Use of Pore Network Models to Simulate Laboratory Corefloods in a Heterogeneous Carbonate Sample
Authors: Xu B. , Kamath J. , Yortsos Y.C. , Lee S.H. 
Published: December 2007

Abstract
Many carbonate rocks have pore features that are large at the core plug scale. Laboratory assessment of recovery behavior in these carbonates can be unreliable. Pore network modeling offers an approach to improve our analysis of complex flows in such heterogeneous rocks. We have discussed earlier our experimental data on a heterogeneous dolomite, the calibration methodology to link pore model inputs and laboratory data, and our initial attempts to use pore network models to match the experiments. This paper reports new results on our attempts to match the coreflood data. We analyze spatial correlation in our core sample using thin section and Computed Tomography (CT) data. We also investigate further the derivation of throat sizes using mercury injection data. Finally, we compare pore network model predictions to coreflood data.

Introduction
Carbonate rocks often display heterogeneities significant at the core plug scale. Conventional analysis is likely to be inadequate, and it is not surprising that the literature contains few references to multiphase flow studies on such rocks. A review of studies on carbonate systems by Espie et al. shows a wide range of process efficiency. They find that the residual oil saturation to water flood varies from 28 to 80% oip and that to tertiary miscible flood varies from 0 to 50% oip.
Pore network (PN) models (figure 1) attempt to relate macroscopic behavior of porous media to simplified representations of the flow physics and pore structure. The general PN models consider capillary, viscous, and gravitational forces. They are useful for understanding multiphase flow mechanisms and the effects of pore scale heterogeneity. However, except for a few cases, pore network models have not been linked directly to data on real rocks. Technological advances in computing and flow visualization are now allowing us to connect laboratory tests and network models more closely. For example, we can now run 500 000 node models to recreate fine scale CT flow images.

Experimental Data on Carbonate Sample
In an earlier paper, we have discussed our experimental data on a dolomite rock from the Beaverhill Lake formation, Canada (k=300md, =14%, L=4.9 Cm, d=3.8cm). Figure 2 is the CT derived porosity distribution along a cross-section parallel to flow. It shows that the porosity is heterogeneously distributed. The thin section picture in figure 3 displays millimeter size pore features. Figure 4 is a CT image of a unit mobility, equal density miscible flood and it reveals that the flow channeled along a connected high porosity path.
Figure 5 contains cross-sectional images of water saturation obtained at Swi, and at the end of different rate water floods. These images correspond closely to the porosity image in figure 2. At the flow rate of 0.5 feet/day, the water mostly invades the high porosity, high permeability channel. High oil saturation of 67% PV remains in large unswept areas. Completely new channels are invaded as the flow rate is increased and the residual oil saturation at the flow rate of 120 feet/day has decreased to 37% PV. This waterflood is a drainage process and new channels are invaded as the flow rate increases. We were unsuccessful in using conventional analysis to extract meaningful information from this data.
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