Recovering acid after a stimulation job can prove to be very challenging in sandstone reservoirs, especially if the well has a potential to produce H2S. When the customer's gathering station is unequipped to handle acid or water, then it becomes impossible to flow the acid via the flow lines. The acid must be disposed at the surface in a separate facility as fast as possible. Allowing the acid to remain downhole for long time carries the risk of damaging the sandstone reservoir due to secondary precipitations. The customer, as per standard operating policy, does not allow flowing back fluids into their pits. Only return tanks are allowed, adding to the difficulty of flowing back stimulation fluids.
With the help of concentric coiled tubing, an innovative technique to produce back acid from H2S wells became the sole solution to a major flowback challenge in South Oman wells. The technology successfully enabled the recovery of stimulation acid from a customer's well that had high H2S content, without producing any H2S at surface. The unique approach consisted of mixing an H2S scavenger with a scale inhibitor. This mixture was used as the power fluid for the concentric coiled tubing tool. With the help of the good shearing energy at the WellVac BHA, combined with the small concentric coiled tubing return annulus, the H2S scavenger solution was perfectly mixed downhole with the return fluids, and no H2S was produced or recorded at the surface return tanks. This new technique will certainly open new possibilities to stimulate many more wells that could not be stimulated previously due to high H2S content. The technique would also eliminate the great costs of a portable flare system.