminous coal in this area. The intensity factor and fractal dimension are the highest in Yuxi area, Puchi, but the way to absorb structural stress is not to form faults, but densely developed folds, and soft coal is the least developed. Fan Zhuang is somewhere in between.
That is to say, under the premise that the nature and magnitude of tectonic stress are basically the same in a local area, the strength and structure of rock mass determine the deformation of coal rock mass, and coal rock mass in different regions can absorb stress through different deformation methods.
4.2 the influence of stress field on coal and rock deformation
4.2. 1 Features of joint development
Through a large number of field observations on outcrops of coal-bearing measures in Fanzhuang block, it is found that coal-bearing measures in this area, especially fine sandstone of Lower Shihezi Formation and siltstone of Upper Shihezi Formation of Permian, have developed many sets of high-angle yoke shear joints, mainly in NE-SW direction and NW-SE direction, with an average dip angle of 82, and even some joints have a dip angle of 90. Joints extend several centimeters to several meters along the strike, and some reach tens of meters. The joint density varies from 2 to 20/m, and the average density is 10/m. Generally speaking, the joint density in brittle rock stratum is higher than that in ductile rock stratum with the same thickness, and the joint density is directly controlled by the tectonic stress on the rock stratum. In areas where tectonic stress is concentrated, such as fold turning parts and fault zones, the joint density is much higher.
There are many cuts between joints, which reflects the diversity and multi-stage formation of mechanical properties. According to the cutting relationship of joints and the results of matching analysis in stages, it is determined to be four groups of * * * yoke shear joints (Figure 7). * * * The first phase of the yoke shear joint consists of group I and group II, and the acute angle indicates near-SN compression, which is the earliest; The second stage consists of group ⅰ and group ⅲ, and the acute angle indicates NW-SE extrusion. The third stage is composed of group ⅰ and group ⅳ, and the acute angle indicates NNE-SW compression; The fourth stage is composed of group II and group V, and the acute angle indicates the extrusion in the NE-SW direction, and the formation time is the latest.
Fig. 7 Segmented matching of joints
4.2.2 Analysis of tectonic stress field
Based on a large number of field observations, the above-mentioned systematic description of the characteristics of coal-bearing joints, and the previous research results [13~ 16], the tectonic stress field period since Mesozoic has been restored:
The compressive stress field near the north-south direction in Indosinian period (1)
During the Indosinian period, the near-north-south compression led to near-east-west folds, and the extension showed near-north-south normal faults, with small folds and faults. At this time, the Sitou fault has begun to develop.
(2) NW-SE horizontal compressive stress field in the early Yanshanian-Himalayan period.
The NW-SE compressive stress field generally existed in Qinshui Basin during the early Yanshanian-Himalayan period. The whole area became a ne syncline due to compression, and the Sitou normal fault in the west was further strengthened, forming a parallel normal fault near NE and NNE directions.
(3) Near-horizontal compressive stress field of NNE-SSW in the late Himalayan period.
The NNE-SSW compression in the late Himalayan period resulted in a large-scale NW fold superimposed on the NE fold in the early Yanshan-Himalayan period. At this time, Sitou fault gradually changed from extensional to compressive.
(4) Near-horizontal compressive stress field in NE-SW direction in Neotectonic period since Quaternary.
In the neotectonic movement since Quaternary, with the continuous uplift of Huoshan Mountain and Taihang Mountain, the nearly horizontal compressive stress field in the NE-SW direction of Qinshui fault depression formed a small fold in the NW direction, and this tectonic stress field has continued to this day.
The rock joints and coal seams in this area are dominated by NE-SW direction, which is consistent with the current principal stress field.
4.2.3 Influence of stress field on coal and rock deformation
The occurrence of exogenous cracks in coal seam is basically the same as that of joints in upper and lower surrounding rocks. The large fault system in Shanxi Formation 3 coal reservoir has obvious directionality, showing two dominant directions: NE-SW and NW-SE, in which NE-SW direction is more developed. This is basically consistent with the dominant direction of rock joints, and the direction of main cracks in coal seam is also basically consistent with the direction of the maximum principal stress in the current stress field.
This coupling relationship between fracture and stress field leads to the gradual increase of fracture opening and the continuous decrease of fluid pressure in the process of continuous drainage of coalbed methane wells, which further leads to the increase of coal seam permeability with the increase of maximum principal stress difference.
This is one of the main controlling factors for stable and high production of coalbed methane wells in this area.
4.3 The influence of faults on coal deformation
According to the exposure of coalbed methane wells near Sitou fault in Guxian area, it is found that the fault has limited influence on the serious deformation of coal body.
(1) The coalbed methane well near the fault shows that the damage of coal body as soft coal is not serious;
(2) The reservoir pressure of Well Gu 7-9 in the north is 1.5 MPa, and the productivity is 2,700 m3/d, and the surrounding wells are 6- 10, 7- 10, 7- 1, 7-/kloc-0-. However, the productivity of Entity 7-8 and Entity 8-8 is relatively low, less than 400 m3/d. According to the productivity analysis, Sitou fault has an impact on coal deformation. In Guxian area, due to the small fault drop, the influence range is limited, generally not exceeding 100m ... With the increase of fault drop, the influence range will increase;
(3) Sitou fault affects the distribution of tectonic stress field in this area, and then controls the formation and distribution of structures in this area. The influence on coal deformation is that a soft coal belt is formed along the fault, and its width is related to the fault drop. The greater the drop, the greater the width of soft coal, generally not exceeding 500; It gradually transits to the east into a fractured coal distribution area, which is also the area with the best reservoir permeability. It is difficult to determine the width of this area accurately. According to the current test and production data of coalbed methane wells, the width of this zone is about1~ 2 km; Further east, the belt is basically unaffected, and primary structural coal is developed.
conclusion and suggestion
(1) The macroscopic deformation of coal body in Qinnan area is mainly brittle deformation, and cleavage is filled with calcite, which makes little contribution to reservoir permeability. Signs of ductile deformation include folds, residual spots and SC structure.
(2) The strength and structure of rock mass determine the deformation of coal rock mass. The development degree of soft coal is closely related to NW fold, and the spatial distribution of its thickness and ratio is related to strength factor and fractal dimension.
(3) NE-SW is the main trend of coal-bearing series joints and coal seam fractures, which is consistent with the current principal stress field, and Sitou fault has limited influence on the serious deformation of coal body.
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