Pore Scale Simulation of NMR Response in Porous Media

Rock properties are usually predicted using 3D images of the rock’s microstructure. While single-phase rock properties can be computed directly on these images, twophase properties are usually predicted using networks extracted from these images. To make accurate predictions with networks, they must be topologically similar to the porous medium of interest. In this work, NMR response is simulated using a random walk method. The simulations were performed on 3D images obtained from micro-CT scanning and in topologically equivalent networks extracted from these images using a maximal ball algorithm. A comparison of the NMR simulations on a 3D image and an extracted network helps to ascertain if the network is representative of the underlying 3D image. Single-phase NMR simulations are performed on 3D images and extracted networks for different porous media including sand packs, poorly consolidated sandstones, consolidated sandstones and carbonates, and are compared successfully with experimental measurements. The algorithm developed for the simulation of NMR response in networks was validated using a tuned Berea network that reproduced experimental capillary pressure data in Bentheimer sandstone. Simulation results of the sand packs and poorly consolidated media show that the T2 distributions of the networks are narrower than those of the corresponding micro-CT images and experimental data. This is attributed to the loss of some fine details of the pore structure in the network extraction algorithm. The algorithm developed for singlephase NMR response in networks was extended to two-phase fluids in order to study the effect of wettability on simulated NMR response in networks. While NMR behaviour is influenced pore structure, wettability and phase saturation, it is not possible to determine each of these influences uniquely. The ultimate goal is to have a two-phase simulator that predicts relative permeability, electrical resistivity, NMR response and capillary pressure which will be used to determine the wettability of a porous medium