Wettability characterization of low-permeability reservoirs using nuclear magnetic resonance: An experimental study

Influence of wettability to petrophysical responses in low permeability reservoirs is increasingly evident, but research of nuclear magnetic resonance (NMR) technology on wettability calibration in such reservoirs is very limited even though it has been used promisingly. The challenge is that the empirical interpretation model only applies to simple pore structure. It is urgent to develop a relatively simple, quick, and practicable model for wettability evaluation in low permeable reservoirs. This paper presents an experimental NMR study, integrating with Amott tests, X-ray diffraction, and scanning electron microscope (SEM) measurements, on wettability characterization of five typical core samples from different low-permeability reservoirs. NMR relaxation mechanisms and the effect of the aging were analyzed in detail. The study showed that all the core samples had a shift of T2 (spin-spin relaxation time) spectra between before and after aging. According to the shift characteristics, the core samples were divided into two types. The first type is that the wettability alteration relied on the aging and it displaced crude oil into original water-wet movable pore spaces to restore reservoir wettability. This type of formation is generated in distant-source reservoirs, which mainly involves fluid transport through buoyancy. Another type is that the wettability alteration was attributed to the natural oil-wet property of the rock matrix. This formation is mainly found in inner-source or near-source reservoirs, which may generate abundant oil-wet materials. Thus, the wettability alteration is the result of hydrocarbon accumulation modes. Based on the observations, a potential method for obtaining the wettability index was proposed. The method uses both the ratio of oil-wetted to water-wetted pore surface areas and diffusion-relaxation diagrams. The estimated results corresponded reasonably to the independent Amott tests. This study demonstrates that NMR relaxation is an effective solution to analyze and quantify wettability in low permeability reservoirs.