Two wastewater disposal wells in a carbonate reservoir in Saudi Arabia suffered loss of injectivity due to severe formation damage. A thorough experimental study was conducted to evaluate the use of acid-in-diesel emulsions to stimulate these wells, which had several tight zones. The emulsified acid consisted of 70 vol% of 15 wt% HCl, 30 vol% diesel, and an emulsifier. This is the first time emulsified acid has been used to stimulate disposal wells.
Experimental results indicated that the acid-in-diesel emulsion behaved as a shear-thinning fluid. The stability and reaction rate of the acid with reservoir rocks were found to be a function of emulsifier concentration. Coreflood results showed that the emulsified acid formed wormholes in tight carbonate cores. Permeability ratio (final/initial) of reservoir cores increased exponentially with the acid injection rate.
The acid-in-diesel emulsion was used to acidize both disposal wells. The treatment included an in-situ gelled acid stage for acid diversion, and an emulsified acid stage to create deep wormholes in the reservoir. The treatment was very successful and the injectivity of both wells has significantly increased. Field data indicated that longer soaking times were needed to stimulate disposal wells. This is to ensure complete acid spending.