Study on the Brittleness Evaluation in the Naturally Fractured Carbonate Formation and Its Application in the Ordos Basin

Abstract

Rock brittleness is a key factor to influence the fracture behavior in the formation. Therefore, it is important to evaluate the brittleness when doing the hydraulic fracturing. Previous studies provided various methods for rock brittleness evaluation. Few evaluations, however, could be applied for the naturally fractured carbonate formation because those methods did not integrate the influence from lithology, natural fractures and vugs. Hence, this paper indicated an integration evaluation method to investigate the brittleness in the naturally fractured carbonate formation. The rock in this study was from the formation in the Ordos Basin. The brittleness evaluation method asked the experiment studies that included triaxial compression test, continuous strength test, Kaiser test and X-Ray Diffraction analyze. Based on the results, the influence of substrate properties and fractures-vugs in fractured carbonate rock are analyzed. Then a method to evaluate the brittleness of fractured carbonate rock is raised in which the stress-strain curves of rock mechanics tests, geologic microcharacter and the characteristics of fractures are considered. The method can give a better application in Ordos Basin. The results show that the failure mode of fractured carbonate rock under the effect of confining pressure is mainly the shear failure. The facture will have an obvious effect on the strength of rock. The brittleness of fractured carbonate rock appears as the ability for resisting inelastic deformation before rupture and losing rate of bearing capacity after rupture, besides the minerals of rock and the development of fracture will influence the brittleness. With the increasing of confining pressure, the fractures tend to be closed which leads to the increasing of brittleness. However, the carbonate in high confining pressure is characterized by plasticity, the brittleness would reduce. The brittleness was used to design hydraulic fracturing work in the naturally fractured carbonate formation of Ordos Basin. Hydraulic fracturing is necessary to guarantee a successful development in the naturally fractured carbonate formation. Therefore, the brittleness evaluation method is worth to study when designing the hydraulic fracturing.