Novel Single Phase Retarded Acid System for Effective Stimulation of Limestone Reservoirs

Abstract

Abstract HCl based stimulation fluids occupy the dominant space in matrix acidizing of limestone reservoirs. To initiate wormhole propagation at high temperature/low injection rates and provide deeper penetration, the reactivity of HCl is generally retarded by emulsifying it with an external hydrocarbon phase like diesel. Although it solves the issue of high HCl reactivity, its performance is found to compromised for reservoirs with bottom hole temperature more than 135°C where the emulsion suffers from stability issues. The high viscosity of emulsified acid restricts the pumping rate especially for the cases where the well is having ESP completion or the job is being done through CTU. Also, around 30% of emulsion comprises of diesel which poses health hazard, reduces rock dissolution capacity and incurs extra cost. To counter these challenges, this paper presents a new single phase/aqueous retarded acid formulation that consists of special water-soluble acid retarders which slow down the rate of consumption of HCl. The retardation effect observed is even more than what is observed with emulsified acids while maintaining a high dissolution capacity and low fluid viscosity. A comparative analysis of acid retarders on HCl reactivity has been shown through bench top reaction studies. Suitable corrosion inhibitors compatible with the acid retarders have been identified and the final formulation has been tested through core flooding experiments coupled with Micro CT scan for its effectiveness. Results from lab study indicate that the single-phase retarded acid system requires lesser pore volumes to breakthrough than the conventional acids over wide range of injection rates. Also, dominant wormhole regime has been observed with its use under the conditions where facial dissolution and conical wormholes were found with plain acid and emulsified acid respectively. The developed formulation will be very helpful in increasing the effectiveness of stimulation in limestone reservoirs with special applications in high temperature, horizontal and injectivity compromised wells. Replacing the emulsified acid application with the developed formulation will help in reducing carbon footprint and make job execution simpler, safer and easier.