Evaluation of light hydrocarbon composition, pore size, and tortuosity in organic-rich chalks using NMR core analysis and logging

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 n-decane. 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 T2app 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 Padé approximation to estimate the mean pore size, heterogeneity length scale, and tortuosity of the light-hydrocarbon filled porosity. We show how these new techniques could in principle be used to evaluate the shale-rock reservoirs.