The effect of pre-existing microcavities on mechanical rock performance in sedimentary and crystalline rocks

Abstract

Diamond and percussion drill performances, and mechanical rock properties in 11 sedimentary and crystalline rock types are reported. Pre-existing or natural microcavities in sandstones (high porosity materials) and crystalline rocks (low porosity materials) each have a significant and different effect on drilling rates and strength properties in their respective rock types. A gradual change in sedimentary porosity produces a similar gradual change in drilling rates and strength properties. A small change in crystalline rock porosity produces a profound change in strength properties and a lesser effect on drill penetration rates. Brazilian disc fracture modes in the sandstones were predominantly inter-granular and through the phyllosilicate matrix. Crystalline rocks exhibited three fracture modes: inter-granular along grain boundaries, intra-granular along cleavage direction and intra-granular chonchoidal fractures. The mechanical performance of sandstones can be directly related to porosity. The mechanical performance of crystalline rocks cannot be directly related to porosity as measured in this paper. It is assumed other mechanisms control fracture processes in these rock types.