Mixing Rules and Correlations of NMR Relaxation Time With Viscosity, Diffusivity, and Gas/Oil Ratio of Methane/Hydrocarbon Mixtures
- Sho-Wei Lo (Rice U.) | George J. Hirasaki (Rice U.) | Waylon V. House (Rice U.) | Riki Kobayashi (Rice U.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2002
- Document Type
- Journal Paper
- 24 - 34
- 2002. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 5.6.1 Open hole/cased hole log analysis, 4.1.9 Tanks and storage systems, 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 4.1.5 Processing Equipment, 4.3.3 Aspaltenes, 4.6 Natural Gas, 1.2.3 Rock properties, 5.4.2 Gas Injection Methods, 4.1.2 Separation and Treating
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Viscosity, diffusivity, relaxation time, and gas/oil ratio are important properties in the characterization of reservoirs by nuclear magnetic resonance (NMR) well logging and in prediction of production performance. For the past few years, NMR well logging has been used to estimate formation properties and hydrocarbon liquid/ vapor characterization. Previous work has shown that pure alkanes, alkane mixtures, viscosity standards, and stock tank crude oils have NMR relaxation times that vary linearly with viscosity/ temperature and diffusivity on a log-log scale. However, pure methane at some temperatures and pressures does not follow the same trend. Thus, the linear correlation may not be valid for live crude oils that contain a significant amount of methane. Therefore, the study of methane-hydrocarbon mixtures is of interest.
An NMR spectrometer equipped with a high-pressure probe was used to study the relationship between NMR T1 relaxation time and viscosity/temperature, diffusivity, and gas/oil ratio of methane-hydrocarbon mixtures. Relaxation time and diffusivity measurements of three mixtures were made: methane-n-hexane, methane-n-decane, and methane-n-hexadecane. It was found that unlike stock tank oil, relaxation times do not depend linearly on viscosity/temperature on a log-log scale. Each of the mixtures forms a different curve.
Generalized correlations between viscosity, diffusivity, gas/oil ratio, and NMR relaxation times were developed. First, the relaxation time mixing rule was developed by studying the theory of NMR relaxation mechanism. From the mixing rule, it was found that departure of relaxation times of methane-n-alkane mixtures from linear correlations on a log-log scale can be correlated with the proton fraction of methane, expressed as gas/oil ratio. Thus, correlations between relaxation time, viscosity/temperature, and gas/oil ratio were developed. Correlations between relaxation time, diffusivity, and gas/oil ratio were also developed. There is a linear relation between diffusivity and viscosity/temperature that is independent of composition. From these correlations, viscosity and gas/oil ratio can be estimated from NMR T1 relaxation time and diffusivity.
There are existing correlations between NMR relaxation time and viscosity for pure alkanes, alkane mixtures, and crude oils. In 1961, Brown made relaxation time measurements on a number of crude oils and showed that relaxation time was closely correlated with viscosity.1 Recently, measurements of T2 relaxation times of dead crude oils were made, and it was found that T2 depends linearly on viscosity on a log-log plot for crude oils.2-4 There was also work done on deoxygenated pure alkane and alkane mixtures, and it was found that relaxation times of pure alkane and alkane mixtures could also be linearly correlated with viscosity/ temperature.5-8 However, there were no existing correlations between viscosity, diffusivity, and NMR relaxation time for live oils.
Previous publications of this work have shown that methane-decane mixtures do not follow the same correlation of pure alkanes and alkane mixtures.9,10 The objective of this work was to develop correlations between transport properties (viscosity, diffusivity), gas/ oil ratio, and NMR relaxation time of methane-hydrocarbon mixtures.
Two NMR spectrometers were used to measure relaxation times. One was a low-field spectrometer, which operates at 2 MHz with a permanent magnet, the MARAN-2 (Resonance Instruments, Inc., Skokie, Illinois). This spectrometer was used for relaxation time measurements of pure alkanes at 30°C and ambient pressure.
Relaxation times of methane-hydrocarbon mixtures and pure hexane, decane, and hexadecane at elevated temperatures and pressures were measured with an integrated superconducting NMR spectrometer. This spectrometer is connected with a high pressure vapor-liquid equilibrium (VLE) apparatus and a temperatureregulated air bath that maintains a constant temperature of the fluid as it is introduced to the NMR probe. The magnet was a superconducting magnet made by Oxford with a proton frequency of 90 MHz. The probe was made specifically for high-pressure fluids by constructing the sample chamber and sensing coils inside the pressure vessel.
The alkane samples were obtained from Fisher Scientific and Aldrich Chemicals. The purities stated by the manufacturers were 99% for all alkanes. The primary impurities were estimated to be other alkanes of similar boiling points, which would not be expected to have a significant effect on relaxation times. Therefore, no further purification of the alkanes was performed except deoxygenation.
Pure methane gas was obtained from Matheson Gas Products. The quality was ultrahigh purity (99.97% minimum). The sum of impurities, N2, O2, CO2, C2H4, C3+, and H2O, was less than 300 ppm. No further purification of methane was attempted except further removal of oxygen.
Oxygen presence affects relaxation time significantly because it is paramagnetic. Therefore, deoxygenation was performed. For the alkane samples, the freeze-thaw method was used to remove oxygen. The sample was frozen in liquid nitrogen, and the solid was evacuated for fifteen minutes. The sample was then thawed and backfilled with nitrogen gas. According to previous investigation, 5 oxygen was completely removed after the first cycle. However, the method was performed three or four times for each sample to ensure complete removal of oxygen. The oxygen contained in methane gas was removed by passing methane gas through an oxygen absorbing purifier, Matheson Model 6411, which gives an oxygen content of less than 0.1 ppm after purifying.
The samples were introduced to a clean probe. The apparatus was filled with toluene for one day, and then the toluene was flushed out. Then the apparatus was heated to 50°C and evacuated for at least 8 hours to ensure complete removal of toluene. The cleaning procedure was performed three times before introducing a new sample.
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