Abstract: Although alkali sensitivity of reservoirs has been discovered for several decades, it has not been paid much attentions by petroleum engineers in China. It began to attract considerations from the petroleum engineering sector when the fact that alkali sensitivity damage from salted kill fluids with higher Ph was recognized to be a critical factor causing drastic decrease of oil production for well Donghe 1 which was an important discovery well in Tarim Basin occured. Petrographic analysis indicates that kaolinite kominates clay minerals while the content of swelling clays is much lower than that of kaolinte. Most of the clay microstructures are relatively stable. These facts determine that the critical factors of potential damage must be alkali sensitivity. Core flow tests show that alkali sensitivity is the most severe sensitivity among flow rate, fresh water, HCL acid, salted water sensitivity. Simulations of working fluid - core interations proved that working fluids with high Ph (higher than 9) can
cause decrease of core permeability in a great magnitude. Analysis of operations and well test information further provide evidences to verify that salted kill fluid with high pH is a critical factor for the decrease of oil production. Mechanisms of alkali sensitivity include changes of electric property on the edge of clay crystals that lead to failure
of clay microstructures, cation exchange, and deposition of new minerals. It is a curious coincidence for the example given in this paper that salted kill fluid with high PHseriously damages the reservoir; at the same time,
the most severe factor of potential damage is also alkali sensitivity. Finally, a few techniques to alleviate damages from alkali sensitivity are presented.

Key words: Formation damage, Clay minerals, Core analysis, Reservoir sensitivity, Clay microstructure