Nuclear Magnetic Resonance (NMR) response of gas in gas shale is different from that of bulk gas, where relaxation is dominated by spin rotation. Shale gas exists as an adsorbed phase on the pore surface and as free gas phase in the pore interior at high pressure. Thus, relaxation properties of gas in gas shale are controlled by the surface relaxation due to heteronuclear dipolar coupling and homonuclear dipolar coupling. This is different from conventional sandstone and carbonate.
NMR responses of methane in Changqing shale core were researched in laboratory using a 4.8MHz independently developed on-line NMR detecting system under high temperature and high pressure. Through a number of experiments, the NMR measuring method of free gas and adsorbed gas was established at high pressure and constant temperature, which could also be used to explore seepage, diffusion, adsorption and desorption rule of shale gas. The measurement method of adsorbed gas saturation could be applied to NMR logging.
This study shows that transverse relaxation time T2 of bulk methane ranges from 193ms to 896ms when the pressures were from 5MPa to 15MPa at a temperature of 28°. The distribution of T2 in the Changqing shale samples is bimodal. Peak 1 represented free gas located at over or equal to 5 miliseconds and Peak 2 represents adsorbed gas at less or equal to 0.5 miliseconds. The adsorbed gas T2 cutoff between free gas and adsorbed gas for this core is 1.3 miliseconds.
Nuclear Magnetic Resonance is used more and more in shale reservoirs. It is different from sandstone and carbonate reservoirs. The majority of dipolar interactions arise from not only heteronuclear dipolar coupling between fluid molecules and paramagnetic impurities, but also homonuclear dipolar coupling between existing fluids and the organic matrix(Washburn et al., 2013).
Ambrose and Rai(2010) studied Micro-Structural Studies of Gas Shales. They found that NMR logging measurements should be able to provide quantitative estimates of the free gas storage and information on the percentage of free gas storage in organic pores. Sigal and Odusina (2011) studied the T2 distributions of methane saturated Barnett shale samples. They found that the amount of methane present in the pores at elevated pressure can be quantified with NMR. Kausik and minh studied characterization of gas dynamics in kerogen nanopores by NMR. They found that relaxation and diffusion properties of gas in gas shales are controlled by the combined effects of adsorption, enhanced surface relaxation, restricted diffusion and molecular exchange between the adsorbed and free phases with the help of 2D NMR experiments. Tinni (2014) researched NMR response of brine, oil and methane in organic rich shales. They found that the NMR response of methane and oil is similar. Washburn(2013) studied updated methodology for nuclear magnetic resonance characterization of shales. The used solid echoes in the measurement of T1 and T1-T2 correlations and found the presence of components undergoing homonuclear dipolar coupling. The studies mentioned above are useful in understanding different aspects of NMR responses in shale. However they did not find a contribution of adsorbed gas to the measured NMR signal. The present study investigates the T2 distribution of methane in saturated shale cores under laboratory condition in order to measure adsorbed gas saturation and free gas saturation.
