On the Characterization of Porosity and Permeability in Limestones

About half of all producing oil reservoir are limestones and dolomites. While porosity and some other physical properties are routinely evaluated from logging, measuring in situ permeability is usually not workable at low cost. Furthermore, it is well admitted that permeability is a scale-dependent property, so that its measurement on core plugs cannot be directly used for assessing reservoir scale permeability. The ability to estimate the permeability of a reservoir rock from other more readily measurable parameters would be of great value to the oil industry. Empirical permeability models such as the Kozeny-Carman equations that make use of the porosity and a specific surface parameter or Katz-Thomson equations relating permeability to porosity and the electrical formation factor are usually used for sandstone reservoirs. However, such models are insufficiently accurate for limestone reservoir characterization purposes because of the complex pore and pore-network structure of these rocks. Indeed, the pore network structure, porosity and permeability of limestone reservoirs as they are observed on cores are the result of complex mass transfer mechanism during deposition, diagenesis and hydro-chemical alteration periods. The effects of the hydro-chemical alteration processes, mainly dissolution and precipitation, are probably the most difficult to assess because of large variability of the forcing mechanisms and the intrinsic mineralogical and structural heterogeneity of the rock. For the same type of limestone, distinctly different pore network morphologies and hydrodynamic properties are usually observed at less than meter scale. Moreover, it is often observed that post-depositional hydro-chemical alteration processes act as increasing irreversibly the heterogeneity due feedback effects of the flow, transport and reactions.