Residual oil saturation measurements in carbonates with pulsed NMR logs

John P. Horkowitz, Donald E. Hartman, Edward A. Clerke, Harold J. Vinegar, George R. Coates

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

A new technique has been developed for determining residual oil saturation using pulsed nuclear magnetic resonance (NMR) logging. This technique retains the accuracy and reduces the costs and operational problems of the MnEDTA inject-log technique, which is considered to be the most accurate method of determining residual oil saturation in open holes. The NMR log is unique because, unlike other logging tools, the signal measured is derived only from the pore fluids. In the inject-log technique the brine is doped with a paramagnetic ion to "kill" the water signal (i.e., shorten its relaxation time below the dead time of the tool) so that only oil is detected. The new generation of pulsed NMR logging tools has greatly simplified this method, both by eliminating the need for doping the drilling mud with magnetite and by reducing the tool dead time from 30 ms to about 1 ms. Using a new NMR doping method, we have found that in carbonate reservoirs in West Texas the starch-based drilling mud can be successfully doped with MnCl 2, which is an order of magnitude less expensive than chelated Mn-EDTA. The Mn ion also has greater relaxivity for water protons than Mn-EDTA so that less dopant is required. The drilling mud is doped before drilling the target zone so that spurt-loss is primarily responsible for Mn ++ invasion to the measurement annulus. There is, therefore, no need to pack off and inject in the target interval as done in the inject-log technique. The NMR measurement annulus is sufficiently deep that the oil saturation is not stripped below waterflood residual. The Mn ++ concentration is chosen to separate widely the oil and brine T 2 signals but not to shorten the brine signal below detectability as in the older technique. Thus, the pulsed NMR tool can measure porosity and oil saturation in the same pass without the need to log-inject-log. Moreover, the oil viscosity is determined by either relaxation or diffusion measurements. The Mn ++ ion also serves as a tracer for flushing in core. The result is an accurate and virtually painless measurement of residual oil saturation.

Original languageEnglish
Pages (from-to)73-83
Number of pages11
JournalLog Analyst
Volume38
Issue number2
StatePublished - 1 Mar 1997
Externally publishedYes

ASJC Scopus subject areas

  • Process Chemistry and Technology

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