Abstract: It is critical to understand the fluid sensitivity of reservoir and its corresponding evaluation method to optimize the selection of working fluids in the wells and their properties.The fluid sensitivity,which is influenced by mineral composition,pore throat characteristics,high-temperature fluid environment,etc,refers to damage of ultra-deep tight sandstone reservoirs.So the conventional evaluation methods in the industry seem to be not applicable any more.The ultra-deep tight sandstone gas reservoirs located in Tarim Basin are taken as the objects in this paper,and a modified fluid sensitivity evaluation method is proposed,which requires the simulation of the formation temperature and maintaining high back pressure on the outlet throughout the experiments.Twelve typical ultra-deep tight sandstone base blocks are chosen to carry out water,salt and alkali sensitivity experiments.The results show that the index of water,salt and alkali sensitivity is 0.41-0.52,0.72-0.73 and 0.83-0.92,respectively.By comparison,the degree of damage derived through this method,which maches well with the field measured data,is higher than that obtained at the room temperature experiments in the past.After analysis the paper indicates that (1) the modified evaluation method could reflect the actual condition of reservoirs and also reduce experimental errors, (2) the narrow pore throat and abundant clay minerals are the fundamental causes of fluid sensitivity, (3) the major mechanisms for increased fluid sensitivity for ultra-deep tight sandstone gas reservoirs,can be summarized as follows:in higher temperature,the thickness of the water film on mineral surface increases,reducing the effective pore throats' radius,and the intensified clay hydration and swelling accelerate the expansion/migration of formation particles and the dissolution/deposition of minerals.

Key words: high temperature, high back pressure, fluid sensitivity, water film, ultra-deep reservoir, tight sandstone, Tarim Basin