It is well known that millimeter to meter scale sedimentary heterogeneities can affect the displacement efficiency of immiscible flooding by the process of capillary entrapment. There are, however, few practical methods that adequately assess the importance of field-scale capillary entrapment. As a result, the effects of capillary entrapment in small scale sedimentary heterogeneity (e.g. cross stratification) are generally ignored.
In this paper, we aim to take a practical approach towards the evaluation of trapping on the field scale. We characterize the most common sedimentary structure found in sandstone reservoirs, the cross bed, by use of a limited amount of parameters. The extent of trapping in any type of sedimentary structure is quantified as a function of the magnitude and direction of the applied pressure gradient in the reservoir. This approach facilitates the calculation of the cumulative effect of a variety of different sedimentary structures present in the vertical succession of reservoir strata. An important aspect of the physical model is that it permits trapping on laminae-scale, as well as trapping on bed-scale. We evaluate the extent of trapping for three different sedimentary flow units; the braided fluvial, the meandering fluvial, and the shallow marine system. The results show a minimal importance of trapping in the shallow marine system due to a low content of trough cross-stratification and very well sorted, reworked sandstone. In contrast, trapping under realistic operational conditions in fluvial systems can easily reach 10-40% of the non-residual oil (I-Swc-Sor). The influence of capillary entrapment in cross stratified reservoirs must be seriously considered during field evaluation.
The scientific challenge of modern-day reservoir technology is to evaluate the physical aspects of multi-phase flow in porous media taking full account of the large-, as well as the small-scale geological features of the reservoir. Large-scale heterogeneity effects the sweep efficiency while heterogeneity on a smaller (millimeter to meter) scale reduces the displacement efficiency. We illustrate these effects schematically in Fig. 1.
Current methodologies in reservoir evaluation are largely dependent on numerical simulation. Simulation grid resolution is sufficient to give a good representation of large-scale reservoir architecture. Therefore, the sweep process can usually be analyzed with sufficient accuracy. For the description of multiphase flow on the smaller (sub-gridblock) scale, which determines the displacement efficiency, numerical simulators rely heavily on the input of relative permeability curves. These curves result from displacement experiments on small core samples. When these samples are sedimentologically heterogeneous, the measured relative permeability curves can vary drastically depending on the experimental boundary conditions. The heterogeneity gives rise to uncertainty which can by far exceed other aspects of the numerical simulations.
The most important effect of small scale heterogeneity on immiscible displacement is capillary entrapment. Laboratory experiments, small scale simulations and simplified analytical calculations 6 all show a significant effect of capillary entrapment on oil recovery. However, the importance of capillary entrapment on field scale has not been established, and is heavily debated. Practical methods to asses the effects of capillary entrapment in centimeter- to meterscale heterogeneities on field scale are presently not available. Consequently, it is impossible to evaluate the economic viability of remedies like surfactant flooding, WAG injection, polymer flooding and in-fill drilling.
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