We quantify the impact of fracture-pore diffusional coupling on permeability assessment using nuclear magnetic resonance (NMR) measurements in complex formations. NMR T2 (spin-spin relaxation time) distribution has been conventionally considered insensitive to the presence of micro-fractures. In a recent publication (Chi and Heidari, 2015), we demonstrated that NMR relaxometry can be influenced by the presence of micro-fractures and a phenomena named fracture-pore diffusional coupling. If the diffusional coupling effect is not taken into account in the interpretation of NMR measurements, the T2 relaxation times of intergranular pores or micro-fractures can be underestimated by up to 29%, which causes underestimation of pore size, or micro-fracture aperture, by NMR measurements. Consequently, the distortion of T2 distribution can influence NMR-based permeability assessment. However, the impact of fracture-pore diffusional coupling on NMR-based permeability estimates has not yet been investigated.
The objectives of this paper are (a) to quantify the impact of micro-fractures and fracture-pore coupling on NMRbased permeability estimates and (b) to evaluate the reliability of conventional NMR permeability models (e.g., the Schlumberger Doll Research (SDR) and Coates models) and a new NMR directional permeability model in fractured formations. We compared the estimated permeability of fractured carbonate rock samples using different NMR permeability models. Results show that the fracture-pore coupling in multiple-porosity systems may cause underestimation or overestimation of permeability when using the Coates model, which relies on differentiation of movable fluids and irreducible fluids. This underestimation or overestimation is influenced by aperture of micro-fractures. However, when using NMR permeability models relying on geometric mean of T2 distribution (T2GM), such as the SDR model and the recently introduced NMR directional permeability model, the influence of fracture-pore diffusional coupling can be negligible. We observed that in multiple-porosity systems where fracture-pore diffusional coupling is significant, the SDR and NMR directional permeability models are more reliable than the Coates model. The outcomes of this paper enable improved permeability assessment in formations with complex pore structure, such as fractured carbonate and organic-rich mudrock formations.
