(China University of Mining and Technology Xuzhou 22 1008)
About the author: Cui Ruofei, born in 1954, male, born in Luoyang, Henan Province, is a professor and doctoral supervisor of China University of Mining and Technology, and has been engaged in the teaching and scientific research of applied geophysics for a long time. E-mail :rfcui@cumt.edu.cn, mailing address: School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou City, Jiangsu Province, postal code: 22 1008.
Three-dimensional longitudinal wave seismic exploration is one of the key technologies of coalbed methane exploration and development, which belongs to the category of lithologic seismic exploration. Based on the successful experience of oil and gas exploration at home and abroad and the characteristics of coalbed methane exploration, the "two theories and six technologies" are proposed to guide coalbed methane seismic exploration. Two theories are two-phase medium theory and anisotropic medium theory, and six technologies are seismic attribute technology, seismic inversion technology, azimuth AVO technology, azimuth anisotropy technology, nonlinear inversion technology of coal seam thickness and multi-source information prediction technology based on MAPGIS. The prediction model of coalbed methane (gas) enrichment zone is established by using geological means such as coalbed methane seismic exploration technology, which provides scientific geological basis for coalbed methane development.
Coalbed methane exploration; Lithology; Seismic exploration; Seismic inversion azimuth AVO azimuth anisotropy
Seismic exploration technology of coalbed methane
Cui Refei, Chen,,, Li Renhai, Mao Xinrong
(China University of Mining and Technology, Xuzhou 22 1008)
Abstract: 3D P-wave seismic exploration is a lithologic seismic method and one of the key technologies for coalbed methane exploration. According to the successful oil and gas exploration experience at home and abroad and the characteristics of coalbed methane exploration, it is pointed out that seismic exploration of coalbed methane should be guided by two theories and six technologies. These two theories refer to two-phase and anisotropic medium theory. Six technologies include seismic attributes, seismic inversion, azimuth AVO, azimuth anisotropy, nonlinear inversion of coal seam thickness and multi-source information prediction based on MAPGIS. Using seismic exploration of coalbed methane combined with other geological methods, the prediction model of coalbed methane enrichment area is established, which provides scientific geological basis for coalbed methane development.
Keywords: coalbed methane exploration; Lithologic seismic survey; Seismic inversion; Azimuth AVO azimuthal anisotropy
1 the significance of coalbed methane exploration
Coalbed methane (gas) is a mixed gas mainly composed of methane, which is formed by coalification and exists in coal seams. China is a big energy consuming country, so it is of great significance to speed up the exploration and development of coalbed methane.
First of all, as a new clean energy, the development and utilization of coalbed methane can make up for the shortage of conventional energy in China. China is a country rich in coalbed methane resources, ranking second in the world. In recent years, the genesis, reservoir characteristics, occurrence state and accumulation theory of coalbed methane have been systematically studied, and a lot of achievements have been made. However, the corresponding exploration and development technology is relatively backward. Today, geologists and geophysicists have concentrated their research on the field of exploration and development technology.
Secondly, the gas outburst problem is a disastrous problem that has long plagued the safety production of coal mines. According to the statistics of the State Administration of Work Safety, in 2005, there were 2 157 coal mine gas accidents in China, accounting for 36% of all coal mine accidents. In a serious coal mine accident with more than one death 10, gas accidents accounted for 69%. In fact, gas has become the "first killer" of coal mine safety production in China. There are many reasons for this situation, both management reasons and technical reasons. The key point is that the coal mine knows nothing about underground gas enrichment before and during mining. In this way, it is impossible to formulate targeted measures according to the gas distribution in the process of coal mine production and mining.
At present, if Jincheng mining area wants to extract gas, it can only arrange drilling holes evenly according to a certain density, hoping to extract gas before mining in this way. However, this will face a dilemma. If the gas is to be discharged as cleanly as possible, the drilling holes must be arranged quite densely, and the cost will increase. If we want to control the cost, we must reduce the drilling density, which may not ensure that the gas concentration is lower than the safety index, which may easily lead to gas accidents. Therefore, only by relying on scientific and technological progress and adopting new technologies and methods can we find gas-rich areas in coal seams for coal mines, which is an important issue to be solved urgently in current coal mine production.
Finally, the utilization of coalbed methane can effectively protect the ecological environment. The absorption capacity of methane to infrared rays is 25 ~ 30 times that of carbon dioxide, which is one of the culprits of the greenhouse effect. Methane emissions during coal mining account for half of the global total emissions, which shows that the development and utilization of coalbed methane can effectively reduce the greenhouse effect.
In a word, the exploration, development and utilization of coalbed methane can improve China's energy structure, promote coal mine safety production and effectively protect the ecological environment, which is a major event that benefits the country and the people.
