Case Study: MR Scanner Service Sorts Out Producible Hydrocarbon from Oil Filtrate

Single-pass radial NMR profiling distinguishes mixed fluids in West Africa shaly sand reservoirs

Challenge: Interpret the invasion pattern of oil-base mud (OBM) filtrate to identify producible oil saturation in shaly sands.

Solution: Acquire MR Scanner expert magnetic resonance data at multiple depths of investigation (DOIs) in a single pass to identify bound water, formation hydrocarbon, and OBM filtrate in the reservoirs. Inversion of the continuous radial data determines the relative changes in saturation for estimating producible oil properties.

Result: Untangled the complex fluid distribution to show that the bottom sand had watered out, whereas the higher two sands were oil saturated.

Complex mixing of hydrocarbon, OBM, and water

An operator needed to sort out the mixing of formation hydrocarbon, OBM, and water in three sand reservoirs in West Africa. The reservoirs were bounded by shales above and below and separated by tight streaks.

Single-pass acquisition by MR Scanner service at multiple DOIs

A single pass of MR Scanner expert magnetic resonance service obtained continuous nuclear magnetic resonance (NMR) data at multiple DOIs. The total water saturation (Swt) in Track 4 of the log was computed from the NMR total porosity and resistivity. It shows that the top two sands remain at irreducible water saturation, whereas separation of the curves for Swt and irreducible water saturation (Swirr) in the bottom sand shows depletion.

Simultaneous 4D inversion of the data was then conducted. With the bound-fluid component constrained, variation in fluid saturation can be attributed to OBM invasion to distinguish it from formation hydrocarbon.

Formation hydrocarbon distinguished from OBM

The processed data from MR Scanner service in the plots to the right of the log show a complex hydrocarbon-OBM mixture in the top sand at X,262 m. At shallow DOI, the longitudinal relaxation time (T1) has three peaks, representing bound water, hydrocarbon, and OBM filtrate. At deep DOI, the hydrocarbon and OBM merge to one broad peak. The light components of the hydrocarbon are mixing with the OBM filtrate at shallow DOI, separating out the heavier components. This effect is also evident in the DOI-diffusion (D) plot, where the broader deep DOI spans the oil range left of the water line.

The bottom sand at X,314 m shows a different situation. The OBM signal at shallow DOI virtually disappears at deep DOI, and the free-fluid T1 peak at deep DOI is water, with the signal trending right, toward the water line. The D-T1 plots for each DOI also show the bottom sand has watered out, progressing with increasing DOI from OBM to water.

Download: Differentiate Producible Formation Hydrocarbon from Oil Filtrates (0.51 MB PDF)

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Untangling Complex Fluid Saturations

Conventional logs indicate the zones of interest, but the MR Scanner 4D processing plots to the right are key to understanding OBM filtrate invasion to improve pay quantification. The lines overlying the DOI-D plots represent gas (red), water (blue), and oil (dashed green for 1, 10, and 100 cp).Diffusion-relaxation maps of the 4D processed data clearly identify zones of oil and mixed oil-water.
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Measuring Properties Independently from Rock Type

Investigate the formation at multiple depths in a single pass.
MR Scanner expert magnetic resonance service measures 3D profiles of porosity, permeability, and fluid type and movability that are unbiased by the reservoir matrix, which is typically highly variable in carbonates.
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