Injection rate optimization for acidizing process of carbonate rocks based on dissolution morphology

During acidizing process of carbonate rocks, it is important to generate several main acid dissolution paths (i.e. wormholes), which can provide favourable oil and gas flow channels through the contaminated zone near the wellbore. The distribution and characteristics of the wormholes play a critical role on acid penetration distance and acidification effect, and the development of wormholes is directly influenced by the injection rate of acid fluid. In this study, typical carbonate rock cores were used for acid injection and displacement experiments, during which the effect of injection rate on the formation of wormholes was investigated. In the experiments, the breakthrough volume ratio was used to determine the optimum injection rate of acid fluid, the pressure drop crossing the core can be used for analyzing the development process of the wormholes. Meanwhile, a nuclear magnetic resonance (NMR) imaging technique was used to analyse the surface and internal structures of the cores before and after acidification, so as to reveal the acid dissolution types and morphology of the wormholes. For the coring injection testing, the acid breakthrough volume ratios at different injection rates were calculated and compared when a hydrochloric acid solution was used (with HCl of 20%), and the experimental results show that an injection rate of 2 mL/min can initiate the formation of wormholes, and injection rate of 3-4 mL/min can result in the optimum acidification effect. © 2017, Periodical Office of China University of Petroleum. All right reserved.