Hydrocarbon Composition From NMR Diffusion And Relaxation Data
- M.D. Hürlimann (Schlumberger - Doll Research) | D.E. Freed (Schlumberger - Doll Research) | L.J. Zielinski (Schlumberger - Doll Research) | Y.Q. Song (Schlumberger - Doll Research) | G. Leu (Schlumberger - Doll Research) | C. Straley (Schlumberger - Doll Research) | A. Boyd (Schlumberger - Doll Research) | C. Cao Minh (Schlumberger)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- April 2009
- Document Type
- Journal Paper
- 2009. Society of Petrophysicists & Well Log Analysts
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- 498 since 2007
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The detailed knowledge of fluid composition is a key ingredient for the successful management of oilfield reservoirs. Here we show that measurements of diffusion – relaxation distribution functions (D-T2) contain detailed information on the composition of hydrocarbon fluids. We present results on a suite of stock tank oils to demonstrate that D-T2 measurements provide fingerprints of the fluids that correlate with the chemical composition of the oils, i.e. the SARA analysis. For oils that have a high fraction of saturate or aromatic hydrocarbons, the D-T2 signal lies close to the standard oil line, but shows systematically different slopes that depend on the chemical composition. Asphaltenes act as relaxation contrast agents and shift the oil signals to shorter relaxation times. Oils below the wax appearance temperature show restricted diffusion effects. In addition, for oils without asphaltene or waxing effects, the measured diffusion or relaxation time distributions can be quantitatively related to the chain length distributions of the oil molecules, extending to molecular sizes beyond those measureable with gas chromatography. For samples with significant methane content, diffusion is a useful probe to identify the gas. We show that it can be used to distinguish gas dissolved in the oil from free gas.
Crude oils are complicated mixtures of fluids. For the understanding and prediction of fluid properties viscosity and the onset temperatures for transitions such as gelling, waxing or asphaltene drop-out, it is essential to have an accurate knowledge of the distribution of molecular sizes and the chemical composition. Standard laboratory techniques that give information on the distribution of molecular sizes include gas chromatography, mass spectroscopy, and NMR relaxation and diffusion measurements. Information on chemical composition can be obtained by standard chemical analysis and various spectroscopies, including optical, infrared and NMR spectroscopy. NMR relaxation and diffusion measurements of crude oils have long been used to infer viscosity information of crude oils (Kleinberg 1996, Morriss 1997, Hirasaki 2003, Straley 2006). Here we take advantage of newly developed two-dimensional NMR techniques that allow the extraction of D-T2 and T1-T2 distribution functions (Hürlimann 2002b, Song 2002). In addition, we extend the analysis of relaxation and diffusion measurements to the extraction of chain length distributions and chemical properties of the oil.
TWO-DIMENSIONAL NMR MEASUREMENTS
It is well known that the logarithmic mean relaxation time of the oil is related to its viscosity. In this paper, we demonstrate that additional information can be extracted from the hydrocarbon signal in a D-T2 measurement. The shape and position of the oil signal in a D-T2 map provide unique information about hydrocarbon composition.
In order to isolate effects due to hydrocarbon composition from the other effects mentioned above, we have performed measurements on bulk fluid samples. Most measurements have been performed on stock tank oils. Since oxygen is paramagnetic, dissolved air in the oils shortens the longest relaxation times (Mutina 2005). For this reason, samples were deoxygenated by repeated freeze-thaw cycles. We also performed measurements on hexadecane – methane mixtures pressurized to 5500 psi.
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