Complex heterogeneous reservoir exhibits special characteristics, such as multiple storage types, wide range of storage sizes, complicated storage combinations, and various flow behaviors. Most fracture-matrix dual-porosity models in current simulators have difficulties to capture the complicated fluid flow in those complex reservoirs and are not well suitable to be used directly. Some numerical and experimental studies are in process but they have not been available in field practice yet.

This paper considers various impact complex reservoir characteristics and proposes an approximate numerical simulation method on the basis of dual-porosity theory and current simulators. The simulators are extended to more accurately model flow behaviors in the complex reservoirs. Several steps developed in this paper are as follows: First, reservoir storage is categorized into continuous and non-continuous media according to storage scale characteristics, distribution and intensity. In general, small cavities, small fractures, and pores in the rock are continuous while large-scale fractures, big cavities or caves are non-continuous. Second, an M-medium model is determined and applied to the continuous region which has N storage types. M is less than or equal to N. The determination of N is illustrated through a dual-porosity model: a continuous medium with two different storage types (such as vug and fracture) or different scales should be treated by a dual-porosity model only if the direction of the flux is ignorable. A new calculation for the flux transfer is then derived for such fracture-vug dual-porosity system. On the other hand, if the flux is directional, a single-medium model is sufficient to the reservoir simulation. Third, flow in a filled non-continuous medium is described by Darcy's law. Free flow in large unfilled cavities or caves follows Navier-Stokes equation and an approximate approach to using Darcy flow simulation is provided. The determination of the critical value for the cavity permeability is illustrated through a cross-sectional model. Lastly, the complex fracture-vug reservoir is simplified into four types of dual-medium models.

The proposed method is applied to a naturally fractured carbonate reservoir with cavities in the Tarim Basin in China. The simulation results match production data and meet the accuracy required in engineering.

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