computation of fluid saturations, but also helps infer fluid viscosity from the T2 contribution of the fluid (right).
Diffusion-editing sequences from the new MR Scanner service supply a water-saturation output that is independent of that traditionally derived from resistivity and porosity measure - ments. In contrast to a saturation derived from Archie’s equation, NMR-based saturation measurement techniques are useful in fresh water or formation waters of unknown salinity. Wettability can also be inferred from NMR data. One drawback to using NMR measurements for fluid characterization is that the measurement comes from a near-well region referred to as the flushed zone, where mud-filtrate effects are strongest.
The MR Scanner Tool
Although NMR measurements may come from only a few inches into the formation, they can still provide formation-fluid properties. To measure continuous in situ fluid characteristics— including fluid type, volume and oil viscosity— much more information is needed than was provided by previous-generation NMR tools.9
For
this reason, fluid characterization was a key driver in the development of the MR Scanner service. In the past, there were two basic designs for NMR tools: pad-contact tools and centralized concentric-shell tools. The pad device, represented by the CMR tool, measures NMR properties of a cigar-sized volume of the reservoir fluids at a fixed depth of investigation (DOI) of approxi - mately 1.1 in. [2.8 cm]. The NUMAR MRIL Magnetic Resonance Imager Log tool measures concentric cylindrical resonant shells of varying thickness and at fixed distances from the tool, with the DOI determined by hole size and tool position in the wellbore.
The MR Scanner design offers the fixed DOI of a pad device with the flexibility of multiple DOIs of resonant shells.10
It consists of a main
antenna optimized for fluid analysis and two shorter high-resolution antennas best suited for acquiring basic NMR properties (right). The main antenna operates at multiple frequencies corresponding to independent measurement volumes (shells) at evenly spaced DOIs.
9. Heaton NJ, Freedman R, Karmonik C, Taherian R, Walter K and DePavia L: “Applications of a New- Generation NMR Wireline Logging Tool,” paper SPE 77400, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, September 29–October 2, 2002.
10. DePavia L, Heaton N, Ayers D, Freedman R, Harris R, Jorion B, Kovats J, Luong B, Rajan N, Taherian R, Walter K, Willis D, Scheibal J and Garcia S: “A Next- Generation Wireline NMR Logging Tool,” paper SPE 84482, presented at the SPE Annual Technical Conference and Exhibition, Denver, October 5–8, 2003.
Winter 2008/2009
High-resolution antennas
Antenna Main antenna
10 T2 (TE = 0.2 ms) 1 0.1 T1 0.01 T1 T2 0.001 0.0001 0.1 1 10 100 Viscosity, cP
> Viscosity transform. The T2 (or T1) relaxation time for crude oil is a function of viscosity. The relaxation time can be converted to viscosity using an empirically derived transform. Because of diffusion effects, the viscosity measurement for heavy oils below 3 cP [0.003 Pa.s] is influenced by the echo spacing (TE) of the measurement. Thus, T2 times may be tool dependent for heavy oils if the tool is not capable of shorter TEs. As a consequence of diffusion, T1 and T2 values in light oils diverge above 100 cP [0.1 Pa.s].
1,000 10,000 100,000 T2 (TE = 0.32 ms)
T2 (TE = 1 ms) T2 (TE = 2 ms)
Sensed region Permanent magnet
> MR Scanner service. The MR Scanner tool has three antennas. The main antenna operates at multiple frequencies and is optimized for fluid-property acquisition. The sensed region consists of very thin shells that form arcs of approximately 100° in front of the 18-in. [46-cm] length of the antenna. The thickness of the individual shells is 2 to 3 mm. The two high-resolution antennas are 7.5 in. [19 cm] long and provide measurements with a DOI of 1.25 in. [3.17 cm]. The MR Scanner tool is run eccentered with the antenna section pressed against the borehole.
13
T1 or T2, s
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