The acid sensitivity test has been carried out to evaluate the ultra-low permeability reservoir in structure-B of Bohai Bay. Combined with the analysis and test data such as casting thin sections

scanning electron microscope

nuclear magnetic resonance and X-ray diffraction

the reason of rock pore structure changes before and after the acid-rock reaction has been analyzed

and the applicability of the mud acid acidizing reservoir has been discussed. The results showed that the reservoirs of the glutenite and gneiss in the study area are basically unharmed after reacting with the soil acid（7.2 %HCl+3 %HF）. After the acidrock reaction of the samples in the study area

the micropores and the dissolution pores of feldspar particles develop

no carbonate or other soluble minerals are found

the effective face rate and data of large and small pores increase

the cut-off value of T2 spectrum and irreducible water saturation decrease

and the pore structure become better. Meanwhile

the clastic grains of the glutenite are still in linear-convexity contact

the mineral metamorphic grains of gneisses reservoir are still in granular close contact

and the reservoir framework is still stable. The acidification and dissolution of soluble carbonate cement is the key factor to change the pore structure of reservoir in the study area. Secondary precipitation caused by acid-sensitive mineral chlorite and blockage of pore throat by insoluble particles have little damage to the reservoir. After the reaction of soil acid with the reservoir in the study area

the pore structure is improved

showing a constructive role

which has a certain applicability to improve the reservoir permeability.