By: Schlumberger

Avoiding chemical sources saves time and money logging a directional well in Egypt

New sourceless measurement capabilities have become available in a unique logging-while-drilling (LWD) service based on pulsed neutron generator (PNG) technology. The NeoScope formation evaluation while drilling service provides a comprehensive sourceless petrophysical description of the formation in the shortest multifunction LWD collar available. Eliminating the need for chemical sources reduces drilling risk and handling costs, and also avoids potential delays related to local permitting and regulations.

Nuclear logging

PNGs have been successfully used by the well logging industry to acquire a variety of measurements. This radioisotope-free source of high-energy neutrons has been in use for decades 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).

Radiation sources

Radioactive materials present risks to health, safety, and environment (HSE), 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 relating to compliance with local legislation.

Non-chemical alternative

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 well site 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 U.S. 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 (thermal nuclear capture cross-section) 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 they 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 they are a safe distance below the surface. The NeoScope LWD tool and its PNG can only be powered when the turbine from the measurement-while-drilling (MWD) system is energized by mud circulation. 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 removes the need to conform to stringent regulations in transportation and storage that can lead to delays. It also 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 resumed closer to the originally planned trajectory.

New sourceless neutron-gamma density measurements

Schlumberger has developed new technology that, for the first time, provides reliable SNGD measurements without the use of a chemical nuclear source. 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. Through 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 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 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 rate-of-penetration up to 200 ft/h while providing accurate formation evaluation measurements. High real-time data transmission rate, provided by the TeleScope high-speed telemetry-while-drilling service and its Orion II* data compression telemetry platform, ensures that the full suite of measurements is available in real time to improve decisions and mitigate risk.

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