In naturally fractured reservoirs, the gas-oil gravity drainage of stacked blocks may result in an oil flow through the matrix blocks or within the fractures network. This laboratory study outlines factors affecting the distribution of oil fluxes in the fractures and in the matrix blocks.

Gravity drainage and spontaneous reimbibition tests were carried out on single blocks and on stacked blocks with capillary continuity. Experiments were performed on blocks with sealed (ID case) and open (3D case) lateral interfaces. The oil can be expelled out of the matrix and then either flows in the fracture or is reimbibed in the lower blocks. Both streaming and drainage oil rates were separately measured for various flow rates on the upper block of the stack.

Causes of the flow out of the matrix and reimbibition are discussed. They are mainly due to heterogeneity of the petrophysical properties along the stack: permeability, capillary pressures for drainage and imbibition, saturation history, fractures capillarity. The spontaneous flow rate of oil through a matrix block is not only controlled by the block permeability. It also depends on boundary conditions imposed by the upper and lower blocks due to the capillary continuity through the fractures. All the oil flowing in the fractures is not systematically reimbibed in the lower matrix blocks, as previously thought. This is essential for the production forecast of fractured reservoirs.

Numerical simulations were conducted using a finely gridded single porosity model. Comparisons with experimental results have outlined the sensitivity on some basic parameters, i.e. the capillary pressure curves and their hysteresis.

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