Anisotropic rock physics modeling and its application for deep coalbed methane reservoirs

Rock physics modelling, as an important method to quantitatively characterize the properties of oil and gas reservoirs, is seldom applied to coalbed methane reservoirs, and most of the established rock physics models are targeted at shallow and medium coalbed methane reservoirs, which mainly take into account the influence of the storage state of coalbed methane and the horizontal fracture, and neglect the influence of the intrinsic anisotropy of coalbed methane reservoirs. Based on the characteristics of deep coalbed methane reservoirs and microphysical features, focusing on the problems of difficult to characterize the complex pore structure of deep coalbed methane reservoirs and the unknown cause of the intrinsic anisotropy, a set of dual-pore anisotropic rock physics models for deep coalbed methane reservoirs is proposed. The validity and reliability of the model are verified through ultrasonic experimental data and actual well logging data. The microscopic observation results show that the coal body of the deep coal samples in the study area is relatively dense in structure, dominated by circular pores, but still contains a small number of microscopic cracks with random fracture distribution. The organic matter constitutes the rock skeleton of the deep coal samples, which is banded and oriented, and some clay minerals are horizontally oriented in the process of diagenesis, which makes the deep coalbed methane reservoirs show strong intrinsic anisotropy properties. The constructed rock physics model of deep coalbed methane reservoir was applied to a well in a work area in the southeast edge of Ordos Basin, and the inversion of fracture structure parameters and lamination factors as well as the quantitative prediction of velocity anisotropy parameters were realized, and the predicted P- and S-wave velocities matched well with the actual logging data with an error of less than 3%. The fracture porosity of the target reservoir inversion ranges from 0.88% to 3.15%, the fracture aspect ratio is 0.01–0.1, the lamination factor is 1–120, and the predicted velocity anisotropy values are relatively great, which indicates that the target reservoir has strong seismic anisotropy. In the target depth range, the velocity anisotropy parameter is roughly positively correlated with the organic matter and clay content, and negatively correlated with the lamination factor. The results can provide theoretical support for revealing the seismic rock physics mechanism of deep coalbed methane reservoirs and subsequent high-precision prediction of key parameters in the engineering ‘sweet spot’.