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Study on transmitted channel wave based, horizontal multilayer 3 D velocity model inversion and quantitative coalbed thickness detection method

Hu Zean, Zhang Pingsong, Ji Guangzhong, Su Xiaoyun

Acta Geophysica

2020

Vol. 68, no. 6

1703--1713

Most methods using transmitted channel wave (TCW) prospecting to quantitatively detect the thickness of coal seams based on the statistic relationship of group velocity in certain wave bands to the thickness of coal seams cannot be applied universally. To establish a universal applicable method, we frst obtained the theoretical dispersion curve of TCW using the generalized refection–transmission coefcient method and the 1-D horizontal multilayer velocity model, performed iteratively match calculation using the inversion model and the genetic algorithm and analyzed the distributive characteristics of shear wave velocity of coal and rock formations at a certain depth. We then obtained the 3-D velocity images of the coal seam working face based on TCW data using the 3-D back-projection technology. According to the changes of shear wave velocity at the coal–rock interface and the rate of inversion velocity change, we further proposed the quantitative discriminant model for coalbed thickness. Based on the model, we quantitatively interpreted the thickness of the coal seam by computing the depths corresponding to the extremes of the positive and negative rate of the shear wave velocity change and obtained the distribution characteristics of the coal thickness in the working surface. To verify the feasibility and validity of the proposed model for coalbed thickness, we conducted a 3-D physical similarity model experiment and subjected the collected two-component TCW data to inversion calculation and compared the obtained coal seam thickness with the known model parameters. Overall, our study achieved the universal 3-D quantitative detection of coalbed thickness and provided technical supports for intelligentized coalbed mining.

Brittleness index analysis of coal samples

Wu Haibo, Zhang Pingsong, Dong Shouhua, Huang Yaping, Zhang Min

Acta Geophysica

2019

Vol. 67, no. 3

789--797

The brittleness index (BI), which serves as a key reference for reservoir fracturing, is also an important quantitative index for the evaluation of coal-bed methane (CBM) reservoirs. To address the lack of research regarding this application of the BI, we measured the ultrasonic wave velocity of 10 coal samples collected from the Qinshui Basin, China. We then calculated the BI in three test directions, i.e., BI(90°), BI(45°), and BI(0°), as well as the BI anisotropy value (ABI) using the dynamic elastic method. Analysis of the calculated results showed that BI(90°) generally had the highest values and that BI(45°) was close to BI(0°). The ABI showed a positive correlation with the dynamic Young’s modulus anisotropy value, dynamic Poisson’s ratio anisotropy value, S-wave velocity anisotropy value, and the ratio of P-wave and S-wave velocity anisotropy values. However, the ABI had an unclear correlation with the P-wave velocity anisotropy value. Further analysis of the correlation between the BI and two other reservoir parameters (coal structure type and fracture development) revealed that samples with high BI values generally corresponded to primary or fragmented types of coal and also had low Poisson’s ratios, which indicates undeveloped fractures, while samples with low BI values corresponded to granulated types of coal and had high Poisson’s ratios, which indicates developed fractures. We investigated these correlations in order to understand the multiparameter constraints and their combined application in brittleness evaluations, which could reduce risk and improve the precision of ideal brittleness identification in CBM reservoirs.