Radiation Sources in Drilling Tools: Comprehensive Risk Analysis in the Design, Development and Operation of LWD Tools

Abstract

Abstract Nuclear logging while drilling (LWD) methods containing chemical sources of ionizing radiation have become commonplace in the past 15 years. However, introducing radioactive sources into a drillstring's bottomhole assembly (BHA) involves a level of risk greater than similar tools deployed on wireline. Such tools must meet acceptable standards not only in terms of performance, but environmental and personnel safety. In addition, stringent regulatory requirements are imposed on the transport, storage, handling, abandonment and eventual disposal of chemical radioactive sources. Comprehensive risk analysis has driven the development of two generations of nuclear LWD tools at one oilfield service company. We discuss these tools, their Health, Safety and Environmental (HSE) performance and the process of continuous improvement, both incrementally and in fundamental design changes, as risks were better quantified. Substituting a pulsed neutron generator (PNG) in place of chemical sources can eliminate many of the risks, while increasing the measurements available for petrophysical analysis. PNGs have been used successfully in wireline-deployed tools but, until recently, have not been sufficiently robust for LWD application. Prototype LWD tools are successfully proving the viability of a new generation of PNG. These PNG-based LWD tools open the door to an enhanced suite of LWD measurements without the HSE risks of chemical source-based tools. Introduction Radioactive materials present two distinct risks. The first is irradiation: the effect of the radiation on the body, where no direct contact with the material is involved. Irradiation is reduced by minimizing time of exposure, maximizing distance from the material and introducing shielding to cut the level of radiation. The second risk is contamination: radioactive material comes directly in contact with persons or the environment. It can then enter the body where the effects can be much more severe because of proximity to vital organs. Containing the material in sealed capsules, designed to withstand mechanical damage and corrosion, reduces the contamination risk. Here we discuss the deployment of sealed sources in logging tools. In normal operations, sealed sources present no contamination hazard. Only in abnormal circumstances, such as physical damage to the capsule causing a release of the source material, would any environmental contamination risk exist. Government regulators rightly consider that the industrial use of radioactive material requires stringent control. Public opinion is influenced by incidents such as Chernobyl and the common perception is that any activity involving radiation is prone to accidents with severe consequences. Trained service company personnel control operations with radioactive sources at wellsites. Other personnel, untrained in the specific risks and safety precautions involved, also work on these rigs. Therefore, it is essential that the service companies ensure that operations with radioactive sources are carried out with minimum risk. The best time to start the process of ensuring safe operations is in the tool design phase, where risk analysis can be applied and the tool system can be designed to maximize the HSE benefits. Sources of radiation have been deployed in wireline tools for approximately 50 years and safe systems for their use have evolved over time. The use of radioactive sources in LWD tools began much more recently. Because of the severity of the drilling environment and because rig personnel make up these tools in the BHA, a serious reevaluation of the risks involved was required. Comprehensive HSE risk analysis drove the company's LWD tool design process in parallel with the need for meaningful quantitative measurements. This process is described using data from the design and field use of two generations of commercial nuclear LWD tools.