Trapping of spent acid after acidizing job is a major problem in carbonate rocks. Spent acid needs to be fully mobilized and recovered in order to enhance gas production. Wettability of the rocks and capillary forces are the main reason behind fluid trapping in carbonate formations. These forces can be weakened by decreasing surface tension and increasing contact angle. Additives that are injected along with the acid can have an impact on changing the surface tension, contact angle and possibly the wettability of the rocks. Previously, separate studies were conducted on the effect of acid additives on contact angle and surface tension. However, there is still a need to investigate the overall impact of acid additives on these two parameters and wettability on the trapping of spent acid in the carbonate rocks.
In this paper, the full impact of two additives, formic acid and methanol, on trapping and wettability is investigated. Irreducible fluid saturation is compared before and after the exposure of rocks to spent acid. Formic acid decreased irreducible fluid saturation whereas methanol increased irreducible fluid saturation. Spontaneous water imbibition was conducted in each case to ensure that there is no permanent effect on rock wettability as a result of using these additives. It is very important to understand that the effect of various additives on wettability must be studied before any acid job, as their impact on wettability can consequently affect well productivity.