Nuclear Magnetic Resonance in Unconventional Rocks: What Do the Data Tell Us?

Abstract:

Nuclear magnetic resonance (NMR) is a well-known tool to measure porosity and pore size distribution for high-permeability rocks at laboratory and at downhole conditions. In low (nanoDarcy) permeability rocks, although downhole tools lack the resolution to measure small pores, low-field laboratory instruments are able to measure fast relaxation times (down to 40-60 µs) and can be used as to assess their porosity and pore size distributions. However, the full potential of NMR data to measure small, µm-sized pores is undervalued. It remains unclear whether NMR transverse relaxation (T₂) distributions represent the entire pore structure in low-permeability rocks. In this study, we discuss important acquisition parameters for reliable NMR measurement in mudrocks. We present the challenges and limitations of NMR data inversion and interpretation by simulating the echo train data for two synthetic rocks, a sandstone and a mud rock. We show examples of analytical solutions for simplified geometries to investigate the contribution of different relaxation mechanisms on T₂distributions in nanometer-sized pores and to understand NMR diffusion measurements.

Our results for mud rocks show that even data with high signal to noise ratio fail to capture fast relaxation of the hydrogen nuclei in small pores and, consequently, do not measure the multi-modal pore size distributions. NMR data do capture the dominant pore size range through logarithmic average of the time distribution that is independent of signal to noise ratio and inversion algorithm. Analytical solutions of T₂relaxations in mud rocks shows that the response is dominated by the surface relaxation. Diffusion induced relaxation, even at high magnetic susceptibility contrasts, is not significant. Analytical modeling of diffusion coefficient, assuming Gaussian phase distribution, reveals that diffusion of spins in nano-meter pore sizes requires a very large applied gradient for the spins to diffuse. The results of this work lead us to a better understanding of the NMR response in mudrocks and consequently the complex pore structure of low-permeability rocks.