Infuence of crack on acoustic propagation characteristics in low porosity and low permeability gas reservoir

Key words:  acoustic logging  petrophysics  crack  gas reservoir  dispersion  attenuation

Abstract:

Based on the unified elastic wave theory which describes the effect of both pores and cracks, the influence of crack strength on the acoustic velocity of cracked low-porous gas reservoir was studied. According to the compressional and shear wave velocity obtained by measuring the core samples, the estimation method of formation crack density was investigated. The results of numerical calculation show that the elastic wave velocities decrease with the crack density increasing, and the velocity dispersions become obvious. When the porosity is 1.1％, the compressional velocity of gas-saturated rock is lower than that of water-saturated rock. When the crack density is 0.15and 0.35, the compressional velocity for 500kHz decreases by 7.6％ and 27％ respectively. The attenuation of borehole mode waves is more sensitive to gas saturation than the velocity, which provides a new way of identifying pore-fluid nature by acoustic logging technology.