The worlds hydrocarbons reserves have been exploited for over a century, and the amount of undiscovered reserves is dwindling, while the worlds need for energy is increasing (Owen et al., 2010). This pushes hydrocarbon exploration into unconventional reservoirs, often in complex geological structures. Making reliable discoveries in challenging conditions requires an increasing level of detail in seismic interpretation. Advanced seismic processing as Amplitude-Versus-Offset (AVO) analysis, have become commonplace in seismic interpretation. These techniques involves comparison with synthetic seismograms generated from velocity versus depth trends recorded as a sonic log in a borehole. Such a comparison is referred to as a well-tie. A high-quality well-tie requires a highquality sonic log, but shallow depth of penetration makes sonic logs sensitive to invasion of mud-filtrate from the borehole. The pressure in the borehole is kept higher than in the surrounding formation to control fluid production from the borehole. The over pressure in the borehole drives fluid from the borehole into the formation, whereby a sonic log measures velocities at a different saturation than the formation saturation, which the seismic data probes. For a proper well-tie sonic logs therefore require correction.Gassmann (1951) fluid substitution relates elastic velocities of a porous medium at one saturation to the elastic velocities at another saturation. Fluid substation can then predict a sonic log at the undisturbed formation saturation from the invaded saturation. Fluid substitution is exact for a uniform, high-permeable porous medium saturated by a single fluid. Rocks does not necessarily conform to these requirements, and both the formation saturation and invaded saturation involve two fluids. Sonic log correction therefore requires extension of Gassmann fluid substitution to the complex nature of an invaded rock formation.Only sandstones are considered in the thesis. Measurements and model for fully saturatedsandstones quantify divergences from fluid substitution arising from the first few percent of wetting phase saturation, and the clay content. When the water saturation is at the irreducible water saturation or higher only the effect of clay on the elastic velocities have a differential effect on the elastic velocities. Mixed saturations are fluid substituted using effective fluid moduli formulated as a set of bounds by Mavko and Mukerji (1998). Ultrasonic velocity data from the literature shows that these bounds applies most accurately to sandstones with a simple pore-space, as reflected in a high permeability and low clay fraction.
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