T2 (or T1) distributions
Gas Oil Water Diffusion distribution
Gas diffusion coefficient
Gas
Water diffusion coefficient
D
Water
T2, which contains important complementary information about composition, such as gas/oil ratio (GOR) and pore-size information for water, cannot be measured. This limitation is overcome by invoking a third dimension to combine with diffusion, T1 relaxation times.6 The 3D NMR measurements acquire echo- train data at multiple WTs (for T1) and multiple TEs (for diffusion). Sufficient information is then available to create 3D maps of D-T1-T2, in which the T2 axis refers to a transverse relaxa tion time with diffusion effects removed (next page, left). The map is therefore a 3D correlation of intrinsic fluid properties for T1, T2 and D. In practice, NMR fluid maps are typically presented in a 2D format, plotting D with either T1 or T2, or on occasion plotting T1 with T2.
Oil T2
> D-T maps. Diffusion plotted with T2 (or T1) provides 2D reservoir-fluid maps that can resolve oil, gas and water. In this example, the diffusion dimension (right) is the key to identifying the fluids, which otherwise overlap in the T2 dimension (top left). The amplitudes of the signals along one direction of the two-dimensional map result in 1D distributions, which then can be converted to fluid saturations. As an aid to interpreting the 2D maps, fluid-diffusion coefficients are superimposed on the map (bottom left). The gas line (red) is computed using downhole pressure and temperature inputs. The water line (blue) is calculated using the downhole formation temperature. The oil line (green) shows the position of oil at different viscosities, with the lower left being heavy oil, trending to light oil and condensate at the upper right. The interpretation of this map is that the reservoir contains gas, oil and water.
which are a graphical means to identify fluid type and quantify saturations (above).5 Although the two-dimensional D-T2 measure ment is effective in separating the oil signal from that of the water, it is less robust for distinguishing between highly diffusive
5. Hürlimann MD, Venkataramanan L, Flaum C, Speier P, Karmonik C, Freedman R and Heaton N: “Diffusion- Editing: New NMR Measurement of Saturation and Pore Geometry,” Transactions of the SPWLA 43rd Annual Logging Symposium, Oiso, Japan, June 2–6, 2002, paper FFF.
6. Freedman and Heaton, reference 4, main text.
7. Cao Minh C, Heaton N, Ramamoorthy R, Decoster E, White J, Junk E, Eyvazzadeh R, Al-Yousef O, Fiorini R
fluids, such as gas, condensate or water at high temperatures. The problem arises because diffusion can dominate the T2 relaxation mechanism for these fluids, even at the shortest echo spacing available from the logging tools. The underlying “diffusion-free”
and McLendon D: “Planning and Interpreting NMR Fluid-Characterization Logs,” paper SPE 84478, presented at the SPE Annual Technical Conference and Exhibition, Denver, October 5–8, 2003.
8. For more on wettability: Abdallah W, Buckley JS, Carnegie A, Edwards J, Herold B, Fordham E, Graue A, Habashy T, Seleznev N, Signer C, Hussain H, Montaron B and Ziauddin M: “Fundamentals of Wettability,” Oilfield Review19, no. 2 (Summer 2007): 44–61.
Overlain on the maps are default fluid- response lines for the D of gas and water computed from their diffusion coefficient at formation temperature and pressure. The oil line is derived from the estimated dead-oil response at downhole conditions. The fourth dimension in NMR logging, the radial distance from the borehole wall, results from acquisition at multiple depths of investigation (DOIs). Data from two or three DOIs are simultaneously inverted. Results from the shallow DOI are used to correct data from deeper DOIs, improving the outputs affected by missing information and poorer signal-to-noise ratios.
NMR logging tools acquire data from a region often affected by filtrate invasion, which alters the original fluid distribution. The 4D NMR processing is based on the assumptions that bound-fluid volume and immovable hydrocarbon volume are invariant to DOI. The shallow-measurement data are used to constrain the inversion for the deeper measurements by fixing the bound-fluid components. (For examples of 4D NMR processing, see pages 16 and 17.) Diffusion and T1 (or T2) data from 4D NMR are used to produce fluid maps at multiple DOIs. Fluid changes that take place as filtrate invades the reservoir rock are graphically displayed and
10
Oilfield Review
Oil diffusion coefficient viscosity correlation
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