In 1948 Well Surveys Inc., a small oilfield service company in Tulsa, Okla., began a difficult task: the design and construction of a pulsed-neutron logging tool that could locate and evaluate the oil-producing capacity of a reservoir in wells with casing in the borehole.

Completing this task took more than 12 years and required novel miniaturization techniques, clever and brilliant engineers, and plenty of testing. The result was the Neutron Lifetime Log service — the world’s first commercial pulsed-neutron logging tool service.

The goal for the tool was straightforward: direct a large number of highly energetic neutrons into a formation as far as possible, then, after the neutrons slowed down and were absorbed by the formation atoms, record the resulting gamma rays from the absorption process.

The tool at work
The typical logging run started by lowering the tool on a standard wireline into the borehole.

The pulsed-neutron logging tool, in its 60th year after the start of development, has become an essential part of the developer’s toolbox. (Photo courtesy of Baker Atlas)
When the desired depth was reached, a command from the surface powered the tool.
Additional commands caused the tool to generate huge numbers of neutrons in short pulses — some early estimates predicted about 100,000 neutrons per pulse. Each pulse lasted only 30 microseconds, (one microsecond = one millionth of a second), and the tool pulsed 1,000 times per second.

Many of the neutrons generated by the tool collided with atoms in the drill casing and the rock surface immediately surrounding the borehole. These collisions resulted in a shower of short-lived gamma rays (called “inelastic scattering”). This radiation, though, was not the type recorded by the tool.

Although many neutrons were stopped by the casing or the rock surface, a significant number of remaining neutrons penetrated into the formation. These neutrons contacted or passed by water and hydrocarbons molecules and atoms of the rock elements, reducing the neutron energy level substantially. From an initial energy level of 14 million electron volts as they left the tool, the neutron energy level quickly fell to about 0.025 electron volts.

When the neutrons reached such low energy levels, (called “thermal neutron levels”) the atoms in the rock formation quickly absorbed them and then gave off gamma rays.
This posed a challenge: How could the tool detect the gamma rays from thermal neutron absorption in a way that indicated the unique neutron absorption rate of the formation atoms? The answer was in timing and synchronization, controlled by electronics which — at the time — were very advanced.

Timing and synchronization
When the tool generated a pulse, the detector in the tool was not enabled. The electronics delayed the detector enabling until after a specific time had elapsed. This delay was called
the borehole die-away time. Previous calculations and experimentation revealed that waiting for a short time allowed most of the neutrons from the inelastic scattering to die away (to be absorbed) and enabled the gamma rays from the thermal neutron absorption to reach the detector.

After the tool emitted a neutron pulse, electronics within the device delayed enabling the detector for 400 microseconds. The detector collected gamma ray data for 200 microseconds, and then it was disabled. After another time interval of 100 microseconds, the detector was enabled again for another 200 microseconds, and then disabled before the next pulse.
This gating technique provided two measurement intervals. During the time between the intervals, the gamma ray quantities declined as the limited number of neutrons were absorbed. Data from the detector measurements enabled the determination of the gamma ray decline rate — a rate which was directly related to the absorption rate (the neutron lifetime) in the formation.

Calculations performed on the decline rate computed the neutron capture cross-section, or Sigma (S) of the formation. By comparing this to a list of known Sigma values of
elements, many formation elements and fluid (water) saturation levels were determined.

High-voltage power generator
The tool had more than 3,500 parts, mostly made from stainless steel, and three main components: the high-voltage power generator, the neutron generator and the gamma ray detector.

In the early US patent descriptions of neutron logging tools, the high-voltage power generator was quickly recognized as a crucial component. Only the constant, direct-current power it produced could drive the neutron generator. Fortunately, American industry had devices which these produced high-voltages: Van de Graaff generators.

The basic Van de Graaff generator design contained a high-speed electric motor which ran a continuous looped belt over two rollers. As the belt brushed against a comb of metal needles possessing a positive electric charge, the belt picked up the charge. Further along, the belt brushed against another set of needles that removed the charge, accumulated it and transferred it to an electrode. Increasing the electric motor speed increased the electric charge.

In the early 1950s the smallest available Van de Graaff generator was 4 ft (1.2 m) in diameter, 6 ft (1.83 m) long, and weighed several hundred pounds. The power generator on the tool had to fit inside a 31¼2-in.-diameter housing and then supply at least 125,000 volts continuously. No company had ever designed and built a generator to such unique and demanding requirements.

The assistance of specialists in power-generator design was sought. High Voltage Engineering Inc. in Cambridge, Mass., was contacted. This company had produced several Van de Graaff generators and led the industry in particle generation equipment for medical, scientific and military research.

Several months of design and testing work resulted in a functioning high-voltage power generator, which was the first of its kind.

Neutron generator
Proceeding in a parallel design path was the neutron generator. It also had to fit within a diameter of less than 31¼2 in. Wellbore temperatures of 300°F (148.7°C) and pressures of 20,000 psi were the anticipated conditions for the neutron generator during operation. In addition, the device had to generate precisely timed pulses of neutrons at a rate of 1,000 bursts per second.

The neutron generator was a vacuum tube containing special electrodes in a low-pressure atmosphere of deuterium gas. As high-voltage current passed through an electrode, it produced deuterium ions. Additional electrodes accelerated the ions toward a target — a titanium ring. The surface of the target ring contained a coating of tritium, a hydrogen isotope.

When the deuterium ions struck the target, the tritium atoms absorbed the deuterium ions and then emitted numerous high-energy neutrons.

Gamma ray detector
The gamma ray detector also presented design challenges. After performing research and experimentation, the decision was made to use crystals from existing detector technology. These crystals, called “scintillators,” gave off small flashes of light when hit by gamma rays. An additional device, a photomultiplier, was attached to the scintillator. This device magnified the intensity of the minute light flashes so they could be converted into electric signals and processed.

To make matters more challenging, available commercial scintillators could not withstand the high temperatures and pressures within the well bore. Experiments also revealed these devices became radioactive when exposed to radiation, resulting in detection errors.
Small, custom-made detectors from specially grown crystals along with new, stringent testing procedures and the use of transistorized controls produced detectors which could survive the rigors of the wellbore environment.

Assembly and testing
Several challenges appeared during the assembly and test of the tool. To simulate some of the wellbore conditions, much of the testing was performed in high-temperature ovens. The high-voltage generator frequently experienced belt failure during testing. Extreme heat made the belt so pliable that it slipped off the rollers. Carbon from high-voltage arcing
frequently accumulated on the belt, causing it to slip. To meet these challenges, a stringent maintenance program was performed by skilled workers.

The neutron generator assembly also required extreme measures to purge the device of impurities and contaminants. A custom-built vacuum chamber was used. After heating the assembly with an induction coil to white-hot temperatures, vacuum pumps attached to the chamber ran for more than 2 days to remove particles.

Final tool assembly necessitated heating the components to very high temperatures before welding to minimize contaminants.

Commercial release
These assembly and testing challenges were eventually overcome, and the first commercially available neutron lifetime log service was released by Dresser Atlas in 1963. The first commercial runs of the tool were made in Texas in that year.

Field operations proved that the tool provided accurate, quantitative measurement through drill casing, and it differentiated between oil-bearing and water-bearing zones in a wide variety of conditions. Logging measurements were made independently of the casing, the borehole size, the borehole fluid or the tool position in the borehole.

The tool also measured small changes in fluid saturation and in-situ changes of gas properties. Trapped and by-passed oil were also located. Some estimates concluded that millions of barrels were discovered by the use of the tool.