The Accuracy And Precision Of Femwd Density Tools

The accuracy of the density measurement is critical to formation evaluation. In order for FEMWD (Formation Evaluation Measurement While Drilling) technology to provide a true standalone formation evaluation service, a consistent and accurate density measurement must be provided, This consistency and accuracy must be demonstrated for more than a few ideal conditions. It must be shown for the full range of formations and muds encountered downhole. While the formations and mud systems encountered by FEMWD and wireline density tools are the same, there are significant operational, environmental and design differences between wireline and FEMWD density logging. FEMWD tools are usually run in boreholes that are in better condition than those available to wireline tools. The formation viewed by the FEMWD tools is routinely less invaded. FEMWD tools must be able to acquire accurate data whether rotating or sliding unlike the mode of acquisition of wireline density data, which is run only in sliding mode. With these facts in mind, a laboratory program was developed to fully characterize two density tool sizes that have recently undergone a significant upgrade in both precision and accuracy: the 6 ?" and the 8 ¬" FEMWD density. For this characterization, formations with densities ranging from 1.7 g/cc to 3.0 g/cc and Pe spanning 2.2 to 6.0 barns/electron are used. Water standoffs of 0" to 1/2", as well as mudcakes ranging from 1/8" to 3/8" thickness were used to determine the standoff response. The density and Fe in these mudcakes vary from 1.0 to 2.4 g/cc, and 0.3 to 120 barns/electron (b/c), respectively. Data were taken for individual standoffs and were also integrated over a range of standoffs in order to properly emulate downhole conditions. In addition to laboratory data, field data are required to verify the accuracy of any formation evaluation tool. In this paper, several field examples are used to demonstrate the tool accuracy. These data also span a wide range of formation conditions. Formation densities range from less than 2.00 g/cc to over 2.70 g/cc. Mud weights vary from about 1.2 g/cc to 1.9 g/cc. In addition, both sliding and rotating data are considered. This analysis includes consideration of both the accuracy and precision of the measurement. Accuracy is determined by comparison with other measurements such as wireline logs and cores. Precision will be determined by the consistency of the measurement. In this manner, the response of the tool is verified in fields under conditions that are representative of those encountered by the European formation evaluation community.