Impact of Shale Anisotropy on Seismic Wavefield

During unconventional resources exploration, ignoring shale anisotropy may lead to wrong seismic interpretations, thus affecting the accuracy and credibility of sweet spots prediction and reservoir characterization. In order to investigate the impact of shale anisotropy on the seismic wavefield, we propose a quantitative evaluation method by calculating the waveforms’ amplitude and phase deviations. Based on the 3D elastic wave equation and the staggered-grid finite-difference method, the forward modeling theory with the consideration of shale anisotropy is established. Then, we use the envelope misfit (EM) and phase misfit (PM) parameters to illustrate the differences in waveforms’ amplitude and phase morphology, which are caused by anisotropy. Lastly, by comparing the waveforms of the models with/without anisotropy and calculating their EM and PM values, a practical and quantitative evaluation method is constructed. We used synthetic models of different complexity and oilfield models to validate the proposed method. Through the research, we also gained some new insights about the anisotropy’s effects. For a certain medium model, the impact of shale anisotropy on seismic wavefield is complicated and needs specific analysis. The proposed method provides a useful and quantitative tool for the evaluation of shale anisotropy’s impact.

Effect of Shale Anisotropy on Hydration and Its Implications for Water Uptake

Water uptake induced by fluid–rock interaction plays a significant role in the recovery of flowback water during hydraulic fracturing. However, the existing accounts fail to fully acknowledge the significance of shale anisotropy on water uptake typically under in situ reservoir temperature. Thus we investigated the shale-hydration anisotropy using two sets of shale samples from the Longmaxi Formation in Sichuan Basin, China, which are designated to imbibe water parallel and perpendicular to shale bedding planes. All the samples were immersed in distilled water for one to five days at 80 °C or 120 °C. Furthermore, samples’ topographical and elemental variations before and after hydration were quantified using energy-dispersive spectroscopy–field-emission scanning electron microscopy. Our results show that shale anisotropy and imbibition time strongly affect the width of pre-existing micro-fracture in hydrated samples. For imbibition parallel to lamination, the width of pre-existing micro-fracture initially decreases and leads to crack-healing. Subsequently, the crack surfaces slightly collapse and the micro-fracture width is enlarged. In contrast, imbibition perpendicular to lamination does not generate new micro-fracture. Our results imply that during the flowback process of hydraulic fracturing fluid, the shale permeability parallel to bedding planes likely decreases first then increases, thereby promoting the water uptake.

Study of the quantitative effect of the depositional layering tendency of inclusions on the elastic anisotropy of shale based on two-step homogenization

SUMMARY Shale anisotropy is related to numerous small-scale factors, including the transverse isotropy of clay particles, clay shape aspect ratio, shape preference orientation, pore/crack alignment, infilling materials and the depositional distribution and shape aspect ratio of inclusion minerals. Although a depositional layering tendency of inclusions due to sedimentation has been observed, few studies of its effect on shale anisotropy have been carried out. In this work, the effect of the depositional layering tendency of inclusions on the elastic anisotropy of shale is quantitatively analysed along with three other factors (i.e. the inclusion fraction, the orientation dependence of clay elastic properties, where ‘oriented’ refers to transverse isotropic clay and ‘non-oriented’ refers to isotropic clay, and the clay elastic moduli) based on analysis of variance and the two-step homogenization of shale by assuming that shale is a composite of clay and inclusions. The results show that the depositional layering tendency of inclusions of this type of shale has a relatively limited effect on the elastic anisotropy of intact shale, while the orientation dependence of the elastic properties of the clay has a predominant influence.