SIMULATION OF BOREHOLE STONELEY WAVES AND THEIR APPLICATION IN FRACTURED TIGHT SANDSTONE FORMATIONS.

The target stratum of gas reservoirs in the Ku-che area, Western China, is a fractured tight sandstone reservoir with a fracture width of less than 100 jam, which are difficult to identify with conventional and imaging logging methods. The sensibility of Stoneley wave energy to fluid motion in a well makes it possible to evaluate the effectiveness of fractures through Stoneley wave energy (amplitude) information. In this study, the 3-D staggered grid stress-velocity finite difference (FD) method was applied to simulate the acoustic fields of the borehole in the formation with tilt fractures. The effects of different fracture widths and tilt angles on the amplitude of the Stoneley wave were investigated. The results show that the existence of fractures causes the attenuation of Stoneley waves, which is well correlated with the fractures' width. Accordingly, the Stoneley wave energy extracted from the array acoustic data was normalised, and its attenuation coefficient was calculated. According to the simulation results, the fracture width and permeability can be inverted. This method was applied to evaluate the ef-fectiveness of the fractured tight sandstone reser-voirs in the Kuche area, and good results were ob-tained. [ABSTRACT FROM AUTHOR]