Surface relaxivities and bulk relaxation times from nuclear magnetic resonance T1-T2 logging in unconventional reservoirs

Abstract

In borehole nuclear magnetic resonance (NMR) logging of unconventional reservoirs, important fundamental parameters such as surface relaxation, bulk fluid relaxation, pore size, and wettability often are ignored because their controls are nontrivial. We developed a data-driven workflow to estimate surface relaxivities and bulk relaxation times from borehole wireline NMR [Formula: see text]-[Formula: see text] measurements for practical modeling of petrophysical properties in unconventional reservoirs. The workflow, in a series of analytical steps, initially implements petrophysical multimodal analysis of [Formula: see text]-[Formula: see text] maps to identify naturally clustered responses of key relaxation effects. Subsequently, it then jointly derives apparent bulk relaxation times ([Formula: see text]) and surface relaxivities, i.e., transverse ([Formula: see text]) and longitudinal ([Formula: see text]), per distinct petrophysically relevant pore and fluid types. The results, and associated uncertainties, in an unconventional field are within ranges of published experiments in the literature for siliciclastic clay- and organic-rich formations. This approach for in situ, in contrast to laboratory, derivations of surface relaxivities in unconventional reservoirs creates novel opportunities to estimate NMR-based pore-size distributions and permeability in the subsurface.