NMR quality control

Quality control (QC) procedures are especially important during the creation of integrated computed products and to ensure optimal nuclear magnetic resonance (NMR) data acquisition. NMR tools have calibration standards and real-time QC indicators. Standard techniques include:

• Examination and verification of service company calibrations and QC curves
• Calibrations and examination of the repeatability of the computed porosities

Processed results should agree with other data from logs, core, and/or formation test results. NMR QC includes a series of prejob and post-job checks and calibrations.

Prejob calibration and quality checks during logging

Several prejob QC steps are necessary to ensure reliable results.

Frequency check

The amplitude of the NMR measurement is proportional to the square of the magnetic field strength in the measurement region. To attain maximum signal strength, the tool must operate at the correct frequency. Because temperature affects magnetic field strength, a frequency sweep is recorded before the calibration and logging run and is repeated downhole to account for changes in temperature. The combinable magnetic resonance (CMR) tool includes sensors that monitor temperature and changes in the magnetic field strength and adjusts frequency automatically to changes in sonde temperature. The accumulation of metal drilling debris on the magnets can adversely affect the NMR measurements, and the tool may require occasional retuning. The gradient tools, such as magnetic resonance imaging tool (MRIL), Magnetic Resonance Explorer (MReX), and MRScanner, create a self-correcting effect, and the tool rarely requires retuning after the initial downhole tuning. Drifting off frequency may present itself as a loss of porosity (CMR) or loss of precision (MRIL, MReX, and MRScanner). Field calibration should be included on the log.

Statistical calibration

A statistical calibration to 100% porosity (water) should be made at the shop and/or wellsite before and after every logging job for all combinations of T_E frequency and for expected quality factor (Q level); calibration should be done at the shop at least monthly. Primary calibration involves either a flask of fluid (for the CMR tool) or a water tank (for the MRIL tool).

HI

NMR-tool calibration uses water where HI = 1. However, when the HI of the formation pore fluids is expected to be less than one, the NMR-porosity readings will be proportionally lower, and correction is necessary. An NMR tool’s processing software will, therefore, compute a correction based on the salinity of the mud filtrate and on the maximum formation pressure and temperature. In general, a mud-filtrate relaxation time of < 200 ms may suppress the long T₂ components in a T₂ distribution. A specific HI value may be used when residual hydrocarbons or OBM filtrates are present.

System gain

CMR and MRIL tools monitor and calibrate in real time for downhole changes in system gain. A gain measurement is made as a part of each pulse sequence. Gain indicates the amount of loading applied to the NMR tool’s transmitter circuit by borehole fluid and formation resistivity. Gain is affected by changes in temperature, mud conductivity, and borehole size. Because gain is frequency dependent, the operating frequency of a tool should be set to achieve maximum gain. Abrupt changes or spikes in gain should not be present. For a particular MRIL tool, the gain value determines the appropriate acquisition power (Q) level.

System noise (ringing)

Noise contamination of the spin-echo signal, known as ringing, may be a remnant of the RF pulsing process. Noise can interfere with the echo trains and, when present, is usually evident in the NMR porosity readings, either as poor repeatability or lack of agreement with other tools. Both tools (CMR tool and MRIL tool) monitor ringing.

χ

χ is a measure of the quality of fit between the calculated decay curve and the recorded echo amplitudes. Problems with the echo data are likely to be reflected in the χ curve. In general, the value of χ should be less than two but may average slightly higher in certain situations. Spikes in χ may correlate with spikes in porosity and usually indicate tool problems, even if χ remains lower than two. χ serves as is a primary MRIL log-quality indicator and is monitored while logging.

Correction for RF-tipping pulse=

The strength of the CMPG RF pulse (B₁) that produces proton tipping and rephasing is measured as part of every pulse sequence and requires correction for changes in borehole temperature. If the pulse angles are either < 90° or > 90°, the magnetization will be undertipped or overtipped, respectively. The measured amplitude will, thus, be too small, and porosity will be underestimated. The B₁ curve should be relatively constant but should show some variation with changes in borehole and formation conductivity. B₁ will decrease across conductive washouts and conductive formations. Changes in the B 1 values should track changes in total conductivity and vary together in the same direction as gain. The need for excessive correction (i.e., > 5% of the optimum shop-peak value) may indicate a tool problem and can result in undertipping or overtipping of the protons, a reduced S/N ratio, and a loss of precision in determining porosity. Sudden changes in the B₁ correction curve may also indicate a tool problem.

γ

Regularization methods are used to select a smooth T₂ distribution that is consistent with the spin-echo sequence. These methods require a parameter γ that is automatically computed from the raw echo data. Values of γ are dependent on the S/N ratio and the shape of the underlying T₂ distribution. In high-S/N environments (i.e., medium-to-high-porosity formations), typically γ < 5; in low-S/N environments (i.e., tight sands and shales), γ > 10.

Repeatability

Whenever possible, a repeat pass should be recorded with parameters identical to those used in the main pass. These parameters include T_W, N_E, and T_E, as well as computation parameters such as the echoes selected for processing, and for T_2cutoff. The generally accepted goal for porosity is a standard deviation of 1 porosity unit (p.u.). Repeatability of BVI is usually > 1 p.u., and repeatability of FFI is usually >> 1 p.u.

If repeatability is a concern, decreasing logging speed or increasing the degree of echo data stacking during logging in post-job processing can improve S/N ratio and repeatability. Typically, fluid-typing applications using dual-T_W or dual-T_E methods are more sensitive to data repeatability than a standard NMR log acquired for porosity, bound fluid, and permeability.

A given data set should agree with similar data acquired by other logs, formation tests, and/or core analysis.

Environmental corrections

NMR logs are similar to neutron logs in that they respond to the hydrogen volume present in a sample. Because the hydrogen volume changes with temperature and pressure, NMR logs require environmental corrections for temperature and pressure similar to those applied to neutron logs. Also, the magnitude of NMR resonance (and S/N ratio) varies inversely with temperature. NMR-logging tools include a temperature sensor to acquire the data needed for this correction. At high temperatures, data stacking can be increased or logging speeds reduced to compensate for a lower S/N ratio.

Porosity check

NMR porosity can serve as an important diagnostic of data quality. High porosity readings may result from washouts (e.g., borehole fluid in the sensed volume), tool problems, improper environmental corrections or calibration issues, loss of pad contact (for the CMR tool), tool eccentering, or borehole ellipticity (for the MRIL tool). Low porosity readings may result from insufficient wait time, light-hydrocarbon effects, the presence of heavy oil, improper frequency tuning, and/or calibration error.

Post-logging quality check

NMR-log responses should be checked against conventional logs when they are available (see NMR applications).

Nomenclature

B1	=	amplitude of the oscillating magnetic field perpendicular to
NE	=	number of echoes
T2	=	transverse relaxation time, seconds
T2cutoff	=	T2 cutoff value, seconds
TE	=	CMPG interecho spacing, seconds
TW	=	polarization (wait) time, seconds
γ	=	gyromagnetic ratio—the ratio of the magnetic dipole moment to the mechanical angular momentum, Hz/gauss
χ	=	goodness of fit

Noteworthy papers in OnePetro

Stambaugh, B., Svor, R., & Globe, M. (2000, January 1). Quality Control of NMR Logs. Society of Petroleum Engineers. doi:10.2118/63212-MS

Ehigie, S.O. - NMR-Openhole Log Integration: Making the Most of NMR Data Deliverables 136971-MS SPE Conference Paper - 2010

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See also

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