Quantitative relationship between shale NMR transverse relaxation time and pore size distribution and its application

Abstract

Nuclear magnetic resonance （NMR） transverse relaxation time T2 is commonly used to characterize the full-scale pore size distribution characteristics of shale. In order to determine the quantitative relationship between T2 and the pore size of shale， seven shale samples from Shahejie Formation in Jiyang Depression are selected to perform low-temperature nitrogen adsorption and NMR experiments. The surface relaxivity， the critical parameter to calculate the pore size distribution by T2， is obtained according to the equation reflecting the relationship between the logarithmic mean of T2 and specific surface area and pore volume. For these samples， the surface relaxivity ranges from 1.52 nm/ms to 3.06 nm/ms， with an average value of 2.53 nm/ms. The pore size distribution results calculated by surface relaxivity are more similar to the calculation results of the NLDFT model for low-temperature nitrogen adsorption， confirming the rationality of the determination method and values of the shale surface relaxivity. The pore size distribution of typical shale thin layers in Jiyang Depression is determined by the above method. Based on the reservoir properties and geochemical results， it is believed that the argillaceous thin layer in the shale mainly plays the role of generating and storing shale oil， while the fibrous calcite thin layer， powdered calcite thin layer， and silty sand thin layer can be used as the storage and flow channel. It is necessary to analyze the pore size distribution characteristics of different thin layers and their generation， storage， and flow when studying the micro-enrichment and flow mechanism and evaluating the sweet spot of shale oil resources.

Keywords

• shale
• nmr transverse relaxation time
• surface relaxivity
• pore size distribution
• porous structure
• jiyang depression