Abstract: Acidification is one of the most effective injection stimulation methods in the ultra-low permeability reservoirs.These reservoirs however,are usually characterized with poor physical property and high heterogeneity,leading to an unnecessarily long operation period,low efficiency,and insufficient effectively-operating distance for conventional acidification measures,as well as fluid fingering and secondary damage to the reservoir during the acidizing process.In this regard,we studied the Triassic ultra-low permeability reservoir in the Huanjiang oilfield,Ordos Basin,proposed the online diversion acidizing technology and developed COA-1S achelating acid system and COA-1P,a new water soluble diversion agent.After a large number of laboratory experiments,we found that,compared with the conventional acidizing method,this new technology is more retardant,compatible,water-soluble,and better at chelating and temporary blocking.In-situ pilot test has shown that the homogeneity of the water absorption profile has increased from 28.5% to 50.9%,that is,the water injectivity index doubled after the implementation of this technology.Meanwhile,its characteristics of "zero well shut-off and depressurization,zero impact on the tubing strings,and zero flow-back",allows this technology to simplify the operation process of diversion and acidification,reduce the risks in security and environment protection,and thus can be recommended as a new acidification technology for stimulating injection of wells in the ultra-low permeability reservoirs.

Key words: chelating acid, diversion agent, depressurization and injection stimulation, ultra-low permeability reservoir, diversion acidification online