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.
Logging while drilling (LWD) and wireline logs are important formation evaluation tools, and often form the bases for initial economic evaluation of a well. Recent improvements in LWD sensors have led to increased use of LWD, and even make it feasible for oil companies to replace wireline with LWD (Brami, 1993). In many cases LWD services can provide the same formation evaluation parameters as the wireline triple combo (resistivity density, neutron porosity, gamma ray, caliper); however. the logs are not directly interchangeable (Evans, 1991), and some times, differ significantly in the same well. These differences will also be present during multiwell studies, and the geoscientist must know if differences in log responses between two wells are from LWD-vs-wireline effects, or from a real difference in formation properties.
First, there are the differences in LWD and wireline sensor designs. Most of these result from the requirement that LWD tools must first function as drill collars, and then as logging tools. (The cost of failure in the former is far greater than the cost of failure in the latter.) These differences in sensor design can produce different logs under identical borehole and formation conditions.