Mechanical characterization of shale through instrumented indentation test

Elastic modulus, hardness and brittleness are important mechanical parameters for hydraulic fracturing design. Conventional procedure to obtain these parameters consumes a large amount of rock material and has a difficult sample processing flow. Recently-developed instrumented indentation test is ideal for obtaining the three parameters in one run. However, its applicability to brittle materials needs to be justified, and a specific brittleness quantification method is needed for the characterization of anisotropic and heterogeneous shale. In this work, instrumented indentation test is performed on the shale outcrop from Longmaxi formation to study the cracking characteristics of shale under indentation. The hardness and elastic modulus of shale obtained under different loads are recorded. The results show that shale has different cracking characteristics compared to ceramic and glass. Moreover, both Martens hardness and indentation hardness values of shale decrease as indentation load increases. Thus, only the hardness value obtained at appropriate load can be used to predict proppant embedment depth. In contrary, Indentation modulus is hardly influenced by the test load and comparable to Young's modulus. A brittleness quantification method is proposed based on the cracking characteristics of shale. This method has sound physical basis as it reflects the relationship between the energy used to create new fracture surfaces and the total energy exerted by the indenter. In summary, this paper proposes a comprehensive characterization method for shale in Longmaxi formation, it has the advantages of easy specimen processing, simple experiment procedure and attractive cost.