Assessment of the elastic response of shale using multiscale mechanical testing and homogenisation

Abstract

Robust geomechanical characterisation of shale reservoirs is necessary for safe and economic resource exploitation but there is still a lack of mechanical data on well-characterised shale, partly due to the difficulties of obtaining high quality core samples for laboratory testing. The composition of shale also presents challenges when attempting to constrain the mechanical response. Multi-scale homogenisation techniques have recently been used to predict the macroscopic behaviour of shales based on quantitative mineralogical descriptions. However, there is a considerable amount of uncertainty associated with some key inputs into these homogenisation schemes. In particular, the organic matter of shale encompasses a range of scales, from nanometre to micrometre-size material, and its mechanical properties are not well understood. Here, PeakForce Quantitative Nanomechanical Mapping (PF-QNM), a recently developed form of atomic force microscopy (AFM), is combined with nanoindentation testing to characterise the mechanical response of the organic matter and clay phases of Posidonia shale from north-west Germany. The nanoscale testing revealed a clear peak in the histograms of the reduced elastic modulus, which can be attributed to kerogen in the shale matrix. Upscaling of the mechanical properties through homogenisation showed a reasonable prediction when compared with experimental data, including capturing the inherent anisotropy of the shale response. The influence of factors such as the volume fraction of silt inclusions and the applicability of different homogenisation formulations warrant further investigation.