Experimental and numerical analyses of rock damage behavior and fragmentation mechanism under enlarged hole impact conditions

Abstract

The cluster down-the-hole hammer reverse-circulation drilling technology is an attractive approach for achieving a high rate of penetration (ROP) through the "small hole drilling, large hole enlarged" construction method. However, limited research has been conducted on the mechanism of button rock-breaking mechanism under enlarged impact conditions. In this paper, the rock crater morphology, debris size, and debris removal volume of sandstone, limestone, and granite with different ratio (k) of enlarged-hole to pilot-hole diameter were investigated by means of drop hammer impact test under the impact of a nine-button drill bit. Subsequently, a three-dimensional dynamic damage numerical model of button bit-enlarged hole rock based on the plastic damage model was established to investigate the impact stress distribution, damage evolution characteristics, impact depth and other parameters under enlarged hole impact loading. The laboratory tests showed that the limestone has the largest crater projection area, with a maximum impact depth of 1.396 mm at k = 2 and a maximum volume of rock debris removed of 3.925 cm³; the granite has the largest volume of debris removal at k = 3; the sandstone has a significantly higher depth of fragmentation than the other two types of rock, and has the largest crater size and volume of rock debris removed at k = 2. Numerical simulation indicated that the enlargement condition is more favorable to concentrate the stress around the borehole wall, resulting in rapid rock fragmentation. Under the enlargement condition, the peak impact force gradually increases with increasing k-value, and the impact depth declines with the increase of k-value. The peak impact force at k = 5 is similar to that of the non-enlargement condition, and the peak impact force will be smaller than that of the non-enlargement condition when it is lower than this enlargement ratio. Well wall damage in granite, limestone and sandstone under enlargement conditions (k = 2–3) is significantly less than in non-enlargement conditions and favors fragmentation of the rock while maintaining the stability of the borehole wall. Although larger reaming ratios k are conducive to rock fragmentation, the fracture effect and time consumption of k-values need to be comprehensively accounted for in practical engineering.