2 Key technologies of coalbed methane exploration and development
Nowadays, three-dimensional seismic exploration technology has become an indispensable means in coal mine production, which has largely replaced the traditional geological exploration methods.
At present, the coal field seismic exploration technology mainly uses the kinematic characteristics of reflected waves to solve structural problems, while coalbed methane (gas) seismic exploration belongs to lithologic seismic exploration. Among the five main factors affecting coalbed methane accumulation, the thickness of coal seam, the distribution of faults and other structures, the buried depth of coal seam, the dip angle of coal seam and the outcrop position can be found out by using seismic data and other geological data. However, it is impossible to evaluate the permeability of coal seam and surrounding rock, that is, it is impossible to determine the nature of fractured media (distribution and thickness of structural coal).
As a kind of gas, if gas is to be stored and transported in coal seam, there must be interconnected cracks in coal seam and its roof and floor. In a word, the existence of cracks is a necessary condition for the existence of gas, and it is also the key to find out the gas-rich zone in coal seam. Therefore, it is extremely important for coal mining to study the distribution and connectivity of cracks in coal seam and its roof and floor. The chief culprit of gas outburst explosion is the cracks in coal seam and its roof and floor. Because cracks and fissures are places for gas enrichment, storage and migration, finding out the distribution of faults and fissures in the mining area can make a correct evaluation of the permeability of coal seam and its roof and floor (surrounding rock). Therefore, the core of coalbed methane (gas) seismic exploration is to find out the development direction and density of cracks in coal seam and roof.
As early as 1990s, geophysicists attached great importance to studying the direction and density of fracture development by using seismic data. The main reason is that carbonate rock is a favorable high-yield oil and gas reservoir, and about 60% of the world's oil and gas comes from carbonate rock reservoirs, which are closely related to fractures. A lot of research work and observation data show that the characteristics of fractured media can be explained by two-phase media theory and anisotropic media theory. Therefore, using seismic data to study the anisotropy of two-phase media and detect cracks has become the focus of domestic and foreign scholars. There are three main methods: ① multi-wave and multi-component crack detection technology; ② Shear wave flaw detection technology; ③ Longitudinal wave crack detection technology. Because of the low cost of P-wave seismic exploration, geophysicists have turned their attention to P-wave exploration since 1990s, and it has become an important research topic to use P-wave instead of S-wave/converted wave to detect fractures.
The same is true of coal field seismic exploration. P-wave three-dimensional seismic exploration technology was applied in 1993, and three-dimensional three-component seismic exploration technology was introduced into coal field in 198, and experiments were carried out in many coal mines in 10, hoping to comprehensively use P-wave and converted wave to solve the mining technical conditions in coal mine production. However, contrary to expectations, no breakthrough has been made so far. Looking back today, when analyzing the gains and losses of converted wave seismic exploration in coal fields, we can't ignore the remarkable characteristics of shallow coal seam, high signal-to-noise ratio and high resolution of P wave. The signal-to-noise ratio of converted wave is two orders of magnitude different from that of P wave. Therefore, the exploration and development of coalbed methane should be based on geological means such as three-dimensional longitudinal wave seismic technology.
3 characteristics of coalbed methane seismic exploration technology
The purpose of longitudinal wave seismic exploration of coalbed methane is to study the small-scale compressive and compressive-torsional structures and lithology of coal seam by using the kinematic and dynamic characteristics of seismic waves, especially to find out the direction and density of cracks in coal seam and roof (the degree of structural damage of coal body) and the thickness of structural coal.
Based on the successful experience of oil and gas exploration at home and abroad and the characteristics of coalbed methane exploration, the "two theories and six technologies" are proposed to guide coalbed methane seismic exploration.
Two theories are two-phase medium theory and anisotropic medium theory, and six technologies are seismic attribute technology, seismic inversion technology, azimuth AVO technology, azimuth anisotropy technology, nonlinear inversion technology of coal seam thickness and multi-source information prediction technology based on MAPGIS.
3. 1 seismic attribute technology
Seismic attributes refer to the geometric, kinematic, dynamic and statistical characteristics of seismic waves obtained from pre-stack or post-stack seismic data through mathematical transformation. Seismic attribute technology refers to the technology of extracting, displaying, analyzing and evaluating seismic attributes, including extracting and analyzing seismic attributes, distinguishing structures and lithology by using seismic attributes, and predicting target layers.
CBM reservoir is a typical two-phase medium. Compared with single-phase medium, after seismic wave propagates in two-phase medium, the energy distribution of each frequency component changes, mainly because the seismic wave energy moves to the low frequency direction. This change of dynamic characteristics of seismic wave field provides a theoretical basis for predicting gas-rich layers. Dr. Yang Shuang 'an verified this theory through numerical simulation. Figure 1 is a six-layer medium model, in which the middle of the fourth layer is a two-phase medium, representing a gas-rich area. The synthetic record is shown in Figure 2.
