Stimulation of water-disposal wells using acid-in-diesel emulsions: Case histories

Two wastewater disposal wells in a carbonate reservoir in Saudi Arabia suff ered loss of injectivity due to severe formation damage. A thorough experim ental study was conducted to evaluate the use of acid-in-diesel emulsions t o 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. Thi s is the first time emulsified acid has been used to stimulate disposal wel ls. Experimental results indicated that the acid-in-diesel emulsion behaved as a shear-thinning fluid. The stability and reaction rate of the acid with re servoir rocks were found to be a function of emulsifier concentration. Core flood results showed that the emulsified acid formed wormholes in tight car bonate cores. Permeability ratio (final/initial) of reservoir cores increas ed exponentially with the acid injection rate. The acid-in-diesel emulsion was used to acidize both disposal wells. The tr eatment included an in-situ gelled acid stage for acid diversion, and an em ulsified acid stage to create deep wormholes in the reservoir. The treatmen t was very successful and the injectivity of both wells has significantly i ncreased. Field data indicated that longer soaking times were needed to sti mulate disposal wells. This is to ensure complete acid spending.