Abstract
A generalized network extraction workflow is developed for parameterizing three-dimensional (3D) images of porous media. The aim of this workflow is to reduce the uncertainties in conventional network modeling predictions introduced due to the oversimplification of complex pore geometries encountered in natural porous media. The generalized network serves as a coarse discretization of the surface generated from a medial-axis transformation of the 3D image. This discretization divides the void space into individual pores and then subdivides each pore into sub-elements called half-throat connections. Each half-throat connection is further segmented into corners by analyzing the medial axis curves of its axial plane. The parameters approximating each corner-corner angle, volume, and conductivity-are extracted at different discretization levels, corresponding to different wetting layer thickness and local capillary pressures during multiphase flow simulations. Conductivities are calculated using direct single-phase flow simulation so that the network can reproduce the single-phase flow permeability of the underlying image exactly. We first validate the algorithm by using it to discretize synthetic angular pore geometries and show that the network model reproduces the corner angles accurately. We then extract network models from micro-CT images of porous rocks and show that the network extraction preserves macroscopic properties, the permeability and formation factor, and the statistics of the micro-CT images.
Highlights
Pore-scale modeling has wide applications in petroleum engineering, hydrology, and environmental engineering [1,2]
The results presented cannot be used as a proof that the extracted networks provide a good parametrization of other element properties which are relevant to multiphase flow through porous media
We have presented a network extraction workflow which discretizes the pore space by subdividing it into pores bounded by throat surfaces, and further into half-throat corners
Summary
Pore-scale modeling has wide applications in petroleum engineering, hydrology, and environmental engineering [1,2]. Pore-network modeling divides the void space of the rock into pores representing wider regions that are connected through narrower restrictions called throats This representation is used to model flow by computing and tracking the pore-scale configuration of fluid phases for different displacement sequences using empirical equations derived normally from semianalytical expressions for pores of a simple geometry. The watershed approach divides the void space into pore regions where the distance map increases to a pore center: throats are surfaces of minimal distance separating two pore regions These methods of network extraction have produced reasonable representations of the pore space and have been used to successfully reproduce multiphase flow properties for a variety of rock types [59,62,69,70,71]. The values assigned to the flow properties, deduced from the intermediate parameters (G, R, V , A, L), may not provide a good representation of flow through the original pore space image and may produce significantly different results depending on the algorithms used to calculate them [73]
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