Effect of waterflooding history on the efficiency of fully miscible tertiary solvent injection and optimal design of water-alternating-gas process

• Corefloods were performed for proper WAG design.
• When a WAG project starts with a solvent, more efficient oil recovery is obtained than starting with water.
• WAG injections with larger slug sizes ultimately recovered more oil (while lowering cost).
• The highest solvent retrieval was obtained when the process was started with solvent.

Miscible gas injection is one of the most common tertiary recovery methods applied at the mature stage of a reservoir, usually after severe waterflooding. This application is typically done in the form of water alternating gas (WAG) rather than continuous injection of expensive gases. In this process, the performance of miscible gas, especially if it is water insoluble hydrocarbon gas, injection is affected by the amount of water exsiting in the reservoir.To study this effect of preceding water injection on-fully-miscible displacement in water-wet media and propose optimal WAG patterns, a set of experiments were performed on a sand pack model. The results showed that the amount of water existing in the system significantly reduces the miscibility of gas with oil and, therby, the ultimate recovery. However, a critical amount of water was found at which this effect becomes more prominent. It was also observed that a WAG cycle starting with gas as the first injection phase results in more effective recovery than the same WAG cycle with water as the first injection phase. The effect of water in the system during the following cycles is not as critical as in the WAG cases that start with waterflooding. Finally, the optimal design constrained by the injection scheme design (gas injection whether on its own or with water), slug sizes, time to switch to gas injection after first waterflooding, and injection patterns were presented. Considerations were also given to solvent retrieval in proposing the optimal injection schemes.