Seismic Evaluation Method Research On Exploitation Condition Of Coal-bed Methane Reservoir

The coal-bed methane(CBM) is an important component of the unconventional energy resources. Its reasonable exploitation and utilization can not only alleviate the shortage of oil and gas resources as well as decrease the environmental pollution caused by fossil fuels, but also reduce the threat of coal gas outburst in underground mining. However, the problem of “high gas content & low permeability” was detected during exploitation of most CBM reservoirs in China. The insufficient permeability combined with the brittleness due to the reservoir fracturing are the main limiting factors of the CBM exploitation, given the perfect resource conditions. In this study, we use seismic methods in an attempt to evaluate the exploitation potential of CBM reservoirs with respect to these two factors.The permeability of a CBM reservoir depends on complex interactions between multiple parameters. Since it is not feasible to simultaneously evaluate all parameters, our study focused on the analysis of the seismic response of only two closely related factors: fracture density and tectonic stress. First, we collected multigroup coal samples from the survey area, an ultrasonic test was conducted on coal samples under normal temperature and various confining pressures in order to acquire the reference data, which could be used in further modeling and seismic inversion. Comprehensive analysis on the changing rules for the reference data variables was used to reveal the relationship between the coal sample’s anisotropy and fracture characteristics, as well as the different main controlling factors and the changing rules of coal sample’s anisotropy. Thereafter, a HTI equivalent medium model for the CBM reservoir was built to study the connection between the seismic response of velocities, dynamic elastic parameters, anisotropic AVO gradient and the fracture’s porosity and aspect ratio, to provide a theoretical basis for seismic inversion. During the modeling, two features of CBM reservoir, namely, absorbed gas and dual-porosity system, were equivalent calculating by differential equivalent medium theory(DEM) and Mori-Tanaka model respectively.Fracture density and tectonic stress parameters of CBM reservoir were calculated using the seismic inversion. We derived the P-wave reflection coefficient approximate formula directly expressed by the fracture density. Thereafter, the anisotropic AVO gradient, inverted by the pre-stack azimuthal AVA method, could be directly implemented to indicate the fracture density and strike of the CBM reservoir. According to the thin plate theory, the tectonic stress field of the CBM reservoir was calculated by volumetric curvature extracted from post-stack seismic data and dynamic elastic parameters, which in turn were obtained from the pre-stack data inversion. Finally, considering the results on the tectonic stress field and fracture density, the permeability of the CBM reservoir was estimated.The brittleness index of CBM reservoir was calculated by dynamic elastic parameters. We derived the P-wave reflection coefficient approximate formula directly expressed by dynamic elastic parameters, and constructed an inversion frame based on Bayes theorem and on the assessment of pre-stack AVO simultaneous inversion. The dynamic elastic parameters of the CBM reservoir could be inverted, with the brittleness index being calculated in a high precision. Thereafter, referring to the characteristics of CBM reservoir brittleness classified into three classes and their corresponding ranges of dynamic elastic parameters, we could accept the classification of CBM reservoir brittleness reasonably.Finally, the discussed evaluation methods were applied to the case study in Qinshui Basin. Results on the permeability and brittleness of 3# CBM reservoir showed a perfect agreement with the daily gas production data. This finding indicates the feasibility of the proposed evaluation methods, as well as their capability to facilitate the CBM exploitation.