Low recovery factor is identified as the main problem encountered in the heavy oil production from a strong bottom-water-drive reservoir. Unlike for conventional oils, where the expected recovery from such reservoirs could be very high — in excess of 50 percent, the expected recovery factor in heavy oil water-driven reservoirs is less than 20 percent. In this study, a qualitative analysis of the well productivity mechanisms specific for heavy oil reservoirs with bottom water is provided. The objective is to understand what make the production of heavy oil different to that of lighter oils, identify the mechanism that mostly hamper the well’s productivity and recovery efficiency.
Many believe the by-passed oil due to water coning is the major cause of low ultimate oil recovery in heavy oils underlain by strong bottom water. However, in this paper, we identify another important parameter affecting recovery efficiency in such reservoirs, which hasn’t been recognized by others and its effect on recovery process is significant. The mathematic modeling and numerical study lead to a new finding: due to the aquifer’s influence on pressure response in reservoir, a no-flow boundary at xi is established, where xi is often much smaller than that of the actual reservoir size xe. The oil out to the distance xi is immobile and become bypassed oil, which accounts for large amount of the OOIP. Even the water coning can be effective controlled; the ultimate oil recovery factor will not be improved significantly if the small mobilized oil zone can’t be enlarged. An analytical solution is derived in this paper to calculate the actual drainage radius. The validity of this analytical solution is confirmed by numerical simulation runs.