Pulsed neutron generator (PNG) technology has been successfully used by the well logging industry for years to acquire a variety of measurements. This radioisotope-free source of high-energy neutrons has been used in wireline-conveyed logging tools for various commonly used measurements such as neutron porosity, spectroscopy, and thermal neutron capture cross section (also known as sigma). These are all acquired without the need for chemical sources such as americium-beryllium. Recent developments have extended the suite of measurements to include a radioisotope-free bulk density measurement, eliminating the need for cesium sources. This density is referred to as the sourceless neutron-gamma density (SNGD).
Radioactive materials present HSE risks, whether through direct contamination or extended close contact with the human body. Abandonment of a chemical source downhole can present a potential environmental risk that can last hundreds or thousands of years. Government regulations rightly consider that the industrial use of these materials requires stringent control. Service companies apply tight controls and comprehensive training programs to ensure strict adherence to procedures that minimize operational risk. These are particularly important in LWD operations due to the severity of the drilling environment and because rig personnel often assemble the tools in the bottomhole assembly (BHA) onsite.
Several technologies have been implemented to minimize human exposure to radioactive materials and maximize the ability to safely retrieve sources when tools become stuck downhole. Nevertheless, the use of chemical sources inherently poses a risk, and the opportunity to entirely eliminate the need for them provides benefits not only for HSE risk reduction but also for improving operational efficiency and avoiding restrictions and delays related to compliance with local legislation.
A PNG can provide a safe alternative to chemical sources. A PNG is, in effect, a miniature particle generator producing neutrons by accelerating hydrogen ions across a minitron. PNGs do not emit any external radiation when not electrically energized, so there are no restrictions on wellsite manipulation of unpowered PNG-based tools. There is no need to take any special precautions when assembling the tool into a BHA and no operational delays for source loading and unloading. Authorities such as the US Nuclear Regulatory Commission have exempted them from any special precautions for abandonment in oilfield wells.
PNGs have been used in wireline tools since the late 1980s to provide sourceless openhole neutron porosity measurements, and the technology has subsequently been used to provide measurements of physical properties beyond those that can be made with chemical source-based tools. Neutrons are generated in carefully timed pulses, and the measurements are acquired in specific time-gates using an interlaced timing sequence, enabling measurement of sigma and spectroscopy (for elemental concentrations and formation mineralogy). These measurements are now available in both LWD and wireline applications.
The principal operational hazard of PNG-based tools is the emission of large numbers of high-energy neutrons when these are powered. Wireline tools have software locks on the power supply that can only be released by human control and also only when depth measurements confirm these are a safe distance below the surface. Schlumberger's NeoScope LWD tool and its PNG can only be powered when the turbine from the MWD system is energized by mud circulation (Figure 1). Therefore, the PNG generates neutrons on demand and ceases emission when turbine power stops. Additional hardware and software safety control mechanisms include pressure-controlled shutdown, a physical field neutron plug required to enable the PNG, passwords, and sequential logic. The combination of safety interlocks prevents accidental operation at the rig floor and ensures zero emissions if the tool has to be abandoned downhole.
The use of the sourceless LWD service reduces risk if tools are lost in-hole as chemical sources incur costly abandonment measures and require sidetracks to deviate a safe distance from the lost tool. With a PNG source, tools can be handled or abandoned without special safety considerations, and sidetracks can be resumed closer to the originally planned trajectory.
New sourceless neutron-gamma density measurements
Using the PNG and a suite of detectors, the NeoScope tool determines SNGD from the gamma rays induced by the interaction of high-energy neutrons with the formation. The SNGD can be used to replace the traditional gamma-gamma-density measurement. Throughout the past decade, extensive studies have characterized the SNGD measurement in a wide range of formations and environments. The new measurement enables a comprehensive sourceless petrophysical description of the formation using the shortest multifunction LWD tool available. A single 7.6-m (25-ft) collar provides azimuthal natural gamma ray; array resistivity; dual ultrasonic calipers; a full drilling mechanics package, including annular pressure and near-bit inclination; and sourceless PNG-based measurements such as capture spectroscopy, sigma, neutron porosity, and neutron-gamma density. All of these measurements are acquired within 5 m (16 ft) of the bottom of the tool, providing the full suite of data close to the bit.
This compact design reduces the amount of rathole by providing comprehensive formation evaluation measurements close to the bit. Collocation ensures that the measurements are acquired under the same environmental and formation invasion conditions, reducing uncertainty in data interpretation. Since there is only one collar, flat time associated with making up and breaking down the BHA is reduced. Having fewer connections also enhances BHA reliability. The service's large memory capacity allows the recording of two samples per foot at an ROP of up to 61 m/hr (200 ft/hr) while providing accurate formation evaluation measurements. High real-time data transmission rate, provided by a high-speed telemetry-while-drilling service and its data compression telemetry platform, ensures that the full suite of measurements is available in real time to improve decisions and mitigate risk.
Case study: Egypt
While running an exploration program to evaluate the West Kanayes concession in the Western Desert of Egypt, Apache Corp. encountered a zone with wellbore stability issues in a directional well. Wireline formation evaluation tools were able to log the upper target zone but were prevented from reaching the lower interval of interest. Deploying LWD tools was selected as the best alternative for evaluation of the complete interval; however, due to local permitting requirements, shipping chemical sources to the rig usually takes seven days. It was decided to use the sourceless LWD service to acquire a full suite of petrophysical measurements, primarily because it could be mobilized to the well site faster than a chemical nuclear source.
Using PNG-based neutron porosity and its unique SNGD measurement, the NeoScope service provided high-quality data over a 540-m (1,774-ft) interval, which included the oil-bearing sandstone reservoir section. The PNG-based measurements were compared to wirelineconveyed conventional chemical nuclear source-based neutron porosity and density measurements in the shallower section. The SNGD data and other LWD measurements compared favorably in the interval where both sets of data were available, providing confidence for use of the PNG-based measurements for formation evaluation over the entire interval logged with the NeoScope tool.
Laterolog resistivity measurements also were acquired, providing quantitative, high-resolution array resistivities in addition to high-quality images. The images were used to pick bedding planes and identify natural and drilling-induced fractures and faults.
The combined LWD services provided detailed formation evaluation across the full openhole section despite poor borehole conditions. In addition to eliminating the expected seven-day delay related to nuclear source transportation, the sourceless service also saved rig time related to loading and unloading of a chemical nuclear source.
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