High-Field Nuclear Magnetic Resonance Observation of Gas Shale Fracturing by Methane Gas

Abstract

We monitor the fracturing of gas shale using high-pressure methane gas, by studying the changes in gas transport using high-field nuclear magnetic resonance (NMR). This helps us understand the fundamental relation between the newly created pathways and the enhanced gas transport. The ability to make such correlations is challenging, partially because of the difficulty in monitoring the gas transport during the transient fracturing process. Here, we demonstrate a methodology for fracturing gas shale core samples inside a high-pressure, high-field NMR sample tube and studying its effect on gas transport kinetics by tracking the time-dependent NMR signal intensity. The ultralow permeability of shale makes the transient gas transport slow enough to be monitored by a series of NMR signals. The time constant that characterizes the transient process toward equilibrium is directly related to the permeability of core samples. The signatures of permanent fractures in the shale core plugs created by a sudden pressure release are identified by the shorter equilibrium time constant. Although these permanent fractures are not visible to the naked eye, they are ex-situ-verified by microcomputed tomography (microCT). These results demonstrate a NMR methodology to characterize the gas transport and fracturing property of gas shale, promising a better understanding of their relationships.

Methane Storage in Nanoporous Media as Observed via High-Field NMR Relaxometry

“…However, at pressure values of P > 39. 6 MPa we observe that the quantity N excess starts to slowly increase again as shown in the inset of Fig. 10.…”

“…For these reasons, NMR has become an invaluable tool, especially in oil and gas exploration applications where it is routinely applied for downhole logging and laboratory rock core analysis. NMR has also found broader applications in energy research including investigations of methane gas in shale rocks [6][7][8] and hydrogen storage in carbon nanotubes and other nanoporous materials [9,10].…”

Responses of chemically active and naturally fractured shale under time‐dependent mechanical loading and ionic solution exposure

“…This assumption is reasonable because shale has ultralow permeability usually in the range of nD . Many other authors also make the same assumption of quasi‐static state for shale, for example, previous studies . Body force is also ignored in this paper.…”

“…82,83 Many other authors also make the same assumption of quasi-static state for shale, for example, previous studies. 84,85 Body force is also ignored in this paper. Therefore, the terms with body force (ρ g !…”

NMR spin-rotation relaxation and diffusion of methane

“…This is in stark contrast to the 1 H-1 H dipole-dipole relaxation T 1RT which increases with T , for all hydrocarbons, including methane. Also of interest recently is the influence of pore confinement on the NMR response of methane [25][26][27][28][29][30][31][32][33][34], which has practical applications for characterizing the light hydrocarbons in the organic nano-pores of kerogen and bitumen in organic-rich shale. One of the current mysteries is why the T 1S /T 2S ratio for surface-relaxation of methane in organic-shale is typically T 1S /T 2S 2, while for higherorder alkanes it is typically higher T 1S /T 2S 4.…”