A new method to evaluate the brittleness for brittle rock using crack initiation stress level from uniaxial stress–strain curves

Abstract

Brittleness has been a prevalent descriptor in rock mass engineering and formation fracturing stimulation. Here a new definition of mechanical brittleness index as crack initiation stress level (σ ci/σ c) is proposed and verified with diorite, granite, marble, sandstone, and shale samples using uniaxial compressive strength (UCS) test. Regression analysis reveals obvious correlation between σ ci/σ c and four brittleness definitions based on UCS and Brazilian tensile strength (BTS) (B19 = σ c/σ t, B20 = (σ c − σ t)/(σ c + σ t), B21 = (σ c × σ t)/2, B22 = (B21)1/2). For diorite, granite, marble, and sandstone, significant relationships exist between B19, B20, and B23; however, for anisotropic shale, obvious relationships were found between B21, B22, and B23. It is suggested that σ ci/σ c can well reflect the heterogeneity and anisotropy of rock and the correlation between B19, B20, B21, B22, and σ ci/σ c depends on the rock structural fabric. In addition, rock with high brittleness generally has a lower σ ci/σ c value and fracture easily occurs during sample deformation. It is not the case that formation with higher brittleness is considered as good fracturing candidates. Fracture pattern was obtained for shale samples from X-ray CT scanning, and the results reveal that fracture density is the maximum for sample with the lowest σ ci/σ c value. The most striking finding is that there exists a good correlation between the stimulated fracture density and σci/σc, and it implies that a good formation for hydraulic fracturing is not of high brittleness. The proposed brittleness index would be helpful to evaluate the formation fracability and screen hydraulic fracturing candidates.