RIASSUNTO
Abstract
After several decades of use, resistivity, formation density and neutron porosity measurements are considered fundamental to formation evaluation. In the last decade, logging-while-drilling (LWD) formation density and neutron porosity tools utilizing chemical radioactive sources have been added to resistivity, gamma ray, directional and other drilling parameter tools run as part of the bottomhole assembly (BHA) during drilling. These drill-collar mounted formation evaluation tools are now safely and reliably delivering high-quality data.
Field experience with one such tool, an LWD formation density-neutron porosity tool with wireline-retrievable logging sources, is discussed. The paper gives statistics that show the success in retrieving sources from LWD tools in stuck bottomhole assemblies. A number of operational safety techniques have been enhanced, resulting in a reliable and safe system. A new source retrieval and communications tool significantly enhances the tool's operational efficiency.
In the unlikely event that downhole conditions prevent retrieval of the chemical radioactive sources from the LWD tool, safe, well-established and accepted abandonment procedures are given.
Introduction
Radioactive material is introduced into openhole (uncased) oil and gas wells in the form of sealed sources in various measuring instruments (logging tools) for the purpose of defining the lithology and fluid content of reservoirs. If tools containing radioactive sources become stuck in the hole, the operator is required, by regulation and the need to protect both the safety of operations and the environment, to make every effort to recover them.
Logging tools containing radioactive sources are most commonly conveyed on wireline cable. Similar tools, run as part of the BHA (bottomhole assembly) and performing logging measurements while drilling (LWD) have been recently introduced. While retrieving stuck wireline tools is routinely successful, retrieving a stuck BHA may present greater difficulties. The ability to recover radioactive sources from LWD tools stuck downhole makes it easy for the operator to fish for or abandon the BHA without regulatory or environmental problems.
One currently available nuclear LWD tool, the CDN* Compensated Neutron Density tool (which exists in 6 1/2-in. and 8-in. sizes), is equipped with retrievable logging sources. After retrieval of the logging sources, extremely small amounts of radioactive material are left in the tool in the form of reference sources for the measurement system, but these pose little regulatory problem in most countries outside the United States, and no significant environmental problem.
Nuclear tools in Well Logging
For more than 30 years, wireline well logging measurements using nuclear techniques have become fundamental to the evaluation of hydrocarbon reservoirs.
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