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2 Acta Geophysica

2 2019

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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.

3D-RVSP experimental study above a carbonate outcrop for coal resource exploration

Hu Mingshun, Pan Dongming, Li Juanjuan, Zhang Hui, Dong Shouhua, Chen Shenen, Xu Yongzhong

Acta Geophysica

2019

Vol. 67, no. 1

95--107

In the areas where carbonate rocks expose to the near surface, there are several intractable issues in conventional surface seismic, including (1) weak reflection energy, (2) complex wave field and (3) serious static correction. Therefore, the seismic imaging result suffers significantly. However, RVSP is able to achieve reflected data with high quality since it generates seismic waves in borehole and receives seismic waves at the surface. In order to verify the applicability of RVSP technique in complex areas, this study carried out a 3D-RVSP seismic experiment in Wulunshan coal field, southwest China. Compared with the surface seismic data, RVSP data show higher signal-to-noise ratio, wider frequency band and weaker surface wave interference. In addition, two imaging methods (conventional CDP transform stack and novel equivalent-surface conversion) were implemented for RVSP data imaging. The imaging results show that the smaller and deeper structures can be revealed better by equivalent-surface conversion method than by CDP transform stack method. Hence, this study demonstrates that RVSP is an efficient method applied in the area with complex surface condition.