Figure 1 model schematic diagram
Fig. 2 Synthetic seismic records
As can be seen from Figure 2, there are two groups of reflected waves. The reflection wave near 100ms is the reflection wave formed by 1 interface, and the reflection wave near 200ms is the composite wave formed by 2 interfaces, 3 interfaces, 4 interfaces and 5 interfaces. The composite reflected wave of about 200ms is divided into different frequencies, and the energy of different frequency components is obtained. In Figure 3, the middle two-phase medium region shows the following kinematic characteristics: ① Time delay and good continuity of reflected waves; ② Frequency characteristics of low-frequency energy enhancement and high-frequency energy attenuation; (3) Phase characteristics opposite to normal reflected waves. In a word, the gas-rich area with two-phase medium characteristics is obviously different from that with single-phase medium.
Fig. 3(a) Standard low-frequency component energy (1~10hz); (b) Energy of high frequency component (35 ~ 45 Hz) (according to Yang Shuang 'an)
Huainan Zhangji Coal Mine West Third Mining Area 13- 1 energy percentage of main frequency zone of coal seam.
Fig. 4 shows the percentage of energy in the main frequency zone of 13- 1 coal seam in the west third mining area of Zhangji coal mine in Huainan, and the changing law of energy in the main frequency zone can be found.
3.2 Seismic inversion technology
Wave impedance inversion technology is one of the important means of lithologic seismic exploration. According to the favorable condition of high vertical resolution of borehole logging data, the seismic data near the borehole are constrained inversion, and on this basis, the seismic data between boreholes are inverted, and the lithologic changes of coal measures strata on the plane are inferred. In this way, the known logging data with high vertical resolution are linked with the continuously observed seismic data to learn from each other, which greatly improves the vertical and horizontal resolution of 3D seismic data and the exploration and research degree of underground geological conditions.
Through wave impedance inversion, the lithologic characteristics of coal seam and roof and floor can be predicted. Fig. 5 shows the comparison between conventional seismic profile and wave impedance inversion profile of 13- 1 coal in a certain area. By comparison, it is found that Figure 5(b) can clearly show not only the coal seam, but also the lithology of the roof and floor of the coal seam. Therefore, we can invert the azimuth seismic data volume, extract the relevant section attributes from the azimuth inversion data volume and analyze the anisotropy.
Fig. 5 Comparison between conventional seismic profile and wave impedance inversion profile of 13- 1 coal in a certain area.
3.3 azimuth AVO technology
AVO (amplitude versus offset) technology is an important technology to analyze the law of amplitude versus offset on prestack gathers, estimate rock elastic parameters and detect oil and gas by using the principle that reflection coefficient varies with incident angle. Azimuth AVO analysis is a technique that divides macro bins into equal parts in multiple directions, and then carries out AVO analysis in different directions.
The most important factor affecting the variation of reflection amplitude with offset is Poisson's ratio of medium, followed by velocity. Therefore, AVO response is actually a reflection of abnormal Poisson's ratio in formation. Usually, the Poisson's ratio of coal is 0.37 ~ 0.45, and that of gas-bearing sandstone can be reduced to 0. 1. Therefore, we can explore gas reservoirs according to the relationship between CDP gather amplitude and offset. Fig. 6 is a schematic diagram of azimuth AVO analysis, fig. 6(a) is a macro bin azimuth division method, and fig. 6(b) is a macro bin azimuth AVO curve.
Fig. 6 schematic diagram of azimuth AVO analysis
Because the AVO curve can be approximated by the following formula:
AP(θ)=P+G*sin(θ)
Therefore, the AVO curve of each orientation of each macro bin can be fitted with the above formula, that is, the P attribute value and G attribute value of each orientation can be obtained. Similarly, the P value and G value in each direction in each macro bin can be elliptically fitted to calculate the directional anisotropy (Figure 7).
Fig. 7 Azimuth anisotropy of P-wave properties
3.4 Azimuth Anisotropic Technology
Anisotropic media theory can explain the properties of fractured media with cracks, while traditional seismic theory only studies isotropic media.
