Shifting the Stimulation Approach in Carbonate Fields Towards the Use of Single-Phase Retarded Acid: Case Study

Abstract

In carbonate reservoirs, reactive matrix stimulation creates highly conductive channels, which improves hydrocarbon production. Hydrochloric acid is widely used as stimulation fluid; however, its efficiency at high temperature ranges is limited. Other systems, such as organic acids or chelating agents, are used due to their slow reaction rate in hot carbonates; however, they cannot achieve the same volume of rock dissolution per unit of treatment. Seeking to optimize the stimulation process in "T" field in Mexico, the impact delivered by the introduction of a single-phase retarded acid in the stimulation workflow is presented in this paper. A change in completion architecture in "T" wells using slotted liners in the production pay zone is leaving exposed lengths in the carbonates in excess of 200 m, triggering early reaction with the treatment systems. This situation requires the use of reactive and divergent systems that will not spend quickly when in contact with the formation wall to ensure their full strength is used in the productive zones. Based on compatibility and reaction-rate laboratory tests, it was considered to add single-phase retarded acid as the main treatment system to improve the dissolution profile in the producing zone and reduce the reaction in the formation face. With the help of advanced simulation software, longer complex wormholes were modeled for given reservoir conditions, increasing confidence in the fluid selection described in the case study. The single-phase retarded acid showed improvements in production results in "T" field wells with new architecture. The production increase—approximately 25% after treatments in selected wells—were higher when compared to organic acid treatments. This is in part derived from improved wormhole efficiency (as tested and observed in laboratory), coupled with the higher dissolution power. The selected fluids have better solubility and controlled reaction rates, which allowed optimizing the treatment volume, thus helping to increase the profitability of the stimulation treatments in this field. Single-phase retarded acid exhibits a dominant wormhole regime for a wide range of injection rates due to its slow reaction rate. This greatly helps to accomplish the goal of being in a dominant wormhole regime. Having a dissolution capacity practically equal to hydrochloric acid and higher than that of the organic acids or chelating agents allows the optimization of stimulation treatment volumes. The retardation efficiency of this system is equal to that of emulsified acids, while it has a significantly less friction pressure profile and many other operational advantages, allowing more flexibility during job execution to pump at the optimum injection rate. Additionally, the ease of mixing and simple quality control result in faster well delivery and increased system stability at ambient conditions.