Rock physical characterisation of microstructural fabrics and elastic anisotropy for a shale oil reservoir

Abstract

Rock physics models are constructed to describe elastic anisotropy of various fabrics in shales. A method is developed to invert elastic properties of the clay mixture in shales. Inversion results indicate that the clay mixture has abnormally low Vs and consequently leads to a very high Vp/Vs ratio, which is related to the presence of the soft interparticle medium in the clay mixture. Accordingly, a model is constructed to describe anisotropy clay mixture that consists of layering distributed illite-smectite particles, and a more compliant interparticle medium that has a bulk modulus similar to that of water and smaller nonzero shear modulus. Rock physical modelling indicates that an increase in the fraction of the soft interparticle medium (f-soft) leads to a dramatic increase in Vp/Vs ratios of the clay mixture. Meanwhile, the fraction of illite (f-illite) also shows a significant impact on elastic anisotropy of the clay mixture. Accordingly, a rock physical inversion scheme is further proposed to estimate the parameters f-soft and f-illite. Finally, based on these inverted parameters, elastic anisotropy values of various fabrics at different scale, including illite-smectite platelet, clay mixture, solid matrix of shale and shale rock, are computed by the constructed rock physics model. The estimated anisotropy parameters reveal lamination textures and can help in the evaluation of mechanical properties of shale reservoirs. Also, the obtained anisotropy parameters can provide an accurate velocity model for seismic modelling, seismic data processing and inversion.