Evaluation of multi-scale mechanical properties of conglomerate using nanoindentation and homogenization methods: A case study on tight conglomerate reservoirs in southern slope of Mahu sag

Because the unconsolidated cores drilled from some of the tight conglomerate reservoirs in southern slope of Mahu sag indicate a strong water sensitivity, it is difficult to accurately evaluate their rock mechanical properties by using conventional mechanical tests. In this study, a cross-scale evaluation method for the macroscopic rock mechanical properties of conglomerate was proposed. Specifically, the elastic modulus, indentation hardness and fracture toughness of gravel and matrix were characterized at meso-scale through nanoindentation experiments. Afterward, the macroscopic mechanical properties of conglomerate were obtained by applying the homogenization method. Results show that the mechanical responses of the located indentations into gravels indicate a feature of clustering, reflecting the mechanical properties of quartz and feldspar. Different from that, the grid indentation in matrix exhibits complex mechanical responses, which primarily represent the mechanical properties of quartz particles, clay matrix and composite phase. The four-component model of matrix is able to consider the effects of the interface phase, the mineral alteration phase and the substrate-effect phase, such that the simple mechanical responses of the quartz particle and the clay matrix and the complex mechanical responses of the composite phase can be unified. The predicted macroscopic elastic modulus of conglomerate by using the Mori-Tanaka homogenization method tends to be higher than the measured value of rock cores. However, the correction by the deviation factor can give more accurate prediction ranges of the elastic moduli for the conglomerates with variable mineral contents and litho-facies, providing a valuable guidance for the mechanical parameter characterization of tight conglomerate reservoirs in Mahu areas. © 2022, Science Press. All right reserved.