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A major offshore operator in the East China Sea wanted to identify the oil and gas zones in a low-porosity, low-permeability sand reservoir with significant mud/oil invasion. The operator drilled a well with oil-based mud (OBM) with 12 ¼-in. and 8 ½-in. borehole sections. In order to reduce risks, the mud density needed to be high, which resulted in a deeper OBM invasion than was typical. The company needed to distinguish the formation oil and gas from the mud/oil invasion to get accurate porosity measurements.

The operator deployed the MR eXplorer™ (MReX™) nuclear magnetic resonance (NMR) service from Baker Hughes, a GE company (BHGE). NMR logging reveals precise porosity and identifes the reservoir fluid properties, even in reservoirs with complex geologies.

The NMR 2D plot showed oil signals for all measured intervals, whether naturally oil-bearing or not. The operator needed to isolate the reservoir oil-bearing zones.

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Challenges & Results
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Challenges

  • Low-porosity, low-permeability sand reservoir with mud/oil contamination
  • Deep OBM invasion
  • OBM-contaminated signals distributed across the measured interval requiring isolation of the formation’s actual oil zones

Results

  • Produced accurate fluid identification plots
  • Obtained accurate porosity measurements
  • Identified gas and oil reserves