What’s new in exploration

 Vol. 232 No. 11

Neutron sources are a mainstay in the geophysical toolbox; neutron-based tools are used to determine several formation parameters, particularly porosity. For the last 70 years, the industry has relied on the alpha reaction to produce neutrons downhole, requiring the use of highly radioactive elements with potential radiation hazards for operators. Project cost can rise tremendously if a radionuclide source is lost downhole, requiring a well to be plugged and abandoned. For example, 426 sources were reported as lost downhole in Texas alone between 1956 and 2001, according to a 2010 report from the Pacific Northwest National Laboratory.

PNNL reviewed state-of-the-art nuclear and non-nuclear well logging methods, with a view toward radiogenic source replacement. Recent actions by the US Department of Energy (DOE) have reduced the availability of sealed, chemical isotope radiation sources in the US, heightening interest in alternative technologies.

Sealed sources. Long-lived radionuclides decay slowly, emit radiation slowly, and remain in the environment for many years. They are the basis for sealed sources used in current neutron tools.

Radium-beryllium (RaBe) was the original radioactive source used in well logging. Radium-226 has a half-life of 1,601 years, and the first byproduct of its alpha decay sequence is radon, a heavy gas that is harmful if ingested because it replaces calcium in bones. Americium (Am) is a synthetic, transuranic metallic element first produced in 1944 at the University of California, Berkeley, by bombarding plutonium with alpha particles. It is now produced from either uranium or plutonium in nuclear reactors, and its most common use is in smoke detectors.

Americium-beryllium (AmBe) has become the most common neutron source for well logging, and contains a mixture of americium oxide and beryllium metal powders compacted at high pressure. Americium acts as the alpha source, and beryllium produces neutrons.

Am-241 decays to neptunium-237 and emits alpha particles; daughter products emit gamma rays and neutrons. The long half-life of Am-241, 458 years, means there is a very slow decline in neutron production and the sealed source has a longer useful life.

According to Brian Jurczyk, president of Champaign, Illinois-based Starfire Industries, there are about 9,000 AmBe units with capacity under 16 Ci (curie, a unit of decay rate) used in well logging operations in the US. Small well logging tools range from 3 Ci to 5 Ci each.

Sealed-source manufacturers used to obtain americium from DOE laboratories, but the US government is no longer producing it. Currently, there is only a single supplier of new AmBe neutron well logging sealed sources, QSA Global, which uses americium of Russian origin.

Exclusive reliance on imported americium sourced through a single vendor is a major drawback to continued use of this technology. Fortunately, there are other ways to generate neutrons.

Spontaneous fission. Californium-252 is a radioactive neutron source produced in high-energy research reactors. Most is produced in the High Flux Irradiation Reactor at Oak Ridge National Laboratory (ORNL), at an annual cost of about $6 million. Cf-252 is also produced at the Research Institute of Atomic Reactors (RIAR) in Dimitrovgrad, Russia, and used in Russia and China.

Spontaneous fission of Cf-252 is several orders of magnitude more efficient at neutron production than AmBe sources. This element is more readily available, and at lower cost than AmBe sources. However, Cf-252 has a short half-life of 2.645 years, requiring a steady supply.

The transuranic test program at Oak Ridge ended a few years ago, but Cf-252 production continues under private funding. In May 2009, a consortium of source manufacturers agreed to fund the Cf-252 program at ORNL through 2012, according to the Nuclear Energy Institute.

Plasma generators. The most promising sea change is sealed-tube plasma neutron generation, sometimes referred to as “electronic” or “accelerator” neutron sources. Reluctance to move to this new technology may actually be rooted in the fear of losing the extensive interpretation experience with, and investment in, conventional neutron logging tools.

There are two fusion processes useful for neutron generation: D-D (deuterium-deuterium) and D-T (deuterium-tritium). Existing neutron generators use the D-T process; Schlumberger and Halliburton each offer a form of D-T accelerator-based neutron tool for logging while drilling. However, the technology has limits for AmBe replacement due to the very high (14.1 MeV) neutron energy.

At the Oil & Gas Innovation Center’s 2011 Innovation Showcase held in The Woodlands, Texas, last month, Starfire Industries’ Jurczyk discussed a new neutron generation system that is potentially more efficient and lower in cost, enabling a 2.5-MeV D-D neutron reaction for porosity measurements. The nGen plasma neutron generator output is 10 million D-D neutrons/sec. at 100 W, equivalent to a 4-Ci AmBe source, but it contains no radioactive materials, therefore avoiding export and supply restrictions as well as regulatory uncertainty. It is designed to operate at 150°C and 20,000 psi and produces low-energy neutrons (2.5 MeV) that are similar to AmBe-generated neutrons (4.2 MeV average). High-temperature testing will be completed in mid-2012.

The takeaway is that AmBe chemical neutron sources are not truly “irreplaceable.” Innovative designs are already at hand and, with sufficient investment, the industry can further develop and commercialize tools to efficiently log wells without using radionuclides.