Available online at www.sciencedirect.com
Procedia Earth and Planetary Science 3 (2011) 210 – 216
2011 Xican International Conference on Fine Geological Exploration and Groundwater & Gas Hazards Control in Coal Mines
Application of AVO Inversion to the Forecast of Coalbed Methane Area Yuan Wang a* , Ruofei Cui a, Shaoqing Zhang a, Fangyao Daia ,Wenhui Jiab a
Coal bed methane resources and accumulation of Ministry of Education Key Laboratory of course, China University of Mining and Technology,Xuzhou 221116, China b China's General Administration of Coal Geology Prospecting Bureau first, China National Administration of Coal Geology,Xingtai 054000, China
Abstract Based on the different elastic parameters and AVO attributes of the different types of coal, seismic attributes can be inversed and geological basis for the delineation of coal bed methane enriched area can be provided by the appropriate AVO formula. In this paper, the theoretical basis and practical process of AVO inversion are introduced. Through the combination of the G value and P value, the 'V value is obtained which is sensitive to lithology variation. The application features of the AVO inversion in the coal bed methane exploration are analyzed.
© 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of China Coal Society Keywords : AVO inversion; Zoeppritz equation; Poisson's ratio; coal reservoir; coalbed methane
1. Introduction Traditional inversion is based on the stack seismic data, the reflection amplitude only considers the case of vertical reflection and loses a lot of pre-stack information. AVO inversion utilizes the pre-stack reflection amplitude variation with offset and considers the reflections with different incident angles,
* Corresponding author. Tel.: +86-516-83590995; fax: +86-516-83590995. E-mail address:
[email protected].
1878–5220 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of China Coal Society doi:10.1016/j.proeps.2011.09.085
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which uses more realistic pre-stack information and researches the lithology properties and physical parameters of the overlying and underlying media of reflection boundaries. Therefore, the detection of oil/gas, coal bed methane and lithology is realized using seismic reflection amplitude information [1]. 2. AVO inversion principle A complete Zoeppritz equation describes the relation between the P-wave and S-wave reflected and transmitted by interface, which is the foundation of AVO theory. In practice, however, the analytic solution of Zoeppritz equation is very complicated and difficult to analyze directly the influence of medium parameters on the reflection coefficients. Many scholars simplified Zoeppritz equation from different aspects to make it more obvious physical meaning. Aki & Richards[2] assumed that the changes of the elastic parameters between the adjacent two mediums are small, and 'VP / VP ǃ 'VS / VS ǃ 'U / U are smaller than the other values. Then, the high order terms of the analytical solution can be omitted, and the reflection coefficient of the P-wave becomes approximately 1§ 'V 'U · § 1 'VP VS2 'VS VS2 'U · 2 1 'VP 2 RP (T) | ¨¨ P ¸¸ ¨¨ 4 2 2 2 ¸¸sin T (tg T sin2 T) 2© VP U ¹ © 2 VP 2 VP VP VS VP U ¹
(1)
where VP , VS and U are the averages of the P-wave velocity, S-wave velocity and density of the media on both sides of the reflecting interface respectively VP1 VP 2 2
VP
,
VS
VS1 VS 2 2
,
U
U1 U 2 2
'VP , 'VS and 'U are the differences of P-wave velocity, S-wave velocity and density of the media on both sides of the reflecting interface respectively 'V P T
V P 2 V P1 ,
'VS
VS 2 VS 1 ,
'U
U 2 U1
is the average of the incidence angle and the transmission angle of P-wave T
T1 T 2 2
There is no S-wave in the first term of the equation (1), that is T P
R P ( 0)
1 § 'V P 'U · ¨ ¸ 2 ¨© V P U ¸¹
0 . Thus
V P 2 V P1 U 2 U1 U 2V P 2 U1V P1 | V P 2 V P1 U 2 U1 U 2V P 2 U1VP1
