An oil producing well (Well-A) in a carbonate reservoir was dead due to severe formation damage. The well was acidized by regular 15 wt% HCl, but remained dead. A thorough experimental study was conducted to evaluate using acid-in- diesel emulsions, 70 vol% HCl (15 wt%):30 vol% diesel, to stimulate this well which had several tight zones.
Experimental results indicated that the acid-in-diesel emulsion was a viscous and non-Newtonian fluid. The thermal stability of the acid decreased as temperature was increased. The stability of emulsified acid also decreased in the presence of reservoir rock. The emulsified acid was found to be compatible with native crude oil and acid additives, except mutual solvents and demulsifiers. The reactivity of the emulsified acid with reservoir rock was slower than that of the 15 wt% HCl by a factor of 45 at 24 C. The reaction rate of the emulsified acid increased as temperature was increased.
Coreflood results showed that the emulsified acid formed deep wormholes in tight carbonate cores (< 50 md). At the same acid volume and flow rate, the emulsified acid penetrated deeper into the core compared to the regular acid which caused face dissolution. 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 injection rate.
The acid-in-diesel was successfully applied in Well-A using a coiled tubing unit, whereby the well productivity has been increased by several orders of magnitude. After the treatment, a flowmeter test indicated that the percentage production of tight zones has increased by a factor of 2.