Experimental Study of Stress Dependent P-and S-wave Velocities in Both Dry and Saturated Sand
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Vp/Vs ratio is known as lithological and fluid prediction method as well as key part of AVO analysis. This diploma thesis is based on the based on the theoretical and mainly experimental studies to evaluate the behavior of velocities with stress and strain with different stress path. As the physical and chemical behavior of the sediments change with change in overburden stress which has not only influence on the sediments grain structures, sizes, angularity and sorting but it has also a strong influence on elastic and acoustic properties of the rocks. The main idea of the present study was to find that at what extent pore fluid has an effect on the acoustic properties of the sand and sandstone. So for this different experiments has been performed in the laboratory in an oedometer setup with different percentage of brine and also with different grainsized samples both for dry and saturated cases to see the clear effect of fluid on velocities and also the velocities behavior with stress. Data included both the P- and S- wave velocities, strain sensitivity with stress and also porosity change with stress. In the present study the for case#01 in which Ottawa (OT) sand with different percentage of brine were tested in uniaxial conditions performed in an oedometer setup. The vertical P- and S- wave velocities are found much higher with high concentration of the brine which could possibly because of the sample stiffness which increases with an increase in brine percentage. Moreover the changes in velocities are also found because of an increase in net vertical stress which needs to be considered as well. Similarly the Vp/Vs ratio decreased with in an increase in external stress but at lower stress level it seems to be relatively stable. For the first case stress sensitivity with strain, as all the samples were compacted about 11.0 MPa net vertical stress, so it can be termed as consolidation samples, therefore less strain sensitivity would expected. But the samples with high concentration of brine show high strain sensitivity and viceversa. All the samples show the strain hysteresis because of their inelastic behavior as none of the samples returns to its original shape. Porosity reduction with stress was not found higher so it was more or less stable. For the second case#02 in which different samples were used with different grain sizes but with same percentage of brine to see the possible fluid effect on velocities and also grain size. But it was clearly observed that P- wave velocity after fluid substitution went higher as compared to the velocities in dry samples. However there is no such change has been found in case of S- wave as it doesn t have any effect with the fluid. In this case velocities were found also stress sensitive as in first case. Similarly there was not such difference has been observed in velocities due to change in grain sizes. While the behavior of, stress strain sensitivity and porosity reduction with stress was found more or less same as in case#01 as discussed above.