Traditional methods of adsorbed gas quantification in coal seam gas (CSG) exploration rely on a stringent coring program. These programs are costly, time consuming and significant uncertainties exist in the free and adsorbed gas estimates, due to lost gas over the time between initial desorption downhole and sealing in pressurised canisters at surface. The gas content uncertainty means it is infeasible to solely rely on such results for resource appraisal; an in-situ gas content measurement is needed. Advanced nuclear magnetic resonance (NMR) measurements provide a suitable alternative to coring and have been used for independent saturation evaluation methods in conventional oil and gas plays for the past 15+ years. However, such NMR measurements have previously lacked sensitivity to adsorbed gas, which has a unique NMR signature.
A novel magnetic resonance pulse sequence has been developed that allows for the in-situ quantification of adsorbed gas concentration in coals. The stimulated echo diffusion editing pulse sequence involves diffusion timing parameters that have been appropriately optimised to ensure sensitivity to the adsorbed gas phase, along with water and free gas phases. The measurement is obtained using a downhole borehole magnetic resonance tool stationed at the coal of interest, with the pulse sequence repeated to achieve the necessary signal-to-noise ratio.
A series of stationary NMR measurements were performed on several coal seams in two core wells and two stratigraphic wells, located in the western Surat Basin, Queensland. Considering the differences in vertical resolution and assumed gas losses from cores there is excellent agreement between the core total gas contents and the NMR results. Furthermore, the downhole NMR measurement provides a smaller vertical resolution (24 cm) that enables improved characterization of gas concentration heterogeneity across coal seams. Moreover, the NMR technique detects free gas that previously has not been measured directly.