EFFECT OF DRILLING-MUD BASE-OIL CONTAMINATION ON CRUDE OIL T2 DISTRIBUTIONS

摘要

Oil-based drilling fluids can invade formations and contaminate crude oils, hindering logging analysis. This work details 1H NMR T2 measurements with mixtures of one drilling-fluid base oil, NovaPlus (SNP), and crude oils to determine the effect of the contamination. Measurements are made using 2 MHz MARAN bench-top instruments on mixtures having various concentrations of SNP, with each of three crude oils (labeled STNS, SMY, and PBB), whose viscosities range from 13.7 to 207 cp. T2 measurements for mixtures containing SMY and SNP are repeated four times for purpose of statistical analysis. Two approaches are explored to better relate NMR measurements with contamination. In the first approach, a selective contamination index (SCI) is defined that relates the T2 distribution to the contamination. Here, a subset of the T2 data is chosen for analysis based on sensitivity to contamination. In the second approach, the T2 data are fit to a fourparameter, skewed Gaussian model for T2 distributions. Two parameters of the model are combined in a distribution parameter index (DPI), which can be related to contamination. Both the SCI and DPI values can be fit using cubic polynomials, resulting in a functional dependence on concentration. The polynomial functions, used in reverse, yield estimations of the degree of contamination, which for SMY-SNP mixtures are compared to standard T2,LM methods. The comparison of the T2,LM, SCI, and DPI methods is done in terms of the estimated error in the degree of contamination. The SCI method provides the best estimate of contamination.