S. F. Crary and D. J. Smith
Abstract
A common problem facing log analysts is accurately correcting resistivity logs for environmental effects. Much of the development of these corrections depends upon forward modeling. The commercial availability of forward modeling software, allows one to reverse this process and verify environmental correction routines by modeling specific field situations.
Laterolog devices have the same three environmental effects as other resistivity logs: borehole, shoulder, and invasion. A basic assumption in any correction scheme is that these effects are separable and can be made sequentially. Correction routines for shoulder effect on the laterolog tool include additional assumptions, since unlike the latest induction devices there are no additional measurements used to make the correction. These assumptions are reviewed and forward modeling is used to provide an estimate of the magnitude of these effects for various geometries and expose those situations where simplifying assumptions lead to erroneous results.
The technique employed to validate the environmental corrections consists of generating theoretical logs from a forward model, processing these results with the standard environmental correction routines, and then comparing the corrected results with the original model. In the simple case of symmetrical beds the situations studied are squeeze, in which the shoulder beds are more resistive than the center bed, and antisqueeze the opposite condition. Results are presented for a range of bed thicknesses from 30 ft to 2 ft, and resistivity contrasts from 10:1 to 1000:1 in decade steps. This analysis is extended to a more complex case taken from an actual field example that demonstrates how borehole and shoulder effects may lead to difficulties with standard correction routines.