Investigation of Wettability and Fluid Distribution in Organic-rich Mudrocks using NMR Two-Phase Simulation

Interpretation of NMR relaxometry in organic-rich mudrocks still remains a challenge for petrophysicists. A reliable numerical simulation of NMR response in these rocks helps in better understanding of the NMR relaxometry data and the corresponding petrophysical properties (e.g., the wettability and fluid distribution in the pore space). The hydrocarbon-wetting or mixed-wetting characteristics of organic-rich mudrocks have significant impacts on hydrocarbon recovery from the rocks, but quantification of wettability using conventional well-log interpretation methods remains a conundrum. The objectives of this paper are (a) to introduce a NMR two-phase simulation method to model the NMR responses in organic-rich mudrocks and (b) to investigate the effects of wettability alteration and fluid distribution on NMR relaxometry using numerical simulations.

We simultaneously simulate the NMR responses from both water and hydrocarbon phases in organic-rich mudrocks using a random-walk algorithm. The main input to this two-phase NMR simulator is a digital rock matrix reconstructed from 3D pore-scale images of organic-rich mudrock samples, including water, inorganic grains, hydrocarbon, and kerogen. The pore-scale digital images are obtained from FIB-SEM (Focused Ion Beam Scanning Electron Microscope) images. The output of the NMR simulator is the NMR transverse magnetization decay in the rock matrix. The NMR T₂ distribution is estimated by inversion of the simulated magnetization decay using a curvature smoothing method. We also separate the individual T₂ distribution peaks from different pores and compare them against the combined T₂ distribution to demonstrate the reliability of the simulation results.

We successfully simulated NMR T₂ distribution in synthetic and actual organic-rich mudrock samples. We also showed that the alteration of wettability and fluid distribution within the organic-rich mudrock matrix have a measurable impact on NMR T₂ distribution. The introduced simulation method enables further developments on interpretation of NMR measurements in organic-rich mudrocks, which has been challenging in laboratory experiments. The outcomes of this paper can improve the understanding of the parameters affecting NMR relaxometry in organic-rich mudrocks. They also potentially enhance the interpretation of NMR borehole measurements for real-time assessment of porosity, pore-size distribution, and wettability.