Abstract: The original distribution state of formation fluid and the formation resistivity around the borehole will be changed while mud filtrate invades the permeable formations. Based on both of the water-oil phase flow theory and convective diffusion theory, numerical simulation method is applied to build the formation model for the study of the mud invasion characteristics in low permeable reservoir, for studying the distribution characteristics of salinity, water saturation, resistivity and oil/gas reservoir around the borehole during mud invasion. The numerical simulation results of mud invasion in different permeable formation show that mud invasion depth increases as permeability increases in the same invasion time in low permeable reservoir, and the invasion profile of salinity, water saturation, resistivity and remaining oil change obviously as invasion time increases. The numerical simulation analysis of three practical boreholes shows that the array induction logging response characteristics and their resistivity amplitude difference are influenced by mud invasion depth and invasion time. According to the dynamic evolution simulation of different reservoir fluid types around borehole, the distribution of reservoir has obvious relationship with fluid type. The invasion depth calculation formula is obtained based on the results of the case study borehole and the array induction log response, and also the original formation resistivity calculation formula is constructed based on geometrical factor theory according to the invasive step profile for assistant reservoir resistivity correction. Correct understanding the characteristics of mud invasion and the distribution of reservoir around the borehole can improve the accuracy in array induction logging reservoir evaluation.

Key words: low permeable reservoir, mud invasion, numerical simulation, array induction, logging response, resistivity correction