Multiscale Reconstructions, Effective Elastic Properties, and Ultrasonic Responses of Kerogen Matter Based on Digital Organic Shales

Organic shales usually present significant heterogeneities in rock textures and reservoir properties due to differing kerogen contents and morphologies, subsequently impacting shale elastic properties and acoustic responses. Numerical upscaling of digital organic shales to evaluate effective elastic properties and acoustic responses has important implications for source rocks and unconventional reservoir characterization. We propose a modeling framework that includes the multiscale reconstruction of kerogen distributions, the numerical modeling of effective elastic properties, and the acoustic response to evaluate the contribution of organic matter. Based on digitized images of the microstructure of Longmaxi black shale samples obtained by X-ray CT, the kerogen components are identified and decomposed into different-level slices in terms of organic matter sizes and morphologies. Multiscale random media reconstruction is applied to these kerogen slices, with synthetic kerogen distributions validated by original counterparts. A finite-element method is used to model the effective elastic properties of digital organic shales, by which we investigate the effect of different kerogen contents and organic matter morphologies. We use a rotated staggered-grid finite-difference method to simulate ultrasonic wave propagation in digital organic shales to evaluate the response of different kerogen contents and organic matter morphologies. Numerical examples show that the multiscale random media method can be applicable to natural organic shales for the reconstruction of kerogen distributions. The elastic properties mainly depend on kerogen contents, with less influence by organic matter morphologies. The ultrasonic scattering effects become stronger for higher kerogen contents with smaller rounding coefficients. Our results confirm the applicability of the proposed modeling framework to support unconventional reservoir characterization. The purpose of this study is to provide the possibility of indicating the sweet point of shale.