Characteristics and the formation mechanism of the heterogeneous microfractures in the tight oil reservoir of Ordos Basin, China

This study aims to investigate the characteristics and the formation mechanism of the heterogeneous microfractures in the upper Triassic Yanchang Formation tight oil reservoir of Ordos Basin, China with some new perspectives. In this paper, we define two sets of parameters describing the size and geometry of microfractures by using Feret diameter calculation and Legendre ellipse fitting. Then the unidirectional fracture zone and multidirectional fracture zone are also be defined. Results of the microfractures characterization show that when the lengths of microfractures are less than 600 mu m (and the widths are less than 90 mu m), the above two set of data show a good linear positive correlation, but when the lengths of microfractures continue to increase, the widths of microfractures begin to deviate from the linear relationship and gradually diverge. This indicates that the lengths and widths of microfractures are not strictly and steadily correlated, but their correlation has a threshold value. If the lengths of microfractures increase due to the change of internal or external stress, when the length value is higher than a certain value, the opening process of microfractures will begin to lose stability, and the widths will change gradually irregularly. Once the compaction process reach a certain extent, some of the microfractures opened previously will be temporarily closed, which will greatly promote the process of pressure-solution, and thus increase the microfracture tortuosity. The length, width, solidity and boundary tortuosity of microfractures could be used to distinguish two zones include the multi-directional fracture zone from the unidirectional fracture zone. The conclusion would provide the important scientific basis for the EOR applications of tight oil.