Determining the Occurrence of Oil in Micro/Nanopores of Tight Sand: A New Approach Using Environmental Scanning Electron Microscopy Combined with Energy-Dispersive Spectrometry

摘要

The use of fluorescence slices and other regular methods tend to be limited in resolution as a result of optical microscopy issues when they are applied to characterize oil occurrences in tight sand microanopores. To address this, an experimental method that combines the use of environmental scanning electron microscopy (ESEM) and energy-dispersive spectrometry (EDS) is proposed in this paper on the basis of a large number of experiments; oil was observed during these tests using ESEM, and the carbon (C) content was qualitatively evaluated using EDS. Data show that the most appropriate experimental parameters were a sample room pressure of 10 Pa, a working distance of 5 mm, a working voltage of 15 kV, and an electronic beam spot size of 4.5 nm. The experimental analysis of six samples from five wells within the Songliao and Sichuan Basins, China, reveals that oil mainly occurs in the form of oil films and oil droplets within micro-intergranular seams, microano-intergranular pores, and nano-intragranular pores. Observations aslo show that oil films are found mainly within micro-intergranular seams and micro-intergranular pores, while oil droplets, in contrast, are mainly found within nano-intragranular pores. Data show that the occurrence space occupied by oil films is relatively larger, having planar dimensions between 200 nm and 10 mu m by between 1 and 10 mu m. The oil films adhere to the pores just like bonding with the pores. The content of the C element in the oil films is 50-90%. In contrast, the occurrence space occupied by oil droplets is relatively smaller, with intragranular pore plane dimensions predominantly between 200 and 1000 nm by between 200 and 1000 nm. Data also show that oil droplets are controlled by the shape of intragranular pores and occur at scales between 200 and 1000 nm by between 200 and 1000 nm. Thus, although the occurrence space occupied by oil droplets is smaller than that of oil films, the development of numerous intragranular dissolved pores provides the necessary room for oil droplets to occur. The EDS data of these droplets show that the content of the C element of the oil droplets in intragranular pores is relatively small, mainly concentrated between 15 and 30%.