EVALUATION OF LIGHT HYDROCARBON COMPOSITION, PORE SIZE, AND TORTUOSITY IN ORGANIC-RICH CHALKS USING NMR CORE ANALYSIS AND LOGGING

摘要

Laboratory NMR core-analysis integrated with downhole NMR logging has proven to contribute significantly to formation evaluation. In this paper, we integrate laboratory NMR measurements with NMR logging to estimate the hydrocarbon composition in an organic-rich chalk prospect. We also use NMR laboratory-measured restricted diffusion to estimate the mean pore size, heterogeneity length scale, and tortuosity of the hydrocarbon-filled porosity. Our core analysis consists of pressure saturation of the as-received reservoir core-plugs in a NMR overburden cell, followed by in-situ NMR T1-T2 and D-T2 measurements. The saturating fluids in the core-plugs include water and light hydrocarbons, including methane, ethane, propane, n-butane, n-pentane, and ndecane. The laboratory-measured T2 distributions (projected from T1-T2 measurements) of the hydrocarbons in saturated cores are converted to T_(2app) (T2 apparent) distributions by simulating the effects of diffusion in the magnetic-field gradient of the NMR logging tool. The core data indicate a large contrast in T_(2app) distributions between the different hydrocarbons due to different surface relaxivities and diffusivities. This contrast is used to estimate the downhole hydrocarbon composition by minimizing the least-square error in the T_(2app) distributions between core and log data. The laboratory-measured T1/T2 exhibits contrast between water and light hydrocarbons. The magnetic-field gradient of NMR logging tools amplifies the contrast and makes the downhole-measured T1/T_(2app) favorable for fluid typing. We find that methane and natural gas liquids (NGLs) tend to yield higher T1/T_(2app) compared to water and longer alkanes. The laboratory-measured restricted diffusivity indicates that the saturating methane can be distinguished from liquid-state hydrocarbons by the higher diffusivity. In addition, the laboratory-measured restricted diffusivities of different light hydrocarbons are fitted to the Pade approximation to estimate the mean pore size, heterogeneity length scale, and tortuosity of the lighthydrocarbon filled porosity. We show how these new techniques could in principle be used to evaluate the shale-rock reservoirs.