On the interrelationship of electrical and hydraulic conductivities in carbonate rocks

Carbonate reservoir exhibits "low-resistivity and (ultra) low-permeability" anomaly due to complex pore structure. It means that hydraulic conductivity, closely related to permeability ( K ), is not simply equivalent to electrical conductivity in this case. As a result, the conventionally well-established permeability-resistivity relationship does not apply to carbonate reservoirs any longer, and this poses a big challenge for the interpretation and evaluation of carbonate reservoirs. In this study, we found that the relationship between permeability and formation factor is non-monotonic and scattering through direct-current (DC) resistivity and nuclear magnetic resonance (NMR) experiments, and this means that the hydraulic conductivity is not well correlated with the electrical conductivity in carbonate rocks as one expects. There are the two factors, the volume fraction ratio of macro- and micro-pores, and the number of dolomitization-formed micropores, predominantly control hydraulic conductivity and electric conductivity, respectively. Moreover, the non-uniform distribution of intercrystalline pore networks will significantly reduce hydraulic conductivity. To more accurately estimate the permeability of carbonate rocks, the new permeability model based on NMR T 2 spectra rather than formation factor has been proposed. In this model, the characteristic relaxation time ( T 2 c = 83.418 m s ) was defined, which is associated with the macropores that contribute most significantly to permeability and can be used to distinguish between macro- and micro-pores. This model demonstrates a minimal average absolute logarithmic deviation ( d = 0.3667 ) and exhibits generalizability across various carbonate rocks. The study addresses the mechanism analysis of "low-resistivity and (ultra) low-permeability" phenomenon in carbonate reservoir, and has potential to facilitate the interpretation and evaluation of carbonate reservoir. • The ϕNMR_sat is the best choice for discussing the relationship between permeability and resistivity in carbonate rocks. • The hydraulic conductivity is primarily controlled by the porosity and the volume fraction ratio of macro-to micro-pores. • The electrical conductivity is mainly influenced by the number of intercrystalline micropores formed by dolomitization. • The new permeability model based on NMR T 2 spectra has the minimum average absolute logarithmic deviation. • The new permeability prediction model demonstrates generalizability across various types of carbonate rocks.