A Method of Making Two-dimensional Glass Micromodels of Pore Systems

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Abstract
Two-dimensional micro models of pore systems can be made in glass by use of a photo-imaging technique followed by chemical etching of the glass. The method is a modification of one used for making printed circuits in the electronics industry. Pore systems with many thousands of Individual elements of specified geometry and arrangement can be fabricated. Such models are useful for observing the effects of pore and fluid variables on the trapping and subsequent mobilization of residual phases during secondary and tertiary recovery processes. Introduction This paper describes a method for making transparent, two-dimensional micro models of pore systems in glass. The displacement of one fluid by another in a porous medium, say water displacing oil in a reservoir rock, is affected by the geometry and surface properties of the pores in the medium, the properties of the fluids and their interactions with each other and the solid surfaces, as well as on the forces acting on the system. The complex interactions among moving multiphase fluids and porous media can be observed conveniently in transparent, two-dimensional micro models and this can provide a better understanding of the processes. The advantage of two-dimensional models over other idealized porous media, such as packs of glass beads, is that the geometry and topology of the system can be specified precisely. A disadvantage is that three-dimensional networks have topologic properties not shared by two-dimensional networks. For example, bicontinua cannot exist in two dimensions(1). Attempts have been made in the past to mechanically scribe a pattern on wax-coated glass and to etch this pattern with hydrofluoric acid(2). However, such a process relies on the manual dexterity of the model maker and does not lend itself to making complex or repetitive patterns. Others have used mono-layer packs of glass beads successfully(3.4), but such systems do not allow precise control of pore geometry nor wide variability of pore geometry within a single system. Bonnet and Lenormand(5) and Mahers et al.(6) (1981) produced micro models in nylon film using a photo-etching procedure similar to one commonly used for making printing plates. The advantage over previous methods is that large and complex models composed of many thousands of pores of specified geometry and arrangements can be photo-reduced to give pores of sizes approaching those commonly found in reservoir rocks. The technique described in this paper uses photo imaging followed by chemical etching in glass and has the limitation that pores less than about 20 μm in radius are difficult to etch with precision. However, in the case of pores bounded by glass, the roughness and wettability are more easily controlled than in models with surfaces of nylon or resin. The method is similar to one used for making printed circuits in the electronics industry, but modification-is necessary because the available photo resists cannot withstand exposure to hydrofluoric acid and the resultant etch in the glass is shallow and commonly imperfect. Davis and Jones(10) used similar techniques with photo-sensitive resists and acid water.

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