616 Washout Depth ft
Caliper in.
50 50 50
Standard Bound Fluid %
Shell No. 1 0
Shell No. 4 %
Shell No. 8 %
0 0 50 50 50
Shell No. 1 4D Bound Fluid
%
Shell No. 4 %
Shell No. 8 %
More coherence 0 0 0
25 25 25
Standard Free Fluid %
Shell No. 1 0
Shell No. 4 %
Shell No. 8 %
0 0 25 25 25
Shell No. 1 4D Free Fluid
%
Shell No. 4 %
Shell No. 8 %
More coherence 0 0 0 50
Heavy Oil
Free Water Oil
Bound Water
Porosity, Shell No. 1 %
0 50
Standard 2D NMR Processing Heavy Oil
Oil Free Water Bound Water
Porosity, Shell No. 4 %
0 50
Heavy Oil Oil
Free Water Bound Water
Porosity, Shell No. 8 %
0 50
Heavy Oil Oil
Free Water Bound Water
Porosity, Shell No. 1 %
0 50
4D NMR Processing Heavy Oil
Oil Free Water Bound Water
Porosity, Shell No. 4 %
0 50
Heavy Oil Oil
Bound Water Free Water
Porosity, Shell No. 8 %
0
X,100
X,150 Shell No. 1
Standard 2D NMR Processing Shell No. 4
Shell No. 8 Shell No. 1
4D NMR Processing Shell No. 4
Shell No. 8
Oil Water
Oil Water
T1
, ms
T1
, ms
T1
, ms
T1
, ms
T1
, ms
T1
, ms
> 4D NMR processing. Standard processing results in a lack of coherence between bound-fluid volumes measured by Shells No. 1, No. 4 and No. 8 (top, Track 1). The same is true for the free-fluid volumes (Track 3). Using 4D NMR processing, the bound-fluid volumes, which should remain constant across each DOI, are constrained and the porosity contributions are reassigned. The result is improved coherence for both the bound-fluid (Track 2) and free- fluid (Track 4) volumes. The fluid properties are affected by hole conditions from X,120 to X,135 ft (red shading) as evidenced by the increased porosity measured from the shallower shells (Tracks 5, 6, 8 and 9). Shell No. 8 (Tracks 7 and 10) senses from beyond the washout and provides more-accurate data. The D-T1 maps used for saturation computation for each shell demonstrate the effectiveness of 4D processing. The standard 2D NMR processing (bottom left panel) results in similar fluid volumes in Shells No. 1 and No. 4. Shell No. 8 has less bound fluid, but all three shells should have equivalent volumes because bound fluid should not change with DOI. The 4D NMR processing (bottom right panel) constrains the fluid volume to be the same below 30 ms. Reapportioning the porosity to account for the bound-fluid volume delivers a more-accurate measurement from the deepest shell. As a result, the 4.0-in. measurement furnishes fluid properties from a region less influenced by mud-filtrate invasion.
The independent measurements at various DOIs from the MR Scanner tool read deeper into the formation than the CMR tool. Not only has the MR Scanner tool overcome rugosity problems, it also has verified a condition—previously theorized from the CMR data—of partial- or whole-mud invasion effects on the bound-fluid volumes and permeability. These effects were especially noticeable in water zones. The mud solids did not appreciably alter NMR porosity, but the bound-fluid measurement was too high. As an input to the NMR permeability calculation, incorrect bound-fluid volume resulted in permeability outputs that were too low. Deeper-reading NMR measurements over come rugosity but have some trade-offs. Because the
formation signals from the deeper shells are weaker, noise has the potential to corrupt the processed data. Vertical resolution is degraded because data must be averaged or stacked over a longer interval to overcome the noise effects. The measurement from deeper shells is also acquired at longer echo spacings because of tool power limitations. The CMR tool uses 0.2-ms echo spacing so that in 10 ms it generates 50 pulses. This provides sufficient data to resolve some heavy oils such as those found in the Orinoco wells. However, the 1.0-ms echo spacing available from the MR Scanner tool’s 4.0-in. shell provides only 10 pulses and echoes in an equivalent time frame. The result is a decrease in signal-to-noise ratio because there are fewer echoes to work with.
One solution to this dilemma comes in the form of four-dimensional (4D) NMR processing in which DOI is the fourth dimension.15
This
processing simultaneously inverts the NMR data within the portion of the relaxation-time distribution that should be common for all DOIs. For the Orinoco wells, the time interval to allow the oil signal to decay is always below 10 ms. The bad-hole and whole-mud effects begin after 20 ms. Imposing a common solution on each shell for the first 10 ms forces the deeper-reading 4.0-in. shell measurement to be equivalent to the higher-resolution 1.5-in. shell reading in this common-data area. The result is improved coherence between the shells and more-accurate readings from the deeper shell (above). This is
16
Oilfield Review
Amplitude
Diffusion
Amplitude
Diffusion
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