HR: 0800h
AN: GP41B-0877    [Abstracts]
TI: Link Between Resistivity and Acoustic Velocity Revisited
AU: * Hacikoylu, P
EM: phacikoylu@stanford.edu
AF: Stanford Rock Physics Laboratory, Geophysics Department, Stanford, CA 94305-2215 United States
AU: Dvorkin, J P
EM: dvorkin@stanford.edu
AF: Stanford Rock Physics Laboratory, Geophysics Department, Stanford, CA 94305-2215 United States
AB: Seismic modeling at a well is essential to many impedance inversion methods as well as quality control for real seismic data. The three main inputs for seismic modeling are the P- and S-wave velocity and density. A common problem is poor quality of sonic, dipole, and density logs, or an absence of these curves in parts of a well or in older well data sets. As a result, attempts have been made to reconstruct these curves from more reliable measurements, such as resistivity. The earliest attempt is by Faust (1953) where both the velocity and resistivity are empirically related to the geologic age, depth, and lithology. From these two relations an equation follows that links the sonic velocity to the depth and formation factor, where the formation factor is the ratio of the resistivity of water-saturated rock to the resistivity of water. This relation between the resistivity and velocity does not have any apparent physical basis simply because the velocity depends on the elasticity of a material while the resistivity describes its electrical charge transport capability. The observed link is most likely due to the dependence of both material properties on porosity. We analyze this link by using recent rock physics transforms between the velocity, porosity, and mineralogy together with existing empirical (e.g., Archie) and theoretical (Hashin-Shtrikman bounds) resistivity-porosity models. We also use a number of high-quality lab and well data sets to verify the results. We find that Faust's equation is applicable to consolidated cemented sandstones with low clay content with porosity between 5 and 20 percent. It should not be used in shale or unconsolidated and/or uncemented rock. By using rock physics theory we derive a family of new resistivity-velocity equations appropriate for various textures of clastic sediment. Specifically, an analytical solution applicable to unconsolidated shale is a combination of the lower Hashin-Strikman bound for resistivity and the soft sand/shale model for velocity (the modified lower Hashin-Strikman elastic bound). Keywords: formation factor, acoustic velocity, rock physics relations.
DE: 5102 Acoustic properties
DE: 5109 Magnetic and electrical properties (0925)
DE: 5114 Permeability and porosity
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005