A robust NMR method to measure porosity of low porosity rocks

Abstract

Rocks in low porosity reservoirs contain a considerable number of micro-pores which make the accurate measurement of total porosity more and more difficult. Nuclear magnetic resonance (NMR) is wildly used in petrophysical analyses; however, when using traditional NMR porosity measurement method, it is hard to detect signals of micro-pores due to the limitation of echo spacing. In the current research study, we used a robust NMR method, i.e. magic-sandwich echo (MSE) pulse sequence, to measure the porosities of various types of hydrocarbon-bearing rocks, and compared the results with those of the conventional measurement methods. It is worth mentioning that clay minerals have a great influence on NMR porosity measurement results. Therefore, it was needed to measure the magnetic resonance signal of dry rock to reduce the influences of clay minerals on the rock porosity measurement by NMR approach. After conducting experiments on different low porosity rocks, the results showed that the relative error of our method was 13.50%, which was more accurate than traditional NMR porosity measurement method. In addition, based on porosity deviations of shales and homonuclear dipolar coupling, we put forward a novel method to estimate shale total organic carbon (TOC) content which is a helpful tool for evaluation of unconventional shale reservoirs.