At present, scholars at home and abroad have proved that reflected longitudinal waves show directional anisotropy in fractured strata through a large number of forward calculations. It is mainly manifested in the variation of amplitude, velocity and travel time difference of prestack P-wave data with offset or azimuth. The research results show that the reflected P-wave is very sensitive to the azimuthal anisotropy of fractured strata, and all P-wave attribute distribution functions are elliptical, as shown in Figure 7. Fig. 8 shows the azimuth CDP gathers of macro warehouse 4 in a certain area. In Figure 8, the macro bin is divided into 18 regions in equal directions, and the trace sets in each direction are arranged in sequence, and the position of the red arrow is the destination layer. From Figure 8, it can be found that the amplitudes of macro-bins in various directions are different, so they are extracted and fitted by ellipses, and the long axis direction of ellipses is taken as the main direction of cracks. In this way, the crack distribution diagram can be obtained, as shown in Figure 9. In fig. 9, the direction of arrow indicates the direction of crack, the length of arrow indicates the density of crack, and the longer the arrow, the more developed the crack. In addition, by analyzing the CDP gather velocity of macro bin in all directions, the relationship between interval velocity and azimuth angle can be obtained, and the fracture distribution diagram can also be fitted.
Fig. 8 CDP gathers of No.4 macro bin azimuth in a certain area.
Fig. 9 Schematic diagram of fracture distribution obtained by using P-wave attribute.
Expand the above viewpoint, study the law of various seismic attributes changing with incident angle, and extract fracture attributes by using the characteristics of seismic attribute parameters changing with azimuth angle, so as to determine the spatial distribution of karst fracture zone. This technique is called azimuthal anisotropy technique.
3.5 Nonlinear Inversion Technology of Coal Seam Thickness
The traditional calculation of coal seam thickness is obtained by comparing and interpolating borehole data. However, in any exploration area, the number of boreholes is limited, so the reliability of the calculated coal thickness is very low. Therefore, many scholars at home and abroad try to obtain coal seam thickness information from continuously observed seismic data, especially high data density 3D seismic data.
A variety of quantitative interpretation methods of coal seam thickness are put forward, the formation mechanism of coal seam reflection wave is discussed theoretically, and the variation law of its seismic characteristics (including waveform, amplitude and frequency) with coal seam thickness is studied, which provides a theoretical basis for predicting coal seam thickness by using seismic attribute parameters of coal seam reflection wave. However, these methods basically only use one seismic attribute parameter, which has certain limitations. They all require that the relationship between the thickness of coal seam and the attribute parameters of coal seam reflection wave is linear within a certain range, that is, they all belong to the linear inversion method of coal seam thickness. However, there is a nonlinear relationship between the attribute parameters of coal seam reflection wave and coal seam thickness. Therefore, it is urgent to establish a nonlinear inversion method of coal seam thickness.
The nonlinear inversion technology of coal seam thickness belongs to statistical analysis method, that is, the statistical relationship between some seismic attribute parameters and thin layer thickness is used to predict the thickness change of structural coal seam. Firstly, the seismic profile is decomposed by spectral decomposition technology to obtain narrow-band frequency profile, then seismic attribute parameters are extracted from low-frequency profile, and finally the coal seam thickness is inversed by artificial neural network.
3.6 Multi-source information prediction technology based on MAPGIS
Because gas enrichment is related to the development degree of cracks, the thickness of coal seam, the structural distribution of faults, the buried depth of coal seam, the dip angle and outcrop position of coal seam, the degree of coalification and other factors. Therefore, all the above factors must be fully considered in order to accurately predict the gas-rich zone in coal seam. It can be found that after extracting the relevant attributes of the above factors, the attribute data of the above factors will be quite large and the relationship will be quite complicated. In order to effectively and reasonably use the attributes of the above factors, GI S is selected as a platform to fuse various attributes and spatial data, generate various thematic maps, and finally establish a reasonable multi-source information fusion method. On this basis, the prediction model of gas-rich zone serving coal mine production is established. Figure 10 shows the multi-source information fusion method and comprehensive analysis process.
Figure 10 Multi-source Information Fusion Method and Comprehensive Analysis
4 conclusion
The overall goal of coalbed methane seismic exploration is to combine geophysical technology, basic geological exploration means, mathematical geological analysis means and geographic information system technology organically and apply them to the prediction and evaluation of coalbed methane (gas) enrichment zone.
The technical features of coalbed methane seismic exploration are as follows:
(1) Combine the two-phase medium theory and elastic wave propagation theory in anisotropic medium with the characteristics of coalfield seismic data;
(2) Using seismic attribute technology, seismic inversion technology, azimuth AVO technology and azimuth anisotropy technology, the azimuth anisotropy characteristics of seismic P wave to fractured strata are extracted, and the fracture attributes of coal seam and surrounding rock are extracted from the characteristics of seismic attribute parameters changing with azimuth;
(3) Using the nonlinear inversion technology of coal seam thickness to obtain the thickness information of structural coal;
(4) Based on GIS, the prediction model of coalbed methane (gas) enrichment area is established after the fusion and comprehensive analysis of multi-source information such as fracture properties of coal seam and surrounding rock, thickness of coal seam, distribution of faults and other structures, buried depth of coal seam, dip angle of coal seam and outcrop position, which provides scientific geological basis for coalbed methane development.
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