The influence of asphaltene on the interpretation of NMR measurement

Abstract

In the middle south of Sichuan province in China, there are varying degrees of asphaltene found in the marine carbonate reservoir rock of the Gaoshiti-Moxi Longwangmiao formation. This study verifies the effect of asphaltene on the interpretation of nuclear magnetic measurements, and results are reported with 65 samples from the Longwangmiao formation. Furthermore, an additional 48 samples from the same well block in the shallower Xixia formation and the deeper Dengying formation are used in the experiment to study the influence of a different evolutionary degree of asphaltene on the nuclear magnetic signal. The experimental results show that the asphaltene produces a certain amount of nuclear magnetic signals that impact the calculation of the NMR porosity, the bound water saturation, and the T2 cutoff values. Reservoirs at different depths have differing asphalt contents and differing degrees of carbonization due to thermal evolution. At the real reservoir temperature, the asphaltene T2 spectrum form becomes much wider and slower. The effects of asphaltic magnetic signals on the calculation of porosity and the T2 cutoff value are much weaker in the deep Dengying formation than they are in the shallow Xixia formation. In order to determine the bound water T2 cutoff, the asphalt T2 cutoff is introduced to account for the asphaltene effect in NMR measurement. The results also show that the calibrated NMR porosity, the bound water saturation, and the obtained T2 cutoff are closer to the actual values. Asphalting could affect NMR measurement and other parameters interpretation. The method mentioned in this paper provides the basis for NMR measurement to study the asphaltene reservoir more accurately.