Prediction of anisotropic coalbed methane reservoir brittleness index based on rock physics modelling

Abstract

The brittleness index is a significant indicator for forecasting the characteristics of coalbed methane reservoirs. However, the brittleness index of coalbed methane reservoirs is susceptible to a number of influencing factors, including the pore structure, coalbed methane content and pore fluid. Moreover, coalbed methane reservoirs are characterized by vertical transverse isotropy anisotropy, which presents a significant challenge to the accurate prediction of brittleness. Accordingly, this paper presents a new rock physics model for coalbed methane reservoirs, based on experimental test data obtained from coal samples. This model considers the vertical transverse isotropy anisotropy characteristics of coalbed methane reservoirs and the influence of pore structure, adsorbed gas and pore fluid in coal. The accuracy of the model is corroborated by the results of the experimental tests. The model predictions indicate that organic matter and clay content exert a greater influence than the stratification indicator factor. The impact of content is smaller than that of pore structure, and the influence of coalbed methane increases following water saturation. The model may be employed to forecast the elastic anisotropy and brittleness index of vertical transverse isotropy-type coalbed methane reservoirs. The inversion results of the logging data indicate that the brittleness index of coalbed methane reservoirs exhibits some variation in different directions and the brittleness index is greater along the bedding direction. The study assists in elucidating the interrelationship between the rock physical parameters of coalbed methane reservoirs and the brittleness index of coal seams, thereby furnishing a basis for anticipating the anisotropic sweet spot in coalbed methane reservoirs.