Abstract

The main obstacle for a detailed study of a reservoir is the lack of geophysical and petrophysical information between producing wells. Crosshole seismic data can aid reservoir geologists and engineers in (1) estimating the volume of oil in place, (2) mapping permeability/porosity barriers, and ultimately (3) monitoring and designing enhanced oil-recovery experiments. Due to their high-frequency content, crosshole seismic data offer an ideal information link between conventional seismic data (surface reflection, VSPs, etc.) and full-waveform sonic acoustic logs.Using a weightdrop downhole source located at one depth level, acoustic and multicomponent data at two different wells were collected in the Paris basin with interwell distances of about 100 m. The target zone includes three sand reservoir levels between depths of 575 and 600 m. A 2-D elastic migration/inversion (M/I) of the scattered S-S and S-P crosshole field data produced high-resolution S-wave velocity and density depth images of the subsurface, extending information away from wells and identifying reservoirs. The residual energy reduction between synthetic seismograms derived from M/I images and field data is 18 percent, confirming that images contain elastic information. Structural dips obtained are very reasonable, the observed vertical spatial resolution being of the order of 3 m. We believe that this is the first time that such techniques have been applied to crosshole data. Elastic M/I images are generally better than elastic VSP-CDP and migration images and have the advantage of producing global quality measures of images. Such a technique uses as input a background velocity, e.g., a tomogram obtained by traveltime tomography, and complements the background by recovering subsurface discontinuities and changes in elastic parameters within the signal bandwidth.

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