STUDY ON FRACTURE TOUGHNESS OF MODE I OF SHALE BASED ON MICRO-MECHANICAL TEST

Fracture toughness of mode I (K_IC) is one of the important mechanical parameters for hydraulic fracturing of shale gas reservoir. Due to the heterogeneity of shale composition, the conventional mechanical measurement has some problems such as large sample volume, discontinuous mechanical interpretation parameters, and low interpretation accuracy. One of the challenges is to obtain the fracture characteristics of shale in time to ensure the safety and efficiency of engineering construction. In this paper, research on fracture toughness of mode I of shale is performed based on micro-indentation. It can be used to study the mechanism of shale micro-crack initiation, development and formation of macro-crack, and to predict the macro-parameters of shale. Based on the analysis of multi-scale composition of shale, the fracture toughness tests with pyramid indenter (Vickers indenter and Berkovich indenter) were performed by micro-indentation. The relationship between residual indentation and indenter was evaluated, and the effect of experimental load on shale micro-fracture was analyzed. The optimization of indenter parameter also was discussed. The fracture toughness of shale is evaluated at meso-scale. The applicability of the micro-indentation test was evaluated, based on a comparative analysis with the results of the Brazil disc test. The results show that t the fluctuation of fracture toughness obtained by micro-indentation is slight when load is within the effective range. When load is too large, the fracture toughness of meso-scale is gradually reduced due to local drop-cuts on the indentation area. The average value of K_IC obtained by micro-indentation is 0.86 MPa\cdot \sqrtm,and the average value obtained by Brazilian disc test is 0.92 MPa\cdot\sqrtm. The heterogeneity of shale composition results in more dispersed meso-mechancial measurement than macroscopic measurement. Micro-indentation test can be used to characterize shale fracture toughness of mode I and perform macroscopic prediction. It provides a new method for effectively solving shale gas hydraulic fracturing.