A new permeability calculation method using nuclear magnetic resonance logging based on pore sizes: A case study of bioclastic limestone reservoirs in the A oilfield of the Mid-East

It is difficult to accurately obtain the permeability of complex lithologic reservoirs using conventional methods because they have diverse pore structures and complex seepage mechanisms. Based on in-depth analysis of the limitation of classical nuclear magnetic resonance (NMR) permeability calculation models, and the understanding that the pore structure and porosity are the main controlling factors of permeability, this study provides a new permeability calculation method involving classifying pore sizes by using NMR T2 spectrum first and then calculating permeability of different sizes of pores. Based on this idea, taking the bioclastic limestone reservoir in the A oilfield of Mid-East as an example, the classification criterion of four kinds of pore sizes: coarse, medium, fine and micro throat, was established and transformed into NMR T2 standard based on shapes and turning points of mercury intrusion capillary pressure curves. Then the proportions of the four kinds of pore sizes were obtained precisely based on the NMR logging data. A new NMR permeability calculation model of multicomponent pores combinations was established based on the contributions of pores in different sizes. The new method has been used in different blocks. The results show that the new method is more accurate than the traditional ones.