Characterization of Gas Dynamics in Kerogen Nanopores by NMR

Abstract

The Nuclear Magnetic Resonance (NMR) response of gas in gas shale nanopores is different from that of bulk gas, where relaxation is dominated by spin rotation and diffusion is unrestricted. Gas shales are characterized by very low porosity and ultra low permeabilities. Their porosity is dominated by nanometer-scale pores in the organic kerogen that restricts diffusional motion, in addition to having very high surface-to-volume ratios that enhance surface relaxation. At high pressure, the gas exists as an adsorbed phase on the pore surface and as free gas phase in the pore interior. Thus, 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. One of the biggest challenges is the understanding of such effects in order to determine the quantity of free and adsorbed gas from NMR data, and to devise novel techniques to log these unconventional plays. Proper estimation of fluid volumes also requires the knowledge of the hydrogen index for the gas restricted in the gas shale nanopores, which is yet another challenge. The NMR responses of methane gas in Haynesville shale plugs cored from a well in East Texas, USA were studied in laboratory experiments using a 2 MHz NMR spectrometer at elevated pressures up to 5 kpsi. The effects of adsorption, surface relaxation and restricted diffusion have been characterized, and the hydrogen index of the gas has been measured. Mineralogy, elemental analysis and Brunauer-Emmett-Teller (BET) experiments have also been carried out on the same plugs to understand the formation characteristics. In the samples studied, faster relaxation modes (few tens of milliseconds) and slower apparent diffusion coefficients (an order of magnitude less than their bulk values) for the confined gas molecules in comparison to their bulk properties have been observed for the first time with the help of 2D-NMR experiments at high pressure. It has been observed that the relaxation spectra for bound water and the gas in the small pores overlap. Additional information is required to resolve these two fluids. Subsequently, the diffusion dimension is investigated to resolve the various fluids in the nanopores. We formulate new relaxation and diffusion models for the interpretation of the dynamics of gas restricted in gas shale and propose that multi-dimensional NMR logging with pulse sequences optimized for gas shales be further tested in the field, to help quantify the total gas in place.

Improved Method for Measuring the Permeability of Nanoporous Material and Its Application to Shale Matrix with Ultra-Low Permeability

“…A shorter T 2 time corresponds to a smaller amount of fluid. Researchers have demonstrated that a T 2 of around 1 ms contains a signal from nanopores in kerogen [35,36,37].…”

NMR T1-T2 Map of Different Hydrogen Contents of Bakken Formation

“…Analyzing the core samples from which the organic matter [24]. The mobility is small for large T1/T2 ratios, whereas for solid protons, it corresponds to reduced molecular mobility.…”

“…Reference [3] and [24] proposed typical T1-T2 map, which distinguishes between different hydrogen content based on their location in T1-T2 map, Fig. 4.…”

Static Solid Relaxation Ordered Spectroscopy: SS-ROSY

“…The ability to characterize these different components in the lab or at the well-site through the application of quick NMR methods would enable the possibility of improving the exploration and field development plans. While multi-dimensional relaxation–diffusion experiments have already found to be useful for the characterization of shale rocks [28,29,30] the challenge for characterizing the solid and viscous components still exists. Recently the application of solid state NMR techniques has proved to be promising for the understanding of these components [31,32,33].…”