A Study of Shale Wettability Using NMR Measurements

Shale gas and oil are significant unconventional energy resources in the United States. Because of their low porosity and permeability, as well as the complex pore systems in organic rich shale formations, their petrophysical properties are not fully understood. Further, it is well known that wettability influences many aspects of reservoir characteristics, especially hydrocarbon storage and recovery; however, wettability of organic rich shale is an enigma. In this work, rock wettability of various shale formations, including Eagle Ford, Marcellus and Mancos, was studied using Nuclear Magnetic Resonance (NMR) measurements. A special fluid injection device was developed to study the impact of injected fluids on the shale rock properties. The effect of various fluids was studied by monitoring the changes in NMR response caused by fluid injection. After injecting either water or oil into the shale cores, we observed amplitude increases in the T2 relaxation curve at T2 values much faster than those associated with the bulk fluids. The results provide a positive indication that the fluids were successfully injected into and incorporated into the rock. When water is injected, the amplitudes of the NMR signals representing fluids in the water-wet pores are expected to increase, while signals from 100% oil-wet pores should remain unaffected. Similarly, the amplitudes of the oil-wet pore signals increase when oil is injected. Also, NMR signals with relaxation times slower than several milliseconds are considered to be associated with relatively larger pores and micro-fractures. For Eagle Ford and Mancos shale samples, the experiments showed NMR signal amplitudes increase in the T2 range greater than a few milliseconds when oil and water were injected. This increase can be interpreted as having a mixed-wet condition in the relatively large pores and microfractures. In addition, the injection of oil in the Eagle Ford shale cores did not produce a signal amplitude increase within the relaxation time region below 1 ms; therefore, small pores in these Eagle Ford samples are likely water-wet. On the other hand, our study indicated that the small pores are mixed-wet in the Marcellus shale core samples, since a signal amplitude increase was observed after injecting both oil and water.