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Optimum Fluid Injection Rate in Carbonate Acidizing Based on Acid Dissolution Morphology Analysis

Qi, Ning, Li, Boyang, Chen, Guobin, Fang, Mingjun, Li, Xiaqing, Liang, Chong
Energy & fuels 2017 v.31 no.12 pp. 13448-13453
acidification, carbonate rocks, fuels, hydrochloric acid, magnetic resonance imaging, nuclear magnetic resonance spectroscopy, oils, seepage
Successful carbonate rock matrix acidizing should contribute to the formation of several main acid dissolution holes (wormholes), which provide favorable oil and gas seepage channels through the contaminated zone near the wellbore. The distribution and characteristics of wormholes play a critical role in deciding the acid penetration distance and acidification effect, while the development of wormholes is directly influenced by the acid fluid injection rate. In this study, carbonate rock cores are selected for displacement experiments, during which effects of different injection rates on wormhole development are compared. The breakthrough volume is a key parameter in determining the optimum acidizing fluid injection rate, while the pressure drop curve is used to analyze the development process of wormholes. Meanwhile, nuclear magnetic resonance imaging technology is employed to analyze the core end surface and internal structure after acidification to identify acid dissolution types and further determine the corresponding boundary of fluid injection rates for different acid dissolution types. The acid breakthrough volumes with different injection rates are calculated and compared when the hydrochloric acid concentration is 20%, which comes to the conclusion that the injection rate of 2 mL/min initiates the formation of wormholes and the injection rate of 3–4 mL/min results in the optimum acidification effect.