Using rock physics for constructing synthetic logs

Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Abstract The generation of detailed well-to-seismic ties is an essential
part of reservoir geophysics. This kind of geophysical log
analysis requires the existence of sonic and density logs from
seabed to target depth. However, due to practical difficulties
associated with hole size and economical considerations such
logs are rarely available for the entire depth interval. Hence, one
needs to create synthetic sonic and density logs. In this study we
have used a compaction model and a rock physics model for the
generation of porosity and sonic logs, respectively. The
evolution of porosity with depth assumes an exponential form
and it is based on rock mechanical arguments. The sea floor
porosity is computed from an equation that links the shale
volume with mean sediment size. Subsequently, a rock physics
model is utilized for calculating the shear and bulk moduli for
shale from the synthetic porosity log. The rock physics model
assumes that shale is composed of co-centric clay spheres,
bound water and free water. This model can be used for
computing continuous Vp and Vs logs from the synthetic
porosity logs. The modelled velocity logs are compared with the
sonic logs for the North Sea wells. We observe a good
correlation with the synthetic and real data. This kind of
technique can be used for obtaining a realistic p- and s-wave
model for the whole well if accurately calculated shale volume
logs are available. In addition, the comparison of synthetic
porosity logs with a normal compaction trend can provide an
estimate of the pore pressure evolution at the well location.
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