Multi-Frequency NMR and TGA Characterization of TOC in Shales

摘要

The production of unconventional shale assets depends on an understanding of pore structure,fluid content and distribution within the reservoir rock. The nanometer scale pores and range of hydrocarbon types(mobile vs. immobile hydrocarbons)present in shale reservoirs lead to difficulty in using conventional petrophysical measurements to produce quantitative values for important parameters such as porosity,fluid content or saturation. Many existing techniques can be used to characterize these materials,but it can be useful to combine multiple standard measurements to provide a more quantitative understanding of rock properties. T1-T2 NMR relaxation maps can provide insight into the fluids present in the shales,and measurements of the total organic content(TOC)provide an estimate of the amount of organic material. Since the NMR signal in shales is largely due to the hydrocarbons in place,it is probable that magnetic resonance should provide a direct probe of the amount and type of hydrocarbon types in a shale sample. Further clarification can be provided by the addition of thermal analysis methods combined with mass spectroscopy. In this work,we present experimental results showing not only a direct correlation between TOC and T1/T2 ratio as measured by 2D NMR,but also a frequency dependence of this relationship. We further correlate these results with compositional data provided by thermogravimetric analysis with mass spectroscopy(TGA-MS). Our measurements cover several different shale types and span a wide range of TOC. 2D NMR spectra were obtained at multiple frequencies on crushed shale samples that were exposed to the key temperatures indicated in the TGA results. The results helped to correlate compositional changes to regions of the 2D relaxation maps,potentially providing a method for determining the amount of mobile and immobile hydrocarbons that constitute the total organic carbon content in shale.