Velocity-strain relations from hydrostatic measurements on multiple lithologies
Proceeding of the 17th International Congress of the Brazilian Geophysical
We investigate the velocity-strain sensitivity on numerous samples from the entire stratigraphic column in the Santos and Campos basins, Offshore Brazil. The samples all measure P-wave velocity as a function of applied effective pressure during hydrostatic loading. We introduce a transform to derive the sensitivity of vertical P-wave velocity to vertical strain, from the observed velocityeffective stress relationship. This allows to calculation of 𝑅 −factors, a common parameter to integrated 4D
... seismic and 4D geomechanical data, from the velocity measurements. The non-linearity in the relationship between P-wave velocity and applied effective stress causes a non-linearity in 𝑅 −factor as a function of initial stress state at which the 𝑅 −factor is evaluated. This naturally explains a depth dependence of 𝑅 −factor in field data. The derived 𝑅 −factor from laboratory data are found to be an order of magnitude larger than those derived from field observations. This is an effect which is commonly observed, and which to date has not been satisfactorily explained. We propose that the relative magnitudes of 𝑅 −factors between different lithologies are a useful diagnostic. We find that shales, marls and pre-salt carbonates have an increased 𝑅 −factor by a factor of approximately four compared to unconsolidated sandstones. Conversely, evaporites (halites and unhydrites) show low 𝑅 −factors of approximately 0.5 of those observed in unconsolidated sandstones.