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.