Radioactive sources provide the most accurate measurement of porosity, but their use is causing increasing concern.

Radioactive isotopes emitting ionizing radiation are used in a wide range of applications in the petroleum industry, from exploration and production to platforms to refineries. Downhole tools using such sources provide the most accurate estimate of porosity, arguably the most important petrophysical parameter. However, following the tragedy of 9/11, the use of radioactive sources in various industries has come under increased governmental scrutiny due primarily to their potential for radiological dispersal devices (RDD) [IAEA, 2002; USDOE, 2003]. Consequently, the regulatory regimes are being tightened, and industrial users of sources are being urged to develop alternatives. The petroleum industry is actively engaged in this dialog and is assessing alternatives. A recent paper surveyed in detail the use of radioactive sources in petroleum industry in general, the basis of associated radiation protection regimes, concerns raised, and potential for alternatives [Badruzzaman et al., 2009].
This schematic of a formation density tool

This schematic of a formation density tool shows the source well protected from the elements. (Adapted from Ellis, Well Logging from Earth Scientists, 1987).

Radioactive sources in E&P
Cs-137, used in formation density tools, and Am-241, used in the Am-Be source in neutron porosity tools, are the most significant radioactive isotopes in upstream [Ellis, 1987]. Density tools provide the porosity to within 1 porosity unit (pu), while NMR and acoustic tools, often suggested as alternatives, could have a 2 to more than 4 pu error. Neutron porosity tools determine the fluid-filled porosity and help identify the lithology. Two service companies also use the Am-Be sources in their capture spectroscopy tools to generate mineralogical information [Herron and Herron, 1996; Galford et al., 2009]. Density and neutron porosity data together provide the classic gas signature.
In addition, frac-pack monitoring tools are generally based on several hundred microcuries of Am-241, and the Am-Be source used in wireline neutron porosity tools has also been utilized for such monitoring.

Absence of nuclear log data would result in increased uncertainty in reserves estimates, an even greater uncertainty in the deliverability estimate, inaccurate lithology analysis, less desirable well placement, etc. For example, in a 30-pu reservoir with a nominal reserve of one billion barrels, the NMR and acoustic porosity uncertainties noted previously would lead to a reserves uncertainty of 66 million barrels to 148 million barrels if these were the only porosity measurements available. In contrast, the reserves uncertainty would be only 33 million barrels if density-based porosity was used. For lower porosities, the relative uncertainty would be much greater. Increased uncertainty in deliverability estimates would result in completing more uneconomic wells. In the Gulf of Mexico, completion costs could be US $120 MM. Increased uncertainty in lithology can lead to errors in well placement, increase environmental issues, and possibly compromise safety. Thus, non-nuclear alternatives are inadequate.

Well logging source safety and security
Nuclear logging tools are robustly built with almost no chance of radioactivity release under normal oilfield operations. Figure 1 displays the schematic of a wireline density tool. The Cs-137 source is in a glass matrix and is doubly-encapsulated in steel. The tool is designed to withstand pressures of more than 25 Kpsi and 150 deg C. The neutron porosity tool is designed with similar robustness. Logging-while-drilling (LWD) tools contain both the neutron and density sondes in the same assembly (Figure 2), but the Am-Be source in the LWD tool has half the radioactivity of its wireline counterpart. Since these sources are in the drill string, a special mechanism is used to secure the sources in place to prevent them from falling down the hole and being penetrated by the drill bit [Aitken et al., 2002].
International Atomic Energy Agency (IAEA) guidelines, European Union protocols, and national regulatory bodies prescribe standards for the handling of all radioactive sources [IAEA, 2003a; IAEA, 2004; IAEA, 2005; EU, 2009; NRC, 1987; NRC, 1991]. Government licenses are required for access, transport, and use of radioactive sources. In upstream operations, service companies are typically the licensees, while in refinery applications operators are generally the licensees.

Nuclear LWD tools

Nuclear LWD tools have a greater margin of error when measuring porosity. (Holenka et al., “Azimuthal Porosity While Drilling,” SPWLA, 36th Annual Logging Symposium, Paris, France, June 26-28, 1995, Paper BB.)

