ABSTRACT:

Low Rate Of Penetration (ROP) is one of the key technical difficulties in the anisotropic shales reservoir. Rock fragmentation efficiency is improved under coupled dynamic-static load. A new model for estimating the indentation fragmentation process of shale is developed. The damage and failure behaviors of rocks are simulated by using the explicit contact finite element method. According to the macro indentation experiment, a simulation model of rock fragmentation is verified. Rock fragmentation process is studied under three types of load conditions, including static load, dynamic load and coupled dynamic-static load. The results show that rock failure occurs along with the direction of maximum principal stress, mainly depending on tensile and shear stress, followed by the compressive stress. With the increase of static load, indentation depth increases, and the volume of fractured rock is enlarged as well, but there is a non-linear relationship between the static pressure and fragmentation volume. With the increase of dynamic load velocity, the transverse transmission range of stress wave get smaller, the longitudinal transmission depth of stress wave increases, and the transmission depth of the impact energy is improved. When the peak value of load is the same, energy density under dynamic-static combination load is higher than that under static load condition, and rock fragmentation efficiency is also improved significantly, which further explains the indentation fragmentation mechanism of shale. The mechanical behaviors of indentation fragmentation under complex dynamic-static loads conditions should be considered in the design of ROP optimization and drilling tools.

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