Page 143 - Primer on Enhanced Oil Recovery

Water altering gas injection 133

Figure 11.8 Effects of gas/oil miscibility properties during WAG implementation.
blockage of highly permeable volumes with a water gas mixture and better con-
nection to the zones with low permeability. It has been established that the final oil
recovery of a heterogeneous formation, when using any technology of joint injec-
tion of water and gas, is higher than when oil is displaced only by water or gas.
WAG under optimal conditions increases oil recovery by 7 15% compared to the
conventional water flooding. The best effect of WAG application is achieved if the
injected gas is evenly distributed over the waterflood reservoir and the water and
gas breakthrough to the production wells at the same time. The latter is difficult to
achieve and is not always possible. Consequently, in practice, the efficiency of the
resulting process is lowered. The high efficiency is impossible to achieve in highly
heterogeneous formations.
General screening parameters for WAG can be summarized as shown on
Fig. 11.9.
The injection of alternating slugs of water and gas into the formation or simulta-
neous gas-water mixture through the same injection well also reduces the obtained
oil displacement effect. This is due to the fact that after the first cycle of injection
of working agents, the phase permeability of the around the injection well
decreases, and as a result, the injectivity sharply decreases. This effect can be as
high as reduction of injectivity by 8 10 times for gas, and by 4 5 times for water.
The separation of gas and water in reservoir conditions under the influence of
gravitational forces produces negative effect the displacement of oil and the cover-
age of the reservoir by the method. Depending on the heterogeneity of the reservoir
and the ratio of viscosities of reservoir fluids, the efficiency of the process can be
reduced by 10 20%.
Business considerations come into the effect. For instance, for alternating injec-
tion of water and gas, the equipment of each injection well is significantly more
complicated.
In order to eliminate the above drawbacks careful process understanding and
planning are needed. Injection pressure and slugs volumes should be carefully
determined. A significant increase in the number of injection wells with