A Laboratory Study of Seismic Wave Velocities in Sandstones Bearing Different Pore Fluids at Different Pressures

: Seismic properties of rocks have wide applications in regional tectonic research and shallow seismic exploration. The Yanchang oilfield is one of the important production bases for oil and gas in China, but lacks basic data of seismic wave properties of rocks. This work applied the AutoLab 2000 computer-controlled servo hydraulic triaxial test system to measure accurate P- and S-wave velocities at pressures up to 180 MPa on three kinds of core samples of sandstones in dry, water and oil-saturated conditions from the Yanchang oilfield. The results show the wave velocities Vp, Vs1 and Vs2 for the three kinds of samples of sandstones change basically logarithmically with increasing (or decreasing) pressures. The relationships between wave velocities in dry, water and oil-saturated conditions indicates obvious differences for three kinds of core samples of sandstones, mainly because rock elastic modulus, pore fluid property, and rock structure are different. For the different kinds of sandstones, the pressure dependence of the Vp, Vs1 and Vs2 in the same pore fluid condition is controlled by the porosity and grain size, and for the same kind of sandstone, the pressure dependence of the Vp, Vs1 and Vs2 in the different pore fluid conditions is affected by the effective modulus of elasticity and density. Moreover, Vs1 and Vs2 hardly vary with water and oil saturation. The experimental results can provide an important basis for the interpretation of seismic data and comparing with well log data in that region.