Fabrication of a new class of porous media models for visualization studies of multiphase flow processes

The conventional wet-chemistry techniques used for the fabrication of porous media models exhibit some shortcomings with regard to the control of the structural parameters of pores etched either on glass or on plastic plates. A new method of fabrication of pore network micro-models, using an excimer laser LIGA technique, is presented. First, the microstructure is etched on a thin PMMA layer by using as input data the pore depth distribution (10–25 μm) and the pore width-to-depth aspect ratio distribution (∼1–4). Then, the void space is filled with a layer of nickel (total thickness∼300 μm) which, in turn, is covered by a thick layer of copper (∼1700 μm) by using micro-electroforming. Finally, and after a series of mechanical treatments, a metal insert, which is a negative replica of the target microstructure, is produced and used for the printing of a large number of identical structures on PMMA plates with hot embossing. Each plastic model is glued with a thin PMMA cover foil by using a spin coating technique. In this manner, pore network models of well-controlled pore dimensions are produced. It is found that the depth of pores is well-controlled, whereas the depth of each intersection region (node) is almost equal to the sum of the depths of the two intersected capillaries (bonds). The aforementioned features in combination with the broad pore depth and width range (10–100 μm) make the new micro-models representative of real porous media such as natural formations (e.g. sedimentary rocks, soils). Preliminary experiments of two-phase immiscible displacement performed on the new models confirm their applicability to visualization studies of multiphase transport processes in porous media.