An Experimental Investigation Of Methane In Rock Materials Available to Purchase

High-pressure, nuclear magnetic resonance (NMR), laboratory measurements of methane have been made with the goal of improving the understanding of logging measurements made in gas fields. Preliminary measurements of bulk methane at 2k, 3k, and 4k psi agree with literature values for relaxation time, methane density, and diffusion constant. These measurements demonstrate the utility of the pressure cell, the high-pressure gas lines and, the gradient coils, which had to be built in-house. Measurements on dry sandstones and carbonates pressurized with methane show that surface relaxation dominates the T1 relaxation of the methane, and internal magnetic gradients affect the measured T2 relaxation times. Methane in dry carbonate rock shows weaker surface-induced relaxation and has insignificant effects due to internal gradients when compared to sandstones. In partially water-saturated rock systems, surface-relaxation also dominates the relaxation of methane in both sands and carbonates, contrary to the assumptions usually made for theoretical models of gas/rock systems. Additionally, in sandstones the effect of internal gradients on the methane T2 is important in the partially water-saturated rock. The comparison of measurements with and without applied field gradients illustrates differences between internal and applied gradients. The T2 relaxation due to diffusion of methane in the internal gradients is not described by free diffusion in a linear gradient field. In contrast, in applied linear gradient fields, the gas peak in rock is shown to shift due to diffusion as anticipated when the contributions of bulk relaxation, surface relaxation and diffusion in internal field gradients are removed. Laboratory examples of the Differential Spectrum Method and Shifted Spectrum Method for detecting natural gas are provided. However, because of the surface relaxation component of methane relaxation in rock materials, methane resonances will be found at shorter relaxation times than previous estimates based on applied gradient strengths and bulk methane parameters. Qualitatively, the unexpected shifts can lead to confusion when using the current interpretation models, and quantitatively, attempts to compensate for incomplete polarization using only bulk methane relaxation parameters will lead to overestimates of gas porosity.