This is the reflection coefficient of P-wave in the case of vertical incidence, proportional to the impedance difference. There is S-wave information in the second term of the equation (1), which is called as the moderate angle incidence item. When the angle of incidence is 0°< T d 30°, tan 2 T sin 2 T d 0.083 in the third term of the equation (1), and 'VP / VP is smaller and can be omitted. So, the equation (1) becomes V 2 'VS V 2 'U · 2 1 § 'V P 'U · § 1 'V P (2) ¸ sin T ¨¨ ¸¸ ¨¨ 4 S2 2 S2 2 © VP U ¹ © 2 VP V P VS V P U ¸¹ 2 2 If VP /VS 2 , G 1 'VP 4 VS 'VS 2 VS 'U 1 §¨ 'VP 2 'VS 'U ·¸ , the equation (1) is given by 2 2 VP 2 ¨© VP VS U ¸¹ VP VS VP2 U
RP (T ) |
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RP (T ) | P G sin 2 T
(3) 2
Above formula is linear equations of sin T . P is the seismic trace of the zero offset, and means the seismic trace of P-wave superposition, which represents the response on the impedance variation on both sides of the reflected interface. G is called as the gradient stacking trace, which represents the comprehensive response of the S-wave velocity, P-wave velocity and density on both sides of the reflected interface, and is the rate of amplitude variation with the incident angle (or offset). The third term of the equation (1) is called as the wide-angle incident item. When the incident angle is larger ( T ! 30°), tg 2T sin 2 T is quickly increased, and the third term of the equation (1) can not be ignored. In order to link the reflection amplitude with the Poisson ratio, Shuey[3] replaces wave velocity VS with Poisson's ratio V . Then 1 2V VS VP 2(1 V ) So G
'V 1 § 'VP 'U · ¸ ¨¨ 2 S U ¸¹ 2 © VP VS
4 ( P G) 9
'V
P
9 'V 4
V 2 V1
G shows that Poisson's ratio 'V has a great effect on the reflection amplitude variation with the incident angle, when the impedance on both sides of the interface is constant. Variation of 'V is direct proportion to the amplitude variation with the incident angle. 3. Petrophysical basis for AVO inversion
'V is a very important parameter in CBM exploration. Because the coal is relatively soft and easily deformed under the external force, Poisson's ratio of coal is higher than surrounding rocks. Under the action of structure destruction, the primary coal turns into the structural coal, in which P-wave velocity changes little and the S-wave velocity declines greatly, and 'V increases highly. Poisson's ratio of the primary coal can be as low as 0.26, while the structural coal can be as high as 0.45. The possibility of CBM enrichment is different in different types of coal. The possibility of the structural coal, especially with soft layered structure, is higher than that of the primary coal[4 -5]. There are great lithology difference among different types of rocks, thus the lithology differences between the different types of coal and its roof/floor are different, and AVO responses of coal reflections are quite different. AVO analysis techniques can locate CBM enriched area through detecting the structural coal, especially with soft layered structure [7-8]. Table 1 is physical parameters of the coal and its roof/floor in Huainan[6], based on the data in the table, the coal formation model is established, and synthesis AVO records and the negative phase AVO inversion are fulfilled, then P value, G value and 'V value for different types of coal in the Table 2 are obtained. Clearly, the destroy degree of coal structure is greater, the absolute P and 'V values are larger, and G change is not obvious. 4. AVO inversion process AVO inversion based on pre-stack data inverses the variation rate of P, G and 'V values using the amplitude variation versus offset, and predicts CBM using these attributes and their combinations. For desired inversion results, the following steps are needed: (a) high-quality pre-stack AVO preprocessing,
Yuan Wang et al. / Procedia Earth and Planetary Science 3 (2011) 210 – 216
including the true amplitude recovery, surface consistent deconvolution, and fine velocity analysis; (b) accurate logging data, including P-wave velocity, S-wave velocity and density; (c) fine velocity models based on seismic data and controlled by logging data; (d) common angle gather, according to the quality of seismic data, selected as a wider range as possible; (e) fitting the ranges of different angles based on the selected approximate formula . AVO inversion process is shown in Fig 1. Table 1. Physical parameters of the coal and its roof/floor Lithology
Į(m/s)
ȕ(m/s)
ȡ(g/cm3)
ı
primary coalĉ
2400
1259.4
1.500
0.310
primary coalĊ
