Multi-scale, image-based pore network models to simulate two-phase flow in heterogeneous rocks

Over the last couple of years, algorithms have been developed to extract pore network models directly from 3D images of a rock's porosity. These models can be used to simulate capillary pressure curves, (relative) permeabilities and electrical properties during drainage/imbibition cycles. However, heterogeneous rocks with very broad pore-size distributions remain difficult to simulate with these models, as it is difficult to acquire and incorporate information on the different pore scales which are present. Nonetheless, the understanding of the multi-phase flow behaviour of such materials (e.g. many carbonates and tight gas sandstones) is of crucial economic and scientific importance. We build two-scale network models which incorporate microporosity information without taking every individual micropore into account. We start from a micro-CT scan which is segmented into 3 phases (pore, solid and microporous voxels), and extract a maximal ball network [1] from the porous voxels. The microporosity is clustered into 3D connected regions. Pores touching the same microporous cluster are connected by a new type of network element called micro-connections. These connections are assigned conductances based on the local contact surface areas between pores and microporous clusters, as well as on the continuum petrophysical properties of the microporosity it represents. We attempt to assess these properties with mercury intrusion and FIB/SEM experiments. This study shows results of resistivity curves and relative permeability curves calculated from networks based on micro-CT datasets of carbonate rocks. We also show the comparison of results obtained with our networks with averaged microporosity properties to results from a network with individual micropores, described in [2]. The results suggest that our method is a promising tool to simulate core-scale multi-phase flow behaviour of rocks with complex, multi-scale porosities. REFERENCES [1]\0009H. Dong and M. Blunt, textquotedblleftPore-network extraction from micro-computerized-tomography images,textquotedblright Phys. Rev. E, vol. 80, no. 3, p. 036307, Sep. 2009. [2]\0009A. Mehmani and M. Prodanovi, textquotedblleftThe effect of microporosity on transport properties in porous media,textquotedblright Adv. Water Resour., vol. 63, pp. 104--119, Jan. 2014.