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.