This is an experimental study aimed at improving the recovery of spent acid in carbonate gas reservoirs. When acidizing, the spent acid penetrates deeper into the formation beyond the stimulated zone. The spent acid needs to be removed in order to restore the permeability to gas and enhance well performance. One of the most important factors that control spent acid recovery is rock wettability. In this study, we investigate the changes in the wettability of carbonate rocks when invaded by spent acid experimentally. These changes in the wettability are due to the various additives that are added to the acid treatment.

In this paper, we explain quantitatively the effect of wettability on spent acid recovery and we present experimental results on the effect of spent acid on carbonate rock wettability.

Our results show that carbonate rocks are not strongly water-wet; spent acid (HCl) by itself does not change wettability of the rock; some commonly used additives can change the wettability of the rock either towards stronger water-wetting or less water-wetting.

To the best of our knowledge, there is no published data on the effect of spent acid on rock wettability and how that relates to the rate of recovery and its ultimate value.

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