In field drilling, formation rock is saturated and confined. However, investigation into dynamic failure behavior of water saturated rock under confining pressure is still limited. In this study, percussion drilling in borehole was simulated on a pressurized Split Hopkinson Pressure Bar (SHPB). Shale samples with different water content were impacted at different strain rate under confining pressure. The results show that the dynamic compressive strength (DCS) fluctuates with water content under confining pressure. The DCS increases first, then decreases, then increases again, regardless of the strain rate. The shale samples saturated with 35% water have the largest DCS of all. The specific energy assumption (SEA) shares a similar trend with DCS. The interaction of three factors is supposed to account for this variation: the hydration of clay, meniscus effect and Stefan effect. Furthermore, the influence of water content seems to be weakened when the strain rate is increased from 400s−1 to 600s−1. Finally, the variations of DCS and SEA with water content are fitted with sinusoidal curves. The results of this study are expected to reveal the failure behavior and breaking mechanism of formation shale in percussion drilling.

1. INTRODUCTION

A great number of successful field applications of percussion drilling in hard rock layers has been reported these years (Ford et al., 2011; Ziani et al., 2018).

In order to further improve the efficiency of percussion drilling, it is necessary to understand the breaking mechanism of formation rock under dynamic impact.

Since the prototype of Split Hopkinson Pressure Bar (SHPB) firstly proposed by Hopkinson (Hopkinson, 1914), this equipment has been developed for a century by many researchers (Hauser, 1966; Kolsky, 1949; Lindholm, 1964). To date, SHPB is a mature technology to investigate the dynamic properties of various materials, including rocks.

In 1968, Kumar conducted dynamic impact tests on rocks with SHPB for the first time (Kumar, 1968). After that, investigations on the properties of rocks at high strain rate had never stopped. Based on these tests, researchers found that the properties of rock in static, quasi-static and dynamic tests are different.

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