Wireline logs and logging-while-drilling (LWD) logs are important pieces of the formation evaluation puzzle. They are often the basis for the initial economic evaluation of a prospect and serve as references for monitoring reservoir depletion. As the data from the LWD devices approaches wireline quality, it becomes feasible for oil companies to consider replacing wireline data with LWD data.
Unfortunately, these logs are not directly interchangeable. In addition to differences in basic tool responses, changes in logging environment between LWD and wireline logging runs can cause significant variation between similar measurements (resistivity, density, neutron, gamma ray) that affect subsequent reservoir evaluation. The importance of resolving these differences cannot be overemphasized. Small errors in petrophysical parameters can result in a multimillion dollar difference in the calculation of economic reserves.
This examination of the differences between LWD and wireline logs and consideration of how these differences affect formation evaluation focuses on the following topics:
Differences in sensor physics
Basic reference standards
Filtrate invasion effects
The outcome is a consistent procedure for making meaningful comparisons between these logs.