Studying Porosity Domains Using NMR Relaxation and Flow Experiments

Abstract:

Many physical, chemical and biological processes are closely tied to the way in which water moves through a porous geological material. The movement of the water impacts the processes, and the processes, in turn, impact the movement of the water. When a range of pore sizes exists, there can be significant variation in the pore-scale velocity of the water, leading to the concept of mobile and less mobile porosity domains. We explore the use of nuclear magnetic resonance (NMR) to characterize porosity domains by combining NMR relaxation experiments with NMR flow experiments. Of interest in our research is developing an understanding of how the pore-scale velocity distribution evolves over time in response to various physical and chemical changes in the porous material. NMR relaxation experiments and flow experiments both involve monitoring the response of the nuclear spins associated with the hydrogen atoms in the pore water. The former involves monitoring the return of the spins to an equilibrium state after perturbation with a secondary magnetic field; the obtained relaxation time distribution is related to the pore size distribution. The latter, NMR flow experiments, can track the movement of the spins in response to a hydraulic pressure gradient, thus monitoring the movement of the pore water. The final result is a distribution of displacements experienced by the spins, taken as directly equivalent to the pore-scale velocity distribution. In our first proof-of-concept experiment, we imaged the velocity distribution in a sample of coarse-grained sand. We found a distribution with a significant tail at high velocities. After introducing fines to the material, we saw the expected shift to lower pore sizes, and a decrease in the measured flux of water through the sample. In the velocity distribution there was a dramatic drop in the occurrence of high velocity values. We conclude that NMR relaxation and flow experiments can be used to study the effects of a range of processes on pore-scale fluid flow.