CBM Reservoir Rock Physics Model and Its Response Characteristic Study

The rock physics model of coalbed methane (CBM) reservoir is significant for the study of CBM content. However, because of the adsorption and dissociation of the CBM reservoir, it is difficult to establish models. In addition, the studies on rock physics modeling of the CBM reservoir are scarce. This paper proposes the basic modeling process. First, the coal rock minerals and methane in adsorbed state are used to calculate elastic parameters of coal rock matrix. Then, the differential equivalent medium model is used to get elastic parameters of the dry rock skeleton. The free methane is mixed with water, and finally the Gassmann equation is applied to obtain elastic parameters of the CBM reservoir model. The study on the CBM reservoir rock physics model's response characteristics has found that there is a sensitive negative correlation between CBM content and P-wave velocity and density. The higher CBM content goes with larger absolute values of intercept, gradient, and seismic amplitude, because their seismic attributes are more sensitive to higher CBM content, whereas the response characteristics are opposite with the lower CBM content. The relationship among the CBM content and absolute values of intercept, gradient, and seismic amplitude in the real data of Qinshui Basin is largely consistent with the response characteristics of the established rock physics model, indicating that the CBM reservoir rock physics model proposed in this paper has a certain feasibility, and the response characteristics of its intercept, gradient, and seismic amplitude are more sensitive to predicting CMB reservoirs.