NMR logging while drilling

Nuclear magnetic resonance (NMR) imaging has long been applied in the laboratory, and over the past few decades, downhole NMR tools have been developed. The latest entries into NMR logging are logging while drilling (LWD) tools. The development of LWD-NMR is ongoing and significant changes in hardware design, as well as significant changes and improvements in data acquisition and processing, can be expected in the next few years. The general benefits of LWD have been discussed elsewhere—in particular, NMR-LWD offers a nonradioactive alternative for porosity measurement, an NMR alternative to wireline in high-risk and high-cost wells, and enables high-resolution fluid analysis in thin beds and laminated reservoirs.

LWD-NMR tools

By definition, logging tools operating in the drilling environment are built into drill collars and are, therefore, mandrel devices. In contrast to wireline tools, they must be capable of making measurements when the drillstring is stationary, sliding or rotating, centered or eccentered.

LWD-tool measurements are either omnidirectional or azimuthal, depending on tool design. The incorporation of magnetometers allows binning of the data into azimuthal sectors. Omnidirectional measurements can be generated from azimuthal data, but not the reverse. Although current LWD-NMR services provide only omnidirectional data, patents have been issued for tools with azimuthal capability.

A major concern introduced with the advent of LWD-NMR is the effect of drillstring lateral motion on the basic NMR measurement. NMR measurements are not instantaneous, they involve both polarization and decay, time-dependent components. Lateral movement of a wireline tool or LWD drillstring shifts the polarization volume and the sensitive-measurement area, relative to one another, and these shifts may result in incomplete polarization or incomplete measurement of the decay. Low-velocity motion affects only the decay, but high-speed motion can also affect the initial decay amplitude.

The currently available LWD tools offer different solutions to this concern. While both tool designs can operate in either T₁ or T₂ acquisition mode and both incorporate accelerometers and magnetometers for detecting lateral motion for quality control of the NMR measurements, they differ in their choice of primary measurement mode. Operational factors, such as the slower logging speed (i.e., rate of penetration in drilling), compared with typical wireline logging, also affect the choice of measurement mode.

The NMR acquisition sequences are programmable and interchangeable with those used in wireline tools. Switching between these acquisition modes is accomplished by a variety of methods, including elapsed time, counting measurements, and differentiating between drilling and nondrilling conditions. T₁-mode records echo amplitudes as a function of time. Data output consists of porosity, free-fluid, and bound-water volumes, and can provide a quick-look permeability estimate. T₂-mode is a multifrequency mode that records multiple wait-time CMPG spin-echo amplitudes and is capable of using all wireline-pulse sequences. T₂-mode output includes porosity, free fluid, clay- and capillary-bound water, and differences in the multiple wait-time data are used for hydrocarbon indication.

The Halliburton tool (MRI-LWD) uses T₁ as its preferred acquisition mode. Halliburton considers T₁ more robust for determining porosity and free-fluid volume. The anticipated maximum rates of penetration for LWD—1 to 3 ft/min—allow T₁ acquisition. T₁ is motion tolerant compared with T₂. A sequence of interleaved measurements made at different recovery times is used to construct the T₁ relaxation decay (buildup). As long as the sensitive volume (shell) is contained within the much larger volume reached by the saturation pulse, the measurement is valid. During post-processing, drilling and nondrilling periods are identified, and the invalid T₁ data recorded during drilling are discarded.

Schlumberger’s tool (ProVision LWD-NMR) uses T₂ for its primary measurement. While both companies agree that T₁ is motion tolerant, Schlumberger considers T₁ to be less robust for estimating porosity, bound-fluid, and free-fluid volumes because of the poor S/N resulting from the longer time require for equivalent-quality T₁ measurements. A reduced S/N impacts data quality (e.g., statistical repeatability and vertical resolution), logging speed, and, ultimately, the results. T₂ measurement also enables rapid calibration and correlation with the large body of wireline-NMR data. The multiple wait-time acquisition includes a fully polarizing (3 to 12 seconds), a partially polarizing (normally 0.6 to 1 seconds), and a very fast wait time (typically 0.08 seconds). Porosity and bound-fluid volume are calculated for both the long- and the medium-wait-time measurements, and significant differences between the long-wait-time and medium-wait-time porosity provide real-time hydrocarbon indication. When necessary, logs are corrected for incomplete polarization of the hydrogen nuclei, and T₁ distributions are estimated from the measured T₂ relaxation.

Baker Atlas is presently field testing a new LWD device (MagTrak) that operates in T₂ acquisition mode and has a vertical resolution of less than 3 in. This tool design achieves a motion-tolerant T₂ measurement through a combination of a very-low gradient magnetic field, circuitry that permits a T_E as low as 0.6 ms, and the use of special stabilizers.

Nomenclature

T1	=	longitudinal relaxation time, seconds
T2	=	transverse relaxation time, seconds
TE	=	CMPG interecho spacing, seconds

Noteworthy papers in OnePetro

Kruspe, T., Bittner, R., & Kirkwood, A. D. (2006, January 1). Magnetic Resonance While Drilling: a Quantum Leap in Everyday Petrophysics. Society of Petroleum Engineers. doi:10.2118/100336-MS

Reppert, M., Akkurt, R., Howard, J., & Bonnie, R. (2005, January 1). Porosity and Water Saturation from LWD NMR in a North Sea Chalk Formation. Society of Petrophysicists and Well-Log Analysts.

Prammer, M.G., Akkurt, R., Cherry, R.,Menger, S.- A New Direction In Wireline And Lwd Nmr SPWLA Conference Paper - 2002

Akkurt, Ridvan, Marsala, Alberto F., Seifert, Douglas, Al-Harbi, Ahmed, Buenrostro, Carlos, Kruspe, Thomas - Collaborative Development of a Slim LWD NMR Tool: From Concept to Field Testing 126041-MS SPE Conference Paper - 2009

Drack, E.D.,Prammer, M.G., Zannoni, S.,Goodman, G., Masak, P., Menger, S., Morys, M. - Advances in LWD Nuclear Magnetic Resonance 71730-MS SPE Conference Paper - 2001

External links

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See also

Logging while drilling (LWD)

LWD induction tools

Acoustic logging while drilling

Nuclear logging while drilling