Hydrocarbon Composition From NMR Diffusion And Relaxation Data

Abstract

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-T 2 ) contain detailed information on the composition of hydrocarbon fluids. We present results on a suite of stock tank oils to demonstrate that D-T 2 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-T 2 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.