Do you really know Rt*? *True Formation Resistivity
in resistivity logging tool design over the last 40 years, it is well known
in the oil and gas industry that in complex formations traversed by a borehole,
the measured deep-resistivity or apparent resistivity Ra is not
equal to the true formation resistivity Rt.
Environmental effects and tool limitations
Distortions in resistivity log measurements are caused by various complex
environmental effects (borehole, drilling mud, shoulder beds, invasion,
anisotropy, formation dip) and also tool limitations (vertical resolution,
radial depth-of-investigation, nonlinear behavior).
These make interpretation and evaluation of formation resistivity a
hard job even for the most experienced log analyst.
Click image to enlarge
Limits of correction charts
Applying correction charts to resistivity logs may be a first step
towards an improvement of Rt estimation in very simple formation
models. But this leads commonly to inaccurate corrections for Rt
and therefore causes underestimation of hydrocarbon reserves.
The key to recover Rt
The most efficient way to overcome simultaneously all the environmental
effects and the tool limitations is resistivity inversion
processing involving iterative forward
modeling. This resistivity modeling technology is now becoming a key
step in petrophysical log interpretation, since it allows efficient quantitative
evaluation of properties in the most complex formations and borehole environments.
For hydrocarbon exploration, it is definitely a turning point in complex
borehole fluid invasion determination and accurate water saturation estimation.
Using this technology contributes to significantly improve oil saturation
evaluation, commonly up to 20% compared to conventional methods or approximated
Resistivity Modeling Technology allows:
- re-assessment of old resistivity logs,
- recovery of by-passed hydrocarbons,
- study of thin beds and fractures,
- validation of hypothetical earth models,
- analysis and extension of correction charts,
- pre-computation of a data base of tool responses,
- sensitivity studies of tool responses,
- understanding of anomalies and artifacts in logging data,
- understanding of logs in highly-deviated wells and dipping formations,
- planning and steering of horizontal wells.
|Copyright © 1998-2007 ATEMIS Technologies Sarl. All rights reserved.|