A Fractured Reservoir Simulator Capable of Modeling Block-Block Interaction

Abstract

A new simulator has been developed to model displacement processes in fractured reservoirs, in particular the block-block interaction in gravity particular the block-block interaction in gravity drainage processes. This block-block interaction forces oil draining from a matrix block into the fracture system to be absorbed by the underlying matrix block whenever this is possible. Otherwise the oil flows through the fracture network. Block-block interaction influences both the initial production rates and the resulting hydrocarbon distribution in the reservoir. Simulation of fractured reservoirs with this type of interaction between matrix blocks makes high demands upon the simulator, both in terms of memory requirements and speed. The simulator hence uses dynamic memory management and an efficient grid-block partitioning scheme that reduces the memory partitioning scheme that reduces the memory requirements significantly. In addition, this scheme enables a high degree of vectorisation on a Cray computer. The simulator can be used for a variety of reservoir geometries: dual porosity, dual porosity/ permeability, reservoirs with block-block interaction permeability, reservoirs with block-block interaction and unfractured reservoirs. Introduction Numerical simulation of fractured reservoirs is usually based on the dual-porosity concept. A fractured reservoir is modelled using two interacting continua, one for the matrix and one for the fracture system. In this way detailed knowledge about the individual fractures is not required as long as meaningful average fracture and matrix properties can be defined. The matrix contains most of the oil and the fractures contribute most to the fluid flow conductivity. Therefore the transmissibility between matrix blocks is often neglected and the matrix is treated as a source to the fracture system. However, to model the ultimate recovery from a dual porosity system correctly, it is necessary to take the capillary contacts between the matrix blocks into account. When capillary contacts exist, the ultimate recovery is the same as if the blocks form an homogeneous column. The rate at which depletion occurs is, of course, lower than in the homogeneous case, this being a result of the small area of the capillary contacts. This effect is modelled by a dual permeability approach with reduced transmissibilities between the matrix blocks. Fractured reservoir research in Shell has shown that in a vertical stack of oil-saturated matrix blocks surrounded by gas, the blocks do not drain independently: oil draining from one matrix block into the fracture system will be absorbed (by gravity and capillary forces) by the underlying matrix block whenever this is possible. Otherwise the oil flows through the fracture network. This type of block-block interaction is considered a crucial factor in describing the behaviour of fractured reservoirs. P. 373

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