Abstract:

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 acid-rock 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 acidifi cation 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.

Key words: ultra low-low permeability, acid sensitivity, microscopic pore structure, structure-B, Bohai Bay