DOI
Decreasing B0 strength
Borehole
Shell No. 1
Main antenna 1.5 in. 2.7 in. 4.0 in.
ƒ0
Shell No. 4
Shell No. 8
Distance from magnet
Borehole rugosity and thick mudcake can invali - date shallow NMR measurements but rarely affect the readings from the deeper shells. NMR porosity from a deep shell has been used as a substitute for formation density porosity when that measure - ment was compromised by borehole rugosity. Fluid-property changes resulting from mud- filtrate invasion may also be observed and quanti fied using radial profiling. Often more than just filtrate from the drilling mud invades the formation. Whole mud and mud solids can replace existing fluids in the near-wellbore region. NMR-derived porosity and permeability may be reduced by the presence of these solids, but the effects diminish deeper into the formation. Radial profiling identifies and overcomes these effects.
> Gradient tool and DOI. The MR Scanner tool is referred to as a gradient tool because the magnetic-field strength (B0, blue) from the permanent magnet, although uniform over the sample region, decreases monotonically away from the magnet. The tool’s magnet extends along the length of the sonde section. A constant, well-defined gradient simplifies fluid-property measurements. The DOI is determined by the magnetic-field strength and the frequency of RF operation, f0. Although multiple frequencies are available from the tool, standard operating procedure is to acquire data from the 1.5-in, 2.7-in. and 4.0-in. DOI shells, referred to as Shell No. 1, Shell No. 4 and Shell No. 8, respectively. Three shells are shown, with their respective DOI- related operating frequencies. The frequency associated with Shell No. 1 is shown in green.
Although multiple frequencies are available from the main antenna, the three most commonly used are Shells No. 1, No. 4 and No. 8, which correspond to 1.5-in., 2.7-in. and 4.0-in.
Shell No. 8 Shell No. 4 Shell No. 1
[3.8-cm, 6.8-cm and 10.2-cm] DOIs, respectively. A recently introduced three-shell simultaneous acquisition mode eliminates the need for multiple passes to acquire data from all three DOIs.
Profiling Fluid Properties
> Radial profiling. The MR Scanner tool senses fluid from multiple, thin DOI shells. The spacing is optimized to avoid shell interaction. Fluid properties vary in the first few inches into the formation as a result of flushing by mud filtrate. Deeper shells are less affected by filtrate, mud invasion and borehole rugosity than are shallow- reading shells.
The three primary frequencies commonly used by the MR Scanner tool correspond to three independent DOIs, providing measurements at discrete radial steps into the formation. The frequency of the RF pulse, along with the field strength of the magnet, determines the DOI of the shell (above). A key advantage of the MR Scanner shells is that the measurement comes from a thin cylindrical portion of the formation— an isolated slice—and is not generally affected by the fluids between the tool and the measurement volume. This allows interpretation of near- wellbore fluid properties in a manner that is unique in formation evaluation.
Multiple DOIs introduce new concepts for NMR logging—radial profiling and saturation profiling
(left). Profiling incorporates measurements from successive DOIs to quantify changes in fluid properties occurring in the first few inches of formation away from the wellbore.
Saturation profiling delivers advanced fluid- characterization measurements such as oil, gas and water saturations, fluid type and oil viscosity—all at discrete multiple DOIs. One application of saturation profiling is the evaluation of heavy-oil reservoirs.
Viscosity and Rugosity Of the world’s known reserves, 6 to 9 trillion barrels [0.9 to 1.4 trillion m3], are found in the form of heavy- or extraheavy-oil accumulations.11 This is triple the amount of world reserves of conventional oil and gas combined. Heavy-oil reser voirs pose serious operational concerns for proper evaluation of fluids and production potential. Sampling with wireline-conveyed tools or drillstem tests may not be possible because of the difficulties in getting the oil to flow. NMR measurements can provide essential information on in situ fluid properties to evaluate heavy-oil reservoirs.
Located south of the Eastern Venezuelan basin, the Orinoco heavy-oil belt holds an estimated 1.2 trillion barrels [190 billion m3] of heavy oil. NMR logs have been an integral part of evaluation programs of the wells drilled in this region, but with recognized limitations.12
The
relaxation times of high-viscosity oils are very short and may not be fully measurable using NMR tools. Also, borehole conditions in the Orinoco basin are often poor, with rugosity that affects pad-contact tools.
Although the early introduction of NMR fluid characterization with the CMR tool showed promise, the measurements were limited to a single, shallow DOI. Techniques were developed to use NMR data to estimate the oil viscosity throughout a sand interval based on the logarithmic mean of T2 distributions. Encouraging
14
Oilfield Review
0 in. DOI
1.5 in.
2.7 in. 4.0 in.
Magnet
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