1960
1090
1.390
0.276
structural coalĉ
1500
681.39
1.350
0.370
structural coalĊ(Soft layer)
650
195.98
1.250
0.450
sandstone
3601
2172
2.562
0.214
. Table 2. Attributes of different kinds of coal Lithology
P
G
Ƹı
primary coalĉ
-0.49395
0.0807722
-0.1818
primary coalĊ
-0.71776
0.104666
-0.26976
structural coalĉ
-0.99246
0.0994263
-0.39295
structureĊ(Soft layer)
-1.04683
0.0814195
-0.42478
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Fig. 1. AVO inversion process
5. AVO inversion case Seismic data in the mining area, Zhangji coalmine, Huainan were processed and interpreted by AVO. 3D data were collected in the observing system with 8 line, 8 shot, 48 receiving points per line, 24 coverage, 20m channel spacing, 40 m shot spacing, 551.5 m maximum offset and 10 mh 10 m CDP grid. In AVO processing, the macro gather is 20 mh20 m and 96 coverage. target coal seam 13-1 is thick and stable. The impedance differences between the target coal and its roof/floor were significant. The coal reflection was outstanding with intensive energy and obvious waveform characteristics, which is reflection T5 on the seismic profiles. After the pre-stack AVO process, the AVO macro gather is shown in Figure 2. The red event is Tq reflection based on the bottom of Quaternary Period, and the blue event is the T5 wave refection based on 13-1 coal. The target layer has been horizontal in common mid-points by the RNMO. A variety of attribute profiles are obtained using AVO inversion. Fig 3 is the P value profile, which is obviously continual in the coal seam. The negative phase of the waveform is negative, and the positive phase shows positive. The target layer traces the negative phase of T5 refection T5, so P value is negative. P value is large compared to surrounding rocks and evident in the seismic profile. Fig 4 is the G value profile, which is worse in continuity compared to the P value. Local anomaly of the G value became distinct, relatively high values showed in the vicinity of well 6-7-6. Fig 5 is the P * G value profile, in which response is obvious near coal seam, and shows the local anomalies, relatively high values showed in the vicinity of well 6-7-6. Fig 6 is the 'V value profile, in which response is obvious in the coal seam compared to the surrounding rocks. 'V changes rapidly at high absolute value.
Fig. 2. The AVO macro gather On the 'V data cube through AVO inversion, a time window is selected below 10ms along T5 reflection T5, and the obtained slice is shown in Figure 7. In this figure, the dark blue area basically lies in faults and at the edge of exploration area. The absolute value increases gradually from gray to green to yellow area, the 'V variation increases gradually, indicating that differences between the coal and its
Yuan Wang et al. / Procedia Earth and Planetary Science 3 (2011) 210 – 216
roof rock became gradually larger. To this end, if the lateral variation in the roof rock is not severe; the yellow areas can be considered as a potential CBM enriched area.
Fig. 3. The intercept (P value) profile
Fig. 5. P * G value profile
Fig. 7.
'V
slice of reflection T5
Fig. 4. The gradient (G value) profile
Fig. 6.
'V
value profile
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6. Conclusions As a new technology of lithology determination in coal exploration, AVO inversion utilizes the prestack information which is ignored in conventional inversion. It can better reflect the lithology variation of coal and provide the basis for location of CBM enriched area. AVO inversion can provide the data cubes of both P values and G values directly, and the data cube of 'V indirectly. With the increase of coal destroy, 'V value increases gradually. CBM enriched area is basically located using 'V value through AVO inversion with the actual data in the mining area, Zhangji coalmine, Huainan. AVO inversion is based on Zoeppritz equation and its approximate equations. But, some assumptions are not satisfied with the conditions of coal seismic survey, for example, that the differences of elastic parameters on both sides of the interface should be small. So, the applicability to the coal seismic surveying will be further verified.
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