Pore Size Distribution from Nuclear Magnetic Resonance: A Comparison Between LWD and Wireline Estimations

Abstract

Abstract Nuclear Magnetic Resonance (NMR) experiments measure the response of the hydrogen nuclei after perturbed by an external magnetic field. This response consists in the relaxation of the total hydrogen magnetization, which is related with the exponential decay occurring in each pore size. Usually, the Laplace Inverse Transform is used to transform this time decay to a T2 distribution, which represents how is the relaxation inside the pore space and is commonly used in the oil and gas industry to get important petrophysical rock properties, such as total and effective porosity, fluid distribution and rock permeability. The estimation of these rock properties is independent of lithology. The mentioned relaxation of the magnetization occurs for three different mechanisms: interaction of the hydrogen nucleus with the pore surface, interaction between the hydrogen nucleus between them and diffusion in a magnetic field gradient. The biggest contribution to the relaxation in porous media is due to interaction with the pore surface. For this reason, the T2 distribution can be related with the pore size distribution inside the rock. This paper presents the results of calculating the pore size distribution from NMR data obtained through Logging While Drilling (LWD) and wireline technologies. It is shown the complete workflow, including the saturation calculation used to extract the hydrocarbon and/or filtrate signals from the T2 distribution. A comparison between both results is performed, showing similarities and differences of both procedures.