A new application of NMR in characterization of multiphase methane and adsorption capacity of shale

Abstract

Adsorbed and free methane are the primary contributor for gas production from shale. To accurately quantify the multiphase methane in shale is of great significance for shale gas exploration and exploitation. The conventional calculation methods to quantify the multiphase methane in shale are typically mathematical extrapolation from reservoir information, which tend to generate large error and is hard to be applied for shale reservoir because of its heterogeneity characteristics. Meanwhile, the most commonly used volumetric or manometric experimental methods are tedious, time-consuming and indirect measurement that requires assuming adsorbed phase density. Any volumetric effect or impurities can distort the shape of the isotherm measured with volumetric or manometric method. Therefore, it is necessary to explore a new quantitative method that can directly detect the multiphase methane in shale. In this study, several special experimental setups and methods were designed to evaluate the multiphase methane in shale. By introducing the low-field nuclear magnetic resonance (NMR) into the isothermal methane adsorption experiments of shale samples, this study developed a methodology for quantitative identification of multiphase methane gases in shale. Results show that the transverse relaxation time (T2) spectra of methane in shale contain four distinctive peaks, among which the peak (P1) with shortest T2 (<~1 ms) represents adsorbed methane and the one (P2) with longer T2 (~ 1–50 ms) symbolizes free methane in shale. The P2 increased linearly with methane pressure, whereas the P1 increased with increasing pressure to a very limited extent and reached a limiting value, similar to Langmuir equation. The provided method can directly measure the absolute and relative amounts of free and adsorbed methane gases in shale at a certain pressure, which is applicable to measure the in-situ gas content of the pressure coring shale samples, or alternatively, to theoretically estimate maximum adsorbed and free gas content when only the conventional core sample is available for analysis. This study also reports a newly developed technique that directly measures methane adsorption capacity and adsorption isotherm of shale, without the application of the equation of state for methane or volumetric corrections of any kind. The results show that the proposed method that is based on analysis of NMR relaxation time of methane in shale with respect to increasing pressure, is in excellent agreement with the conventional gravimetric method performed paralleled with the NMR method using the same experimental conditions. The developed theory and analytical technique to determine the multiphase methane and methane adsorption isotherm of shale provide an alternative method for in-situ gas content test and monitoring gas transport processes in shale such as desorption, diffusion and seepage.