Drilling in geologically complex or exploratory environments can increase the risk of bottom-hole assemblies (BHA) becoming stuck, with possible loss of chemical sources downhole. Despite this risk the need for accurate logging-while-drilling (LWD) formation evaluation measurements still remains to fully evaluate reserves or as assurance logs.
Several years ago an electronic, switchable Pulse Neutron Generator (PNG) source was implemented in an LWD tool, enabling acquisition of neutron, capture spectroscopy and sigma, in addition to the industry's first radionuclide-source-free Neutron Gamma Density (SNGD).
Three years have passed since commercial introduction of the neutron gamma density measurement. Differing opinions have been voiced as to whether it provides a valid solution to the industry's formation evaluation requirements. This paper addresses the most common questions with lessons learned and examples acquired in the northern Gulf of Mexico and similar deepwater environments. PNG-based measurements, such as SNGD, elemental capture spectroscopy and formation sigma have been analyzed using comparisons with wireline-conveyed logs.
Radionuclide source-free formation evaluation offers a solid answer to industry demands. Extensive analysis of the industry's first neutron gamma density has defined the range of application. Complete formation evaluation is possible using state-of-the-art radionuclide source-free measurements acquired while drilling.
Radionuclide source-free formation evaluation has been discussed extensively in terms of health, safety and environment (HSE) advantages. The absence of a chemical source eliminates the associated risk in transport, handling, storage, abandonment, and disposal (Aitken et al, 2002). Cost of compliance with strict regulations, related to chemical sources, are also eliminated. In addition, operational efficiency is improved as equipment handling, such as loading and unloading sources on the rig floor, is reduced.
