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Water-alternating gas (WAG) displacement is a pronounced technique to improve gas or/and water injection in heterogeneous reservoirs. For the last half century it has been successfully applied to more than 60 oilfields worldwide. However, its mechanism still deserves further investigation. In this work, a series of water, gas, and WAG displacement studies were conducted with a sets of glass micro-models which were reproduced from a target reservoir rock by copying the pore structure images into the glass media. The pore size and geometry structure of the glass micro-model are very similar to the original. The proceeding of WAG displacement was video tape recorded versus time so that the kinetic saturation to each phase in the porous media can be obtained though a PC based image analyzer. Corroborated with the measured pressure drop across the model, the effect of WAG on mobility control is systematically studied. The results show that the mechanism of WAG flow in porous media is much different from two phase (gas-oil and water-oil) flow. For water displacing oil, a piston-like displacement mainly occurred in the small pores (assisted with capillary pressure) and a non piston-like displacement mainly occurred in large pores. For gas displacing oil, the piston-like displacement occurred in both large and small pores as long as the viscous forces can overcome the capillary force. In general, the piston-like displacement gives a higher displacement efficiency than non piston-like displacement. Channeling of displacing phase (water or gas) caused by heterogeneous feature of the pore media was observed. However, when three phase (water-gas-oil) flow is involved, the mechanism of displacement is changed. The oil phase becomes intermediate wetting, and the gas becomes non-wetting phase and water phase remains a wetting phase. Water phase prefer to occupying the small pores due to capillary effect, while the gas prefer to occupying the large pores. The oil phase presents in the porous media in a discontinuous status; the feature of oil flow is mainly in the form of oil slug or oil film at the interfaces between water and gas phases. Accumulation and expansion of gas phase from the large pores expel the oil phase into down stream. With the proceeding of WAG displacement, the oil slug becomes smaller and finally turns into oil film; the residual oil saturation in average after WAG displacement is less than 20% OOIP, benefited from a reduced size of oil drop. It is found that pore geometry has a significant effect on residual oil saturation. "Snap off " effect plays a very important role to cut off the oil slug into oil blobs and form gas bubbles during WAG displacement. Increase in cycles of WAG tends to reduce residual oil saturation. Oil blobs trapped in very tiny pores, pore corners, and dead pores are the main type of the residual oils. At the last, an optimal WAG project to the target reservoir will be presented. |
