Determination of porosity types from NMR data and their relationship to porosity types derived from thin section

Abstract

Pore size frequency distributions can be derived from Nuclear Magnetic Resonance (NMR) longitudinal relaxation measurements ( T 1) on water saturated plugs. This paper discusses the cross validation of NMR T 1 relaxation distributions of porosity and how they are related to distributions obtained from image analysis of porosity in thin section. Both sets of distributions are polymodal and decomposition of each yields porosity types. Each pore type represents a subdistribution of pores with a characteristic size and shape. The mean T 1 values associated with the NMR pore types are linearly related to the mean size of pore types determined from image analysis. The constant of proportionality of this relationship represents the surface relaxivity (ρ), a parameter that represents the enhancement of relaxation caused by nuclei interacting with the pore wall. The major difference between image analysis and NMR methods is that NMR can resolve much smaller pores. The relationships between pore types and throat sizes using NMR generated pore types are the same as that using image analysis generated pore types. The existence of a strong relationship between pore type and throat size ensures the relevance of NMR data in studying the properties associated with single and multiphase flow.