Evaluation method of the micro-occurrence and utilization patterns of gas via a novel method based on nuclear magnetic resonance

Abstract

The micro-occurrence and utilization patterns of coalbed methane and shale gas are complex, which has limited their scientific explorations and exploitation. Based on high-temperature/high-pressure nuclear magnetic resonance (NMR) online detection technology, a T2 cut-off calibration method for adsorbed, pore-confined and free gas was established. The “one-point calibration method” for the NMR T2 cut-off value of the adsorbed gas was established by combining conventional isothermal adsorption and NMR technology, which substantially facilitated and simplified engineering applications. Based on the calibration of T2 cut-off values for different states, the variations in coalbed methane or shale gas under different occurrence states and the impact of this variation on gas production was quantitatively evaluated during production. Coalbed rock samples from the East Mabi block of the Qinshui Basin, Shanxi Province were used to study the aforementioned method. This study shows that approximately 90% of the coalbed methane is in an adsorbed state, and the proportion of adsorbed methane is slightly lower if the samples contain water. In addition, adsorbed gas is the main source of coalbed methane production, followed by pore-confined gas, with only a small contribution from free gas. Approximately 70% of the gas produced from dry coal samples comes from adsorbed gas, and approximately 60% of the gas produced from wet coal samples comes from adsorbed gas. The contribution of pore-confined gas to production is approximately 20%, regardless of the difference in the water content of the samples. At different development stages, different occurrence states vary significantly in their contribution to drainage and recovery. In the early stage, production is mainly from pore-confined gas and free gas, while in the later stage, it is mainly from adsorbed gas. As production proceeds, the output gradually increases. If pressure drops to a certain value, the output gradually decreases, and maximum production is achieved at approximately 3 MPa. This article lays the foundation for more economical and effective exploitation of coalbed methane and shale gas.