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State-of-the-Art ion milling ablation applied to shale gas sample preparation

Abstract

Gas shales are a promising unconventional source of hydrocarbons to meet the increasing demand of energy. However, our scientific understanding of shales is still limited. In the literature, many studies suggest that porosity in shale gas rocks is mainly related to intercrystalline matrix, intraparticles and organic matter with pores dimensions in the range of nanometers. Thus, it is necessary the application of high resolution microscopy techniques as FIB-SEM or FESEM, where sample preparation is critical due to: (i) increase the shale rock sample polishing quality in order to be able to visualize clean nanoporosity; (ii) to avoid generation of artificial pores during grinding, and (iii) to preserve the original microstructure of the shale rock sample.

The ion milling technique consists in Ar+ bombardment onto the rock surface to remove material at the atom level. As a test, several shale gas rock samples were selected and prepared using a LEICA EM TIC 3X with triple ion beam available at the Repsol Technology Centre facilities (Mostoles, Spain). The use of a triple ion beam considerably reduces cost, milling time and artifacts generated when using a single ion beam. Once prepared, the selected samples were studied using a FEI ESEM Quanta FEG 650, focusing on the observation of the general abundance of organic matter in the silty mudstone/shale. Organic matter components typically present abundant porosity ranging from 5 to 1000 nanometers, which are especially important because they can absorb gases and store free gases.

Total organic matter pore volume can be calculated from individually-calculated porosity in macerals and total organic carbon (TOC). Considering this, average porosity from the sample image area can be calculated and then it can transformed TOC values from weight percent to volume percent, thus allowing the porosity value to be applied to the total organic matter in the sample.