Mechanical Characterisation of Kerogen in Black Siliceous Shale via Nano-Indentation

摘要

Nowadays multi-fractured horizontal well has been widely applied in shale gas reservoir. Mechanical properties of the source-rock are especially critical for drilling and hydraulic fracturing. Total organic carbon (TOC) content has significant influence on elastic modulus of bulk samples. However, elastic property of kerogen can not be measured by conventional method without isolation. This paper presents an efficient practice to measure kerogen properties within millimeter scale specimen, which gives an insight to the impact of TOC on the mechanical property of shale. In this paper, nano-indentation technique is used to investigate the mechanical property of kerogen in south China shale formation. The experiment consists of indenting the polished shale specimen with a diamond indenter. The tests are performed on the dark amorphous kerogen and matrix separately and the tested specimen is observed using an Scanning Electron Microscope (SEM). We perform nano-indentation tests on areas previously identified as kerogen via SEM, and measure indentation modulus based on load-displacement curves while the pyramidal indenter is pushed into the sample, and subsequently calculate Young's modulus. Then with an assumption of Poisson's ratio of the kerogen, we calculate shear- and bulk-modulus using the previously calculated Young's modulus. Consequently, we compare these parameters obtained by this advanced method to the results from published data in North American shale plays. Results show that Young's modulus of kerogen is in the range of 2.8-15GPa based on indentation modulus. Combining the results of triaxial mechanics test on core plugs with nano-indentation experiment, it is shown that Young's modulus of shale decrease with increasing TOC, Young's modulus of kerogen increase with increasing thermal maturation, TOC content has the strongest impact on Young's modulus. This paper provides an innovative method to estimate mechanical property of kerogen of shales on nano scale, which is beneficial for fracture design.