Fracture orientation and fluid flow direction recognition in carbonates using diffusion-weighted nuclear magnetic resonance imaging: An example from Permian

The vertical permeability of carbonate reservoir rocks bearing complex, diagenetically altered pore systems may neglect fluid flow possibilities in remaining directions. Therefore, the main aim of this article was to present an alternative method for the potential examination of 3D fluid flow possibilities using diffusion-weighted NMR imaging (DWI). A cylindrical plug sample was withdrawn from the Permian Zechstein Limestone's drill core (West Poland). The plug was selected upon microscopic research, according to which it contained numerous, bendy dissolution channels formed after former foraminifer enaustations surrounding the bryozoans (ideal for DWI tests). Firstly, the diffusion coefficients (D) were calculated by comparing the signal components with (S) and without (S-0) the diffusion weighting. The former was obtained under the presence of three, separately activated gradients, acting perpendicularly to each other (x, y, z). The diffusion coefficients were examined within xy, xz and yz planes (in 3 mm-thick slices) and utilized to construct 3D models, presenting their values in space. Diffusion vectors were constructed based on the average D values and helped to resolve channels' orientation. The spatially presented values of diffusion coefficients themselves allowed for drawing preliminary conclusions regarding the fluid flow possibilities. For comparison, this kind of information was supported by Kozeny-Carman's formula-based and nitrogen (N-2) permeability derivation. In case of the former, the surface to volume ratio (SN) was derived via PGSTE experiments, through the linear fitting to the normalized, time-dependent diffusion coefficients, in the short diffusion time regime. The effective NMR porosity (12.1%) was measured using a low-field, 0.05 T spectrometer with the echo time of 100 mu s. The T2 cut-off value was assigned by comparing the 70 DC-dried and saturated sample's response. The potential fluid flow through the sample was found to be chiefly controlled by the z-oriented (nearly vertical) structures, with a significant support of the x-oriented pores, generally confirmed by the vertical permeability of the sample, ranging between 100 (N-2) and 169 mD (DWI). It was also concluded that DWI allows for spatial, quantitative analysis of flow possibilities through fractured or highly dissolved carbonate samples. Despite their relative expensiveness, the 3D models of diffusion coefficients could be considered as an alternative or addition to the standard permeability tests in the case of complex pore systems. It also seems that combining DWI and X-ray microtomography in future work could help to bridge the gap between resolution limitations and dynamically resolved fluid mobility, which can be translated into pore system connectivity. Finally, the construction of diffusion vectors appeared to facilitate magnetic field gradient's direction calibration, which should depend on the trend of pore space elongation, thus giving more reliable, diffusion-based permeability values. (C) 2020 Elsevier B.V. All rights reserved.