A Flow Model Based on the Structure of Heterogeneous Porous Media

Abstract

Abstract Laboratory and field experiments on carbonate core samples suggest that heterogeneities present on a centimeter scale control flow distribution. Heterogeneities on this scale are very important to local mass transfer process in this type of rock. A three-parameter capacitance model called the ‘pore diffusion model’ has been used to simulate the nonideal features observed in carbonate tracer tests in both the laboratory and the field. The results from the lab tests indicate that even though the pore diffusion model can match experimental data very well, the model does not represent all the true mechanisms of mixing which take place in flow through vuggy cores such as those tested. As a result an entirely different approach was pursued. Direct observations of flow structure were made using a fourth generation X-ray CT (Computed Tomography) scanner. Local concentration fluctuations on the centimeter heterogeneity scale are quite observable with a CT scanner. Based on physical observations made using CT, a three-dimensional network cell model is proposed for this carbonate core sample. The model is a convection dominated dispersion model, and is based fundamentally on the heterogeneous nature of the rock. Dispersion behavior qualitatively similar to that predicted by the capacitance-dispersion model is shown to be produced by convection alone. This work confirms that the nature of dispersion in highly heterogeneous porous media, at least on a laboratory scale, is convection-dominated rather than diffusion-dominated. Flow models based on convective mixing are more appropriate to characterize dispersion in laboratory scale systems compared to models that superimpose a diffusional process upon a convective flow field.