Stimulation of Water Disposal Wells Using Acid-In-Diesel Emulsion: Case Histories

Abstract

Abstract Two wastewater disposal wells in a carbonate field in Saudi Arabia suffered loss of injectivity due to severe formation damage. Lab tests conducted on reservoir cores indicated that regular 15 wt% HCl did not form deep wormholes and caused surface wash-out only. A thorough experimental study was conducted to evaluate using 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 (a cationic surfactant). 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 shear-thinning fluid. The stability of the acid was found to be a function of emulsifier concentration. The reaction rate of the emulsified acid with reservoir rocks depended also on emulsifier concentration at the reservoir temperature (55 °C). Very low reaction rates were obtained at emulsifier concentrations greater than 20 gals/1000 gals were of acid. These results indicated that longer soaking times would be needed to stimulate disposal wells. This is to ensure complete acid spending. Coreflood results showed that the emulsified acid formed deep wormholes in tight carbonate cores (< 100 md), where the core permeability increased after the treatment. Permeability ratio (final/initial) of reservoir cores exponentially increased with the acid injection rate. The size and number of the wormholes depended on the acid volume, injection rate and initial core permeability. The acid-in-diesel emulsion (280 cp at 2 s−1 and room temperature) was applied in two wastewater disposal wells. The designed treatment included a 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.