Cyclic water injection is an enhanced oil recovery method in heterogeneous reservoirs with zero additional cost and simple implementation procedure. It has been successfully applied in a number of oilfields in China, USA and Russia. However, although it is effective in conventional heterogeneous reservoirs, it is still controversial whether it is effective in stress-sensitivity reservoirs, the formation permeability of which changes with variation of the reservoir pressure.

X oilfield is a fractured metamorphic reservoir with high-permeability contrast. During the production and injection, the variations of reservoir pressure would alter the apertures of fractures, and then alter the reservoir permeability, which is defined as stress-sensitivity of permeability. In this paper, the correlation between the stress-sensitive permeability and effective stress was obtained by stress-sensitive testing experiments with the fractured metamorphic cores. Cyclic water injection experiments were conducted with the fractured cores as well. To evaluate the impact of the different parameters involved in the cyclic water injection, we make a sensitivity analysis using a numerical model considering the stress-sensitive permeability. Finally, the projects of cyclic water injection were designed by combining the experimental and numerical results for X oilfields.

The experimental results showed that very little oil increments were achieved for cyclic water injection when the shut-in period was long and the rate of oil production decreased rapidly with increasing cycles. However, if the shut-in period was relatively shorter, additional 3.4% of OOIP (oil originally in place) was achieved. The numerical analysis showed the similar results that the reservoirs pressure had an important effect on the effectiveness of cyclic water injection. During the depressurizing half-cycle, the reservoir pressure should keep above 65% of initial reservoir pressure. When the optimal parameters were applied in the X oilfield, about additional 3.9% of OOIP could be achieved.

This work indicated that cyclic water injection is effective in stress-sensitive reservoirs, however, the reservoir pressure could not drop so much.

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