The effect of shale properties on the anisotropic brittleness criterion index from laboratory study

Abstract

Shale anisotropy has a significant impact on the brittleness index and hydraulic fracturing. First of all, we improve the Rickman's brittleness index by introducing the anisotropy parameters and angle parameters based on Rickman's brittleness index (BI). Then the anisotropic brittleness criterion index (BCI) is proposed by introducing failure criteria parameters to balance the shortcomings of the conventional BI index. The new brittleness index is applied to the laboratory measurements of samples of Santa Maria Basin shale formation. The anisotropic BCI might be closer to the actual value of brittleness in the fracturing process by considering the stress of crack expansion. The value of the BCI increases as porosity decreases, thus the porosity log is an effective parameter and can help us establish a good BI or BCI predication rock models. We show that total organic carbon (TOC) is not reliable for predicting BI or BCI because there is no linear relationship between BI, BCI and TOC. It is interesting that delta (δ) is more sensitive to BCI than epsilon (ε) and gamma (γ). It appears to have a good linear relationship with the BCI. It might be reliable to predict the BCI of shale formation of the Santa Maria Basin by δ. By introducing the relationship between pressure and BCI, we find that shale samples with different TOC show regular changes of brittleness index under different pressures. The effect of stress needs to be considered in the analysis of BCI and one must choose the appropriate stress for fracturing.