Abstract
Carbonates (limestones, dolomites) are complex, heterogeneous reservoirs, where traditional logging methods (gamma-ray logging, neutron logging, litho-density logging and acoustics) often poorly determine porosity and, especially, permeability. In these conditions, downhole nuclear magnetic resonance has become a key method for studying fluid saturation and reservoir properties. The article presents the results of research on the use of nuclear magnetic resonance in a well (NMR logging) to assess the reservoir properties of carbonate rocks of various textural and structural organization. The features of the formation of T2 relaxation signals under conditions of multimodal porosity inherent in carbonate reservoirs, including matrix micropores, intercrystalline pores, cavities, and cracks, are considered. The analysis of the influence of mineralogy, surface relaxation, and diffusion processes on the shape of T2 spectra and the determination of fluid mobility parameters is carried out. Based on the comparison of NMR logging data with the results of laboratory core studies and other methods of geophysical well study, the limitations of traditional T2 thresholds and empirical permeability models (Timur-Coates, SDR) in the interpretation of carbonate sections are shown. Improved approaches to the isolation of free and bound fluids are proposed, including adaptive T2 thresholds and combined interpretation schemes. The results obtained demonstrate the high potential of NMR logging to improve the accuracy of porosity, fluid mobility, and permeability estimates, as well as to identify productive intervals in complex carbonate reservoirs.