Geological controls on artificial fracture networks in continental shale and its fracability evaluation: A case study in the Yanchang Formation, Ordos Basin, China

Abstract

Large scale hydraulic fracturing is the primary industrial technique for obtaining shale gas. Previous studies indicated that research on the fracture network forming principles, geological controls, and fracability of the shale reservoir should be performed before hydraulic fracturing. This paper reports a detailed study of the controlling factors of continental shale reservoirs from the Triassic Yanchang Formation, Ordos Basin, China. First, the fracture network formation mechanisms of hydraulic fracturing are discussed systematically with related schematic diagrams. Second, mineralogical factors, lithomechanics factors, in-situ stress and natural fractures are discussed as the controlling factors of the formation of a complex fracture network according to previous research and field fracturing experience; further analysis is based on these factors. Third, the Young's modulus, Poisson ratio (PR), brittle mineral content, quartz content, clay content, differential horizontal stress, fracture development condition and other factors are normalized and given corresponding weights based on a judgment matrix. Then, a quantitative method (fracability index) is used to characterize the fracability of continental shale based on analytic hierarchy process (AHP) mathematical modeling. Finally, under the control of the same evaluation parameters and weight values, different evaluation indices of continental shale in the Ordos Basin and marine shale in the Sichuan Basin are calculated, and with the help of two indices, the fracability of continental shale is intuitively analyzed. The results show that the continental shale in the Yanchang Formation in the Ordos Basin has good fracability and is suitable for hydraulic fracturing to form a complex fracture network.