Acid stimulation is one of the most effective methods to increase the well productivity and injectivity in carbonate formations. Hydrochloric acid (HCl) has been widely used with high success rate; however, the high reaction rate between the acid and the rock and hence short wormholes would limit the effectiveness of the acid stimulation particularly at high temperatures. Therefore, several acid systems were developed to retard reactivity with the rock such as emulsified acid and organic acid. This study details the evaluation of a newly developed aqueous single-phase acid for carbonate stimulation. The retarded acid system is mainly composed of hydrochloric acid and non-ionic surfactant in addition to the common acid stimulation additives such as corrosion inhibitor and iron control agent. Static dissolution tests and coreflood experiments have been performed over a wide range of parameters including temperature, acid, and surfactant concentrations to investigate the performance of the acid. The static carbonate dissolution experiments were performed to measure the amount of rock dissolved in the tested acids at comparable conditions. The acid systems were arranged in descending order according to the amount of dissolved carbonate i.e. plain HCl, 1% surfactant with HCl and 5% surfactant with HCl indicating that the surfactant has a retardation effect. The coreflood results are in good agreement with the dissolution tests. The pore volume to breakthrough i.e. the required volume of acid to create a wormhole in the carbonate core, was found to decrease as the surfactant concentration increases. The addition of 1 wt% surfactant resulted in a 20% reduction in the acid volume required to create wormholes, while as the addition of 5 wt% surfactant resulted in more than 30% reduction in the acid volume required for stimulation. Therefore, the newly developed retarded acid would improve the effectiveness of carbonate stimulation by creating conductive fractures and long wormholes.

Systematic Investigation of retarded acid for carbonate stimulation

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