基于碳酸盐岩酸化溶蚀形态的酸液最优注入速度界限

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

成功的碳酸盐岩基质酸化是在储层中形成几条主要的酸蚀孔道(蚓孔),穿过井筒附近污染带形成油气渗流的优势通道。储层中蚓孔的分布形态决定酸液的穿透距离及其酸化效果,而酸液的注入速度又会直接影响到酸蚀蚓孔的扩展。选取碳酸盐岩岩心,采用岩心驱替试验研究不同注入速度对蚓孔扩展的影响,通过计算突破体积比寻找碳酸盐岩酸化时酸液的最优注入速度,并通过压降曲线分析酸蚀蚓孔的生长过程。同时采用核磁共振成像技术分析酸化前后岩心端面及内部结构特征,用于判断酸液溶蚀类型,寻找到不同溶蚀形态所对应的酸液注入速度界限。结果表明,盐酸质量分数为20%时,开始形成酸蚀蚓孔时的注入速度为2 m L/min,注入速度为3~4 m L/min时酸化效果最佳。

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 of20%),and the experimental results show that an injection rate of2mL/min can initiate the formation of wormholes,and injection rate of3-4mL/min can result in the optimum acidification effect.