Protocols to handle a specific source are determined by the IAEA Categorization of sources based on type, the amount of radioactivity, the application and duration of exposure [IAEA, 2003], and overall health risk. Category 1 and 2 sources may cause death, whereas Category 5 sources may produce minor, temporary injuries. Table 1 shows that sources used in the petroleum industry range from Category 2 to Category 4, with sources used in well logging tools for reservoir characterization posing the most risk [Badruzzaman et al., 2009]. Strict security and safety protocols are used for storing and transporting sources. Special shielded containers such as those displayed in Figure 3 are used for transporting sources. Only authorized personnel using specific protocols can access sources.

Challenges with E&P sources
Despite robust design, regulatory safeguards, and the much lower level of radioactivity than that of sources used in many other industries such as the medical industry, well logging sources pose unique challenges and potential for misuse in an RDD. These sources are shipped around the world and are often used in remote locations. Approximately 9,000 well logging sources are in the field worldwide [Roughan, 2007]. No RDD incident has occurred, but in 2003 a logging tool source that was lost in Africa was found in Europe with an unclear trail [Guardian, 2003]. In 2009, a density source was stolen from a service company facility in South America by an ex-employee who was later apprehended and the source recovered [Rhodes, 2010]. In California in 2006, a stuck density source was breached downhole [NRC, 2006], requiring the appropriate disposal of contaminated mud, lost production, and implementation of a 300-yr long monitoring program by the operator [Badruzzaman et al., 2009].

Government and industry initiatives
Governments have instituted stricter control of sources. The US national source tracking system tracks the movement of Category 1 and 2 sources. Currently, 107 Am-Be sources are being tracked, and discussion has been underway to include Category 3 sources in the system [Browder, 2010].

table

Several radioactive isotopes are used in the petroleum industry. (Ci stands for Curie, the older unit of radioactivity generally used in the US. 1 Ci = 3.7 x 1010 disintegrations/sec of the radioisotope. The newer unit is Becquerel (Bq). 1 Bq = 2.703 x 10-11. Ci, or 1 GBq = 27.03 mCi.) (Figures courtesy of Chevron; table adapted from SPE paper 123593, 2009)

In 2009 the US Department of Energy (DOE) partnered with six petroleum service companies -- Baker-Hughes, Halliburton, Pathfinder, Schlumberger, Tucker, and Weatherford -- to develop a voluntary source security training document and video, “Security and Control of High-activity Well Logging Sources Guidelines” [Schwartzel, 2010]. Additional procedures for access to a source side or storage box were recommended, including two-person access as opposed to the current industry practice of access by a single authorized individual.

Until now, well logging source safety practices have been based on service company protocols. As a lesson learned from the lost- and breached-source incidents noted previously, a recent SPE paper recommended that operating companies adopt their own source safety and security protocols [Badruzzaman et al., 2009]. The protocols should include 1) documented procedures for both source receipt at the operator site and return to vendors, 2) a clear decision chain to handle source incidents, 3) designation of a key contact on source issues at the business unit level, and 4) training of appropriate operating company personnel. The key tenet on stuck sources must be that if a source cannot be retrieved using normal procedures, cement it in; DO NOT take extraordinary measures to retrieve it.

Alternative nuclear sources
Industry and governments are investigating the use of alternative sources that pose lower risk. The SPWLA Nuclear Logging Special Interest Group has been discussing the development of alternatives [Nuclear SIG, 2008]. Two generator-based neutron porosity tools, one wireline and one LWD, have been developed [Mills et al., 1988; Scott et al., 1994; Evans et al., 2000] as an alternative to Am-Be sources and are increasingly being used. A generator-based neutron spectroscopy tool was developed recently [Pemper et al., 2008]. In its 2008 report to US Congress, the National Academy of Sciences recommended consideration of 14-MeV neutron generators (which are switchable and can penetrate deeper) or the lower activity Cf-252 isotope as alternatives to the Am-Be source in neutron porosity tools by all service companies [NAS, 2008]. An LWD tool was developed using Cf-252 with neutron output comparable to that of an Am-Be source and three orders of magnitude lower radioactivity [Valant-Spaight et al., 2006]. National laboratories are developing novel generators which may be of interest to the petroleum industry in the future [PNNL, 2010].

