Assessing mesoscale permeability in unsaturated, fractured, and karstified carbonate reservoirs: a joint geological, petrophysical, and hydraulic approach for the interpretation of injection tests

In the study of subsurface reservoirs, permeability is a key parameter whose evaluation and extrapolation at the desired scale, such as that of the numerical model mesh, are both a necessity and a difficult task. Relating permeability measurements to the geological characteristics of the rock, at intermediate scales seldom characterized, can help understanding the heterogeneity of the medium and correctly determining the permeability at the desired scale. This concern is particularly important in karst reservoirs, which exhibit highly variable permeability at different scales of observation and from one location to another. Here, we study the petrophysical and geological properties of carbonate facies from the centimeter to the meter scale. Several boreholes a few meters apart were cored and exhaustively described. Petrophysical measurements were made on rock samples, while inter-packer injection tests were undertaken to investigate some meter intervals of the medium surrounding the wells. The results show that the investigated medium is a complex multi-medium with a multi-scale heterogeneity. The detailed geological description allowed explaining the differences between the permeability values at different scales and from one interval to another. Relationships were quantified between the texture of the limestone matrix, the density and aperture of the discontinuities, the permeability of the matrix, and the permeability contrast related to the dual medium at the meter scale. A strong correlation between this permeability contrast and the aperture of the discontinuities, itself more correlated to the texture of the rock than to the facies, is highlighted.