Rock-typing and permeability estimation of thin-bedded reservoir rock by NMR in the presence of diffusion coupling

Conventional interpretation approaches to nuclear magnetic resonance (NMR) measurements of fluid-saturated reservoir rock rely on the assumption that the distributions of transverse relaxation time (T2) and pore size directly correlate. In practical scenarios this assumption is found not to apply in numerous multi-scale porosity structures as a result of what is known as “diffusion coupling” occurring between various pores. This problem has been analyzed in the context of individual pores, but less so for larger regions of interest. For gas reservoirs in particular it arises frequently in the case of thinly laminated reservoirs due to their characteristically small layer thickness and the subsequent shorter distances to be covered by mobile spins and the appreciably higher diffusion coefficients that characterize gas reservoirs. In such instances, rock-typing cannot directly be achieved from NMR measurements. This study employs NMR simulations on tomographic images for the interpretation of NMR measurements in the presence of interbed diffusion coupling. Knowledge about the magnetization decay of the coupling region is used together with prior knowledge of the individual rock types forming the layered rocks in a methodical treatment to establish the coupling strength (ξ_R). Following successful completion of rock-typing, the improvement in the estimation of vertical and horizontal permeabilities was evaluated, which relies on a proper definition of T2lm for each rock-type that corrected for diffusion coupling. The Lattice-Boltzmann (LB) method was also used to assess the enhancements in the NMR permeability estimations. Synthetic consolidated and unconsolidated laminated structures with two distinct grain sizes and various layer thicknesses are used to test the approach both numerically and experimentally. A relationship between strengthening pore coupling and reducing bed thickness was noted, together with the increase in the diffusion coefficient and the decrease in surface relaxivity. In instances of strong pore coupling, the T2 distribution was found to inaccurately represent the inherent bimodal distribution relative to various morphologies. Successful rock-typing was attained through the decoupling procedure by applying the value of (ξ_R) that consequently improve the NMR permeability estimation

Rock-typing of laminated sandstones by nuclear magnetic resonance in the presence of diffusion coupling: Rock-typing of laminated sandstones by nuclear magnetic resonance in the presence of diffusion coupling

In this work, the aim is to assess the relative import ance of the impact of diffusional coupling on NMR measurements of saturated laminated sandstone numerically at the layer scale to assess the feasibility of NMR rock-typing approaches. We use two 3D model structures based on a Boolean particle process, providing a range of structural to diffusion length ratios to explore the relationships between pore geometry, surface magnetic properties, and NMR transverse relaxation time. The influence of surface relaxivity and bulk susceptibility contrast on T2 relaxation responses is tested for layered structures to improve the rock-typing methodology. An escalation in pore coupling is observed with decreasing bed thickness as well as decreasing bulk susceptibility contrast and surface relaxivity the latter ones reducing the time available for pore coupling by reducing the effective relaxation rate. When pore coupling is strong, the T2 distribution clearly misrepresents the underlying bimodal distribution of the different morphologies. Consequently, the bimodal relaxation time becomes merged and the relative amplitude of the peaks fails to reflect the true morphologies of the models. Furthermore, we observed that in low noise conditions of numerical simulation the effect of diffusional coupling on transverse relaxation may be misinterpreted for the regularization effect on ILT solution. In such cases, careful selection of Laplace inversion method is essential for effective rock-typing by NMR