Concerned only with the RDD potential, the National Academy did not recommend the replacement of the lower risk Cs-137 source used in density tools. However, the industry should explore alternatives because of the possibility of contamination from a source that is breached during drilling operations. A linear electron accelerator-based photon source borehole density device was built and tested in the early 1980s [King et al., 1987] but was not commercialized. Renewed research on such electronic photon sources for density is underway. In addition, pseudo density, obtainable from photons generated by high-energy neutrons interactions, has been proposed as an alternative by several investigators [Badruzzaman, 1998; Odom 1999]. A generator-based LWD tool can supply pseudo density [Evans et al., 2000], but the concept is complex, and considerable effort would be required to approach the accuracy of the direct photon-source density.

Special containers are used to transport radioactive well logging sources

Special containers are used to transport radioactive well logging sources (neutron, left, and density, right). (Original figure courtesy of Halliburton, adopted from SPE Paper 123593, 2009.)

Challenges and opportunities in source replacement
Efforts to replace sources face a number of technical, logistical, and financial challenges. Radiation generators require an external power source, have a finite life, and are prone to failure. The short half-life (2.65 yrs) of Cf-252 would require frequent replacement of the isotope and recalibration of the tool. Interpretation issues, including changed porosity and lithology sensitivity, would result from replacing current sources because of the physics differences. A physical swap of sources in a tool would be insufficient, and design and interpretation changes would be required [Xu et al., 2010; Fricke et al., 2008]. Despite the ability to calibrate and assess new nuclear tool designs by using computer simulation, considerable laboratory calibration and vendor field tests, followed by user field validation, would still be required and may not guarantee an acceptable performance [Badruzzaman, 2005]. Users would have to adapt to new calibration charts and possibly develop new correlations for the tool’s response to geology. Years of experience with a tool in a given field may be needed, especially if the physics is significantly different. Addressing the above challenges from design to user acceptance could take a decade or more.

Designing new tools using novel hardware would involve complex R&D and be expensive and long-term solutions. A 2010 Interagency Taskforce of the US government has recommended support for such R&D [NRC, 2010].
The above challenges to achieve lower risk nuclear logging tools are also an opportunity for a quantum leap in the technology. New tool concepts with appropriately calibrated novel sources (and detectors) could allow better mineralogy description and simultaneous interpretation of multiple petrophysical parameters [Badruzzaman et al., 2004; Odom et al., 2008; Pemper, 2008]. To expedite the process, a collaborative effort between service companies, operators, national laboratories, and universities is under discussion [PNNL, 2010].

Acknowledgement
The author thanks the following for the discussion on the topic: Eve Sprunt, Barry Reik, Simon Clinch, Jim Logan, Dapo Adeyemo, Larry O’Mahoney, Tom Zalan, Dale Julander, David Barnes, and late Don Seeburger (Chevron); Dale Fitz (ExxonMobil); Ray Wydrinski (BP); Rich Ostermeier (Shell); John Nieto (Canbriam Energy); Allen Gilchrist (Baker Hughes); Larry Jacobson, Gordon Moake, and Jerry Truax (Halliburton); Brad Roscoe, Chris Stoller, Bob Adolph, Mike Evans, RJ Radtke, Jim Hemingway and David Madio (Schlumberger); Ward Schultz, Cornelis Huiszoon, and Libai XU (Pathfinder); Don Oliver (Weatherford); Kate Roughan (QSA-Global); Ka-Ngo Leung (Lawrence Berkeley National Lab); Arlyn Antolak (Sandia National Laboratories); Jeffery Griffin (Pacific Northwest National Laboratory); Richard Liu (University of Houston); Robin Gardner (NC State); Michael Ward and Joe DeCicco (NRC); and Vilmos Friedrich (IAEA).

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