MEAN CARBON NUMBER LOGGING: HYDROCARBON CHAIN LENGTH DETERMINATION USING DIFFUSION DISTRIBUTIONS AT MULTIPLE DEPTHS

摘要

NMR has been used extensively for oilfield exploration, in both laboratory settings and in the borehole.Relaxation time measurements (T_1 and T_2) have proved particularly valuable for measurements of rockpermeability and fluid typing while signal intensity has been correlated to saturation. Recent advancesallow for downhole measurement of diffusion coefficients, and it is known that diffusion coefficients canbe a robust indicator of the hydrocarbon molecular size in a mixture of alkanes (Freed 2005). This workapplies the previous work to actual downhole measurements and produces a running log of averagemolecular size versus depth in oil wells.Multi-dimensional diffusion-relaxation experiments conducted at every depth in a well produce mapswhich are then analyzed by a computer algorithm. Bound fluid shows in the map at fast relaxation times;since this fluid will not flow, signal from this region is ignored. Moveable oil and gas often exhibit longerrelaxation times and oil signals have slower diffusion constants than gas. These signals are identified bythe algorithm to obtain the distribution of diffusion coefficients from which the mean carbon number(MCN) for the different phases can be calculated. Because data is taken at multiple frequencies and thusmultiple depths of investigation (DOI), the chain length at different DOIs can be obtained. This DOIdependentMCN allows for the identification of invading fluids in shallower depths and the true formationfluids in the deeper depths. Furthermore, this method also shows some regions where significant oil andgas saturations are coexistent and independently finds an average hydrocarbon size for each component.We validate this method by comparing calculated chain length data with similar data collected fromlaboratory gas chromatography data and downhole spectroscopy measurements.