Borehole measurements of adsorbed gas content in coals using stimulated diffusion nuclear magnetic resonance

Abstract

We present an NMR measurement technique which has the unique ability to determine the in-situ adsorbed gas concentration within coal seams. The measurement of adsorbed gas content is of critical importance for operational safety and commercial resource evaluation in the development of coal mining operations and coal seam gas (CSG) resources. The current measurement technique involves laboratory core analysis, using a canister desorption measurement which is subject to significant measurement uncertainty due to the gas lost between coring and analysis. The NMR measurement technique enables in-situ measurement of fluid content within the coal seams without disturbing the coal seams. The technique utilises the stimulated echo diffusion editing (SEDE) pulse sequence which simultaneously captures relaxation and diffusion behaviour of fluids contained within coal seams. In this work, we have optimised the SEDE pulse sequence by using two diffusion times which are appropriate for capturing the diffusive behaviour of water and adsorbed gas. The measured signal captured by the SEDE sequence is inverted to provide a T2-StimD distribution. The appropriate interpretation of the T2-StimD distribution enables the determination of the free and adsorbed gas concentrations. The adsorbed gas concentration measured using the optimised SEDE sequence shows very good agreement with core canister desorption measurements. The sequence also enables the detection of free gas which is important for understanding the depressurisation of the coal seams and the impact on development strategies.