Abstract: The way to evaluate the fracturing characters and brittleness of tight clastic reservoir is of significant value for optimizing hydraulic fracturing design of tight oil reservoir.We performed tri-axial compression experiments on rock samples of various lithologies taken from the tight clastic reservoir in the Shahejie Formation,Liaohe Basin,analyzed the macroscopic fracturing behaviours and the types of microscopic hydraulic fractures.After considering such brittleness evaluation methods as conventional elastic parameter method and mineral composition method,a new method to evaluate rock brittleness was proposed based on the conservation of energy during triaxial compression.The experimental results show that the fracturing characteristics of rocks are controlled by rock brittleness and its confining pressure.On the one hand,rocks with higher brittleness usually develop transgranular tension cracks,and as the brittleness declines,the number of cracks decreases and the cracks are mainly intragranular.On the other hand,the confining pressure greatly refrains the propagation of micro-fractures,since the transgranular tension cracks turn into intragranular cracks as the confining pressure increases.The brittleness obtained with conventional elastic parameter method is just opposite to that obtained through experiments (i.e.the brittleness is high,but the fracturing degree of rock is low),while the mineral composition method cannot be used to differentiate the degrees of rock brittleness under different confining pressures.The new method proposed in this study can describe the complete mechanical evolution process of rocks transforming from super brittleness to ductility.The brittle indexes are correlated not only with pre-peak Young’s modulus but also with post-peak stress gradient(softening modulus).All in all,the method can be used to describe the tight sandstone brittleness under different confining pressures,and the results coincide with the fracturing behaviours of rock obtained through experiments,verifying the feasibility of the method.

Key words: brittleness index, triaxial compression experiment, confining pressure, rock fracturing, tight reservoir, tight oil