Is API Enough for Gamma Ray Logs, or Do We Need More?

Abstract

Abstract Gamma-ray logs are so widely used in the industry that they can easily be classified as "commodity" services provided by large and very small service companies. In addition, gamma-ray tools are designed and produced by many companies. The wireline and logging-while-drilling (LWD) variants of this equipment present a large variability of these tools, their types and services. Although the standardization effort through the "API" definition helped end the chaotic outlook in the gamma-ray log world, it did not do much to elevate gamma-ray logs from the qualitative world to the quantitative world. Frequent complaints are still expressed about wireline and LWD tools not agreeing, different size LWD tools measuring differently, and logs from different service companies providing different results. The major issue with gamma-ray logs is that the "API" definition is valid only if the tool is run in a 4.89-in. borehole filled with fresh water. Although all gamma-ray tools are supposed to provide the same results for such a well, in the real world there is no single one-size-fits-all concept. The measurements provided by tools characterized with the University of Houston (UH) GR pit standard will stray from a reading that can be relied on to be quantitatively correct. Although this is a well-known fact and there are environmental corrections available, no standard has been defined on how to develop the corrections. Service companies have their own internal correction approaches, resulting in a large variability from company to company. The gamma-ray logs obtained from different service companies are likely to differ from each other, even though they are corrected with their own correction algorithms. The gold standard in LWD gamma-ray logging is the agreement between LWD gamma-ray logs and wireline logs. However, under what conditions they should agree with each other is far from clear. Can one expect a 6¾-in. LWD tool to deliver comparable logs to a wireline tool when both are run in 8½-in. holes with heavy mud? There should be clear-cut definitions of the conditions under which gamma-ray logs are compared to each other to qualify the agreement between the LWD and wireline gamma-ray tools. In this paper we discuss the characterization process of the gamma-ray tools and how they behave in boreholes different than the UH GR characterization pit. Following that, we outline the proposals for developing gamma-ray correction strategies so that gamma-ray logs become quantitative logs rather than the qualitative logs of the past. This approach provides a second-level characterization of gamma-ray logs after the "API" standardization and provides insight for petrophysicists to understand the differences between the logs from different sources and to bring those logs together in a quantitative manner.